Android绘制链图：

网上很多讲Android  view的绘制流程往往只讲到了Measure - Layout - Draw。

但是，这只是一个大体的流程，而我们需要探讨的是Android在我们调用setcontentView（）之后，系统给我们干了什么事情，这个完整的逻辑是什么样的，却很少有人讲，还是先看下系统代码吧。

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

<span style="font-size: 16px;"> </span><span style="font-size:14px;"> private void initWindowDecorActionBar() {
        Window window = getWindow();
        // Initializing the window decor can change window feature flags.
        // Make sure that we have the correct set before performing the test below.
        window.getDecorView();

        if (isChild() || !window.hasFeature(Window.FEATURE_ACTION_BAR) || mActionBar != null) {
            return;
        }

        mActionBar = new WindowDecorActionBar(this);
        mActionBar.setDefaultDisplayHomeAsUpEnabled(mEnableDefaultActionBarUp);

        mWindow.setDefaultIcon(mActivityInfo.getIconResource());
        mWindow.setDefaultLogo(mActivityInfo.getLogoResource());
    }</span>

window.getDecorView();

<span style="font-size:14px;">private void performTraversals() {  
    final View host = mView;  
    ...  
    host.measure(childWidthMeasureSpec, childHeightMeasureSpec);  
    ...  
    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());  
    ...  
    draw(fullRedrawNeeded);  
} </span>

调用然后系统再调用Measure - Layout - Draw实现了View的绘制。

我们看一下完整的绘制流程，直接上一张图，或许更能说明这个意思：

到这里，系统会调用我们之前的比较熟悉的几个方法:Measure - Layout - Draw

Measure

public final void measure(int widthMeasureSpec, int heightMeasureSpec)  
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight)  
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)  

measure 源码：

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {  
    if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||  
            widthMeasureSpec != mOldWidthMeasureSpec ||  
            heightMeasureSpec != mOldHeightMeasureSpec) {  
  
        // first clears the measured dimension flag  
        mPrivateFlags &= ~MEASURED_DIMENSION_SET;  
  
        if (ViewDebug.TRACE_HIERARCHY) {  
            ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);  
        }  
  
        // measure ourselves, this should set the measured dimension flag back  
        onMeasure(widthMeasureSpec, heightMeasureSpec);  
  
        // flag not set, setMeasuredDimension() was not invoked, we raise  
        // an exception to warn the developer  
        if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {  
            throw new IllegalStateException("onMeasure() did not set the"  
                    + " measured dimension by calling"  
                    + " setMeasuredDimension()");  
        }  
  
        mPrivateFlags |= LAYOUT_REQUIRED;  
    }  
  
    mOldWidthMeasureSpec = widthMeasureSpec;  
    mOldHeightMeasureSpec = heightMeasureSpec;  
}  

 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;
        }
        setMeasuredDimensionRaw(measuredWidth, measuredHeight);
    }

public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }

Layout

public void layout(int l, int t, int r, int b) {
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        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;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
    }

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

比如Linearlayout:

 protected void onLayout(boolean changed, int l, int t, int r, int b) {  
      if (mOrientation == VERTICAL) {  
          layoutVertical();  
      } else {  
          layoutHorizontal();  
      }  
  } 

而在最后无论是layoutVertical还是layoutHorizontal都会掉一个setChildFrame方法来控制显示位置。

private void setChildFrame(View child, int left, int top, int width, int height) {          
    child.layout(left, top, left + width, top + height);  
}  

Draw

*      1. Draw the background
*      2. If necessary, save the canvas' layers to prepare for fading
*      3. Draw view's content
*      4. Draw children
*      5. If necessary, draw the fading edges and restore layers
*      6. Draw decorations (scrollbars for instance)

1，绘制背景

2，保存画布图层

3，调用了onDraw方法，子类中实现onDraw方法

4，使用的dispatchDraw方法

View或ViewGroup的子类不用再重载ViewGroup中该方法，因为它已经有了默认而且标准的view系统流程。dispatchDraw()内部for循环调用drawChild()分别绘制每一个子视图，而drawChild()内部又会调用draw（）函数完成子视图的内部绘制工作。

有兴趣的可以看看onDraw的源码。