图+源码，读懂View的Draw方法（二）-阿里云开发者社区

图+源码，读懂View的Draw方法（二）

2022-12-02 103

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简介： 本文将继续讲述 View 绘制三大方法的最后一个方法——Draw 方法。该方法不会很复杂，相信大家很快可以弄懂。

首先查看 step1 的 drawBackground() 方法，在下面的注释1处，我们可以其对偏移量进行了判断，当偏移量为0的时候，就直接绘制；当偏移量不为0的时候，我们先进行偏移再执行背景的绘制。注意此处执行的 draw 方法是在 Drawable 里面的。

/**
 * Draws the background onto the specified canvas.
 *
 * @param canvas Canvas on which to draw the background
 */
@UnsupportedAppUsage
private void drawBackground(Canvas canvas) {
    final Drawable background = mBackground;
    if (background == null) {
        return;
    }
    setBackgroundBounds();
    ...
    final int scrollX = mScrollX;
    final int scrollY = mScrollY;
    if ((scrollX | scrollY) == 0) {//1
        background.draw(canvas);
    } else {//2
        canvas.translate(scrollX, scrollY);
        background.draw(canvas);
        canvas.translate(-scrollX, -scrollY);
    }
}
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然后我们看 step 3 的 onDraw() 方法，我们发现这是个空实现的方法，在注释处标注了需要自定义 View 的时候实现它。

/**
 * Implement this to do your drawing.
 *
 * @param canvas the canvas on which the background will be drawn
 */
protected void onDraw(Canvas canvas) {
}
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我们继续看 step 4 的 dispatchDraw() 方法，这也是一个空方法。注释处标明：需要在派生类(继承类)绘制了自身之后，绘制子类之前，对该方法进行重写调用。在 ViewGroup 中，有对这个方法进行重写，我们可以继续到 ViewGroup 查看。

/**
 * Called by draw to draw the child views. This may be overridden
 * by derived classes to gain control just before its children are drawn
 * (but after its own view has been drawn).
 * @param canvas the canvas on which to draw the view
 */
protected void dispatchDraw(Canvas canvas) {
}
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在 ViewGroup 中，我们在下列注释1处看到 dispatchDraw() 方法对子元素执行了遍历，在注释2 3 处，可以看到其对每一个子 View 都执行了 drawChild() 方法。

@Override
protected void dispatchDraw(Canvas canvas) {
    ...
    for (int i = 0; i < childrenCount; i++) {//1
        while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
            final View transientChild = mTransientViews.get(transientIndex);
            if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                transientChild.getAnimation() != null) {
                more |= drawChild(canvas, transientChild, drawingTime);
            }
            transientIndex++;
            if (transientIndex >= transientCount) {
                transientIndex = -1;
            }
        }
        final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
        final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
        if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
            more |= drawChild(canvas, child, drawingTime);
        }
    }
    ...
}
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而点开 drawChild() 方法，我们发现它是直接调用了子 View 中 boolean 类型的 draw() 方法(并非我们上文提到的执行最终绘制的 draw() 方法)

/**
 * Draw one child of this View Group. This method is responsible for getting
 * the canvas in the right state. This includes clipping, translating so
 * that the child's scrolled origin is at 0, 0, and applying any animation
 * transformations.
 *
 * @param canvas The canvas on which to draw the child
 * @param child Who to draw
 * @param drawingTime The time at which draw is occurring
 * @return True if an invalidate() was issued
 */
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
    return child.draw(canvas, this, drawingTime);
}
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我们继续查看 View 中 boolean 类型的 draw() 方法，在注释1处检测是否无缓存，如果无缓存就直接执行绘制；如果有缓存，就在注释2处利用缓存显示。

/**
 * This method is called by ViewGroup.drawChild() to have each child view draw itself.
 *
 * This is where the View specializes rendering behavior based on layer type,
 * and hardware acceleration.
 */
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
  ...
    if (!drawingWithDrawingCache) {//1
        if (drawingWithRenderNode) {
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            ((RecordingCanvas) canvas).drawRenderNode(renderNode);
        } else {
            // Fast path for layouts with no backgrounds
            if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                dispatchDraw(canvas);
            } else {
                draw(canvas);
            }
        }
    } else if (cache != null) {//2，有缓存
        mPrivateFlags &= ~PFLAG_DIRTY_MASK;
        if (layerType == LAYER_TYPE_NONE || mLayerPaint == null) {
            // no layer paint, use temporary paint to draw bitmap
            ...
        } else {
            // use layer paint to draw the bitmap, merging the two alphas, but also restore
            ...
        }
    }
  ...
    return more;
}
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最后，我们来看 step 6 的 onDrawForeground() 方法，我们可以在下方的注释1处可以看到，它绘制的是 ScrollBar 类型的装饰。并在注释2处调用 foreground.draw() 方法绘制视图内容的上层视图。

/**
 * Draw any foreground content for this view.
 *
 * <p>Foreground content may consist of scroll bars, a {@link #setForeground foreground}
 * drawable or other view-specific decorations. The foreground is drawn on top of the
 * primary view content.</p>
 *
 * @param canvas canvas to draw into
 */
public void onDrawForeground(Canvas canvas) {
    onDrawScrollIndicators(canvas);
    onDrawScrollBars(canvas);//1
    final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
    if (foreground != null) {
        ...
        foreground.draw(canvas);//2
    }
}
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