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我有一个梦想

设计模式

随笔 2021/05/31

本文于 909 天之前发表，文中内容可能已经过时。

[TOC]

单例模式

定义

确保某个类中只有一个实例，而且子性实例化并向整个系统提供整个实例

例子

饿汉：
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public class Singleton {
    //在静态初始化器中创建单例实例，这段代码保证了线程安全
    private static final Singleton instance = new Singleton();
    //Singleton类只有一个构造方法并且是被private修饰的，所以用户无法通过new方法创建该对象实例
    private Singleton(){}
    public static Singleton getInstance(){
        return instance;
    }
}
懒汉：
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public class Singleton {
    private static Singleton instance;
    private Singleton (){}
    //没有加入synchronized关键字的版本是线程不安全的
    public static synchronized  Singleton getInstance() {
        //判断当前单例是否已经存在，若存在则返回，不存在则再建立单例
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
双重校验锁（DCL）：
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public class Singleton {
    private static volatile Singleton instance;
    private Singleton() { }
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class){
                if(instance == null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}
  1. 双重判断的原因？
    • 第一个判断减少锁的使用，提升性能
    • 多个线程同时等待锁，当第一个创建后，就不需要其他线程重复重建
  2. volatile的理解
    • 禁止重排序导致instance获取失败。
      • 给Singleton的实例分配内存
      • 调用构造函数，初始化成员
      • 将instance对象指向分配的内存空间

        执行顺序有可能是1-2-3，1-3-2

如果是1-3-2，new Singleton()方法执行时可能导致分配了空间，并指向了内存空间，但是没有赋值，这样另一个线程拿到后会导致出错

缺点：部分情况下，这种单例模式会失效

静态内部类：
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public class Singleton {
    private Singleton() { }
    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }
    /**
     * 静态内部类
     */
    private static class SingletonHolder{
        private static final Singleton instance = new Singleton();
    }
}

首次加载Singleton不会初始化instance，当调用getInstance方法时，初始化SingletonHolder类，这样虚拟机加载该类是线程安全的，保证单例对象的唯一性

枚举单例
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public enum SingletonEnum{
    INSTANCE;
    public void doSomething(){
        ......
    }
}

注意：在反序列化时，会重新创建对象，不符合单例

容器单例
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public class SingletonManager{
    private static Map<String,Object> objMap = new HashMap<>();
    private SingletonManager(){}
    public static void registerService(String key,Object instance){
        if(!objMap.containsKey(key)){
            objMap.put(key,instance);
        }
    }
    public static Object getValue(String key){
        return objMap.get(Key);
    }
}

对单例内容统一管理，放置在map中，但是HashMap线程不安全，所以最好使用ConcurrentHashMap，管理多个单例类

源码应用

  1. LayoutInflater.from(context)
  • 进入main函数
  • 新建ActivityThread，调用attach函数
  • AMS通信最终调用ahndlelauncherActivity
  • 创建Activity
  • 创建Application
  • 获取Context对象
  • 将context对象attach到activity中
  • 调用Activity的onCreate方法

在虚拟机第一次加载ContextImpl时会注册LayoutInflater Service，将这些服务存储在HashMap中，下次直接从缓存中读取，应用的是容器单例形式。

  1. 深入LayoutInflater
    LayoutInflater是抽象类，具体实现类是PhoneLayoutInflater

inflate方法：

  • 解析xml根标签（父布局）
  • 判断merge，如果是merge直接将子布局添加到根布局
  • 如果是普通标签，调用createViewFromTag对该元素进行解析
  • 解析所有子view，将这些子view都添加到根布局temp下
  • 返回解析到根视图

q：为什么自定义view要使用全包名，而内置view（TextView）就不用?

a：因为在布局加载过程中，如果是内置view，直接执行onCreateView方法创建view，并将“android.widget.”加在内置view前（android.widget.TextView），然后执行createView方法。如果是自定义view，从xml中获取全量包名直接执行createView方法即可，默认前缀为null

inflate通过深度优先遍历来构造视图树

优缺点

优点：

  1. 减少内存开支，避免创建和销毁的性能损耗
  2. 减少性能开销，永久驻留内存方式初始化复杂对象和依赖
  3. 避免对同一资源的多重占用，例如-个写文件操作，只有一个实例可以写，避免对这一个资源文件的同时写操作（避免线程不安全）
  4. 设置全局访问点，优化和共享资源访问，便于管理

缺点：

  1. 拓展困难，只能修改代码
  2. 如果持有context对象，会造成内存泄漏

建造者模式

例子

将一个复杂对象的构建与他的表示分离，是的同样的构建过程可以创建不同的表示

核心思想是添加建造者构建主类，减少主类功能，建造者只负责构建主类

简单例子

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public class Computer {
    //显示器
    String display;
    //cpu型号
    String cpu;
    //主板型号
    String mainBoard;
    //显卡型号
    String gpu;

    public Computer(Builder builder) {
        this.display = builder.display;
        this.cpu = builder.cpu;
        this.mainBoard = builder.mainBoard;
        this.gpu = builder.gpu;
    }

    @Override
    public String toString() {
        return "显示器是" + display + "\ncpu是" + cpu + "\n主板是" + mainBoard + "\n显卡是" + gpu;
    }

    static class Builder{
        //显示器
        String display;
        //cpu型号
        String cpu;
        //主板型号
        String mainBoard;
        //显卡型号
        String gpu;

        public Builder(){
            this.display = "三星曲屏";
            this.cpu = "i5 8400";
            this.mainBoard = "华硕Z360-B";
            this.gpu = "GTX 1050Ti";
        }

        public Builder setDisplay(String display) {
            this.display = display;
            return this;
        }

        public Builder setcpu(String cpu) {
            this.cpu = cpu;
            return this;
        }

        public Builder setMainBoard(String mainBoard) {
            this.mainBoard = mainBoard;
            return this;
        }

        public Builder setGpu(String gpu) {
            this.gpu = gpu;
            return this;
        }

        public Computer build(){
            return new Computer(this);
        }
    }

    public static void main(String args[]){
        Computer computer = new Computer.Builder()
                .setcpu("i9 4700u")
                .setGpu("GTX 2060Ti")
                .setMainBoard("华硕Z480")
                .build();
        System.out.println(computer.toString());
    }
}

源码应用

  1. AlertDialog
    内部调用Buidler模式构建，最终通过WindowManager显示在手机屏幕上。

  2. WindowManager源码分析
    所有需要显示到屏幕上的内容都是通过WindowManager来操作的，WM的一个非常重要的子系统为WMS（WindowManagerService）
    在android.app.SystemServiceRegistry中：

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registerService(Context.WINDOW_SERVICE, WindowManager.class,
               new CachedServiceFetcher<WindowManager>() {
           @Override
           public WindowManager createService(ContextImpl ctx) {
               return new WindowManagerImpl(ctx);
           }});

WMS也是初始化的众多服务的一种，所以在ContentImpl中初始化,并将初始化的服务注册到一个map中，需要时通过键获取调用，属于单例模式的容器单例。

以PopupWindow为例，我们获取到popupWindow的初始化：

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mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);

通过以上方法获取WM，显示时使用showAtLocation()方法，
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public void showAtLocation(IBinder token, int gravity, int x, int y) {
   ......
    invokePopup(p);
}

 private void invokePopup(WindowManager.LayoutParams p) {
    ......
    mWindowManager.addView(decorView, p);
    ......
}

通过一系列的调用，最终使用WM的addView方法添加布局，回溯之前的分析WM的实例是WindowManagerImpl，进入addView方法

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@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyDefaultToken(params);
    mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}

而在WMImpl方法中调用WMGlobal.addView方法，进入：
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public void addView(View view, ViewGroup.LayoutParams params,
           Display display, Window parentWindow) {
       ......
       
       ViewRootImpl root;
       View panelParentView = null;

       ......
       //构建ViewRootImpl
       root = new ViewRootImpl(view.getContext(), display);
       //view设置参数
       view.setLayoutParams(wparams);
       
       //将view添加到列表
       mViews.add(view);
       //将ViewRootImpl添加到列表
       mRoots.add(root);
       //将参数添加在列表
       mParams.add(wparams);

       // do this last because it fires off messages to start doingthings
       try {
           //调用viewRootImpl的setView方法添加布局
           root.setView(view, wparams, panelParentView);
       } catch (RuntimeException e) {
           // BadTokenException or InvalidDisplayException, clean up.
           if (index >= 0) {
               removeViewLocked(index, true);
           }
           throw e;
       }
   }

  • 构建ViewRootImpl
  • 将布局参数设置给view
  • 存储ViewRootImpl、View、LayoutParam到列表
  • 调用ViewRootImpl.setView将view显示在窗口

进一步查看ViewRootImpl是什么？

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public ViewRootImpl(Context context, Display display) {
      mContext = context;
      //获取WindowSession，也就是和WMS建立连接
      mWindowSession = WindowManagerGlobal.getWindowSession();
      ......
      //创建线程为当前线程，因为主界面在UI线程，所以在子线程更新UI会抛出异常，但并不是只用UI线程才能更新UI
      mThread = Thread.currentThread();
      ......
  }

进入WMGlobal.getWindowSession方法：

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public static IWindowSession getWindowSession() {
    synchronized (WindowManagerGlobal.class) {
        if (sWindowSession == null) {
            try {
                InputMethodManager imm = InputMethodManager.getInstance();
                //获取WMS
                IWindowManager windowManager = getWindowManagerService();
                //建立一个WindowSession
                sWindowSession = windowManager.openSession(
                        new IWindowSessionCallback.Stub() {
                            @Override
                            public void onAnimatorScaleChanged(float scale) {
                                ValueAnimator.setDurationScale(scale);
                            }
                        },
                        imm.getClient(), imm.getInputContext());
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        }
        return sWindowSession;
    }
}

进入getWindowManagerService，
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public static IWindowManager getWindowManagerService() {
    synchronized (WindowManagerGlobal.class) {
        if (sWindowManagerService == null) {
            sWindowManagerService = IWindowManager.Stub.asInterface(
                    ServiceManager.getService("window"));
            try {
                if (sWindowManagerService != null) {
                    ValueAnimator.setDurationScale(
                            sWindowManagerService.getCurrentAnimatorScale());
                }
            } catch (RemoteException e) {
                throw e.rethrowFromSystemServer();
            }
        }
        return sWindowManagerService;
    }
}

通过ServiceManager.getService()获取WMS，
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public static IBinder getService(String name) {
    try {
        IBinder service = sCache.get(name);
        if (service != null) {
            return service;
        } else {
            return Binder.allowBlocking(getIServiceManager().getService(name));
        }
    } catch (RemoteException e) {
        Log.e(TAG, "error in getService", e);
    }
    return null;
}

通过sCache获取到一个IBinder对象，所以FrameWork与WMS之间也是通过Binder机制进行通信的，回溯到之前，会调用IWindowManager.Stub.asInterface()，将IBinder对象转化为WM对象，然后通过openSession与WMS建立通信绘画，之后通过这个session进行交换信息，但是到现在还是没有显示view，所以继续走ViewRootImpl.setView方法，
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public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
    synchronized (this) {
        if (mView == null) {
            ......
            //请求布局
            requestLayout();
            ......
            try {
                //通过Windowsession和WMS通信，请求显示
                res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
                        getHostVisibility(), mDisplay.getDisplayId(),
                        mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
                        mAttachInfo.mOutsets, mInputChannel);
            } 
    }
}

  • requestlayout：请求布局
  • 向WMS请求显示布局
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@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        //线程校验
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }
}

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        //开启屏障消息，优先处理
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        //通过handler回调执行MTraversalRunnable
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

通过handler回调执行MTraversalRunnable，

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final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

        if (mProfile) {
            Debug.startMethodTracing("ViewAncestor");
        }
        //view的具体操作
        performTraversals();

        if (mProfile) {
            Debug.stopMethodTracing();
            mProfile = false;
        }
    }
}

最终进入performTraversals进行具体操作

  1. 获取Surface对象，用于图形绘制
  2. performMeasure函数，测量整个视图树各个view的大小
  3. performLayout,布局整个view
  4. performDraw，绘制整个view

优缺点

优点：

  • 良好的封装性，使客户端不知道产品内部组成细节
  • 独立，容易拓展

缺点：

  • 产生多余Builder对象和Director对象，消耗内存

原型模式

定义

用原型实例指定创建对象的种类，并通过拷贝这些原型创建新的对象

简单例子

深拷贝和浅拷贝

原型模式实际上是浅拷贝，也称为影子拷贝。拷贝实际不是将所有字段重新构造一份，而是拷贝文档的字段引用原始文档的字段而已，所以会导致修改副本的字段，原始字段也会跟着修改，因为最终修改的是同一个内存单元，所以在原型模式中，要尽量使用深拷贝。

  • 深拷贝：复制引用对象的值
  • 浅拷贝：复制引用

源码分析

  1. ArrayList的clone方法实现

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    public Object clone() {
        try {
            //克隆自身
            ArrayList<?> v = (ArrayList<?>) super.clone();
            //克隆数组对象
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    克隆自身后，在克隆数组对象，并没有对size进行克隆是因为size是值类型，并不是引用类型。

  2. Intent的clone方法分析

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    @Override
    public Object clone() {
        return new Intent(this);
    }

    public Intent(Intent o) {
        this(o, COPY_MODE_ALL);
    }

    private Intent(Intent o, @CopyMode int copyMode) {
        this.mAction = o.mAction;
        this.mData = o.mData;
        this.mType = o.mType;
        this.mPackage = o.mPackage;
        this.mComponent = o.mComponent;

        if (o.mCategories != null) {
            this.mCategories = new ArraySet<>(o.mCategories);
        }

        if (copyMode != COPY_MODE_FILTER) {
            this.mFlags = o.mFlags;
            this.mContentUserHint = o.mContentUserHint;
            this.mLaunchToken = o.mLaunchToken;
            if (o.mSourceBounds != null) {
                this.mSourceBounds = new Rect(o.mSourceBounds);
            }
            if (o.mSelector != null) {
                this.mSelector = new Intent(o.mSelector);
            }

            if (copyMode != COPY_MODE_HISTORY) {
                if (o.mExtras != null) {
                    this.mExtras = new Bundle(o.mExtras);
                }
                if (o.mClipData != null) {
                    this.mClipData = new ClipData(o.mClipData);
                }
            } else {
                if (o.mExtras != null && !o.mExtras.maybeIsEmpty()) {
                    this.mExtras = Bundle.STRIPPED;
                }

                // Also set "stripped" clip data when we ever log mClipData in the (broadcast)
                // history.
            }
        }
    }

在Intent的clone方法中直接使用new方法构建了新intent

所以延伸出一个问题，new和clone怎么选择？

如果对象的构造成本太高或者构造比较麻烦，那么使用clone函数效率高，否则使用new

  1. intent的查找和匹配

intent是怎么查找对应的组件然后跳转的呢？

在ContextImpl初始化的SystemServiceRegistry中初始化各种服务（WMS,AMS等），其中，初始化了PMS（PackageManagerService）

在ContextImpl中执行getPackageManager方法：

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@Override
public PackageManager getPackageManager() {
    if (mPackageManager != null) {
        return mPackageManager;
    }

    IPackageManager pm = ActivityThread.getPackageManager();
    if (pm != null) {
        // Doesn't matter if we make more than one instance.
        return (mPackageManager = new ApplicationPackageManager(this, pm));
    }

    return null;
}

进入ActivityThread.getPackageManager()
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public static IPackageManager getPackageManager() {
    if (sPackageManager != null) {
        //Slog.v("PackageManager", "returning cur default = " + sPackageManager);
        return sPackageManager;
    }
    //获取PackageManagerService
    IBinder b = ServiceManager.getService("package");
    //Slog.v("PackageManager", "default service binder = " + b);
    sPackageManager = IPackageManager.Stub.asInterface(b);
    //Slog.v("PackageManager", "default service = " + sPackageManager);
    return sPackageManager;
}

获取到PackageManagerService服务，这个服务是在系统初始化时通过SystemServer启动的，现在只是获取，查看PackageManagerService类：

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//PMS为IPackageManager.Stub的具体实现类
public class PackageManagerService extends IPackageManager.Stub
    implements PackageSender {
        public PackageManagerService(Context context, Installer installer,boolean factoryTest, boolean onlyCore) {
        // Collect privileged system packages.
        // 扫描特殊系统包
        final File privilegedAppDir = new File(Environment.getRootDirectory(), "priv-app");
        scanDirTracedLI(privilegedAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM
                | SCAN_AS_PRIVILEGED,
                0);

        // Collect ordinary system packages.
        // 扫描普通系统包
        final File systemAppDir = new File(Environment.getRootDirectory(), "app");
        scanDirTracedLI(systemAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM,
                0);

        // Collect privileged vendor packages.
        // 扫描特殊vendor下包
        File privilegedVendorAppDir = new File(Environment.getVendorDirectory(), "priv-app");
        try {
            privilegedVendorAppDir = privilegedVendorAppDir.getCanonicalFile();
        } catch (IOException e) {
            // failed to look up canonical path, continue with original one
        }
        scanDirTracedLI(privilegedVendorAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM
                | SCAN_AS_VENDOR
                | SCAN_AS_PRIVILEGED,
                0);

        // Collect ordinary vendor packages.
        // 扫描普通vendor下包
        File vendorAppDir = new File(Environment.getVendorDirectory(), "app");
        try {
            vendorAppDir = vendorAppDir.getCanonicalFile();
        } catch (IOException e) {
            // failed to look up canonical path, continue with original one
        }
        scanDirTracedLI(vendorAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM
                | SCAN_AS_VENDOR,
                0);

        // Collect privileged odm packages. /odm is another vendor partition
        // other than /vendor.
        // 扫描特殊odm下包/odm时vendor的一部分
        File privilegedOdmAppDir = new File(Environment.getOdmDirectory(),
                    "priv-app");
        try {
            privilegedOdmAppDir = privilegedOdmAppDir.getCanonicalFile();
        } catch (IOException e) {
            // failed to look up canonical path, continue with original one
        }
        scanDirTracedLI(privilegedOdmAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM
                | SCAN_AS_VENDOR
                | SCAN_AS_PRIVILEGED,
                0);

        // Collect ordinary odm packages. /odm is another vendor partition
        // other than /vendor.
        // 扫描普通odm下包/odm时vendor的一部分
        File odmAppDir = new File(Environment.getOdmDirectory(), "app");
        try {
            odmAppDir = odmAppDir.getCanonicalFile();
        } catch (IOException e) {
            // failed to look up canonical path, continue with original one
        }
        scanDirTracedLI(odmAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM
                | SCAN_AS_VENDOR,
                0);

        // Collect all OEM packages.
        // 扫描所有OEM包
        final File oemAppDir = new File(Environment.getOemDirectory(), "app");
        scanDirTracedLI(oemAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM
                | SCAN_AS_OEM,
                0);

        // Collected privileged product packages.
        // 扫描特殊product包
        File privilegedProductAppDir = new File(Environment.getProductDirectory(), "priv-app");
        try {
            privilegedProductAppDir = privilegedProductAppDir.getCanonicalFile();
        } catch (IOException e) {
            // failed to look up canonical path, continue with original one
        }
        scanDirTracedLI(privilegedProductAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM
                | SCAN_AS_PRODUCT
                | SCAN_AS_PRIVILEGED,
                0);

        // Collect ordinary product packages.
         // 扫描普通product包
        File productAppDir = new File(Environment.getProductDirectory(), "app");
        try {
            productAppDir = productAppDir.getCanonicalFile();
        } catch (IOException e) {
            // failed to look up canonical path, continue with original one
        }
        scanDirTracedLI(productAppDir,
                mDefParseFlags
                | PackageParser.PARSE_IS_SYSTEM_DIR,
                scanFlags
                | SCAN_AS_SYSTEM
                | SCAN_AS_PRODUCT,
                0);
        }
}

PMS会加载一系列，在不同目录下的包执行scanDirTracedLI方法：
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 private void scanDirTracedLI(File dir, final int parseFlags, int scanFlags, long currentTime) {
    Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "scanDir [" + dir.getAbsolutePath() + "]");
    try {
        scanDirLI(dir, parseFlags, scanFlags, currentTime);
    } finally {
        Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);
    }
}

private void scanDirLI(File dir, int parseFlags, int scanFlags, long currentTime) {
    //获取目录下所有文件
    final File[] files = dir.listFiles();
    ......
    for (File file : files) {
        final boolean isPackage = (isApkFile(file) || file.isDirectory())
                && !PackageInstallerService.isStageName(file.getName());
        if (!isPackage) {
            // Ignore entries which are not packages
            continue;
        }
        parallelPackageParser.submit(file, parseFlags);
        fileCount++;
    }
        ......
        for (; fileCount > 0; fileCount--) {
            scanPackageLI(parseResult.pkg, parseResult.scanFile, parseFlags, scanFlags,
                            currentTime, null);
        }
        ......
    }
}

扫描目录下的子目录，对apk文件进行解析

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public void submit(File scanFile, int parseFlags) {
       mService.submit(() -> {
           ParseResult pr = new ParseResult();
           Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "parallel parsePackage [" + scanFile + "]");
           try {
               //创建一个包解析器
               PackageParser pp = new PackageParser();
               pp.setSeparateProcesses(mSeparateProcesses);
               pp.setOnlyCoreApps(mOnlyCore);
               pp.setDisplayMetrics(mMetrics);
               pp.setCacheDir(mCacheDir);
               pp.setCallback(mPackageParserCallback);
               pr.scanFile = scanFile;
               //执行apk包解析
               pr.pkg = parsePackage(pp, scanFile, parseFlags);
           } catch (Throwable e) {
               pr.throwable = e;
           } finally {
               Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);
           }
           try {
               mQueue.put(pr);
           } catch (InterruptedException e) {
               Thread.currentThread().interrupt();
               // Propagate result to callers of take().
               // This is helpful to prevent main thread from getting stuck waiting on
               // ParallelPackageParser to finish in case of interruption
               mInterruptedInThread = Thread.currentThread().getName();
           }
       });
   }

   @VisibleForTesting
   protected PackageParser.Package parsePackage(PackageParser packageParser, File scanFile,
           int parseFlags) throws PackageParser.PackageParserException {
       return packageParser.parsePackage(scanFile, parseFlags, true /* useCaches */);
   }

创建一个PackageParser，调用parsePackage函数解析apk

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public Package parsePackage(File packageFile, int flags, boolean useCaches)
        throws PackageParserException {
    Package parsed = useCaches ? getCachedResult(packageFile, flags) : null;
    if (parsed != null) {
        return parsed;
    }
    //如果是文件夹类型
    if (packageFile.isDirectory()) {
        parsed = parseClusterPackage(packageFile, flags);
    } else {
        //解析单个apk
        parsed = parseMonolithicPackage(packageFile, flags);
    }

    cacheResult(packageFile, flags, parsed);

    return parsed;
}

如果是文件夹类型，加载多个apk文件，如果是单个apk，只加载一个apk

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@Deprecated
    public Package parseMonolithicPackage(File apkFile, int flags) throws PackageParserException {
        final AssetManager assets = newConfiguredAssetManager();
        final PackageLite lite = parseMonolithicPackageLite(apkFile, flags);
        if (mOnlyCoreApps) {
            if (!lite.coreApp) {
                throw new PackageParserException(INSTALL_PARSE_FAILED_MANIFEST_MALFORMED,
                        "Not a coreApp: " + apkFile);
            }
        }

        try {
            //执行具体解析
            final Package pkg = parseBaseApk(apkFile, assets, flags);
            pkg.setCodePath(apkFile.getAbsolutePath());
            pkg.setUse32bitAbi(lite.use32bitAbi);
            return pkg;
        } finally {
            IoUtils.closeQuietly(assets);
        }
    }

进入parseBaseApk:
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private Package parseBaseApk(File apkFile, AssetManager assets, int flags)
           throws PackageParserException {
       //获取apk路径
       final String apkPath = apkFile.getAbsolutePath();

       String volumeUuid = null;
       if (apkPath.startsWith(MNT_EXPAND)) {
           final int end = apkPath.indexOf('/', MNT_EXPAND.length());
           volumeUuid = apkPath.substring(MNT_EXPAND.length(), end);
       }

       mParseError = PackageManager.INSTALL_SUCCEEDED;
       mArchiveSourcePath = apkFile.getAbsolutePath();

       if (DEBUG_JAR) Slog.d(TAG, "Scanning base APK: " + apkPath);

       final int cookie = loadApkIntoAssetManager(assets, apkPath, flags);

       Resources res = null;
       XmlResourceParser parser = null;
       try {
           //获取apk资源文件
           res = new Resources(assets, mMetrics, null);
           //解析AndroidManifest
           parser = assets.openXmlResourceParser(cookie, ANDROID_MANIFEST_FILENAME);

           final String[] outError = new String[1];
           final Package pkg = parseBaseApk(apkPath, res, parser, flags, outError);
           if (pkg == null) {
               throw new PackageParserException(mParseError,
                       apkPath + " (at " + parser.getPositionDescription() + "): " + outError[0]);
           }

           pkg.setVolumeUuid(volumeUuid);
           pkg.setApplicationVolumeUuid(volumeUuid);
           pkg.setBaseCodePath(apkPath);
           pkg.setSignatures(null);

           return pkg;
           ......
   }

获取apk路径，解析apk资源文件及AndroidManifest

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private boolean parseBaseApkChild(Package parentPkg, Resources res, XmlResourceParser parser,
        int flags, String[] outError) throws XmlPullParserException, IOException {
    //创建package对象，将版本信息存储进来
    // Go ahead and parse the child
    Package childPkg = new Package(childPackageName);

    // Child package inherits parent version code/name/target SDK
    childPkg.mVersionCode = parentPkg.mVersionCode;
    childPkg.baseRevisionCode = parentPkg.baseRevisionCode;
    childPkg.mVersionName = parentPkg.mVersionName;
    childPkg.applicationInfo.targetSdkVersion = parentPkg.applicationInfo.targetSdkVersion;
    childPkg.applicationInfo.minSdkVersion = parentPkg.applicationInfo.minSdkVersion;

    childPkg = parseBaseApkCommon(childPkg, CHILD_PACKAGE_TAGS, res, parser, flags, outError);
    if (childPkg == null) {
        // If we got null then error was set during child parsing
        return false;
    }

    // Set the parent-child relation
    if (parentPkg.childPackages == null) {
        parentPkg.childPackages = new ArrayList<>();
    }
    parentPkg.childPackages.add(childPkg);
    childPkg.parentPackage = parentPkg;

    return true;
}

创建package对象，将版本信息存储进来

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private Package parseBaseApkCommon(Package pkg, Set<String> acceptedTags, Resources res,
        XmlResourceParser parser, int flags, String[] outError) throws XmlPullParserException,
        IOException {
        .......
    //解析AndroidManifest的深度
    int outerDepth = parser.getDepth();
    while ((type = parser.next()) != XmlPullParser.END_DOCUMENT
            && (type != XmlPullParser.END_TAG || parser.getDepth() > outerDepth)) {
        ......
        if (tagName.equals(TAG_APPLICATION)) {
           ......
           //解析Application标签
            if (!parseBaseApplication(pkg, res, parser, flags, outError)) {
                return null;
            }
        } else if (tagName.equals(TAG_PERMISSION)) {
            //解析权限标签
            if (!parsePermission(pkg, res, parser, outError)) {
                return null;
            }
        }
    return pkg;
}

获取AndroidManifest的深度，然后深度遍历获取所有的标签并依次处理，这里以Application为例：

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private boolean parseBaseApplication(Package owner, Resources res,
        XmlResourceParser parser, int flags, String[] outError)
    throws XmlPullParserException, IOException {
    //获取应用信息
    final ApplicationInfo ai = owner.applicationInfo;
    //获取包名
    final String pkgName = owner.applicationInfo.packageName;
    //获取Application的TypedArray
    TypedArray sa = res.obtainAttributes(parser,
            com.android.internal.R.styleable.AndroidManifestApplication);
    ......
    //获取应用名
    String manageSpaceActivity = sa.getNonConfigurationString(
            com.android.internal.R.styleable.AndroidManifestApplication_manageSpaceActivity,
            Configuration.NATIVE_CONFIG_VERSION);
    ......
    //获取Application的allowBackUp
    boolean allowBackup = sa.getBoolean(
            com.android.internal.R.styleable.AndroidManifestApplication_allowBackup, true);
    //获取主题
    ai.theme = sa.getResourceId(
            com.android.internal.R.styleable.AndroidManifestApplication_theme, 0);
    //获取描述
    ai.descriptionRes = sa.getResourceId(
            com.android.internal.R.styleable.AndroidManifestApplication_description, 0);

    //获取taskAffinity
    ai.taskAffinity = buildTaskAffinityName(ai.packageName, ai.packageName,
            str, outError);
    
    final int innerDepth = parser.getDepth();
    int type;
    //迭代Application元素下的所有子元素
    while ((type = parser.next()) != XmlPullParser.END_DOCUMENT
            && (type != XmlPullParser.END_TAG || parser.getDepth() > innerDepth)) {
        if (type == XmlPullParser.END_TAG || type == XmlPullParser.TEXT) {
            continue;
        }
        //获取标签名
        String tagName = parser.getName();
        //如果是Activity
        if (tagName.equals("activity")) {
            Activity a = parseActivity(owner, res, parser, flags, outError, false,
                    owner.baseHardwareAccelerated);
            if (a == null) {
                mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                return false;
            }

            owner.activities.add(a);
            //如果是receiver
        } else if (tagName.equals("receiver")) {
            Activity a = parseActivity(owner, res, parser, flags, outError, true, false);
            if (a == null) {
                mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                return false;
            }

            owner.receivers.add(a);

        } 
    }

    return true;
}

解析Application的其他属性及其子标签,而后返回方法直到scanDirLI方法，执行scanPackageLI函数

scanPackageLI—>scanPackageInternalLI—>scanPackageLI—>scanPackageDirtyLI—>commitPackageSettings

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private void commitPackageSettings(PackageParser.Package pkg, PackageSetting pkgSetting,
            UserHandle user, int scanFlags, boolean chatty) throws PackageManagerException {
            ......
            //获取activities的大小
            N = pkg.activities.size();
            r = null;
            for (i=0; i<N; i++) {
                PackageParser.Activity a = pkg.activities.get(i);
                a.info.processName = fixProcessName(pkg.applicationInfo.processName,
                        a.info.processName);
                //将activity添加到mActivies
                mActivities.addActivity(a, "activity");
                if (chatty) {
                    if (r == null) {
                        r = new StringBuilder(256);
                    } else {
                        r.append(' ');
                    }
                    r.append(a.info.name);
                }
            }
            //其他标签忽略
            ......
    }

将解析的标签，例activity记载到PMS的缓存中。至此，apk的所有信息就被存储在系统中，当使用intent跳转时会在该信息表中进行查找，然后跳转。

我们使用intent时，一般是这样：

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Intent intent = new Intent(this,MainActivity.class);
this.startActivity(intent);

跟踪源码，最终跳转到startActivityForResult：
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public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
           @Nullable Bundle options) {
       if (mParent == null) {
           options = transferSpringboardActivityOptions(options);
           //启动Activity
           Instrumentation.ActivityResult ar =
               mInstrumentation.execStartActivity(
                   this, mMainThread.getApplicationThread(), mToken, this,
                   intent, requestCode, options);
           if (ar != null) {
               //发送启动请求
               mMainThread.sendActivityResult(
                   mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                   ar.getResultData());
           }
           if (requestCode >= 0) {
               mStartedActivity = true;
           }

           cancelInputsAndStartExitTransition(options);
           // TODO Consider clearing/flushing other event sources and events for child windows.
       } else {
           if (options != null) {
               mParent.startActivityFromChild(this, intent, requestCode, options);
           } else {
               // Note we want to go through this method for compatibility with
               // existing applications that may have overridden it.
               mParent.startActivityFromChild(this, intent, requestCode);
           }
       }
   }

通过instrumentation启动Activity，并向主线程发送启动请求
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public ActivityResult execStartActivity(
     Context who, IBinder contextThread, IBinder token, String target,
     Intent intent, int requestCode, Bundle options) {
     IApplicationThread whoThread = (IApplicationThread) contextThread;
     ......
     try {
         //将Intent中的数据迁移到粘贴板
         intent.migrateExtraStreamToClipData();
         //准备离开当前进程
         intent.prepareToLeaveProcess(who);
         //通过AMS启动Activity
         int result = ActivityManager.getService()
             .startActivity(whoThread, who.getBasePackageName(), intent,
                     intent.resolveTypeIfNeeded(who.getContentResolver()),
                     token, target, requestCode, 0, null, options);
         //检测结果，并且回调给调用端
         checkStartActivityResult(result, intent);
     } catch (RemoteException e) {
         throw new RuntimeException("Failure from system", e);
     }
     return null;
 }

进入AMS方法，调用

startActivityAsUser—>ActivityStarter.startActivityMayWait—>ActivityStackSupervisor.resolveIntent—>AMS.getPackageManagerInternalLocked—>PMS.resolveIntent—>PMS.resolveIntentInternal—>PMS.queryIntentActivitiesInternal

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private @NonNull List<ResolveInfo> queryIntentActivitiesInternal(Intent intent,
        String resolvedType, int flags, int filterCallingUid, int userId,
        boolean resolveForStart) {
    ......
    //获取包名
    final String pkgName = intent.getPackage();
    //获取component对象
    ComponentName comp = intent.getComponent();
    //如果component不为null
    if (comp != null) {
        final List<ResolveInfo> list = new ArrayList<ResolveInfo>(1);
        //直接getActivityInfo获取ActivityInfo对象
        final ActivityInfo ai = getActivityInfo(comp, flags, userId);
        ......
    }
    ......
    //如果是隐式intent
    synchronized (mPackages) {
        //如果包名为null
        if (pkgName == null) {
            List<CrossProfileIntentFilter> matchingFilters =
                    getMatchingCrossProfileIntentFilters(intent, resolvedType, userId);
            // Check for results that need to skip the current profile.
            //获取resolveInfo对象
            ResolveInfo xpResolveInfo  = querySkipCurrentProfileIntents(matchingFilters, intent,
                    resolvedType, flags, userId);
            if (xpResolveInfo != null) {
                List<ResolveInfo> xpResult = new ArrayList<ResolveInfo>(1);
                xpResult.add(xpResolveInfo);
                return applyPostResolutionFilter(
                        filterIfNotSystemUser(xpResult, userId), instantAppPkgName);
            }
        } else {
            //通过包名获取Package对象
            final PackageParser.Package pkg = mPackages.get(pkgName);
            result = null;
            if (pkg != null) {
                //通过package获取ActivityInfo
                result = filterIfNotSystemUser(
                        mActivities.queryIntentForPackage(
                                intent, resolvedType, flags, pkg.activities, userId),
                        userId);
            }
           ......
}

如果intent指明了Component（即MainActivity），直接获取到ActivityInfo并且数量只有一个，直接返回。如果Component为空，会检测发起方所在的包名，如果有包名，通过包名获取ActivityInfo，反之，根据其他信息比如外部拉起，action，Category等判断。

总结：在系统启动时，PMS启动分析所有apk的信息，创建一个信息表，当用胡使用Intent跳转时，会根据intent中包含的信息到PMS的信息表查找，最后跳转到目标组件

优缺点

优点：

  • 内存中二进制流的拷贝，其比直接new一个对象性能会好很多
    缺点：
  • 直接在内存中拷贝，不会执行构造函数，使用过程中对比new和clone方法的区别

原型模式就是要实现深拷贝

工厂模式

定义

用于创建对象的接口，让子类决定实例化那个类，解决对象之间的解耦

例子

简单工厂
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public abstract class Product {
    /**
     * 子类具体实现方法
    */
    public abstract void method();
}

public class ProductA extends Product {
    @Override
    public void method() {
        System.out.println("制造ProductA");
    }
}

public class ProductB extends Product {
    @Override
    public void method() {
        System.out.println("制造ProductB");
    }
}

public class Factory {
    public Product create(String type){
        if("A".equals(type)){
            return new ProductA();
        }else if("B".equals(type)){
            return new ProductB();
        }else{
            return null;
        }
    }
}

public static void main(String args[]) {
    Factory factory = new Factory();
    Product a = factory.create("A");
    a.method();
    Product b = factory.create("B");
    b.method();
}

运行后：

制造ProductA

制造ProductB

工厂方法抽象

产品类不变，工厂变为抽象类

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public abstract class Factory {
    /**
     * 工厂具体实现方法
     * @return
     */
    public abstract Product create();
}

public class FactoryA extends Factory {
    @Override
    public Product create() {
        return new ProductA();
    }
}

public class FactoryB extends Factory {
    @Override
    public Product create() {
        return new ProductB();
    }
}

public static void main(String args[]) {
    FactoryA factoryA = new FactoryA();
    FactoryB factoryB = new FactoryB();
    factoryA.create().method();
    factoryB.create().method();
}

运行后：

制造ProductA

制造ProductB

抽象工厂

需要多个产品最终组成为一个产品

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public abstract class Cpu {
    /**
     * cpu子类的具体执行
     */
    public abstract void method();
}

public class CpuA extends Cpu {
    @Override
    public void method() {
        System.out.println("A型号CPU");
    }
}

public class CpuB extends Cpu {
    @Override
    public void method() {
        System.out.println("B型号CPU");
    }
}

public abstract class Display {
    /**
     * 显示屏的子类执行方法
     */
    public abstract void method();
}

public class DisplayA extends Display {
    @Override
    public void method() {
        System.out.println("A型号显示屏");
    }
}

public class DisplayB extends Display {
    @Override
    public void method() {
        System.out.println("B型号显示屏");
    }
}

public abstract class Factory {
    /**
     * 创建Cpu
     * @return
     */
   public abstract Cpu createCpu();

    /**
     * 创建显示屏
     * @return
     */
   public abstract Display createDisplay();
}

public class FactoryA extends Factory {

    @Override
    public Cpu createCpu() {
        return new CpuA();
    }

    @Override
    public Display createDisplay() {
        return new DisplayA();
    }
}

public class FactoryB extends Factory {

    @Override
    public Cpu createCpu() {
        return new CpuB();
    }

    @Override
    public Display createDisplay() {
        return new DisplayB();
    }
}

    public static void main(String args[]) {
        FactoryA factoryA = new FactoryA();
        FactoryB factoryB = new FactoryB();
        factoryA.createCpu().method();
        factoryA.createDisplay().method();
        factoryB.createCpu().method();
        factoryB.createDisplay().method();
    }

执行结果如下：

A型号CPU

A型号显示屏

B型号CPU

B型号显示屏

抽象工厂的核心是复杂的工厂模式。存在多个工厂，多种产品类型，就会使用抽象工厂，在android源码中使用较少，并没有那么多产品种类，大部分使用简单工厂或者工厂方法就可以解决。

源码分析

  1. ArrayList和HashSet都继承自Collection接口，collection接口继承自Iterator接口。
  2. xxxActivity都继承自Activity，其onCreate方法就是工厂方法，两个Activity构建不同的view，可以将view看成product，Activity为Factory。
  3. xxService都继承自Service，其onBind方法也可以看作是一个工厂方法
    4.

优缺点

优点：

  • 功能逻辑解耦

缺点：

  • 拓展新功能需要添加新类，复杂类结构

抽象工厂的优点:

  • 分离接口与实现,使用方不知道具体的实现是什么,同时使抽象该工厂方法模式在切换产品类时更加灵活,容易

抽象工厂的缺点:

  • 类文件结构复杂
  • 不容易拓展新类

策略模式

定义

定义一系列的算法，并将每一个算法封装起来，而且使他们可以相互替换，策略模式让算法独立于使用他们的客户而独立存在，解决if-else滥用的问题

例子

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public class CalculatePrice {

    public enum TransPortType {
        BUS,
        SUBWAY
    }

    /**
     * 计算公交车车费
     */
    private void calculateBus(int km) {
        System.out.println("公交车" + km + "公里的车票");
    }

    /**
     * 计算地铁车费
     */
    private void calculateSubWay(int km) {
        System.out.println("地铁" + km + "公里的车票");
    }

    private void calculate(int km, TransPortType transPortType) {
        switch (transPortType) {
            case BUS:
                calculateBus(km);
                break;
            case SUBWAY:
                calculateSubWay(km);
                break;
            default:
                break;
        }
    }

    public static void main(String args[]){
        CalculatePrice calculatePrice = new CalculatePrice();
        calculatePrice.calculate(16,TransPortType.BUS);
        calculatePrice.calculate(15,TransPortType.SUBWAY);
    }
}

结果：
公交车16公里的车票

地铁15公里的车票

当需要拓展时，比如多了一个出租车，需要这样做

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public class CalculatePrice {

    public enum TransPortType {
        BUS,
        SUBWAY,
        TAXI
    }

    /**
     * 计算公交车车费
     */
    private void calculateBus(int km) {
        System.out.println("公交车" + km + "公里的车票");
    }

    /**
     * 计算地铁车费
     */
    private void calculateSubWay(int km) {
        System.out.println("地铁" + km + "公里的车票");
    }

    /**
     * 计算出租车车费
     */
    private void calculateTaxi(int km) {
        System.out.println("出租车" + km + "公里的车票");
    }

    private void calculate(int km, TransPortType transPortType) {
        switch (transPortType) {
            case BUS:
                calculateBus(km);
                break;
            case SUBWAY:
                calculateSubWay(km);
                break;
            case TAXI:
                calculateTaxi(km);
                break;
            default:
                break;
        }
    }

    public static void main(String args[]){
        CalculatePrice calculatePrice = new CalculatePrice();
        calculatePrice.calculate(16,TransPortType.BUS);
        calculatePrice.calculate(15,TransPortType.SUBWAY);
        calculatePrice.calculate(17,TransPortType.TAXI);
    }
}

结果：

公交车16公里的车票

地铁15公里的车票

出租车17公里的车票

改动还是比较大的，对源代码的基础上做了更改，并且添加了if-else判断taxi类型。
如果把他改成策略模式，试试效果

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public abstract class CalculateStrategy {
    /**
     * 子类具体执行车票计算
     */
    public abstract void getPrice(int km);
}

public class BusCalculateStrategy extends CalculateStrategy {
    @Override
    public void getPrice(int km) {
        System.out.println("公交车" + km + "公里的车票");
    }
}

public class SubWayCalculateStrategy extends CalculateStrategy {
    @Override
    public void getPrice(int km) {
        System.out.println("地铁" + km + "公里的车票");
    }
}

    public static void main(String args[]){
        CalculateStrategy calculateStrategy = new BusCalculateStrategy();
        calculateStrategy.getPrice(16);
        CalculateStrategy calculateStrategy1 = new SubWayCalculateStrategy();
        calculateStrategy1.getPrice(15);
    }

结果：

公交车16公里的车票

地铁15公里的车票

当需要拓展出租车时

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public class TaxiCalculateStrategy extends CalculateStrategy {
    @Override
    public void getPrice(int km) {
        System.out.println("出租车" + km + "公里的车票");
    }
}   

    public static void main(String args[]){
        CalculateStrategy calculateStrategy2 = new TaxiCalculateStrategy();
        calculateStrategy2.getPrice(17);
    }

只需要添加Taxi类继承自策略类，执行具体车费计算方法，在main中调用即可，这样对代码的侵入性最小，不用修改原有逻辑代码，在咋付逻辑出引用效果优秀

状态模式

定义

当一个对象的内在行为改变时，允许改变其行为，整个对象看起来像是改变了其类

源码解析

  1. wifi状态管理

    责任链模式

    定义

    使多个对象都有机会处理请求，这些对象形成一条链，依次执行，直到结束。

    源码解析

  2. view的事件传递

观察者模式

定义

定义对象间一种一对多的一栏关系，使得每当一个对象改变状态，则所有依赖与他的对象都会得到通知并被自动更新

例子

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public class Mode2 {
    static public interface IObserver {
        void update(int temp);
    }

    static public interface IObservable {
        void register(IObserver iObserver);

        void unregister(IObserver iObserver);

        void notifyObserver();
    }

    //被观察者（发布者）
    static class Observable implements IObservable {
        private ArrayList<IObserver> list = new ArrayList<>();
        private int temp;

        @Override
        public void register(IObserver iObserver) {
            list.add(iObserver);
        }

        @Override
        public void unregister(IObserver iObserver) {
            list.remove(iObserver);
        }

        @Override
        public void notifyObserver() {
            for (int i = 0; i < list.size(); i++) {
                list.get(i).update(temp);
            }
        }

        public void setTemp(int temp) {
            this.temp = temp;
        }
    }

    //观察者1（订阅者）
    static class Observer1 implements IObserver {

        @Override
        public void update(int temp) {
            System.out.println("Observable1更新为 = " + temp);
        }
    }

    //观察者2(订阅者)
    static class Observer2 implements IObserver {
        @Override
        public void update(int temp) {
            System.out.println("Observable2更新为 = " + temp);
        }
    }

    public static void main(String args[]){
        Observable observable = new Observable();
        Observer1 observer1 = new Observer1();
        Observer2 observer2 = new Observer2();
        observable.register(observer1);
        observable.register(observer2);
        observable.setTemp(32131232);
        observable.notifyObserver();
    }
}
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