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前文先后介绍了onMeasure和onLayout的流程，那么接下来就一起看一下onDraw方法吧。

Draw过程比较简单，主要作用是将View绘制到屏幕上面，先看一下draw方法的源码：

/**     * 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.     */    public void draw(Canvas canvas) {        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) {            drawBackground(canvas);        }        // 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;        }        /*         * Here we do the full fledged routine...         * (this is an uncommon case where speed matters less,         * this is why we repeat some of the tests that have been         * done above)         */        boolean drawTop = false;        boolean drawBottom = false;        boolean drawLeft = false;        boolean drawRight = false;        float topFadeStrength = 0.0f;        float bottomFadeStrength = 0.0f;        float leftFadeStrength = 0.0f;        float rightFadeStrength = 0.0f;        // Step 2, save the canvas' layers        int paddingLeft = mPaddingLeft;        final boolean offsetRequired = isPaddingOffsetRequired();        if (offsetRequired) {            paddingLeft += getLeftPaddingOffset();        }        int left = mScrollX + paddingLeft;        int right = left + mRight - mLeft - mPaddingRight - paddingLeft;        int top = mScrollY + getFadeTop(offsetRequired);        int bottom = top + getFadeHeight(offsetRequired);        if (offsetRequired) {            right += getRightPaddingOffset();            bottom += getBottomPaddingOffset();        }        final ScrollabilityCache scrollabilityCache = mScrollCache;        final float fadeHeight = scrollabilityCache.fadingEdgeLength;        int length = (int) fadeHeight;        // clip the fade length if top and bottom fades overlap        // overlapping fades produce odd-looking artifacts        if (verticalEdges && (top + length > bottom - length)) {            length = (bottom - top) / 2;        }        // also clip horizontal fades if necessary        if (horizontalEdges && (left + length > right - length)) {            length = (right - left) / 2;        }        if (verticalEdges) {            topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));            drawTop = topFadeStrength * fadeHeight > 1.0f;            bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));            drawBottom = bottomFadeStrength * fadeHeight > 1.0f;        }        if (horizontalEdges) {            leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));            drawLeft = leftFadeStrength * fadeHeight > 1.0f;            rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));            drawRight = rightFadeStrength * fadeHeight > 1.0f;        }        saveCount = canvas.getSaveCount();        int solidColor = getSolidColor();        if (solidColor == 0) {            final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;            if (drawTop) {                canvas.saveLayer(left, top, right, top + length, null, flags);            }            if (drawBottom) {                canvas.saveLayer(left, bottom - length, right, bottom, null, flags);            }            if (drawLeft) {                canvas.saveLayer(left, top, left + length, bottom, null, flags);            }            if (drawRight) {                canvas.saveLayer(right - length, top, right, bottom, null, flags);            }        } else {            scrollabilityCache.setFadeColor(solidColor);        }        // 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        final Paint p = scrollabilityCache.paint;        final Matrix matrix = scrollabilityCache.matrix;        final Shader fade = scrollabilityCache.shader;        if (drawTop) {            matrix.setScale(1, fadeHeight * topFadeStrength);            matrix.postTranslate(left, top);            fade.setLocalMatrix(matrix);            p.setShader(fade);            canvas.drawRect(left, top, right, top + length, p);        }        if (drawBottom) {            matrix.setScale(1, fadeHeight * bottomFadeStrength);            matrix.postRotate(180);            matrix.postTranslate(left, bottom);            fade.setLocalMatrix(matrix);            p.setShader(fade);            canvas.drawRect(left, bottom - length, right, bottom, p);        }        if (drawLeft) {            matrix.setScale(1, fadeHeight * leftFadeStrength);            matrix.postRotate(-90);            matrix.postTranslate(left, top);            fade.setLocalMatrix(matrix);            p.setShader(fade);            canvas.drawRect(left, top, left + length, bottom, p);        }        if (drawRight) {            matrix.setScale(1, fadeHeight * rightFadeStrength);            matrix.postRotate(90);            matrix.postTranslate(right, top);            fade.setLocalMatrix(matrix);            p.setShader(fade);            canvas.drawRect(right - length, top, right, bottom, p);        }        canvas.restoreToCount(saveCount);        // Step 6, draw decorations (scrollbars)        onDrawScrollBars(canvas);        if (mOverlay != null && !mOverlay.isEmpty()) {            mOverlay.getOverlayView().dispatchDraw(canvas);        }    }

由源码可以看出，View的绘制过程遵循如下几步：

View绘制过程的传递是通过dispatchDraw来实现的，dispatchDraw会便利调用所有子元素的draw方法，如此draw事件就被一层层的传递下去了。

View有一个特殊的方法setWillNotDraw，先看一下源码：

/**     * If this view doesn't do any drawing on its own, set this flag to     * allow further optimizations. By default, this flag is not set on     * View, but could be set on some View subclasses such as ViewGroup.     *     * Typically, if you override {@link #onDraw(android.graphics.Canvas)}     * you should clear this flag.     *     * @param willNotDraw whether or not this View draw on its own     */    public void setWillNotDraw(boolean willNotDraw) {        setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);    }

从注释中可以看出，如果一个View不需要绘制任何内容，那么设置这个标记位为true后，系统会进行相应的优化。默认情况下，View没有启用这个优化标记位，但是ViewGroup会默认启用这个优化标记位。

这个标记位对实际开发的意义：当我们的自定义控件继承于ViewGroup并且本身不具备绘制功能时，就可以开启这个标记位从而便于系统进行后续的优化。

当我们明确知道一个ViewGroup需要通过onDraw来绘制内容时，我们需要显示的关闭WILL_NOT_DRAW这个标记位。

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