自定义View——背景知识

Activity

作用：负责生命周期管理与事件处理，每个Activity组合了一个Window，实际视图控制是交由Winodw管理ui排版。是为了满足多窗口管理和傻瓜式视图管理的需要而诞生的。
起源：在Activity中的onCreate()方法中会调用setContentView()加载自定义视图，实际调用的是Window的setContentView方法

相关源码：

android/app/Activity.java
public void setContentView(@LayoutRes int layoutResID) {
    //调用mWindow.setContentView()
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}

android/app/Activity.java
final void attach(... ...){
   ... ...
    //实例化mWindow
   mWindow = new PhoneWindow(this, window, activityConfigCallback);
    //实现window的接口，键盘触摸 ui策略等
   mWindow.setWindowControllerCallback(this);
   mWindow.setCallback(this);
   mWindow.setOnWindowDismissedCallback(this);
   ... ...
}

Window

作用：唯一实现类PhoneWindow，创建了顶层视图DecorView，DecorView作为父布局用来加载Activity中setContentView()方法传入的layoutRes。是为了管理ui排版，视图控制而诞生的。

相关源码：

com/android/internal/policy/PhoneWindow.java
@Override
public void setContentView(int layoutResID) {
    if (mContentParent == null) {
        //创建DecorView，并实例化DecorView布局内的ViewGroup控件mContentParent
        installDecor();
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        mContentParent.removeAllViews();
    }

    if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                getContext());
        transitionTo(newScene);
    } else {
        //将mContentParent作为父控件，加载layoutResID
        mLayoutInflater.inflate(layoutResID, mContentParent);
    }
   ... ...
}

com/android/internal/policy/PhoneWindow.java
private void installDecor() {
    ... ...
    if (mDecor == null) {
        //创建DecorView
        mDecor = generateDecor(-1);
       ... ...
    }
    ... ...
    if (mContentParent == null) {
        //实例化mContentParent
       mContentParent = generateLayout(mDecor);
        ... ...
    }
}

com/android/internal/policy/PhoneWindow.java
protected DecorView generateDecor(int featureId) {
    ... ...
    //创建DecorView
    return new DecorView(context, featureId, this, getAttributes());
}

com/android/internal/policy/PhoneWindow.java
public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;
protected ViewGroup generateLayout(DecorView decor) {
    ... ...
    int layoutResource;
    if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
        layoutResource = R.layout.screen_swipe_dismiss;
        ... ...
    } 
    ... ...
    else {
       layoutResource = R.layout.screen_simple;
    }
    ... ...
    //加载DecorView布局
    mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
    //从DecorView中获取id为content的控件
   ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
    ... ...
    //将contentParent返回
    return contentParent;
}

android/view/Window.java
public <T extends View> T findViewById(@IdRes int id) {
    //返回DecorView中给定id的视图控件
    return getDecorView().findViewById(id);
}

frameworks/base/core/res/res/layout/screen_simple.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:fitsSystemWindows="true"
   android:orientation="vertical">
   <ViewStub android:id="@+id/action_mode_bar_stub"
       android:inflatedId="@+id/action_mode_bar"
       android:layout="@layout/action_mode_bar"
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:theme="?attr/actionBarTheme" />

   <FrameLayout
       android:id="@android:id/content"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:foregroundInsidePadding="false"
       android:foregroundGravity="fill_horizontal|top"
       android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

DecorView

作用：继承自FrameLayout，是Android视图树的根视图，View层的事件都会先经过DecorView，再分发到下面的View

相关源码：

com/android/internal/policy/DecorView.java
void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
    mDecorCaptionView = createDecorCaptionView(inflater);
    //加载PhoneWindow生成的layoutResource
    final View root = inflater.inflate(layoutResource, null);
    if (mDecorCaptionView != null) {
        if (mDecorCaptionView.getParent() == null) {
            addView(mDecorCaptionView,
                    new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        }
        mDecorCaptionView.addView(root,
                new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
    } else {
       //添加到DecorView中
        addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    }
    mContentRoot = (ViewGroup) root;
}

WindowManager

作用：在Activity生命周期onResume之后，绑定DecorView与ViewRootImpl。

相关源码：

android/app/ActivityThread.java
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
        String reason) {
       ... ...
    //执行activity的onResume方法
    final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
    ... ...
       final Activity a = r.activity;
       if (r.window == null && !a.mFinished && willBeVisible) {
        //获取activity的window实例
       r.window = r.activity.getWindow();
        //获取window中实例化的DecorView
       View decor = r.window.getDecorView();
       decor.setVisibility(View.INVISIBLE);
        //获取windowManager,获取的是WindowManager的子类，WindowManagerImpl
       ViewManager wm = a.getWindowManager();
       WindowManager.LayoutParams l = r.window.getAttributes();
       a.mDecor = decor;
       if (a.mVisibleFromClient) {
           if (!a.mWindowAdded) {
               a.mWindowAdded = true;
                //windowManager调用addView添加DecorView
               wm.addView(decor, l);
           }
            ... ...
       }
   }
    ... ...
}

android/view/WindowManagerImpl.java
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}

android/view/WindowManagerGlobal.java
public void addView(View view, ViewGroup.LayoutParams params,
        Display display, Window parentWindow) {
    ViewRootImpl root;
    //实例化ViewRootImpl
    root = new ViewRootImpl(view.getContext(), display);
    //保存decorView对象
    mViews.add(view);
    //保存viewRootImpl对象
    mRoots.add(root);
    //保存decorView的layout params
    mParams.add(wparams);
    try {
        //绑定ViewRootImpl与DecorView
        root.setView(view, wparams, panelParentView);
    }... ...
}

ViewRoot(实际实现类是android.view.ViewRootImpl)

作用
1. 连接DecorView与WindowManager，也可以说是Window与DecorView的纽带
2. 通过DecorView完成View的三大流程绘制
3. 用责任链模式，向DecorView分发用户的InputEvent事件
注意：在Activity.onResume()之后，ViewRootImpl.setView中将DecorView传入，并在之后执行requestLayout()通过DecorView递归执行View的三大流程，所以View的三大流程都是在Activity.onResume之后。

相关源码：

android/view/ViewRootImpl.java
final IWindowSession mWindowSession;

public ViewRootImpl(Context context, Display display) {
    //WindowManagerGlobal获取IWindowSession
    mWindowSession = WindowManagerGlobal.getWindowSession();
}

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
    synchronized (this) {
        if (mView == null) {
            //这个mView就是DecorView，view的三大绘制流程均是通过mView来递归的
            mView = view;
            ... ...
            //view的三大绘制流程
            requestLayout();
            //创建InputChannel
            mInputChannel = new InputChannel();
            //wms根据当前的window创建了SocketPair用于跨进程通信，并对传入的mInputChannel进行了注册
            //此后ViewRootImpl中的mInputChannel就指向了正确的InputChannel
            //client端与server端就能进行双向通信了
            res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
                           getHostVisibility(), mDisplay.getDisplayId(), mTmpFrame,
                       mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
                       mAttachInfo.mOutsets, mAttachInfo.mDisplayCutout, mInputChannel,
                       mTempInsets);
            //创建WindowInputEventReceiver，处理事件分发
            mInputEventReceiver = new WindowInputEventReceiver(mInputChannel,
                          Looper.myLooper());
            //组装InputStage责任链
            mSyntheticInputStage = new SyntheticInputStage();
            InputStage viewPostImeStage = new ViewPostImeInputStage(mSyntheticInputStage);
            InputStage nativePostImeStage = new NativePostImeInputStage(viewPostImeStage,
                        "aq:native-post-ime:" + counterSuffix);
            InputStage earlyPostImeStage = new EarlyPostImeInputStage(nativePostImeStage);
            InputStage imeStage = new ImeInputStage(earlyPostImeStage,
                        "aq:ime:" + counterSuffix);
            InputStage viewPreImeStage = new ViewPreImeInputStage(imeStage);
            InputStage nativePreImeStage = new NativePreImeInputStage(viewPreImeStage,
                        "aq:native-pre-ime:" + counterSuffix);
            mFirstInputStage = nativePreImeStage;
            mFirstPostImeInputStage = earlyPostImeStage;
        }
    }
}

View的三大绘制流程入口

android/view/ViewRootImpl.java
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        //这里执行view的绘制流程
        scheduleTraversals();
    }
}

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

final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

void scheduleTraversals() {
   if (!mTraversalScheduled) {
        ... ...
       mChoreographer.postCallback(
           Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        ... ...
   }
}

void doTraversal() {
   if (mTraversalScheduled) {
       ... ...
       performTraversals();
        ... ...
   }
}

private void performTraversals() {
    ... ...
    //measure流程
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    //layout流程
    performLayout(lp, mWidth, mHeight);
    //draw流程
    performDraw();
}

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
   try {
        //执行DecorView的mesaure流程
       mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
   } finally {
       Trace.traceEnd(Trace.TRACE_TAG_VIEW);
   }
}

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {
    final View host = mView;
    try {
        //执行DecorView的layout流程
       host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
    }
}

private void performDraw() {
    ... ...
    try {
       boolean canUseAsync = draw(fullRedrawNeeded);
    }
    ... ...
}

private boolean draw(boolean fullRedrawNeeded) {
    ... ...
    if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
        scalingRequired, dirty, surfaceInsets)) {
       return false;
   }
    ... ...
}

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
        boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
    ... ...
    //执行DecorView的draw流程
    mView.draw(canvas);
    ... ...
}

综上
1. 创建DecorView的流程：
  - Activity在attach时，会创建PhoneWindow实例mWindow;
  - Activity在onCreate中调用setContentView方法时，会间接调用mWindow.setContentView;
  - PhoneWindow在setContentView中会创建DecorView的实例，并将Activity传入的布局加载到DecorView中id为content的mContentParent中;
  - DecorView是android视图树的根节点，最底层的View，在PhoneWindow创建布局时generateLayout会根据不同类型的主题创建布局，其中会有一个id为content的ViewGroup，Activity中传入的布局文件就是以content为父布局;
2. 建立DecorView与WindowManager的联系并最终绘制显示的流程：
  - ActivityThread在调用handleResumeActivity，会执行Activity的onResume方法，随后，会创建ViewRootImpl的实例，获取Activity的Window实例，并获取Window中的DecorView实例，使用WindowManager，将ViewRootImpl与DecorView绑定;
  - ViewRootImpl获取到DecorView的实例后，会持有该引用，并分步调用mView的measure,layout,draw方法。
3. 当InputManager监控到硬件层面的输入事件时，会通知ViewRootImpl对输入事件进行底层分发(具体细节查看View的事件分发机制)
  - 创建InputChannel，并通过Binder在SystemServer进程中完成InputChannel的注册。
  - 创建WindowInputEventReceiver来处理事件分发。
  - 组装InputStage责任链，负责不同InputEvent事件的处理。
其他要点
- View ---getLeft() getTop() getRight() getBottom()返回的是View 相对与ViewGroup的左上右下距离
- MotionEvent---getRawX() getRawY()返回的是触摸点相对于屏幕的x/y距离 getX() getY()返回的是触摸点相对于自身控件的x/y距离
系列文章