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前言

• 大家好！本次我們將繼續(xù)學(xué)習(xí)Android之自定義View的死亡三部曲中的最后一部（Draw）：畫出最真實(shí)的自己
• 在此之前，我們?cè)?a target="_blank" rel="nofollow">Android之自定義View的死亡三部曲之（Measure） 中分析了View測(cè)測(cè)量過程境析，獲得了View的三圍數(shù)據(jù)-測(cè)量后獲得高和寬拣播，在Android之自定義View的死亡三部曲之（Layout） 中分析了View的測(cè)量過程荡短，經(jīng)過測(cè)量后桌吃，我們就能拿到View的left、top榄攀、right嗜傅、bottom四個(gè)點(diǎn)的值。那么我們剩下最后一步檩赢，將我的的View繪制出來吕嘀。
• Ok，這次我們依然是以ViewRootImpl的performTraversals方法起點(diǎn)贞瞒。

    private void performTraversals() {
          ...
            if (!mStopped) {
          //1偶房、獲取頂層布局的childWidthMeasureSpec
            int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);  
          //2、獲取頂層布局的childHeightMeasureSpec
            int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
            //3军浆、測(cè)量開始測(cè)量
            performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);       
            }
          } 

          if (didLayout) {
          //4棕洋、執(zhí)行布局方法
            performLayout(lp, desiredWindowWidth, desiredWindowHeight);
            ...
          }
          if (!cancelDraw && !newSurface) {
           ...
          //5、開始繪制了哦
                performDraw();
            }
          } 
        ...
      }

• 這次我們分析到performDraw方法了乒融。好的掰盘，我們一起來看下performDraw里面的代碼吧,我只保留來于本次分析相關(guān)的關(guān)鍵代碼。

    private void performDraw() {
        ......
        //1簇抵、fullRedrawNeeded這個(gè)變量標(biāo)識(shí)了本次繪制是否需要完全重新繪制
        final boolean fullRedrawNeeded = mFullRedrawNeeded;
        try {
            //2庆杜、此處調(diào)用了ViewRootImpl的draw方法
            draw(fullRedrawNeeded);
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }

        ......
    }

• 看1處，既然有完全繪制碟摆，當(dāng)然也會(huì)有局部繪制了，這樣做是為了提高性能
• OK叨橱，我們看下draw這個(gè)方法里面的代碼

    private void draw(boolean fullRedrawNeeded) {
        ......
        //1典蜕、獲得dirty，也就是我們要繪制的區(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;
        }

        //2罗洗、判斷是否需要完全繪制
        if (fullRedrawNeeded) {
            mAttachInfo.mIgnoreDirtyState = true;
            //3愉舔、需要完全繪制時(shí)，將dirty的值設(shè)置為神歌屏幕的大小
            dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
        }
        ......
        
         //3伙菜、調(diào)用drawSoftware進(jìn)行繪制
        if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                    return;
            }
    }

• 從上面的代碼分析中轩缤，我們看到，最后時(shí)通過調(diào)用drawSoftware進(jìn)行繪制贩绕，那么我們看下drawSoftware方法的代碼

    private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
                boolean scalingRequired, Rect dirty) {
  
        //1火的、哈哈，看到了canvas淑倾，是不是感覺里繪制越來越近了
        final Canvas canvas;
        try {
            //2馏鹤、取出繪制區(qū)域的四個(gè)位置的值
            final int left = dirty.left;
            final int top = dirty.top;
            final int right = dirty.right;
            final int bottom = dirty.bottom;
  
            //3、傳入我們的繪制區(qū)域娇哆，創(chuàng)建一個(gè)被鎖定了繪制區(qū)域的canvas
            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;
            }
            //4湃累、設(shè)置畫布的密度
            canvas.setDensity(mDensity);
        } 
  
        try {
  
            if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
                //5勃救、清除畫布的顏色
                canvas.drawColor(0, PorterDuff.Mode.CLEAR);
            }
            
            dirty.setEmpty();
            mIsAnimating = false;
            attachInfo.mDrawingTime = SystemClock.uptimeMillis();
            mView.mPrivateFlags |= View.PFLAG_DRAWN;
  
            try {
                //7、設(shè)置畫布的偏離值
                canvas.translate(-xoff, -yoff);
                if (mTranslator != null) {
                    mTranslator.translateCanvas(canvas);
                }
                canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
                attachInfo.mSetIgnoreDirtyState = false;
  
                //8治力、調(diào)用mView大的draw方法開始繪制
                mView.draw(canvas);
  
            }
        } 
        return true;
    }
  

• Ok蒙秒，我們分析到第8步知道，最終調(diào)用了mView的draw開始繪制了宵统，而mView也就是DecorView晕讲，我們前面分析過DecorView是一個(gè)FrameLayout，而FrameLayout并沒有重現(xiàn)draw方法榜田，ViewGroup也沒有重寫益兄，所以，我們直接看View的draw方法箭券，代碼有點(diǎn)長(zhǎng)净捅，但是思路分清晰，官方給出的解釋也是非常清晰的

    @CallSuper
    public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
         //（1）辩块、dirtyOpaque標(biāo)識(shí)了當(dāng)前View是否時(shí)透明的
        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) */
     //上面的解釋大知識(shí)蛔六，繪制過程中有一系列的步驟，但是有幾個(gè)是必須要執(zhí)行的
     //1废亭、繪制背景2国章、如果有需要，在可以先保存當(dāng)前canvas的層級(jí)數(shù)據(jù)豆村，3液兽、繪制View的內(nèi)容4、繪制View的子類5掌动、如果又需要四啰，在退出此次繪制時(shí)恢復(fù)之前的canvas的層級(jí)數(shù)據(jù)
     //6、繪制一些裝飾的效果
     // Step 1, draw the background, if needed  int saveCount;
        //（2）粗恢、透明時(shí)不需要繪制背景
        if (!dirtyOpaque) {
          //（3）柑晒、不透明時(shí)，繪制背景
            drawBackground(canvas);
        }
        //他說可以跳過第2步和第5步眷射，說明第2和第5步時(shí)很重要
        // 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
            //（4）匙赞、如果不透明，繪制View的內(nèi)容
      if (!dirtyOpaque) onDraw(canvas);
  
            // Step 4, draw the children
        //（5）將canvas傳遞給childView妖碉，將繪制事件傳遞下去
      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;
        }
  
    ......
    }
  

• OK,第2步和第5步是保存canves狀態(tài)和恢復(fù)的操作涌庭，我們這次就分析其他步驟就好

• 首先，我們看第1步嗅绸，在View非透明情況下脾猛，執(zhí)行背景的繪制操作

    private void drawBackground(Canvas canvas) {
        final Drawable background = mBackground;
        //1、背景為null當(dāng)然是直接返回了
        if (background == null) {
            return;
        }
        //2鱼鸠、確認(rèn)背景的邊界值
        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;
            }
        }
        //3猛拴、獲取當(dāng)前的scrollX和scrollY的值
        final int scrollX = mScrollX;
        final int scrollY = mScrollY;
        if ((scrollX | scrollY) == 0) {
            //此時(shí)沒有滾動(dòng)羹铅，開始繪制背景
            background.draw(canvas);
        } else {
            //正在滾動(dòng)，移動(dòng)canvas后繪制
            canvas.translate(scrollX, scrollY);
            background.draw(canvas);
            canvas.translate(-scrollX, -scrollY);
        }
    }
  

• 從上面的分析愉昆，我有又個(gè)意外的發(fā)現(xiàn)职员，當(dāng)scrllX或者scrollY的值不為0時(shí)，先使canvas偏移后在繪制跛溉，這就是為什么如果我們是使用Scoller來實(shí)現(xiàn)View的滑動(dòng)時(shí)焊切，實(shí)際上移動(dòng)的是View的可視區(qū)域，而不是View本身

• 我們看下setBackgroundBounds里面是如何確認(rèn)背景邊界的

    void setBackgroundBounds() {
        if (mBackgroundSizeChanged && mBackground != null) {
        //1芳室、直接根據(jù)layout中獲得的四個(gè)位置的值直接確定
            mBackground.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
            mBackgroundSizeChanged = false;
            rebuildOutline();
        }
    }
  

• 介紹完繪制背景专肪，我們接下來分析繪制內(nèi)容部分，我們看onDraw方法堪侯，沒錯(cuò)嚎尤，又是空的，因?yàn)檫@是我們?cè)谧远xView的時(shí)候需要自己去實(shí)現(xiàn)的

    protected void onDraw(Canvas canvas) {
    }
  

• OK伍宦，那我們看下一步芽死，傳遞繪制事件給child們

• 我們先看View的dispatchDraw，沒錯(cuò)次洼，還是空的关贵，View就是最原始的了，哪里有child嘛卖毁。

    protected void dispatchDraw(Canvas canvas) {
  
    }
  

• 那么我們來看ViewGroup中的吧,源碼優(yōu)點(diǎn)長(zhǎng)揖曾，我保留于本次分析相關(guān)就好

    @Override
    protected void dispatchDraw(Canvas canvas) {
        boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
        
        //1、獲取child的數(shù)據(jù)
        final int childrenCount = mChildrenCount;
        final View[] children = mChildren;
        int flags = mGroupFlags;
  
        ......
        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) {
            
                //2亥啦、調(diào)用drawChild傳遞canvas翩肌、child進(jìn)去繪制child
                more |= drawChild(canvas, child, drawingTime);
            }
        }
        ......
    }
  

• ok，重點(diǎn)是drawChild這個(gè)方法禁悠，我們看下drawChild里面做什么操作

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

• 里面直接調(diào)用了child的draw方法。不過這個(gè)方法跟我們前面的分析的draw有點(diǎn)區(qū)別哦兑宇，沒錯(cuò)碍侦，參數(shù)個(gè)數(shù)不同，那么我們看下到底卻別在哪呢,這個(gè)方法的代碼很長(zhǎng)隶糕，我截取關(guān)鍵代碼

    boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
  
        ......
        if (!drawingWithDrawingCache) {
            if (drawingWithRenderNode) {
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                ((DisplayListCanvas) canvas).drawRenderNode(renderNode);
            } else {
                if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                    mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                    dispatchDraw(canvas);
                } else {
                // 1瓷产、這里調(diào)用子View的draw方法，并將調(diào)整好的canvas傳進(jìn)去
                    draw(canvas);
                }
            }
        } else if (cache != null) 
            // 2枚驻、如果是cache模式濒旦，則利用cache
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            if (layerType == LAYER_TYPE_NONE) {
                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 {
                int layerPaintAlpha = mLayerPaint.getAlpha();
                mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
                canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
                mLayerPaint.setAlpha(layerPaintAlpha);
            }
        }
        ......
  
    }
  

• 上面主要做的事情就是，如果有cache再登，就利用cache進(jìn)行繪制尔邓，沒有則直接調(diào)用View的draw方法晾剖。然后根據(jù)前面的分析，最終會(huì)調(diào)用個(gè)個(gè)View的onDraw進(jìn)行繪制操作

• 接下來到了第六部梯嗽，繪制裝飾物（例如recyclerView的滾動(dòng)條）齿尽，OK，我們來看下onDrawForeground方法

    public void onDrawForeground(Canvas canvas) {
        //1灯节、繪制滾動(dòng)指示器
        onDrawScrollIndicators(canvas);
        //2循头、繪制滾動(dòng)條
        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);
            }
            //繪制foreground
            foreground.draw(canvas);
        }
    }
  

• 通過以上的分析，我們就把View的Draw分析完了炎疆，

總結(jié)：好吧卡骂，直接上一個(gè)收集的時(shí)序圖

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