Android中View绘制流程详细介绍

时间：2021-02-17 14:03:44|栏目：Android代码|点击：次

创建Window

Window即窗口，这个概念在AndroidFramework中的实现为android.view.Window这个抽象类，这个抽象类是对Android系统中的窗口的抽象。在介绍这个类之前，我们先来看看究竟什么是窗口呢？

实际上，窗口是一个宏观的思想，它是屏幕上用于绘制各种UI元素及响应用户输入事件的一个矩形区域。通常具备以下两个特点：

独立绘制，不与其它界面相互影响；

不会触发其它界面的输入事件；

在Android系统中，窗口是独占一个Surface实例的显示区域，每个窗口的Surface由WindowManagerService分配。我们可以把Surface看作一块画布，应用可以通过Canvas或OpenGL在其上面作画。画好之后，通过SurfaceFlinger将多块Surface按照特定的顺序（即Z-order）进行混合，而后输出到FrameBuffer中，这样用户界面就得以显示。

android.view.Window这个抽象类可以看做Android中对窗口这一宏观概念所做的约定，而PhoneWindow这个类是Framework为我们提供的Android窗口概念的具体实现。接下来我们先来介绍一下android.view.Window这个抽象类。

这个抽象类包含了三个核心组件：

WindowManager.LayoutParams:窗口的布局参数；

Callback:窗口的回调接口，通常由Activity实现；

ViewTree:窗口所承载的控件树。

在Activity的attach方法中通过调用PolicyManager.makeNewWindo创建Window,将一个View add到WindowManager时，WindowManagerImpl创建一个ViewRoot来管理该窗口的根View。并通过ViewRoot.setView方法把该View传给ViewRoot。

final void attach(Context context, ActivityThread aThread, 
    Instrumentation instr, IBinder token, int ident, 
    Application application, Intent intent, ActivityInfo info, 
    CharSequence title, Activity parent, String id, 
    NonConfigurationInstances lastNonConfigurationInstances, 
    Configuration config) { 
  attachBaseContext(context); 
 
  mFragments.attachActivity(this, mContainer, null); 
   
  mWindow = PolicyManager.makeNewWindow(this); 
  mWindow.setCallback(this); 
  mWindow.getLayoutInflater().setPrivateFactory(this);

创建DecorView

DecorView为整个Window界面的最顶层View。
Activity中的Window对象帮我们创建了一个PhoneWindow内部类DecorView（父类为FrameLayout）窗口顶层视图，然后通过LayoutInflater将xml内容布局解析成View树形结构添加到DecorView顶层视图中id为content的FrameLayout父容器上面。Activity的content内容布局最终会添加到DecorView窗口顶层视图上面。

protected boolean initializePanelDecor(PanelFeatureState st) { 
  st.decorView = new DecorView(getContext(), st.featureId); 
  st.gravity = Gravity.CENTER | Gravity.BOTTOM; 
  st.setStyle(getContext()); 
 
  return true; 
}

创建ViewRoot并关联View

WindowManagerImpl保存DecorView到mViews，创建对应的ViewRoot；
ViewRoot用于管理窗口的根View，并和global window manger进行交互。ViewRoot中有一个nested class： W，W是一个Binder子类，用于接收global window manager的各种消息，如按键消息，触摸消息等。 ViewRoot有一个W类型的成员mWindow，ViewRoot在Constructor中创建一个W的instance并赋值给mWindow。 ViewRoot是Handler的子类， W会通过Looper把消息传递给ViewRoot。 ViewRoot在setView方法中把mWindow传给sWindowSession。

public void addView(View view, ViewGroup.LayoutParams params, 
    Display display, Window parentWindow) { 
  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); 
  } 
 
  ViewRootImpl root; 
  View panelParentView = null; 
 
  synchronized (mLock) { 
    // Start watching for system property changes. 
    if (mSystemPropertyUpdater == null) { 
      mSystemPropertyUpdater = new Runnable() { 
        @Override public void run() { 
          synchronized (mLock) { 
            for (ViewRootImpl viewRoot : mRoots) { 
              viewRoot.loadSystemProperties(); 
            } 
          } 
        } 
      }; 
      SystemProperties.addChangeCallback(mSystemPropertyUpdater); 
    } 
 
    int index = findViewLocked(view, false); 
    if (index >= 0) { 
      throw new IllegalStateException("View " + view 
          + " has already been added to the window manager."); 
    } 
 
    // If this is a panel window, then find the window it is being 
    // attached to for future reference. 
    if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW && 
        wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) { 
      final int count = mViews != null ? mViews.length : 0; 
      for (int i=0; i<count; i++) { 
        if (mRoots[i].mWindow.asBinder() == wparams.token) { 
          panelParentView = mViews[i]; 
        } 
      } 
    } 
 
    root = new ViewRootImpl(view.getContext(), display); 
 
    view.setLayoutParams(wparams); 
 
    if (mViews == null) { 
      index = 1; 
      mViews = new View[1]; 
      mRoots = new ViewRootImpl[1]; 
      mParams = new WindowManager.LayoutParams[1]; 
    } else { 
      index = mViews.length + 1; 
      Object[] old = mViews; 
      mViews = new View[index]; 
      System.arraycopy(old, 0, mViews, 0, index-1); 
      old = mRoots; 
      mRoots = new ViewRootImpl[index]; 
      System.arraycopy(old, 0, mRoots, 0, index-1); 
      old = mParams; 
      mParams = new WindowManager.LayoutParams[index]; 
      System.arraycopy(old, 0, mParams, 0, index-1); 
    } 
    index--; 
 
    mViews[index] = view; 
    mRoots[index] = root; 
    mParams[index] = wparams; 
  } 
 
  // do this last because it fires off messages to start doing things 
  try { 
    root.setView(view, wparams, panelParentView); 
  } catch (RuntimeException e) { 
    // BadTokenException or InvalidDisplayException, clean up. 
    synchronized (mLock) { 
      final int index = findViewLocked(view, false); 
      if (index >= 0) { 
        removeViewLocked(index, true); 
      } 
    } 
    throw e; 
  } 
}

ViewRoot是GUI管理系统与GUI呈现系统之间的桥梁，需要注意它并不是一个View类型，。
它的主要作用如下：

1、向DecorView分发收到的用户发起的event事件，如按键，触屏，轨迹球等事件；
2、与WindowManagerService交互，完成整个Activity的GUI的绘制。
View绘制基本流程

这里先给出Android系统View的绘制流程：依次执行View类里面的如下三个方法：

measure(int ,int) :测量View的大小
layout(int ,int ,int ,int) ：设置子View的位置
draw(Canvas) ：绘制View内容到Canvas画布上

整个View树的绘图流程是在ViewRoot.java类的performTraversals()函数展开的，该函数做的执行过程可简单概况为根据之前设置的状态，判断是否需要重新计算视图大小(measure)、是否重新需要安置视图的位置(layout)、以及是否需要重绘 (draw)
mesarue()测量过程

主要作用：为整个View树计算实际的大小，即设置实际的高(mMeasuredHeight)和宽(mMeasureWidth)，每个View的控件的实际宽高都是由父视图和本身视图决定的。

具体的调用如下：

ViewRootImpl 的performTraversals方法中，调用measureHierarchy，然后调用performMeasure

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { 
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); 
    try { 
      mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); 
    } finally { 
      Trace.traceEnd(Trace.TRACE_TAG_VIEW); 
    } 
  }

ViewRoot根对象地属性mView(其类型一般为ViewGroup类型)调用measure()方法去计算View树的大小，回调

View/ViewGroup对象的onMeasure()方法，该方法实现的功能如下：

1、设置本View视图的最终大小，该功能的实现通过调用setMeasuredDimension()方法去设置实际的高(mMeasuredHeight)和宽(mMeasureWidth)

2、如果该View对象是个ViewGroup类型，需要重写onMeasure()方法，对其子视图进行遍历的measure()过程。

对每个子视图的measure()过程，是通过调用父类ViewGroup.java类里的measureChildWithMargins()方法去实现，该方法内部只是简单地调用了View对象的measure()方法。

整个measure调用流程就是个树形的递归过程

measure()方法两个参数都是父View传递过来的，也就是代表了父view的规格。他由两部分组成，高2位表示MODE，定义在MeasureSpec类（View的内部类）中，有三种类型，MeasureSpec.EXACTLY表示确定大小，MeasureSpec.AT_MOST表示最大大小，MeasureSpec.UNSPECIFIED不确定。低30位表示size，也就是父View的大小。对于系统Window类的DecorVIew对象Mode一般都为MeasureSpec.EXACTLY，而size分别对应屏幕宽高。对于子View来说大小是由父View和子View共同决定的。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 
  setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), 
      getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); 
}

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) { 
   boolean optical = isLayoutModeOptical(this); 
   if (optical != isLayoutModeOptical(mParent)) { 
     Insets insets = getOpticalInsets(); 
     int opticalWidth = insets.left + insets.right; 
     int opticalHeight = insets.top + insets.bottom; 
 
     measuredWidth += optical ? opticalWidth : -opticalWidth; 
     measuredHeight += optical ? opticalHeight : -opticalHeight; 
   } 
   mMeasuredWidth = measuredWidth; 
   mMeasuredHeight = measuredHeight; 
 
   mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET; 
 }

layout布局过程

主要作用：为将整个根据子视图的大小以及布局参数将View树放到合适的位置上。
具体的调用如下：
ViewRootImpl 的performTraversals方法中，调用performLayout

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, 
    int desiredWindowHeight) { 
  mLayoutRequested = false; 
  mScrollMayChange = true; 
  mInLayout = true; 
 
  final View host = mView; 
  if (DEBUG_ORIENTATION || DEBUG_LAYOUT) { 
    Log.v(TAG, "Laying out " + host + " to (" + 
        host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")"); 
  } 
 
  Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout"); 
  try { 
    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); 
 
    mInLayout = false; 
    int numViewsRequestingLayout = mLayoutRequesters.size(); 
    if (numViewsRequestingLayout > 0) { 
      // requestLayout() was called during layout. 
      // If no layout-request flags are set on the requesting views, there is no problem. 
      // If some requests are still pending, then we need to clear those flags and do 
      // a full request/measure/layout pass to handle this situation. 
      ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, 
          false); 
      if (validLayoutRequesters != null) { 
        // Set this flag to indicate that any further requests are happening during 
        // the second pass, which may result in posting those requests to the next 
        // frame instead 
        mHandlingLayoutInLayoutRequest = true; 
 
        // Process fresh layout requests, then measure and layout 
        int numValidRequests = validLayoutRequesters.size(); 
        for (int i = 0; i < numValidRequests; ++i) { 
          final View view = validLayoutRequesters.get(i); 
          Log.w("View", "requestLayout() improperly called by " + view + 
              " during layout: running second layout pass"); 
          view.requestLayout(); 
        } 
        measureHierarchy(host, lp, mView.getContext().getResources(), 
            desiredWindowWidth, desiredWindowHeight); 
        mInLayout = true; 
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); 
 
        mHandlingLayoutInLayoutRequest = false; 
 
        // Check the valid requests again, this time without checking/clearing the 
        // layout flags, since requests happening during the second pass get noop'd 
        validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true); 
        if (validLayoutRequesters != null) { 
          final ArrayList<View> finalRequesters = validLayoutRequesters; 
          // Post second-pass requests to the next frame 
          getRunQueue().post(new Runnable() { 
            @Override 
            public void run() { 
              int numValidRequests = finalRequesters.size(); 
              for (int i = 0; i < numValidRequests; ++i) { 
                final View view = finalRequesters.get(i); 
                Log.w("View", "requestLayout() improperly called by " + view + 
                    " during second layout pass: posting in next frame"); 
                view.requestLayout(); 
              } 
            } 
          }); 
        } 
      } 
 
    } 
  } finally { 
    Trace.traceEnd(Trace.TRACE_TAG_VIEW); 
  } 
  mInLayout = false; 
}

host.layout()开始View树的布局，继而回调给View/ViewGroup类中的layout()方法。具体流程如下

1 、layout方法会设置该View视图位于父视图的坐标轴，即mLeft，mTop，mLeft，mBottom(调用setFrame()函数去实现)，接下来回调onLayout()方法(如果该View是ViewGroup对象，需要实现该方法，对每个子视图进行布局)。
2、如果该View是个ViewGroup类型，需要遍历每个子视图chiildView，调用该子视图的layout()方法去设置它的坐标值。

protected void onLayout(boolean changed, int left, int top, int right, int bottom) { 
}

public void layout(int l, int t, int r, int b) { 
  int oldL = mLeft; 
  int oldT = mTop; 
  int oldB = mBottom; 
  int oldR = mRight; 
  boolean changed = isLayoutModeOptical(mParent) ? 
      setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); 
  if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) { 
    onLayout(changed, l, t, r, b); 
    mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED; 
 
    ListenerInfo li = mListenerInfo; 
    if (li != null && li.mOnLayoutChangeListeners != null) { 
      ArrayList<OnLayoutChangeListener> listenersCopy = 
          (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone(); 
      int numListeners = listenersCopy.size(); 
      for (int i = 0; i < numListeners; ++i) { 
        listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB); 
      } 
    } 
  } 
  mPrivateFlags &= ~PFLAG_FORCE_LAYOUT; 
}

draw()绘图过程

ViewRootImpl的performTraversals方法中，调用了mView的draw方法

mView.draw()开始绘制，draw()方法实现的功能如下：

1、绘制该View的背景

2、为显示渐变框做一些准备操作

3、调用onDraw()方法绘制视图本身(每个View都需要重载该方法，ViewGroup不需要实现该方法)

4、调用dispatchDraw()方法绘制子视图(如果该View类型不为ViewGroup，即不包含子视图，不需要重载该方法)

值得说明的是，ViewGroup类已经为我们重写了dispatchDraw()的功能实现，应用程序一般不需要重写该方法，但可以重载父类函数实现具体的功能。

dispatchDraw()方法内部会遍历每个子视图，调用drawChild()去重新回调每个子视图的draw()方法。

5、绘制滚动条

刷新视图

Android中实现view的更新有两个方法，一个是invalidate，另一个是postInvalidate，其中前者是在UI线程自身中使用，而后者在非UI线程中使用。