深入理解 Android 之 View 的绘制流程(四)_Draw

前面我们介绍了Measure和layout，下面我们就开始分析Draw。先从ViewRootImpl中开始。
ViewRootImpl#performDraw

private void performDraw() {
        if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
            return;
        }
        //是否需要重绘标志
        final boolean fullRedrawNeeded = mFullRedrawNeeded;
        mFullRedrawNeeded = false;

        mIsDrawing = true;
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
        try {
            draw(fullRedrawNeeded);
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        //省略部分代码
        . . . . . . .
}

ViewRootImpl#draw

private void draw(boolean fullRedrawNeeded) {
        。。。。。。
        //获取mDirty，该值表示需要绘制的区域
        final Rect dirty = mDirty;
        if (mSurfaceHolder != null) {
            // The app owns the surface, we won't draw.
            dirty.setEmpty();
            if (animating) {
                if (mScroller != null) {
                    mScroller.abortAnimation();
                }
                disposeResizeBuffer();
            }
            return;
        }
        //设置绘制区域的大小
        if (fullRedrawNeeded) {
            mAttachInfo.mIgnoreDirtyState = true;
            dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
        }
        //回调OnDrawListener中的onDraw()方法。
        mAttachInfo.mTreeObserver.dispatchOnDraw();

        int xOffset = 0;
        int yOffset = curScrollY;
        final WindowManager.LayoutParams params = mWindowAttributes;
        final Rect surfaceInsets = params != null ? params.surfaceInsets : null;
        if (surfaceInsets != null) {
            xOffset -= surfaceInsets.left;
            yOffset -= surfaceInsets.top;

            // Offset dirty rect for surface insets.
            dirty.offset(surfaceInsets.left, surfaceInsets.right);
        }

        if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
            if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) {
                . . . . . . . .
                mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this);
            } else {
                // 没有开启硬件加速
                . . . . . . . . .

                if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                    return;
                }
            }
        }
                . . . . . . . . .
    }

上面的方法中，可以看到在执行绘制之前，首先获取需要绘制的区域，设置区域的大小后，然后再调用ViewTreeObserver的回调方法dispatchOnDraw，如果用户设置了OnDrawListener，那么此时就会执行里面的onDraw()方法，通知绘制开始。最后调用方法drawSoftware方法来开始绘制。

ViewRootImpl#drawSoftware

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
            boolean scalingRequired, Rect dirty) {

        // Draw with software renderer.
        final Canvas canvas;
        try {
            final int left = dirty.left;
            final int top = dirty.top;
            final int right = dirty.right;
            final int bottom = dirty.bottom;
            // 初始化画布和固定绘制区域
            canvas = mSurface.lockCanvas(dirty);

            // The dirty rectangle can be modified by Surface.lockCanvas()
            //noinspection ConstantConditions
            if (left != dirty.left || top != dirty.top || right != dirty.right
                    || bottom != dirty.bottom) {
                attachInfo.mIgnoreDirtyState = true;
            }

            // TODO: Do this in native
            canvas.setDensity(mDensity);
        } catch (Surface.OutOfResourcesException e) {
            handleOutOfResourcesException(e);
            return false;
        } catch (IllegalArgumentException e) {
            mLayoutRequested = true;    // ask wm for a new surface next time.
            return false;
        }

        try {

            if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
                canvas.drawColor(0, PorterDuff.Mode.CLEAR);
            }

            dirty.setEmpty();
            mIsAnimating = false;
            mView.mPrivateFlags |= View.PFLAG_DRAWN;

            try {
                canvas.translate(-xoff, -yoff);
                if (mTranslator != null) {
                    mTranslator.translateCanvas(canvas);
                }
                canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
                attachInfo.mSetIgnoreDirtyState = false;

                mView.draw(canvas);

                drawAccessibilityFocusedDrawableIfNeeded(canvas);
            } finally {
                if (!attachInfo.mSetIgnoreDirtyState) {
                    // Only clear the flag if it was not set during the mView.draw() call
                    attachInfo.mIgnoreDirtyState = false;
                }
            }
        } finally {
            try {
                surface.unlockCanvasAndPost(canvas);
            } catch (IllegalArgumentException e) {
                Log.e(TAG, "Could not unlock surface", e);
                mLayoutRequested = true;    // ask wm for a new surface next time.
                //noinspection ReturnInsideFinallyBlock
                return false;
            }

            if (LOCAL_LOGV) {
                Log.v(TAG, "Surface " + surface + " unlockCanvasAndPost");
            }
        }
        return true;
}

我们从上面的方法中可以分析出，开始时初始化了一个Canvas对象，设置了绘制区域的大小。然后再调用View的draw方法。由于ViewGroup没有重写draw方法，所以我们直接进入View中的draw方法。

View#draw

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;
        // 根据是否透明来确定是否绘制背景色
        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);

            // 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);

            // 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);

        // 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);
}

从上面可以看到draw方法里执行的操作还是蛮复杂的，但是里面也给我们清楚的注释了每一步的过程。一开始的标记位dirtyOpaque，该标记位的作用是判断当前View是否是透明的，根据标志位，下面将执行不同的操作。
step1 : 如果不是透明的，将执行drawBackground来绘制背景色；
step2：保存当前viewFlags 的信息；
step3：绘制View的内容，执行onDraw；
step4：对View的子View进行绘制，执行dispatchDraw(canvas)；
step5：绘制View的边缘，类似于阴影效果；
step6：绘制View的装饰，执行onDrawForeground。

第一步：绘制View的背景
View#drawBackground

private void drawBackground(Canvas canvas) {
        final Drawable background = mBackground;
        if (background == null) {
            return;
        }
        //设置背景边距
        setBackgroundBounds();
        // Attempt to use a display list if requested.
        if (canvas.isHardwareAccelerated() && mAttachInfo != null
                && mAttachInfo.mHardwareRenderer != null) {
            mBackgroundRenderNode = getDrawableRenderNode(background, mBackgroundRenderNode);

            final RenderNode renderNode = mBackgroundRenderNode;
            if (renderNode != null && renderNode.isValid()) {
                setBackgroundRenderNodeProperties(renderNode);
                ((DisplayListCanvas) canvas).drawRenderNode(renderNode);
                return;
            }
        }

        final int scrollX = mScrollX;
        final int scrollY = mScrollY;
        if ((scrollX | scrollY) == 0) {
            background.draw(canvas);
        } else {
            canvas.translate(scrollX, scrollY);
            background.draw(canvas);
            canvas.translate(-scrollX, -scrollY);
        }
    }

第三步：绘制View的内容
View#onDraw

protected void onDraw(Canvas canvas) {
}

通过该方法来绘制自身内容，这是个空方法需要他的子类来实现具体逻辑。

第四步：对当前View的所有子View进行绘制，如果没有就不需要绘制
View#dispatchDraw

protected void dispatchDraw(Canvas canvas) {
}

如上，这也是一个空方法，需要子类去实现它。我们可以去看ViewGroup的dispatchDraw方法。

ViewGroup#dispatchDraw

@Override
protected void dispatchDraw(Canvas canvas) {
        boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
        final int childrenCount = mChildrenCount; //子View的数量
        final View[] children = mChildren;  //子View的对象保存在数组中
        int flags = mGroupFlags;
        . . . . . . .
        boolean more = false;
         . . . . . . .
        for (int i = 0; i < childrenCount; i++) {
            . . . . . . .
            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);
            }
        }
        . . . . . . .
}

ViewGroup中重写了dispatchDraw方法，方法里面会遍历每个子View，然后调用drawChild方法。

ViewGroup#drawChild

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

如上，drawChild方法中又调用了View的draw方法，如此便完成了对子View的绘制。
第六步：绘制View的滚动条
View#onDrawForeground

public void onDrawForeground(Canvas canvas) {
        onDrawScrollIndicators(canvas);
        onDrawScrollBars(canvas);
        . . . . . . .
}

绘制滚动条不做具体分析，但是我们知道每个View都有滚动条，一般情况下没有让它显示。