浅谈ondraw的前世今身

相信很多人对ondraw都有疑惑，到底他的canvas参数是谁赋值的呢,下面用源码来分析下：
首先我们知道ondraw在view的源码里是一个空方法，具体还是要view去实现，当然调用者是view的draw方法,注意是一个参数的

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);

            drawAutofilledHighlight(canvas);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

            // Step 7, draw the default focus highlight
            drawDefaultFocusHighlight(canvas);

            if (debugDraw()) {
                debugDrawFocus(canvas);
            }

            // we're done...
            return;
        }

这里贴上draw的源码，我们重点看第4步是dispatchDraw（canvas）
也就是传统意义上所说的分发，这个方法一般来说是有viewgroup所重写的，包括继承viewGroup的类都不用重写此方法， 这点和viewGroup的onLayout不同，毕竟分发是固定的，确定子view的位置是不固定的，那我们去到dispatchDraw里看下是如何分发的

@Override
    protected void dispatchDraw(Canvas canvas) {
       。。。
       while (transientIndex >= 0) {
            // there may be additional transient views after the normal views
            final View transientChild = mTransientViews.get(transientIndex);
            if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                    transientChild.getAnimation() != null) {
                more |= drawChild(canvas, transientChild, drawingTime);
            }
            transientIndex++;
            if (transientIndex >= transientCount) {
                break;
            }
        }

可以看到drawchild里可以看到分发的影子了

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

咦，调用的是view三个参数的draw,我们在看下draw的重载方法
这个方法比较重要，重点说一下

/**
     * 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) {
        final boolean hardwareAcceleratedCanvas = canvas.isHardwareAccelerated();
        /* If an attached view draws to a HW canvas, it may use its RenderNode + DisplayList.
         *
         *是否支持硬件加速
         */
        boolean drawingWithRenderNode = mAttachInfo != null
                && mAttachInfo.mHardwareAccelerated
                && hardwareAcceleratedCanvas;

        boolean more = false;
        final boolean childHasIdentityMatrix = hasIdentityMatrix();
        final int parentFlags = parent.mGroupFlags;

        if ((parentFlags & ViewGroup.FLAG_CLEAR_TRANSFORMATION) != 0) {
            parent.getChildTransformation().clear();
            parent.mGroupFlags &= ~ViewGroup.FLAG_CLEAR_TRANSFORMATION;
        }

        Transformation transformToApply = null;
        boolean concatMatrix = false;
        final boolean scalingRequired = mAttachInfo != null && mAttachInfo.mScalingRequired;
        final Animation a = getAnimation();
        if (a != null) {
            more = applyLegacyAnimation(parent, drawingTime, a, scalingRequired);
            concatMatrix = a.willChangeTransformationMatrix();
            if (concatMatrix) {
                mPrivateFlags3 |= PFLAG3_VIEW_IS_ANIMATING_TRANSFORM;
            }
            transformToApply = parent.getChildTransformation();
        } else {
            if ((mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_TRANSFORM) != 0) {
                // No longer animating: clear out old animation matrix
                mRenderNode.setAnimationMatrix(null);
                mPrivateFlags3 &= ~PFLAG3_VIEW_IS_ANIMATING_TRANSFORM;
            }
            if (!drawingWithRenderNode
                    && (parentFlags & ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS) != 0) {
                final Transformation t = parent.getChildTransformation();
                final boolean hasTransform = parent.getChildStaticTransformation(this, t);
                if (hasTransform) {
                    final int transformType = t.getTransformationType();
                    transformToApply = transformType != Transformation.TYPE_IDENTITY ? t : null;
                    concatMatrix = (transformType & Transformation.TYPE_MATRIX) != 0;
                }
            }
        }

        concatMatrix |= !childHasIdentityMatrix;

        // Sets the flag as early as possible to allow draw() implementations
        // to call invalidate() successfully when doing animations
        mPrivateFlags |= PFLAG_DRAWN;

        if (!concatMatrix &&
                (parentFlags & (ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS |
                        ViewGroup.FLAG_CLIP_CHILDREN)) == ViewGroup.FLAG_CLIP_CHILDREN &&
                canvas.quickReject(mLeft, mTop, mRight, mBottom, Canvas.EdgeType.BW) &&
                (mPrivateFlags & PFLAG_DRAW_ANIMATION) == 0) {
            mPrivateFlags2 |= PFLAG2_VIEW_QUICK_REJECTED;
            return more;
        }
        mPrivateFlags2 &= ~PFLAG2_VIEW_QUICK_REJECTED;

        if (hardwareAcceleratedCanvas) {
            // Clear INVALIDATED flag to allow invalidation to occur during rendering, but
            // retain the flag's value temporarily in the mRecreateDisplayList flag
            mRecreateDisplayList = (mPrivateFlags & PFLAG_INVALIDATED) != 0;
            mPrivateFlags &= ~PFLAG_INVALIDATED;
        }

        RenderNode renderNode = null;
        Bitmap cache = null;
        int layerType = getLayerType(); // TODO: signify cache state with just 'cache' local
         /*
         *不支持硬件加速的绘制
         */
        if (layerType == LAYER_TYPE_SOFTWARE || !drawingWithRenderNode) {
             if (layerType != LAYER_TYPE_NONE) {
                 // If not drawing with RenderNode, treat HW layers as SW
                 layerType = LAYER_TYPE_SOFTWARE;
                 buildDrawingCache(true);
            }
            cache = getDrawingCache(true);
        }
         /*
         *支持硬件加速的绘制
         */
        if (drawingWithRenderNode) {
            // Delay getting the display list until animation-driven alpha values are
            // set up and possibly passed on to the view
            renderNode = updateDisplayListIfDirty();
            if (!renderNode.isValid()) {
                // Uncommon, but possible. If a view is removed from the hierarchy during the call
                // to getDisplayList(), the display list will be marked invalid and we should not
                // try to use it again.
                renderNode = null;
                drawingWithRenderNode = false;
            }
        }

我们都知道从3.0开始默认都开启硬件加速的，当然也可以手动关闭，
从源码中看，不支持的话调用的是buildDrawingCache这方法，里面在view的buildDrawingCacheImpl可以看到分发的代码

private void buildDrawingCacheImpl(boolean autoScale) {
        。。。
        //scroller常调用的方法
        computeScroll();
        final int restoreCount = canvas.save();

        if (autoScale && scalingRequired) {
            final float scale = attachInfo.mApplicationScale;
            canvas.scale(scale, scale);
        }
        //scroller参数为啥是负数的原因
        canvas.translate(-mScrollX, -mScrollY);

        mPrivateFlags |= PFLAG_DRAWN;
        if (mAttachInfo == null || !mAttachInfo.mHardwareAccelerated ||
                mLayerType != LAYER_TYPE_NONE) {
            mPrivateFlags |= PFLAG_DRAWING_CACHE_VALID;
        }

        draw的代码
        if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            dispatchDraw(canvas);
            drawAutofilledHighlight(canvas);
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().draw(canvas);
            }
        } else {
            draw(canvas);
        }

        canvas.restoreToCount(restoreCount);
        canvas.setBitmap(null);

        if (attachInfo != null) {
            // Restore the cached Canvas for our siblings
            attachInfo.mCanvas = canvas;
        }
    }

这里我们终于看到了viewGroup调用view的draw的代码了，也知道了canvas的构成和bitmap有着关系当然如果他的子view还是viewGroup的话，就会跳过draw，除非有背景，接下来我们看下如今基本所有默认设置的硬件加速的情况，是调用了一个叫view的updateDisplayListIfDirty的方法

public RenderNode updateDisplayListIfDirty() {
        final RenderNode renderNode = mRenderNode;
        if (!canHaveDisplayList()) {
            // can't populate RenderNode, don't try
            return renderNode;
        }

        if ((mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == 0
                || !renderNode.isValid()
                || (mRecreateDisplayList)) {
            // Don't need to recreate the display list, just need to tell our
            // children to restore/recreate theirs
            if (renderNode.isValid()
                    && !mRecreateDisplayList) {
                mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                dispatchGetDisplayList();

                return renderNode; // no work needed
            }

            // If we got here, we're recreating it. Mark it as such to ensure that
            // we copy in child display lists into ours in drawChild()
            mRecreateDisplayList = true;

            int width = mRight - mLeft;
            int height = mBottom - mTop;
            int layerType = getLayerType();

            final DisplayListCanvas canvas = renderNode.start(width, height);

            try {
                if (layerType == LAYER_TYPE_SOFTWARE) {
                    buildDrawingCache(true);
                    Bitmap cache = getDrawingCache(true);
                    if (cache != null) {
                        canvas.drawBitmap(cache, 0, 0, mLayerPaint);
                    }
                } else {
                    computeScroll();

                    canvas.translate(-mScrollX, -mScrollY);
                    mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
                    mPrivateFlags &= ~PFLAG_DIRTY_MASK;

                    // Fast path for layouts with no backgrounds
                    if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                        dispatchDraw(canvas);
                        drawAutofilledHighlight(canvas);
                        if (mOverlay != null && !mOverlay.isEmpty()) {
                            mOverlay.getOverlayView().draw(canvas);
                        }
                        if (debugDraw()) {
                            debugDrawFocus(canvas);
                        }
                    } else {
                        draw(canvas);
                    }
                }
            } finally {
                renderNode.end(canvas);
                setDisplayListProperties(renderNode);
            }
        } else {
            mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
        }
        return renderNode;
    }

很明显他这里又判断了一遍是否是硬件加速，当然我们也知道了canvas是有renderNode.start(width, height);方法得到的在else里面我们看到了分发的代码.

好了，基本都看完了，那现在有个问题就是谁第一个调用了draw方法呢，这得从viewRootImpl说起了，所有的分发都是从viewRootImpl的performTraversals方法开始的里面很明显可以看到调用了performDraw的代码

private void performDraw() {
      if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
          return;
      } else if (mView == null) {
          return;
      }

      final boolean fullRedrawNeeded = mFullRedrawNeeded || mReportNextDraw;
      mFullRedrawNeeded = false;

      mIsDrawing = true;
      Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");

      boolean usingAsyncReport = false;
      if (mReportNextDraw && mAttachInfo.mThreadedRenderer != null
              && mAttachInfo.mThreadedRenderer.isEnabled()) {
          usingAsyncReport = true;
          mAttachInfo.mThreadedRenderer.setFrameCompleteCallback((long frameNr) -> {
              // TODO: Use the frame number
              pendingDrawFinished();
          });
      }

      try {
          boolean canUseAsync = draw(fullRedrawNeeded);
          if (usingAsyncReport && !canUseAsync) {
              mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
              usingAsyncReport = false;
          }
      } finally {
          mIsDrawing = false;
          Trace.traceEnd(Trace.TRACE_TAG_VIEW);
      }

而后有调用了draw方法,此时又调用了ThreadedRenderer里的updateRootDisplayList方法

private void updateRootDisplayList(View view, DrawCallbacks callbacks) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Record View#draw()");
        updateViewTreeDisplayList(view);

        if (mRootNodeNeedsUpdate || !mRootNode.isValid()) {
            DisplayListCanvas canvas = mRootNode.start(mSurfaceWidth, mSurfaceHeight);
            try {
                final int saveCount = canvas.save();
                canvas.translate(mInsetLeft, mInsetTop);
                callbacks.onPreDraw(canvas);

                canvas.insertReorderBarrier();
                canvas.drawRenderNode(view.updateDisplayListIfDirty());
                canvas.insertInorderBarrier();

                callbacks.onPostDraw(canvas);
                canvas.restoreToCount(saveCount);
                mRootNodeNeedsUpdate = false;
            } finally {
                mRootNode.end(canvas);
            }
        }
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

我们可以看到onPreDraw也是这里执行回掉的，当然，最终也调用了
updateViewTreeDisplayList这里的view就是decorView，然后又会调用view的updateDisplayListIfDirty就行分发了

 private void updateViewTreeDisplayList(View view) {
        view.mPrivateFlags |= View.PFLAG_DRAWN;
        view.mRecreateDisplayList = (view.mPrivateFlags & View.PFLAG_INVALIDATED)
                == View.PFLAG_INVALIDATED;
        view.mPrivateFlags &= ~View.PFLAG_INVALIDATED;
        view.updateDisplayListIfDirty();
        view.mRecreateDisplayList = false;
    }

关于onDraw的差不多就说完了，也知道了canvas的由来，当然里面牵扯到的native方法和ThreadedRenderer以及DisplayList渲染过程分析都没有细讲(因为本人也了解的不是很清楚),有兴趣的可以看下老罗的android之旅..