Android onDraw()繪制流程解析

文章目錄

- Android onDraw()繪制流程解析
測量與布局流程文章
元件繪制的那些結論
1. ViewRootImpl的performDraw()方法
2. View 類的 draw(Canvas canvas) 方法解析

測量與布局流程文章

Android onMeasure()測量流程解析

Android onLayout()布局流程解析

元件繪制的那些結論

先看結論再看分析

繪制流程開始于performDraw()方法，該方法會調用根View（DecorView，也就是FrameLayout）draw()方法，是以繪制流程開始于ViewGroup的draw()方法。
draw()方法負責元件自身的繪制，元件自身繪制分為7步：

1、）繪制View的背景

2、）如果有必要，儲存目前的繪制圖層，為繪制Fading Edge做準備

3、）繪制View的内容，調用了onDraw(canvas)，因為每個View繪制的内容都有自己要求，是以View類的onDraw(canvas)方法是個空實作。

4、）對目前View的子View進行繪制(元件是ViewGroup并且有子View)，會調用了dispatchDraw(canvas)方法，顧名思義，是向子View分發繪制事件，View類是沒有子View，是以View類的dispatchDraw(canvas)方法是個空實作，ViewGroup類可以有子View，是以ViewGroup類真正實作了dispatchDraw(canvas)方法：

該方法會循環周遊了所有的子View，并調用drawChild（）方法，drawChild（）方法最後還是調用了子View的draw(）方法進行子View自身的繪制，如果子View還是ViewGroup,則會重複上述過程，直到所有的View繪制完成。這就是整個繪制流程。

5、）繪制View的Fading Edge，第二步儲存了圖層，為繪制Fading Edge做了準備，這裡會完成Fading Edge的繪制并恢複圖層。

6、）繪制View的裝飾

7、）繪制預設焦點高亮

View三大工作流程開始于ViewRootImpl的performTraversals()方法,在這個方法内部會分别調用performMeasure()，performLayout()，performDraw() 三個方法來分别完成測量，布局，繪制流程。我們檢視performDraw方法看起，

1. ViewRootImpl的performDraw()方法

我們省略大部分代碼，隻看我們需要的：performDraw()方法調用draw()方法，draw()方法調用mView.draw(canvas)，mView就是根View（DecorView，也就是FrameLayout），是以繪制流程傳遞到的ViewGroup。

private void performDraw() {

try {
    boolean canUseAsync = draw(fullRedrawNeeded);
} finally {}
}


private boolean draw(boolean fullRedrawNeeded) {

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

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
        boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
mView.draw(canvas);
}

2. View 類的 draw(Canvas canvas) 方法解析

因為ViewGroup類沒有draw(Canvas canvas)方法，是以調用的是

ViewGroup類的父類View類的draw(Canvas canvas)方法，我們檢視該方法，該方法代碼比較多，但注釋非常的清楚，源碼如下：

public void draw(Canvas canvas) {
    final int privateFlags = mPrivateFlags;
    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)
     *      7. If necessary, draw the default focus highlight
     */

    // Step 1, draw the background, if needed
    int saveCount;

    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
        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 (isShowingLayoutBounds()) {
            debugDrawFocus(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 topSaveCount = -1;
    int bottomSaveCount = -1;
    int leftSaveCount = -1;
    int rightSaveCount = -1;

    int solidColor = getSolidColor();
    if (solidColor == 0) {
        if (drawTop) {
            topSaveCount = canvas.saveUnclippedLayer(left, top, right, top + length);
        }

        if (drawBottom) {
            bottomSaveCount = canvas.saveUnclippedLayer(left, bottom - length, right, bottom);
        }

        if (drawLeft) {
            leftSaveCount = canvas.saveUnclippedLayer(left, top, left + length, bottom);
        }

        if (drawRight) {
            rightSaveCount = canvas.saveUnclippedLayer(right - length, top, right, bottom);
        }
    } else {
        scrollabilityCache.setFadeColor(solidColor);
    }

    // Step 3, draw the content
    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;

    // must be restored in the reverse order that they were saved
    if (drawRight) {
        matrix.setScale(1, fadeHeight * rightFadeStrength);
        matrix.postRotate(90);
        matrix.postTranslate(right, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        if (solidColor == 0) {
            canvas.restoreUnclippedLayer(rightSaveCount, p);

        } else {
            canvas.drawRect(right - length, top, right, bottom, p);
        }
    }

    if (drawLeft) {
        matrix.setScale(1, fadeHeight * leftFadeStrength);
        matrix.postRotate(-90);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        if (solidColor == 0) {
            canvas.restoreUnclippedLayer(leftSaveCount, p);
        } else {
            canvas.drawRect(left, top, left + length, bottom, p);
        }
    }

    if (drawBottom) {
        matrix.setScale(1, fadeHeight * bottomFadeStrength);
        matrix.postRotate(180);
        matrix.postTranslate(left, bottom);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        if (solidColor == 0) {
            canvas.restoreUnclippedLayer(bottomSaveCount, p);
        } else {
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }
    }

    if (drawTop) {
        matrix.setScale(1, fadeHeight * topFadeStrength);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        if (solidColor == 0) {
            canvas.restoreUnclippedLayer(topSaveCount, p);
        } else {
            canvas.drawRect(left, top, right, top + length, 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);

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

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

一上來就給我們标注了繪制的7個步驟：

1、繪制View的背景，調用的是drawBackground(Canvas canvas)方法，代碼如下：

private void drawBackground(Canvas canvas) {
    final Drawable background = mBackground;
    if (background == null) {
        return;
    }

    setBackgroundBounds();

    if (canvas.isHardwareAccelerated() && mAttachInfo != null
            && mAttachInfo.mThreadedRenderer != null) {
        mBackgroundRenderNode = getDrawableRenderNode(background, mBackgroundRenderNode);

        final RenderNode renderNode = mBackgroundRenderNode;
        if (renderNode != null && renderNode.hasDisplayList()) {
            setBackgroundRenderNodeProperties(renderNode);
            ((RecordingCanvas) 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);
    }
}
void setBackgroundBounds() {
    if (mBackgroundSizeChanged && mBackground != null) {
        mBackground.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
        mBackgroundSizeChanged = false;
        rebuildOutline();
    }
}

mBackground是我們設定的背景參數，然後用setBackgroundBounds确定背景的邊界，最後擷取目前View的mScrollX和mScrollY值判斷是否滾動，如果scrollX和scrollY有值，則對canvas的坐标進行偏移，再繪制背景。

2、如果有必要，儲存目前的繪制圖層，為繪制Fading Edge做準備，

Fading Edge 也就是我們經常用的邊緣漸變陰影（蒙版），比如給RecyclerView的頭部尾部增加半透明的蒙版效果。

效果示例如下：

<androidx.core.widget.NestedScrollView
    android:layout_width="match_parent"
    android:layout_height="200dp"
    android:fadingEdgeLength="50dp"
    android:requiresFadingEdge="vertical">
</androidx.core.widget.NestedScrollView>

Android onDraw()繪制流程解析測量與布局流程文章元件繪制的那些結論1. ViewRootImpl的performDraw()方法2. View 類的 draw(Canvas canvas) 方法解析

我們在NestedScrollView的屬性中添加fadingEdgeLength和requiresFadingEdge後就能得到下面的效果，其中

fadingEdgeLength表示陰影部分的高度，requiresFadingEdge表示陰影的方向。方向可以是水準的，也可以是垂直的。

可以看到代碼中用了一個if判斷if (!verticalEdges && !horizontalEdges) 來判斷是否有水準或豎直的Fading Edge，如果沒有直接省略了第二步和第五步，否則會走完完整的七步。

3、繪制View的内容，調用了onDraw(canvas)，因為每個View繪制的内容都有自己要求，是以View類的onDraw(canvas)方法是個空實作。

protected void onDraw(Canvas canvas) {
}

4、對目前View的子View進行繪制(如果有子View)

調用了dispatchDraw(canvas)方法，顧名思義，是向子View分發繪制事件，View類是沒有子View，是以View類的dispatchDraw(canvas)方法是個空實作，

protected void dispatchDraw(Canvas canvas) {

}

而ViewGroup是有子View的，我們檢視ViewGroup類的dispatchDraw(canvas)方法，該方法比較長，我們照例省略大部分代碼，隻看我們需要的：

protected void dispatchDraw(Canvas canvas) {
for (int i = 0; i < childrenCount; i++) {
    while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
        final View transientChild = mTransientViews.get(transientIndex);
        if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                transientChild.getAnimation() != null) {
            more |= drawChild(canvas, transientChild, drawingTime);
        }
        transientIndex++;
        if (transientIndex >= transientCount) {
            transientIndex = -1;
        }
    }

    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);
    }
}
}

可以看到，這裡循環周遊了所有的子View，并調用了drawChild（）方法，

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

可以看到，調用了子View的draw(Canvas canvas, ViewGroup parent, long drawingTime)方法，該方法不是上面我們檢視過的draw（）方法，這個方法的作用是ViewGroup.drawChild() 調用以讓每個子視圖繪制自身。

該方法代碼很長，照例檢視關鍵代碼：

boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
if (!drawingWithDrawingCache) {
    if (drawingWithRenderNode) {
        mPrivateFlags &= ~PFLAG_DIRTY_MASK;
        ((RecordingCanvas) canvas).drawRenderNode(renderNode);
    } else {
        // Fast path for layouts with no backgrounds
        if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            dispatchDraw(canvas);
        } else {
            draw(canvas);
        }
    }
} else if (cache != null) {
    mPrivateFlags &= ~PFLAG_DIRTY_MASK;
    if (layerType == LAYER_TYPE_NONE || mLayerPaint == null) {
        // no layer paint, use temporary paint to draw bitmap
        Paint cachePaint = parent.mCachePaint;
        if (cachePaint == null) {
            cachePaint = new Paint();
            cachePaint.setDither(false);
            parent.mCachePaint = cachePaint;
        }
        cachePaint.setAlpha((int) (alpha * 255));
        canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
    } else {
        // use layer paint to draw the bitmap, merging the two alphas, but also restore
        int layerPaintAlpha = mLayerPaint.getAlpha();
        if (alpha < 1) {
            mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
        }
        canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
        if (alpha < 1) {
            mLayerPaint.setAlpha(layerPaintAlpha);
        }
    }
}
}

可以看到最後還是調用了draw(canvas)方法進行子View自身的繪制，不過增加了對繪制緩存的判斷。

到這裡我們就能了解整個繪制流程：

從ViewGroup的draw(canvas)開始繪制ViewGroup自身，draw(canvas)中會調用dispatchDraw(canvas)方法調用子View的

的draw(canvas)進行子View的繪制，如果子View還是ViewGroup，則會重複上述過程，知道整個View樹完成繪制。

5、繪制View的Fading Edge，第二步儲存了圖層，為繪制Fading Edge做了準備，這裡會完成Fading Edge的繪制并恢複圖層。

6、繪制View的裝飾（例如：滾動條、foreground前景），調用onDrawForeground(canvas)方法

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

    final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
    if (foreground != null) {
        if (mForegroundInfo.mBoundsChanged) {
            mForegroundInfo.mBoundsChanged = false;
            final Rect selfBounds = mForegroundInfo.mSelfBounds;
            final Rect overlayBounds = mForegroundInfo.mOverlayBounds;

            if (mForegroundInfo.mInsidePadding) {
                selfBounds.set(0, 0, getWidth(), getHeight());
            } else {
                selfBounds.set(getPaddingLeft(), getPaddingTop(),
                        getWidth() - getPaddingRight(), getHeight() - getPaddingBottom());
            }

            final int ld = getLayoutDirection();
            Gravity.apply(mForegroundInfo.mGravity, foreground.getIntrinsicWidth(),
                    foreground.getIntrinsicHeight(), selfBounds, overlayBounds, ld);
            foreground.setBounds(overlayBounds);
        }

        foreground.draw(canvas);
    }
}

可以看到該方法先繪制了滾動條，又繪制了foreground

7、繪制預設焦點高亮，調用drawDefaultFocusHighlight(canvas)方法繪制

Android onDraw()繪制流程解析測量與布局流程文章元件繪制的那些結論1. ViewRootImpl的performDraw()方法2. View 類的 draw(Canvas canvas) 方法解析

Android onDraw()繪制流程解析

文章目錄

測量與布局流程文章

元件繪制的那些結論

1. ViewRootImpl的performDraw()方法

2. View 類的 draw(Canvas canvas) 方法解析

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