Android View 绘制机制的详解

/**   * 请求所有子 View 去 measure 自己，要考虑的部分有对子 View 的测绘要求 MeasureSpec 以及其自身的 padding   * 这里跳过所有为 GONE 状态的子 View，最繁重的工作是在 getChildMeasureSpec 方法中处理的   *   * @param widthMeasureSpec 对该 View 的 width 测绘要求   * @param heightMeasureSpec 对该 View 的 height 测绘要求   */  protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {    final int size = mChildrenCount;    final View[] children = mChildren;    for (int i = 0; i < size; ++i) {      final View child = children[i];      if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {        measureChild(child, widthMeasureSpec, heightMeasureSpec);      }    }  }  protected void measureChild(View child, int parentWidthMeasureSpec,      int parentHeightMeasureSpec) {    final LayoutParams lp = child.getLayoutParams();//获取 Child 的 LayoutParams    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,// 获取 ChildView 的 widthMeasureSpec        mPaddingLeft + mPaddingRight, lp.width);    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,// 获取 ChildView 的 heightMeasureSpec        mPaddingTop + mPaddingBottom, lp.height);    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);  }  /**    * 该方法是 measureChildren 中最繁重的部分，为每一个 ChildView 计算出自己的 MeasureSpec。   * 目标是将 ChildView 的 MeasureSpec 和 LayoutParams 结合起来去得到一个最合适的结果。   *   * @param spec 对该 View 的测绘要求   * @param padding 当前 View 在当前唯独上的 paddingand，也有可能含有 margins   *   * @param childDimension 在当前维度上（height 或 width）的具体指   * @return 子视图的 MeasureSpec    */  public static int getChildMeasureSpec(int spec, int padding, int childDimension) {      .........    // 根据获取到的子视图的测量要求和大小创建子视图的 MeasureSpec    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);   }  /**   *   * 用于获取 View 最终的大小，父视图提供了宽、高的约束信息   * 一个 View 的真正的测量工作是在 onMeasure(int, int) 中，由该方法调用。   * 因此，只有 onMeasure(int, int) 可以而且必须被子类复写   *   * @param widthMeasureSpec 在水平方向上，父视图指定的的 Measure 要求   * @param heightMeasureSpec 在竖直方向上，控件上父视图指定的 Measure 要求   *   */  public final void measure(int widthMeasureSpec, int heightMeasureSpec) {   ...   onMeasure(widthMeasureSpec, heightMeasureSpec);   ...  }  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),        getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));  }

@Override  protected void onLayout(boolean changed, int l, int t, int r, int b) {    if (mOrientation == VERTICAL) {      layoutVertical(l, t, r, b);    } else {      layoutHorizontal(l, t, r, b);    }  }  /**   * 遍历所有的子 View，为其设置相对父视图的坐标   */  void layoutVertical(int left, int top, int right, int bottom) {  for (int i = 0; i < count; i++) {        final View child = getVirtualChildAt(i);        if (child == null) {          childTop += measureNullChild(i);        } else if (child.getVisibility() != GONE) {//不需要立即展示的 View 设置为 GONE 可加快绘制          final int childWidth = child.getMeasuredWidth();//measure 过程确定的 Width          final int childHeight = child.getMeasuredHeight();//measure 过程确定的 height          ...确定 childLeft、childTop 的值          setChildFrame(child, childLeft, childTop + getLocationOffset(child),              childWidth, childHeight);        }      }  }  private void setChildFrame(View child, int left, int top, int width, int height) {        child.layout(left, top, left + width, top + height);  }    View.java  public void layout(int l, int t, int r, int b) {    ...    setFrame(l, t, r, b)  }  /**   * 为该子 View 设置相对其父视图上的坐标   */   protected boolean setFrame(int left, int top, int right, int bottom) {    ...   }

 /**   * Manually render this view (and all of its children) to the given Canvas.   * The view must have already done a full layout before this function is   * called. When implementing a view, implement   * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.   * If you do need to override this method, call the superclass version.   *   * @param canvas The Canvas to which the View is rendered.    *   * 根据给定的 Canvas 自动渲染 View（包括其所有子 View）。在调用该方法之前必须要完成 layout。当你自定义 view 的时候，   * 应该去是实现 onDraw(Canvas) 方法，而不是 draw(canvas) 方法。如果你确实需要复写该方法，请记得先调用父类的方法。   */  public void draw(Canvas canvas) {    / * Draw traversal performs several drawing steps which must be executed     * in the appropriate order:     *     *   1. Draw the background if need     *   2. If necessary, save the canvas' layers to prepare for fading     *   3. Draw view's content     *   4. Draw children (dispatchDraw)     *   5. If necessary, draw the fading edges and restore layers     *   6. Draw decorations (scrollbars for instance)     */  // Step 1, draw the background, if needed    if (!dirtyOpaque) {      drawBackground(canvas);    }     // skip step 2 & 5 if possible (common case)    final int viewFlags = mViewFlags;    if (!verticalEdges && !horizontalEdges) {      // Step 3, draw the content      if (!dirtyOpaque) onDraw(canvas);      // Step 4, draw the children      dispatchDraw(canvas);      // Step 6, draw decorations (scrollbars)      onDrawScrollBars(canvas);      if (mOverlay != null && !mOverlay.isEmpty()) {        mOverlay.getOverlayView().dispatchDraw(canvas);      }      // we're done...      return;    }    // Step 2, save the canvas' layers    ...    // Step 3, draw the content    if (!dirtyOpaque)       onDraw(canvas);    // Step 4, draw the children    dispatchDraw(canvas);    // Step 5, draw the fade effect and restore layers    // Step 6, draw decorations (scrollbars)    onDrawScrollBars(canvas);  }

dispatchDraw(Canvas canvas){... if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {//处理 ChildView 的动画  final boolean buildCache = !isHardwareAccelerated();      for (int i = 0; i < childrenCount; i++) {        final View child = children[i];        if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {//只绘制 Visible 状态的布局，因此可以通过延时加载来提高效率          final LayoutParams params = child.getLayoutParams();          attachLayoutAnimationParameters(child, params, i, childrenCount);// 添加布局变化的动画          bindLayoutAnimation(child);//为 Child 绑定动画          if (cache) {            child.setDrawingCacheEnabled(true);            if (buildCache) {              child.buildDrawingCache(true);            }          }        }      }  final LayoutAnimationController controller = mLayoutAnimationController;      if (controller.willOverlap()) {        mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;      }  controller.start();// 启动 View 的动画} // 绘制 ChildView for (int i = 0; i < childrenCount; i++) {      int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;      final View child = (preorderedList == null)          ? children[childIndex] : preorderedList.get(childIndex);      if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {        more |= drawChild(canvas, child, drawingTime);      }    }...}protected boolean drawChild(Canvas canvas, View child, long drawingTime) {    return child.draw(canvas, this, drawingTime);}/**   * This method is called by ViewGroup.drawChild() to have each child view draw itself.   * This draw() method is an implementation detail and is not intended to be overridden or   * to be called from anywhere else other than ViewGroup.drawChild().   */  boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {    ...  }