第十二章 View绘制流程

2019-01-02  本文已影响0人  唔笛plk

一、Android View树结构

7690104-d1f298425e08b617.png QQ截图20181226160721.png

二、View绘制机制

1.View绘制的流程：measure(测量)->layout(布局)->draw(绘制)

view_draw_method_chain.png
2397836-19c08de6439514a7.png

2. measure(测量):判断是否需要重新计算View的大小，需要的话则计算

3. measure 核心方法

measurechildflow.png 
 /**
     * 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));
    }

3. layout(布局)： 判断是否需要重新计算View的位置，需要的话则计算

@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) {
         ...
     }

4. draw(绘制)：判断是否需要重新绘制View，需要的话则重绘制

4.1 绘制流程相关概念及核心方法

 /**
     * 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);
    }

由上面的处理过程，我们也可以得出一些优化的小技巧：当不需要绘制 Layer 的时候第二步和第五步会跳过。因此在绘制的时候，能省的 layer 尽可省，可以提高绘制效率

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) {
        ...
    }

转自：http://www.codekk.com/blogs/detail/54cfab086c4761e5001b253f

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