Android中Window添加View的底层原理

一、WIndow和windowManager
Window是一个抽象类，它的具体实现是PhoneWindow，创建一个window很简单，只需要创建一个windowManager即可，window具体实现在windowManagerService中，windowManager和windowManagerService的交互是一个IPC的过程。
下面是用windowManager的例子:

mFloatingButton = new Button(this);
      mFloatingButton.setText( "window");
      mLayoutParams = new WindowManager.LayoutParams(
          LayoutParams. WRAP_CONTENT, LayoutParams.WRAP_CONTENT, 0, 0,
          PixelFormat. TRANSPARENT);
      mLayoutParams. flags = LayoutParams.FLAG_NOT_TOUCH_MODAL
          | LayoutParams. FLAG_NOT_FOCUSABLE
          | LayoutParams. FLAG_SHOW_WHEN_LOCKED;
      mLayoutParams. type = LayoutParams. TYPE_SYSTEM_ERROR;
      mLayoutParams. gravity = Gravity. LEFT | Gravity. TOP;
      mLayoutParams. x = 100;
      mLayoutParams. y = 300;
      mFloatingButton.setOnTouchListener( this);
      mWindowManager.addView( mFloatingButton, mLayoutParams);

flags和type两个属性很重要，下面对一些属性进行介绍，首先是flags：
FLAG_NOT_TOUCH_MODAL表示不需要获取焦点，也不需要接收各种输入，最终事件直接传递给下层具有焦点的window。
FLAG_NOT_FOCUSABLE：在此window外的区域单击事件传递到底层window中。当前的区域则自己处理，这个一般都要设置，很重要。
FLAG_SHOW_WHEN_LOCKED ：开启可以让window显示在锁屏界面上。
再来看下type这个参数：
window有三种类型：应用window，子window，系统window。应用类对应一个Activity，子Window不能单独存在，需要附属在父Window上，比如常用的Dialog。系统Window是需要声明权限再创建的window，如toast等。
window有z-ordered属性，层级越大，越在顶层。应用window层级1-99，子window1000-1999，系统2000-2999。这此层级对应着windowManager的type参数。系统层级常用的有两个TYPE_SYSTEM_OVERLAY或者TYPE_SYSTEM_ERROR。比如想用TYPE_SYSTEM_ERROR，只需
mLayoutParams.type = LayoutParams.TYPE_SYSTEM_ERROR。还要添加权限<uses-permission andorid:name="android.permission.SYSTEM_ALERT_WINDOW"/>。
有了对window的基本认识之后，我们来看下它底层如何实现加载View的。
二、window的创建
其实Window的创建跟之前我写的一篇博客LayoutInflater源码分析有点相似。Window的创建是在Activity创建的attach方法中，通过PolicyManager的makeNewWindow方法。Activity中实现了Window的Callback接口，因此当window状态改变时就会回调Activity方法。如onAttachedToWindow等。PolicyManager的真正实现类是Policy，看下它的代码：

public Window makeNewWindow(Context context) {
    return new PhoneWindow(context);
  }

到此Window创建完成。
下面分析view是如何附属到window上的。看Activity的setContentView方法。

public void setContentView(int layoutResID) {
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
  } 

两部分，设置内容和设置ActionBar。window的具体实现是PhoneWindow，看它的setContent。

public void setContentView(int layoutResID) {
    // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
    // decor, when theme attributes and the like are crystalized. Do not check the feature
    // before this happens.
    if (mContentParent == null) {
      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 {
      mLayoutInflater.inflate(layoutResID, mContentParent);
    }
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
      cb.onContentChanged();
    }
  }

看到了吧，又是分析它。
这里分三步执行：
1.如果没有DecorView，在installDecor中的generateDecor()创建DecorView。之前就分析过，这次就不再分析它了。
2.将View添加到decorview中的mContentParent中。
3.回调Activity的onContentChanged接口。
经过以上操作，DecorView创建了，但还没有正式添加到Window中。在ActivityResumeActivity中首先会调用Activity的onResume，再调用Activity的makeVisible，makeVisible中真正添加view ，代码如下：

void makeVisible() {
   if (!mWindowAdded) {
     ViewManager wm = getWindowManager();
     wm.addView(mDecor, getWindow().getAttributes());
     mWindowAdded = true;
   }
   mDecor.setVisibility(View.VISIBLE);
 }

通过上面的addView方法将View添加到Window。
三、Window操作View内部机制
1.window的添加
一个window对应一个view和一个viewRootImpl,window和view通过ViewRootImpl来建立联系，它并不存在，实体是view。只能通过 windowManager来操作它。
windowManager的实现类是windowManagerImpl。它并没有直接实现三大操作，而是委托给WindowManagerGlobal。addView的实现分为以下几步：
1）.检查参数是否合法。

if (view == null) {
      throw new IllegalArgumentException("view must not be null");
    }
    if (display == null) {
      throw new IllegalArgumentException("display must not be null");
    }
    if (!(params instanceof WindowManager.LayoutParams)) {
      throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
    } 

    final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;
    if (parentWindow != null) {
      parentWindow.adjustLayoutParamsForSubWindow(wparams);
    } else {
      // If there's no parent and we're running on L or above (or in the
      // system context), assume we want hardware acceleration.
      final Context context = view.getContext();
      if (context != null
          && context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.LOLLIPOP) {
        wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
      }
    } 

2）.创建ViewRootImpl并将View添加到列表中。

root = new ViewRootImpl(view.getContext(), display); 

      view.setLayoutParams(wparams); 

      mViews.add(view);
      mRoots.add(root);
      mParams.add(wparams); 

3）.通过ViewRootImpl来更新界面并完成window的添加过程 。
root.setView(view, wparams, panelParentView); 
上面的root就是ViewRootImpl，setView中通过requestLayout()来完成异步刷新，看下requestLayout：

public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
      checkThread();
      mLayoutRequested = true;
      scheduleTraversals();
    }
  }

接下来通过WindowSession来完成window添加过程，WindowSession是一个Binder对象，真正的实现类是 Session，window的添加是一次IPC调用。

 try {
          mOrigWindowType = mWindowAttributes.type;
          mAttachInfo.mRecomputeGlobalAttributes = true;
          collectViewAttributes();
          res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
              getHostVisibility(), mDisplay.getDisplayId(),
              mAttachInfo.mContentInsets, mAttachInfo.mStableInsets, mInputChannel);
        } catch (RemoteException e) {
          mAdded = false;
          mView = null;
          mAttachInfo.mRootView = null;
          mInputChannel = null;
          mFallbackEventHandler.setView(null);
          unscheduleTraversals();
          setAccessibilityFocus(null, null);
          throw new RuntimeException("Adding window failed", e);
} 

在Session内部会通过WindowManagerService来实现Window的添加。

public int addToDisplay(IWindow window, int seq, WindowManager.LayoutParams attrs,
     int viewVisibility, int displayId, Rect outContentInsets, Rect outStableInsets,
     InputChannel outInputChannel) {
   return mService.addWindow(this, window, seq, attrs, viewVisibility, displayId,
       outContentInsets, outStableInsets, outInputChannel);
 } 

在WindowManagerService内部会为每一个应用保留一个单独的session。
2.window的删除
看下WindowManagerGlobal的removeView：

public void removeView(View view, boolean immediate) {
    if (view == null) {
      throw new IllegalArgumentException("view must not be null");
    } 

    synchronized (mLock) {
      int index = findViewLocked(view, true);
      View curView = mRoots.get(index).getView();
      removeViewLocked(index, immediate);
      if (curView == view) {
        return;
      } 

      throw new IllegalStateException("Calling with view " + view
          + " but the ViewAncestor is attached to " + curView);
    }
  } 

首先调用findViewLocked来查找删除view的索引，这个过程就是建立数组遍历。然后再调用removeViewLocked来做进一步的删除。

private void removeViewLocked(int index, boolean immediate) {
    ViewRootImpl root = mRoots.get(index);
    View view = root.getView(); 

    if (view != null) {
      InputMethodManager imm = InputMethodManager.getInstance();
      if (imm != null) {
        imm.windowDismissed(mViews.get(index).getWindowToken());
      }
    }
    boolean deferred = root.die(immediate);
    if (view != null) {
      view.assignParent(null);
      if (deferred) {
        mDyingViews.add(view);
      }
    }
  } 

真正删除操作是viewRootImpl来完成的。windowManager提供了两种删除接口，removeViewImmediate，removeView。它们分别表示异步删除和同步删除。具体的删除操作由ViewRootImpl的die来完成。

boolean die(boolean immediate) {
    // Make sure we do execute immediately if we are in the middle of a traversal or the damage
    // done by dispatchDetachedFromWindow will cause havoc on return.
    if (immediate && !mIsInTraversal) {
      doDie();
      return false;
    } 

    if (!mIsDrawing) {
      destroyHardwareRenderer();
    } else {
      Log.e(TAG, "Attempting to destroy the window while drawing!\n" +
          " window=" + this + ", title=" + mWindowAttributes.getTitle());
    }
    mHandler.sendEmptyMessage(MSG_DIE);
    return true;
  } 

由上可知如果是removeViewImmediate，立即调用doDie，如果是removeView，用handler发送消息，ViewRootImpl中的Handler会处理消息并调用doDie。重点看下doDie:

void doDie() {
    checkThread();
    if (LOCAL_LOGV) Log.v(TAG, "DIE in " + this + " of " + mSurface);
    synchronized (this) {
      if (mRemoved) {
        return;
      }
      mRemoved = true;
      if (mAdded) {
        dispatchDetachedFromWindow();
      } 

      if (mAdded && !mFirst) {
        destroyHardwareRenderer(); 

        if (mView != null) {
          int viewVisibility = mView.getVisibility();
          boolean viewVisibilityChanged = mViewVisibility != viewVisibility;
          if (mWindowAttributesChanged || viewVisibilityChanged) {
            // If layout params have been changed, first give them
            // to the window manager to make sure it has the correct
            // animation info.
            try {
              if ((relayoutWindow(mWindowAttributes, viewVisibility, false)
                  & WindowManagerGlobal.RELAYOUT_RES_FIRST_TIME) != 0) {
                mWindowSession.finishDrawing(mWindow);
              }
            } catch (RemoteException e) {
            }
          } 

          mSurface.release();
        }
      } 

      mAdded = false;
    }
    WindowManagerGlobal.getInstance().doRemoveView(this);
  }

主要做四件事:
1.垃圾回收相关工作，比如清数据，回调等。
2.通过Session的remove方法删除Window,最终调用WindowManagerService的removeWindow

3.调用dispathDetachedFromWindow，在内部会调用onDetachedFromWindow()和onDetachedFromWindowInternal()。当view移除时会调用onDetachedFromWindow，它用于作一些资源回收。
4.通过doRemoveView刷新数据，删除相关数据，如在mRoot，mDyingViews中删除对象等。

void doRemoveView(ViewRootImpl root) {
    synchronized (mLock) {
      final int index = mRoots.indexOf(root);
      if (index >= 0) {
        mRoots.remove(index);
        mParams.remove(index);
        final View view = mViews.remove(index);
        mDyingViews.remove(view);
      }
    }
    if (HardwareRenderer.sTrimForeground && HardwareRenderer.isAvailable()) {
      doTrimForeground();
    }
  }

3.更新window
看下WindowManagerGlobal中的updateViewLayout。

public void updateViewLayout(View view, ViewGroup.LayoutParams params) {
    if (view == null) {
      throw new IllegalArgumentException("view must not be null");
    }
    if (!(params instanceof WindowManager.LayoutParams)) {
      throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
    } 

    final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params; 

    view.setLayoutParams(wparams); 

    synchronized (mLock) {
      int index = findViewLocked(view, true);
      ViewRootImpl root = mRoots.get(index);
      mParams.remove(index);
      mParams.add(index, wparams);
      root.setLayoutParams(wparams, false);
    }
  }

通过viewRootImpl的setLayoutParams更新viewRootImpl的layoutParams,接着scheduleTraversals对view重新布局，包括测量，布局，重绘，此外它还会通过WindowSession来更新window。这个过程由WindowManagerService实现。这跟上面类似，就不再重复，到此Window底层源码就分析完啦。

以上就是本文的全部内容,希望对大家的学习有所帮助。