View的draw过程

View的Draw相比就比较简单了，它的作用是将View绘制到屏幕上面。View的绘制过程遵循如下几步。

1)绘制背景，通过background.draw方法。

2）绘制自己，通过onDraw方法

3）绘制子元素，dispatchDraw方法

4）绘制装饰，通过onDrawDScrollBars方法

可以通过源码来验证一下：

    public void draw(Canvas canvas) {        if (mClipBounds != null) {            canvas.clipRect(mClipBounds);        }        final int privateFlags = mPrivateFlags;        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;        /*         * Draw traversal performs several drawing steps which must be executed         * in the appropriate order:         *         *      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)         */        // Step 1, draw the background, if needed        int saveCount;        if (!dirtyOpaque) {            final Drawable background = mBackground;            if (background != null) {                final int scrollX = mScrollX;                final int scrollY = mScrollY;                if (mBackgroundSizeChanged) {                    background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);                    mBackgroundSizeChanged = false;                }                if ((scrollX | scrollY) == 0) {                    background.draw(canvas);                } else {                    canvas.translate(scrollX, scrollY);                    background.draw(canvas);                    canvas.translate(-scrollX, -scrollY);                }            }        }        // skip step 2 & 5 if possible (common case)        final int viewFlags = mViewFlags;        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;        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;        }

从源码中的注释就可以看得出来，step1-step5的一个过程。

View的绘制过程的传递是通过dispatchDraw来实现的，dispatchDraw会遍历调用所有子元素的draw方法，如此draw事件就一层一层传递下去了。
实际上View的onDraw，dispatchDraw都没有实现，交给子类去实现。原因很简单，具体的绘制过程和具体的内容有关，所以交给子类去实现。

    /**     * Implement this to do your drawing.     *     * @param canvas the canvas on which the background will be drawn     */    protected void onDraw(Canvas canvas) {    }

    protected void dispatchDraw(Canvas canvas) {    }

下面以LinearLayout为例分析，它的onDraw方法如下：

    @Override    protected void onDraw(Canvas canvas) {        if (mDivider == null) {            return;        }        if (mOrientation == VERTICAL) {            drawDividersVertical(canvas);        } else {            drawDividersHorizontal(canvas);        }    }

分为水平和垂直两个方向。以drawDividersVertical为例。drawDividersVertical的源码如下：

    void drawDividersVertical(Canvas canvas) {        final int count = getVirtualChildCount();        for (int i = 0; i < count; i++) {            final View child = getVirtualChildAt(i);            if (child != null && child.getVisibility() != GONE) {                if (hasDividerBeforeChildAt(i)) {                    final LayoutParams lp = (LayoutParams) child.getLayoutParams();                    final int top = child.getTop() - lp.topMargin - mDividerHeight;                    drawHorizontalDivider(canvas, top);                }            }        }        if (hasDividerBeforeChildAt(count)) {            final View child = getVirtualChildAt(count - 1);            int bottom = 0;            if (child == null) {                bottom = getHeight() - getPaddingBottom() - mDividerHeight;            } else {                final LayoutParams lp = (LayoutParams) child.getLayoutParams();                bottom = child.getBottom() + lp.bottomMargin;            }            drawHorizontalDivider(canvas, bottom);        }    }

    drawDividersVertical方法的大致流程如下，其核心代码主要是for循环中的代码，它会遍历每一个子元素，但是这里并没有真正绘制，因为LinearLayout属于ViewGroup，真正的绘制在于其子元素，LinearLayout只是简单实现了一下分割线的处理而已。

下面看一下ViewGroup的dispatchDraw方法，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。

    @Override    protected void dispatchDraw(Canvas canvas) {        final int count = mChildrenCount;        final View[] children = mChildren;        int flags = mGroupFlags;<span style="white-space:pre"></span>        if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {            final boolean cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;            final boolean buildCache = !isHardwareAccelerated();            for (int i = 0; i < count; i++) {                final View child = children[i];                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {                    final LayoutParams params = child.getLayoutParams();                    attachLayoutAnimationParameters(child, params, i, count);                    bindLayoutAnimation(child);                    if (cache) {                        child.setDrawingCacheEnabled(true);                        if (buildCache) {                                                    child.buildDrawingCache(true);                        }                    }                }            }            final LayoutAnimationController controller = mLayoutAnimationController;            if (controller.willOverlap()) {                mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;            }            controller.start();            mGroupFlags &= ~FLAG_RUN_ANIMATION;            mGroupFlags &= ~FLAG_ANIMATION_DONE;            if (cache) {                mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE;            }            if (mAnimationListener != null) {                mAnimationListener.onAnimationStart(controller.getAnimation());            }        }        int saveCount = 0;        final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;        if (clipToPadding) {            saveCount = canvas.save();            canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,                    mScrollX + mRight - mLeft - mPaddingRight,                    mScrollY + mBottom - mTop - mPaddingBottom);        }        // We will draw our child's animation, let's reset the flag        mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;        mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;        boolean more = false;        final long drawingTime = getDrawingTime();        if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {            for (int i = 0; i < count; i++) {                final View child = children[i];                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {                    more |= drawChild(canvas, child, drawingTime);                }            }        } else {            for (int i = 0; i < count; i++) {                final View child = children[getChildDrawingOrder(count, i)];                if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {                    more |= drawChild(canvas, child, drawingTime);                }            }        }        // Draw any disappearing views that have animations        if (mDisappearingChildren != null) {            final ArrayList<View> disappearingChildren = mDisappearingChildren;            final int disappearingCount = disappearingChildren.size() - 1;            // Go backwards -- we may delete as animations finish            for (int i = disappearingCount; i >= 0; i--) {                final View child = disappearingChildren.get(i);                more |= drawChild(canvas, child, drawingTime);            }        }        if (debugDraw()) {            onDebugDraw(canvas);        }        if (clipToPadding) {            canvas.restoreToCount(saveCount);        }        // mGroupFlags might have been updated by drawChild()        flags = mGroupFlags;        if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {            invalidate(true);        }        if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&                mLayoutAnimationController.isDone() && !more) {            // We want to erase the drawing cache and notify the listener after the            // next frame is drawn because one extra invalidate() is caused by            // drawChild() after the animation is over            mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;            final Runnable end = new Runnable() {               public void run() {                   notifyAnimationListener();               }            };            post(end);        }    }

drawChild的源码如下，仅仅是调用子元素的draw方法而已：

    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {        return child.draw(canvas, this, drawingTime);    }