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日夜间及换肤（二）-原理分析

移动端 2021/07/28

本文于 851 天之前发表，文中内容可能已经过时。

[TOC]

原始地址：日夜间及换肤（二）-原理分析

官方原理

img

在官方的推荐方法中，我们发现了每次设置完Mode或者Theme都需要recreate()才会生效，这是为什么呢？

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/**
 * Cause this Activity to be recreated with a new instance.  This results
 * in essentially the same flow as when the Activity is created due to
 * a configuration change -- the current instance will go through its
 * lifecycle to {@link #onDestroy} and a new instance then created after it.
 */

使用新实例重新创建此活动。 这导致与由于配置更改而创建 Activity 时的流程基本相同————当前实例将通过其生命周期到达onDestroy ，然后在它之后创建一个新实例

即当前Activity进入onDestroy，重新创建一个Activity，所以会执行新Activity的生命周期

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 @Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    final AppCompatDelegate delegate = getDelegate();
    delegate.installViewFactory();
    delegate.onCreate(savedInstanceState);
    super.onCreate(savedInstanceState);
}

进入delegate.onCreate()方法中

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@Override
public void onCreate(Bundle savedInstanceState) {
    // attachBaseContext will only be called from an Activity, so make sure we switch this for
    // Dialogs, etc
    mBaseContextAttached = true;

    // Our implicit call to applyDayNight() should not recreate until after the Activity is
    // created
    applyDayNight(false);

    ...
}

点击进入applyDayNight方法中

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private boolean applyDayNight(final boolean allowRecreation) {
       if (mIsDestroyed) {
           if (DEBUG) {
               Log.d(TAG, "applyDayNight. Skipping because host is destroyed");
           }
           // If we're destroyed, ignore the call
           return false;
       }

       @NightMode final int nightMode = calculateNightMode();
       ...
   }

进入calculateNightMode()中

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 @NightMode
private int calculateNightMode() {
    return mLocalNightMode != MODE_NIGHT_UNSPECIFIED ? mLocalNightMode : getDefaultNightMode();
}

public static int getDefaultNightMode() {
    return sDefaultNightMode;
}

public static void setDefaultNightMode(@NightMode int mode) {
    ...
    switch (mode) {
        case MODE_NIGHT_NO:
        case MODE_NIGHT_YES:
        case MODE_NIGHT_FOLLOW_SYSTEM:
        case MODE_NIGHT_AUTO_TIME:
        case MODE_NIGHT_AUTO_BATTERY:
            if (sDefaultNightMode != mode) {
                sDefaultNightMode = mode;
                applyDayNightToActiveDelegates();
            }
            break;
        default:
            Log.d(TAG, "setDefaultNightMode() called with an unknown mode");
            break;
    }
}

获取mode类型后进行加载日夜间效果，所以在该方式下要达到动态切换，势必要重新执行onCreate方法

androidx中sdk中已经将recreate方法放入setDefaultNightMode()方法中，所以不需要手动调用

无论通过AppCompatDelegate.setDefaultNightMode还是delegate.setLocalNightMode，都会执行到updateForNightMode方法

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private boolean updateForNightMode(@ApplyableNightMode final int mode,
        final boolean allowRecreation) {
  			...
        ActivityCompat.recreate((Activity) mHost);
        handled = true;
   	...

    return handled;
}

执行recreate重新构建Activity

第三方库实现原理

Android-skin-support

原理分析（SdkVersion = 29）

img

怎么做到的呢？

我们回顾一下setContentView()的加载过程：

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public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

    //androidx.appcompat.app.AppcompatActivity
    @Override
    public void setContentView(@LayoutRes int layoutResID) {
        getDelegate().setContentView(layoutResID);
    }

进入代理中查看实现：

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//androidx.appcompat.app.AppcompatDelegateImpl
@Override
public void setContentView(int resId) {
  	//初始化DecroView
    ensureSubDecor();
  	//获取Content根布局
    ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content);
  	//移除所有布局
    contentParent.removeAllViews();
 		//加载布局
    LayoutInflater.from(mContext).inflate(resId, contentParent);
  	//布局状态接口通知
    mAppCompatWindowCallback.getWrapped().onContentChanged();
}

进入inflate方法中：

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//android.view.LayoutInflater
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
    return inflate(resource, root, root != null);
}

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
    final Resources res = getContext().getResources();

  	...
// 根据XML预编译生成compiled_view.dex, 然后通过反射来生成对应的View，从而减少XmlPullParser解析Xml的时间
		// 需要注意的是在目前的release版本中不支持使用
    View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
    if (view != null) {
        return view;
    }
  	//使用xml进行解析返回一个XmlResourceParser，内部最终会进入native方法进行解析
    XmlResourceParser parser = res.getLayout(resource);
    try {
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

进入inflate中：

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//android.view.LayoutInflater
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
		...

        try {
          	//找到START_TAG，否则报异常，说没有入口
            advanceToRootNode(parser);
            final String name = parser.getName();

           	...
              
          	//如果是merge标签，查看merge是不是根节点，不然的话抛出异常
            if (TAG_MERGE.equals(name)) {
                if (root == null || !attachToRoot) {
                    throw new InflateException("<merge /> can be used only with a valid "
                            + "ViewGroup root and attachToRoot=true");
                }

                rInflate(parser, root, inflaterContext, attrs, false);
            } else {
                // Temp is the root view that was found in the xml
              	//获取根节点View
                final View temp = createViewFromTag(root, name, inflaterContext, attrs);
               	...
                //渲染根节点的孩子们
                rInflateChildren(parser, temp, attrs, true);
              	...

        return result;
    }
}

进入rInflateChildren()中：

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final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
        boolean finishInflate) throws XmlPullParserException, IOException {
    rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}

void rInflate(XmlPullParser parser, View parent, Context context,
        AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {

    final int depth = parser.getDepth();
    int type;
    boolean pendingRequestFocus = false;

  	//循环便利布局，直到布局完成
    while (((type = parser.next()) != XmlPullParser.END_TAG ||
            parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {

       			...
            //获取当前布局的View
            final View view = createViewFromTag(parent, name, context, attrs);
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
      			//渲染当前布局View的孩子们
            rInflateChildren(parser, view, attrs, true);
      			//组合为ViewGroup
            viewGroup.addView(view, params);
        }
    }

    if (pendingRequestFocus) {
        parent.restoreDefaultFocus();
    }

    if (finishInflate) {
        parent.onFinishInflate();
    }
}

进入createViewFromTag():

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//android.view.LayoutInflater
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
            boolean ignoreThemeAttr) {
        if (name.equals("view")) {
            name = attrs.getAttributeValue(null, "class");
        }

        // Apply a theme wrapper, if allowed and one is specified.
        if (!ignoreThemeAttr) {
            final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
            final int themeResId = ta.getResourceId(0, 0);
            if (themeResId != 0) {
                context = new ContextThemeWrapper(context, themeResId);
            }
            ta.recycle();
        }

        try {
            View view = tryCreateView(parent, name, context, attrs);

            if (view == null) {
                final Object lastContext = mConstructorArgs[0];
                mConstructorArgs[0] = context;
                try {
                    if (-1 == name.indexOf('.')) {
                        view = onCreateView(context, parent, name, attrs);
                    } else {
                        view = createView(context, name, null, attrs);
                    }
                } finally {
                    mConstructorArgs[0] = lastContext;
                }
            }

            return view;
        } catch (InflateException e) {
          ...
        }
    }

进行tryCreateView方法，如果view为null，则执行onCreateView或者createView方法，

无论是onCreateView还是createView最终都会进入如下函数：

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@Nullable
public final View createView(@NonNull Context viewContext, @NonNull String name,
        @Nullable String prefix, @Nullable AttributeSet attrs)
        throws ClassNotFoundException, InflateException {
    Objects.requireNonNull(viewContext);
    Objects.requireNonNull(name);
    Constructor<? extends View> constructor = sConstructorMap.get(name);
    if (constructor != null && !verifyClassLoader(constructor)) {
        constructor = null;
        sConstructorMap.remove(name);
    }
    Class<? extends View> clazz = null;

    try {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);

        if (constructor == null) {
            // Class not found in the cache, see if it's real, and try to add it
            clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                    mContext.getClassLoader()).asSubclass(View.class);

            if (mFilter != null && clazz != null) {
                boolean allowed = mFilter.onLoadClass(clazz);
                if (!allowed) {
                    failNotAllowed(name, prefix, viewContext, attrs);
                }
            }
            constructor = clazz.getConstructor(mConstructorSignature);
            constructor.setAccessible(true);
            sConstructorMap.put(name, constructor);
        } else {
            // If we have a filter, apply it to cached constructor
            if (mFilter != null) {
                // Have we seen this name before?
                Boolean allowedState = mFilterMap.get(name);
                if (allowedState == null) {
                    // New class -- remember whether it is allowed
                    clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                            mContext.getClassLoader()).asSubclass(View.class);

                    boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                    mFilterMap.put(name, allowed);
                    if (!allowed) {
                        failNotAllowed(name, prefix, viewContext, attrs);
                    }
                } else if (allowedState.equals(Boolean.FALSE)) {
                    failNotAllowed(name, prefix, viewContext, attrs);
                }
            }
        }

        Object lastContext = mConstructorArgs[0];
        mConstructorArgs[0] = viewContext;
        Object[] args = mConstructorArgs;
        args[1] = attrs;

        try {
            final View view = constructor.newInstance(args);
            if (view instanceof ViewStub) {
                // Use the same context when inflating ViewStub later.
                final ViewStub viewStub = (ViewStub) view;
                viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
            }
            return view;
        } finally {
            mConstructorArgs[0] = lastContext;
        }
    } catch (NoSuchMethodException e) {
       ...
    }
}

简单看一下可以确认是通过反射实现的

然后我们进入tryCreateView()方法：

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//android.view.LayoutInflater
public final View tryCreateView(@Nullable View parent, @NonNull String name,
    @NonNull Context context,
    @NonNull AttributeSet attrs) {
    if (name.equals(TAG_1995)) {
        // Let's party like it's 1995!
        return new BlinkLayout(context, attrs);
    }

    View view;
    if (mFactory2 != null) {
      	//系统一般会创建Factory2的对象
        view = mFactory2.onCreateView(parent, name, context, attrs);
    } else if (mFactory != null) {
        view = mFactory.onCreateView(name, context, attrs);
    } else {
        view = null;
    }

    if (view == null && mPrivateFactory != null) {
        view = mPrivateFactory.onCreateView(parent, name, context, attrs);
    }

    return view;
}

那这个mFactory2/mFactory.onCreateView()要走哪里呢？Factory实例在哪里呢？

接下来

我们需要分析setContenView()之前都做了什么

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@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

点击super.onCreate(saveInstanceState)方法：

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//androidx.appcompat.app.AppCompatActivity
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    final AppCompatDelegate delegate = getDelegate();
    delegate.installViewFactory();
    delegate.onCreate(savedInstanceState);
    super.onCreate(savedInstanceState);
}

进入installViewFactory()方法：

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//androidx.appcompat.app.AppCompatDelegateImpl
@Override
public void installViewFactory() {
    LayoutInflater layoutInflater = LayoutInflater.from(mContext);
    if (layoutInflater.getFactory() == null) {
        LayoutInflaterCompat.setFactory2(layoutInflater, this);
    } else {
        if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImpl)) {
            Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
                    + " so we can not install AppCompat's");
        }
    }
}

首先通过获取layoutInflater.getFactory()进行获取，如果没有就会setFactory2进行设置。

在其中进行setFactory初始化，注意第二个参数，传入的是this，那么，我们看一下这个类：

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class AppCompatDelegateImpl extends AppCompatDelegate
        implements MenuBuilder.Callback, LayoutInflater.Factory2

很好，他是一个实现Factory2接口的类，那么我们直接找onCreateView的实现：

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@Override
public View createView(View parent, final String name, @NonNull Context context,
        @NonNull AttributeSet attrs) {
    		...
        mAppCompatViewInflater = new AppCompatViewInflater();	
		...
    		return mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext,
            IS_PRE_LOLLIPOP, /* Only read android:theme pre-L (L+ handles this anyway) */
            true, /* Read read app:theme as a fallback at all times for legacy reasons */
            VectorEnabledTintResources.shouldBeUsed() /* Only tint wrap the context if enabled */
    );
}

其实调用的是AppcompatViewInflater.createView()方法：

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final View createView(View parent, final String name, @NonNull Context context,
          @NonNull AttributeSet attrs, boolean inheritContext,
          boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
    	...
      switch (name) {
          case "TextView":
              view = createTextView(context, attrs);
              verifyNotNull(view, name);
              break;
          ...
        	default:
              // The fallback that allows extending class to take over view inflation
              // for other tags. Note that we don't check that the result is not-null.
              // That allows the custom inflater path to fall back on the default one
              // later in this method.
              view = createView(context, name, attrs);
      }
		...
      return view;
  }

  @NonNull
  protected AppCompatTextView createTextView(Context context, AttributeSet attrs) {
      return new AppCompatTextView(context, attrs);
  }

通过new的方式新建view，如果switch没有得到处理，则返回view为null，通过反射处理即可

我们要怎么做？

Factory和Factory2的区别是什么？

可惜我们不能从文档说明上获得什么，我们就直接看代码吧

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    public interface Factory {
    @Nullable
    View onCreateView(@NonNull String name, @NonNull Context context,
            @NonNull AttributeSet attrs);
}

    public interface Factory2 extends Factory {
    
    @Nullable
    View onCreateView(@Nullable View parent, @NonNull String name,
            @NonNull Context context, @NonNull AttributeSet attrs);
}

字段中多了一个View的父类，可以认为Factory2是对Factory的重载

在分析过程中，我们发现

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    //androidx.appcompat.app.AppCompatDelegateImpl
    @Override
    public void installViewFactory() {
    LayoutInflater layoutInflater = LayoutInflater.from(mContext);
    if (layoutInflater.getFactory() == null) {
        LayoutInflaterCompat.setFactory2(layoutInflater, this);
    } else {
        if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImpl)) {
            Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
                    + " so we can not install AppCompat's");
        }
    }
}

系统的处理是默认会取Factory，如果没有，才会新建，走AppCompatDelegateImpl，如果我们在这个super.onCreate()方法之前设置Factory就可以让系统走我们的Factory实现

具体实现

最终我们在Activity的onCreate方法之前执行我们的设置

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@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    LayoutInflaterCompat.setFactory(getLayoutInflater(), getSkinDelegate());
    super.onCreate(savedInstanceState);
}

同时在createView方法中生成自己的view，在view中设置刷新接口，通过观察者模式在模式变化的时候通知所有的view，变更背景色，字体色等，不需要重新构建Activity，避免闪屏

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