一、繪製的起點 - performTraversals

和測量、佈局的過程類似，繪製的起點也是從performTraversals開始的：

private void performTraversals() {
    ......
    final Rect dirty = mDirty;
    ......
    canvas = mSurface.lockCanvas(dirty);
    ......
    mView.draw(canvas);
    ......
}
複製程式碼

二、繪製的關鍵方法：draw(Canvas canvas) onDraw(Canvas canvas) dispatchDraw()

和繪製過程的前兩個階段不同的是，與draw過程相關的函式，不僅有draw、onDraw還有一個dispatchDraw，下面我們分析一下這幾個函式在View和ViewGroup中的定義：

• View
• 首先是draw(Canvas)方法，它是public的，我們通過註釋可以看到它最多會執行6步操作，分別是：繪製背景、儲存canvas，繪製View本身內容，繪製子View，恢復canvas，繪製裝飾類（如滾動條）。其中第二、五步不是必要的。 對於整個繪製事件的分發，我們需要關注的是第三、四步：呼叫onDraw(Canvas)繪製View本身的的內容，呼叫dispatchDraw(Canvas)繪製子View，也就是我們上面說的其它兩個方法。

@CallSuper
    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);

            // 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);
    }
複製程式碼

• onDraw(Canvas canvas)方法是一個空實現，因為繪製應當由具體的控制元件來實現。
• 由於單純的View來說，它沒有子View，因此dispatchDraw方法也是一個空實現。
• ViewGroup
• 沒有重寫draw(Canvas canvas)方法。
• 沒有重寫onDraw(Canvas canvas)方法。
• 由於ViewGroup要負責將繪製的訊息通知子View，所以它重寫了dispatchDraw方法，並在裡面呼叫drawChild一級級把繪製的訊息傳遞下去。

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;
               }
           }
           int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
           final View child = (preorderedList == null)
                   ? children[childIndex] : preorderedList.get(childIndex);
           if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
               more |= drawChild(canvas, child, drawingTime);
           }
       }
       //...
}
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
       return child.draw(canvas, this, drawingTime);
}
複製程式碼

通過分析View/ViewGroup的這三個方法的定義，那麼整個由DecorView到個葉節點View的繪製過程就很清楚了：

• 首先呼叫了mView的draw方法，之前我們分析過它其實就是型別為FrameLayout的DecorView，由於FrameLayout和ViewGroup都沒有重寫draw方法，它其實是呼叫了View當中的draw執行前面分析過的那些步驟，首先是呼叫自己的onDraw方法來繪製自己，然後呼叫ViewGroup的dispatchDraw來繪製子View，最終會呼叫到這些子View的draw方法。
• 當第一步的這些子View的draw方法被呼叫時，都會先執行它的onDraw方法，這個大家都是一樣的，但是注意到對於每個具體的子View，由於dispatchDraw方法決定了事件是否會被分發，所以我們會有以下幾種情況：
• 繼承於ViewGroup的控制元件，但是沒有下一級子View，雖然ViewGroup重寫了dispatchDraw方法，但是此時childCount==0，因此drawChild不會被呼叫，整個傳遞過程終止。
• 繼承於View的控制元件，此時dispatchDraw方法是一個空實現，因此不會繼續傳遞下去。
• 繼承於ViewGroup的控制元件，並且有子View，由於ViewGroup重寫了dispatchDraw方法，因此會繼續呼叫子View的draw方法傳遞下去。

三、小結

對於這三個關鍵的方法，簡單的總結一下它們的作用：

• draw：作為整個父容器和子View之間繪製事件傳遞的紐帶。
• onDraw：負責View本身的具體繪製。
• dispatchDraw：負責將繪製分發給下一級的子View。