前言

前幾篇文章卡儒，筆者分別講述了DecorView，measure俐巴，layout流程等骨望，接下來將詳細(xì)分析三大工作流程的最后一個(gè)流程——繪制流程。測(cè)量流程決定了View的大小欣舵，布局流程決定了View的位置擎鸠，那么繪制流程將決定View的樣子，一個(gè)View該顯示什么由繪制流程完成缘圈。以下源碼均取自Android API 21劣光。

從performDraw說起

前面幾篇文章提到，三大工作流程始于ViewRootImpl#performTraversals,在這個(gè)方法內(nèi)部會(huì)分別調(diào)用performMeasure糟把，performLayout绢涡，performDraw三個(gè)方法來分別完成測(cè)量，布局遣疯，繪制流程雄可。那么我們現(xiàn)在先從performDraw方法看起，ViewRootImpl#performDraw:

private void performDraw() {
    //...
    final boolean fullRedrawNeeded = mFullRedrawNeeded;
    try {
        draw(fullRedrawNeeded);
    } finally {
        mIsDrawing = false;
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

    //省略...
}

里面又調(diào)用了ViewRootImpl#draw方法，并傳遞了fullRedrawNeeded參數(shù)数苫，而該參數(shù)由mFullRedrawNeeded成員變量獲取聪舒，它的作用是判斷是否需要重新繪制全部視圖，如果是第一次繪制視圖虐急，那么顯然應(yīng)該繪制所以的視圖过椎，如果由于某些原因，導(dǎo)致了視圖重繪戏仓，那么就沒有必要繪制所有視圖疚宇。我們來看看ViewRootImpl#draw：

private void draw(boolean fullRedrawNeeded) {
    ...
    //獲取mDirty，該值表示需要重繪的區(qū)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為真赏殃，則把dirty區(qū)域置為整個(gè)屏幕敷待，表示整個(gè)視圖都需要繪制
    //第一次繪制流程，需要繪制所有視圖
    if (fullRedrawNeeded) {
        mAttachInfo.mIgnoreDirtyState = true;
        dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
    }

    //省略...

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

這里省略了一部分代碼仁热，我們只看關(guān)鍵代碼榜揖，首先是先獲取了mDirty值，該值保存了需要重繪的區(qū)域的信息抗蠢，關(guān)于視圖重繪举哟，后面會(huì)有文章專門敘述，這里先熟悉一下迅矛。接著根據(jù)fullRedrawNeeded來判斷是否需要重置dirty區(qū)域妨猩，最后調(diào)用了ViewRootImpl#drawSoftware方法，并把相關(guān)參數(shù)傳遞進(jìn)去秽褒，包括dirty區(qū)域壶硅，我們接著看該方法的源碼：

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;

        //鎖定canvas區(qū)域，由dirty區(qū)域決定
        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;

            //正式開始繪制
            mView.draw(canvas);

        }
    } 
    return true;
}

可以看書销斟，首先是實(shí)例化了Canvas對(duì)象庐椒，然后鎖定該canvas的區(qū)域，由dirty區(qū)域決定蚂踊，接著對(duì)canvas進(jìn)行一系列的屬性賦值约谈，最后調(diào)用了mView.draw(canvas)方法，前面分析過犁钟，mView就是DecorView棱诱，也就是說從DecorView開始繪制，前面所做的一切工作都是準(zhǔn)備工作特纤，而現(xiàn)在則是正式開始繪制流程军俊。

View的繪制

由于ViewGroup沒有重寫draw方法，因此所有的View都是調(diào)用View#draw方法捧存，因此，我們直接看它的源碼：

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;
    }
    ...
}

可以看到，draw過程比較復(fù)雜昔穴，但是邏輯十分清晰镰官，而官方注釋也清楚地說明了每一步的做法。我們首先來看一開始的標(biāo)記位dirtyOpaque吗货，該標(biāo)記位的作用是判斷當(dāng)前View是否是透明的泳唠，如果View是透明的，那么根據(jù)下面的邏輯可以看出宙搬，將不會(huì)執(zhí)行一些步驟笨腥，比如繪制背景、繪制內(nèi)容等勇垛。這樣很容易理解脖母，因?yàn)橐粋€(gè)View既然是透明的，那就沒必要繪制它了闲孤。接著是繪制流程的六個(gè)步驟谆级，這里先小結(jié)這六個(gè)步驟分別是什么，然后再展開來講讼积。

繪制流程的六個(gè)步驟：
1肥照、對(duì)View的背景進(jìn)行繪制
2、保存當(dāng)前的圖層信息(可跳過)
3勤众、繪制View的內(nèi)容
4舆绎、對(duì)View的子View進(jìn)行繪制(如果有子View)
5、繪制View的褪色的邊緣们颜，類似于陰影效果(可跳過)
6亿蒸、繪制View的裝飾（例如：滾動(dòng)條）
其中第2步和第5步是可以跳過的，我們這里不做分析掌桩，我們重點(diǎn)來分析其它步驟边锁。

Step 1 :繪制背景

這里調(diào)用了View#drawBackground方法，我們看它的源碼：

private void drawBackground(Canvas canvas) {

    //mBackground是該View的背景參數(shù)波岛，比如背景顏色
    final Drawable background = mBackground;
    if (background == null) {
        return;
    }

    //根據(jù)View四個(gè)布局參數(shù)來確定背景的邊界
    setBackgroundBounds();

    ...

    //獲取當(dāng)前View的mScrollX和mScrollY值
    final int scrollX = mScrollX;
    final int scrollY = mScrollY;
    if ((scrollX | scrollY) == 0) {
        background.draw(canvas);
    } else {
        //如果scrollX和scrollY有值茅坛，則對(duì)canvas的坐標(biāo)進(jìn)行偏移，再繪制背景
        canvas.translate(scrollX, scrollY);
        background.draw(canvas);
        canvas.translate(-scrollX, -scrollY);
    }
}

可以看出则拷，這里考慮到了view的偏移參數(shù),scrollX和scrollY贡蓖，繪制背景在偏移后的view中繪制。

Step 3:繪制內(nèi)容

這里調(diào)用了View#onDraw方法煌茬，View中該方法是一個(gè)空實(shí)現(xiàn)斥铺，因?yàn)椴煌腣iew有著不同的內(nèi)容，這需要我們自己去實(shí)現(xiàn)坛善，即在自定義View中重寫該方法來實(shí)現(xiàn)晾蜘。

Step 4: 繪制子View

如果當(dāng)前的View是一個(gè)ViewGroup類型邻眷，那么就需要繪制它的子View，這里調(diào)用了dispatchDraw剔交，而View中該方法是空實(shí)現(xiàn)肆饶，實(shí)際是ViewGroup重寫了這個(gè)方法，那么我們來看看岖常，ViewGroup#dispatchDraw:

protected void dispatchDraw(Canvas canvas) {
    boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
    final int childrenCount = mChildrenCount;
    final View[] children = mChildren;
    int flags = mGroupFlags;

    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);
        }
    }
    //省略...
    
}

源碼很長(zhǎng)驯镊，這里簡(jiǎn)單說明一下，里面主要遍歷了所以子View竭鞍，每個(gè)子View都調(diào)用了drawChild這個(gè)方法板惑，我們找到這個(gè)方法，ViewGroup#drawChild：

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

可以看出偎快，這里調(diào)用了View的draw方法冯乘，但這個(gè)方法并不是上面所說的，因?yàn)閰?shù)不同滨砍，我們來看看這個(gè)方法往湿，View#draw：

boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {

    //省略...
    
    if (!drawingWithDrawingCache) {
        if (drawingWithRenderNode) {
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            ((DisplayListCanvas) 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) {
            // 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();
            mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
            canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
            mLayerPaint.setAlpha(layerPaintAlpha);
        }
    }

}

我們主要來看核心部分，首先判斷是否已經(jīng)有緩存惋戏，即之前是否已經(jīng)繪制過一次了领追，如果沒有，則會(huì)調(diào)用draw(canvas)方法响逢，開始正常的繪制绒窑，即上面所說的六個(gè)步驟，否則利用緩存來顯示舔亭。
這一步也可以歸納為ViewGroup繪制過程些膨，它對(duì)子View進(jìn)行了繪制，而子View又會(huì)調(diào)用自身的draw方法來繪制自身钦铺，這樣不斷遍歷子View及子View的不斷對(duì)自身的繪制订雾，從而使得View樹完成繪制。

Step 6 繪制裝飾

所謂的繪制裝飾矛洞，就是指View除了背景洼哎、內(nèi)容、子View的其余部分沼本，例如滾動(dòng)條等噩峦，我們看View#onDrawForeground:

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);
    }
}

可以看出，邏輯很清晰抽兆，和一般的繪制流程非常相似识补，都是先設(shè)定繪制區(qū)域，然后利用canvas進(jìn)行繪制辫红，這里就不展開詳細(xì)地說了凭涂，有興趣的可以繼續(xù)了解下去祝辣。

那么，到目前為止导盅，View的繪制流程也講述完畢了较幌，希望這篇文章對(duì)你們起到幫助作用揍瑟，謝謝你們的閱讀白翻。

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