Android View 绘制原理

我想了想，如果直接从ViewGroup里面的方法谈起，可能和网上很多博客一样了，但是如果只是向framework开发者分析哪些，又分析不到应用层，所以我觉得应该从performDraw()这个方法开始分析绘制，因为如果在向framework层深入，那就会接触到WindowManagerService，这个过程需要掌握Binder知识，但是Binder知识很多人一时半会掌握不了，尤其是对于application开发者，不关注这些，所以从performDraw()说起。

private void performDraw() {
    ...
    try {
        //fullRedrawNeeded是否全部绘制
        draw(fullRedrawNeeded);
    } finally {
        mIsDrawing = false;
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    ...
}

private void draw(boolean fullRedrawNeeded) {
    ...
    //获取mDirty，该值表示需要重绘的区域
    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;
    }

    //如果fullRedrawNeeded为真，则把dirty区域置为整个屏幕，表示整个视图都需要绘制
    //第一次绘制流程，需要绘制所有视图
    if (fullRedrawNeeded) {
        mAttachInfo.mIgnoreDirtyState = true;
        dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
    }
    ...
    if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                return;
        }
}

注意的核心点就是mDirty表示的是需要绘制的区域

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;

        //锁定canvas区域，由dirty区域决定
        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;
        }

        canvas.setDensity(mDensity);
    }

    try {

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

        dirty.setEmpty();
        mIsAnimating = false;
        attachInfo.mDrawingTime = SystemClock.uptimeMillis();
        mView.mPrivateFlags |= View.PFLAG_DRAWN;

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

            //正式开始绘制
            mView.draw(canvas);

        }
    }
    return true;
}

Canvas的使用都是大同小异，首先进行锁定区域，接着对canvas进行一系列的属性赋值最后调用了mView.draw(canvas)方法，下面就进入View的绘制，因为第一个view就是DecorView

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;


    //根据注释我们理解这些步骤
    /*
     * 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;
    //有背景先画背景
    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) {
        //画view自己的content，也就是我们熟知的onDraw方法
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        //画子view，自身的实现是空，但是对于ViewGruop这种需要实现
        dispatchDraw(canvas);
        //画自动填充高度的那些
        drawAutofilledHighlight(canvas);

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

        // Step 6, draw decorations (foreground, scrollbars)
        //绘制任何前景内容，比如ScrollBars，ScrollIndicators
        onDrawForeground(canvas);

        // Step 7, draw the default focus highlight
        //绘制默认焦点高亮
        drawDefaultFocusHighlight(canvas);

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

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

    //保存当前画布的堆栈状态并在该画布上创建Layer用于绘制View在滑动时的边框渐变效果，请参见代码第42-108行 
    //通常情况下我们是不需要处理这一步的
    /*
     * 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);

    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);

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

所以总结绘制步骤就是：

绘制流程的六个步骤：
1、对View的背景绘制
2、保存当前的图层信息(跳过)
3、绘制View的内容
4、对View的子View进行绘制一般情况是ViewGroup
5、绘制View的褪色的边缘，类似于阴影效果(跳过)
6、绘制View的装饰（例如：滚动条）