Android View 的繪製流程之 Layout 和 Draw 過程詳解 (二)

時間  2020-05-15
標籤 android view 繪製 流程 layout draw 過程 詳解 欄目 Android 简体版
原文   原文鏈接

View 的繪製系列文章：

 

在上一篇 Android View 的繪製流程之 Measure 過程詳解 (一)，已經詳細的分析了 DecorView 和其子 View 的測量過程，接下去就要開始講  layout 和 draw 流程。下面開始進入分析：html

DecorView Layout 階段

在 ViewRootImpl 中，調用 performLayout 方法來肯定 DecorView 在屏幕中的位置，下面看下具體的代碼邏輯：canvas

// ViewRootImpl 
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { mLayoutRequested = false; mScrollMayChange = true; mInLayout = true; final View host = mView; if (host == null) { return; } if (DEBUG_ORIENTATION || DEBUG_LAYOUT) { Log.v(mTag, "Laying out " + host + " to (" + host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")"); } Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout"); try {
　　　　　　　// 根據測量結果進行繪製  host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());  mInLayout = false; int numViewsRequestingLayout = mLayoutRequesters.size(); if (numViewsRequestingLayout > 0) { // requestLayout() was called during layout. // If no layout-request flags are set on the requesting views, there is no problem. // If some requests are still pending, then we need to clear those flags and do // a full request/measure/layout pass to handle this situation.
                ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, false); if (validLayoutRequesters != null) { // Set this flag to indicate that any further requests are happening during // the second pass, which may result in posting those requests to the next // frame instead
                    mHandlingLayoutInLayoutRequest = true; // Process fresh layout requests, then measure and layout
                    int numValidRequests = validLayoutRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = validLayoutRequesters.get(i); Log.w("View", "requestLayout() improperly called by " + view +
                                " during layout: running second layout pass"); view.requestLayout(); }  measureHierarchy(host, lp, mView.getContext().getResources(), desiredWindowWidth, desiredWindowHeight); mInLayout = true; host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mHandlingLayoutInLayoutRequest = false; // Check the valid requests again, this time without checking/clearing the // layout flags, since requests happening during the second pass get noop'd
                    validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true); if (validLayoutRequesters != null) { final ArrayList<View> finalRequesters = validLayoutRequesters; // Post second-pass requests to the next frame
                        getRunQueue().post(new Runnable() { @Override public void run() { int numValidRequests = finalRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = finalRequesters.get(i); Log.w("View", "requestLayout() improperly called by " + view +
                                            " during second layout pass: posting in next frame"); view.requestLayout(); } } }); } } } } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } mInLayout = false; }

當在 layout 繪製過程當中，收到了關於從新 layout 的請求，會先判斷這些請求裏面哪些是有效的，若是是有效的，那麼就必須先處理，清除 layout-request flags （View.PFLAG_FORCE_LAYOUT）這些標記，再作一次完全的重繪工做：從新測量，layout。緩存

那麼對於 DecorView 來講，調用 layout 方法，就是對它自身進行佈局，注意到傳遞的參數分別是 0，0， host.getMeasuredWidth, host.getMeasuredHeigh，它們分別表明了一個  View 的上下左右四個位置，顯然，DecorView 的左上位置爲 0，而後寬高爲它的測量寬高，下面來看 layout 的具體代碼：app

// ViewGroup 
public final void layout(int l, int t, int r, int b) { if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) { if (mTransition != null) { mTransition.layoutChange(this); } super.layout(l, t, r, b); } else { // record the fact that we noop'd it; request layout when transition finishes
            mLayoutCalledWhileSuppressed = true; } }

 因爲 ViewGroup 的 layout 方法是 final 類型，子類不能重寫，這裏調用了父類的  View#layout 方法，下面看看該方法是如何操做的：less

// View 
public void layout(int l, int t, int r, int b) { if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) { onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec); mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } int oldL = mLeft; int oldT = mTop; int oldB = mBottom; int oldR = mRight; 　　　　　// 設置界面的顯示大小 boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); 　　　　　// 發生了改變，或者是須要從新 layout，那麼就會進入 onLayout 邏輯 if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
 // 開始  onLayout(changed, l, t, r, b); if (shouldDrawRoundScrollbar()) { if(mRoundScrollbarRenderer == null) { mRoundScrollbarRenderer = new RoundScrollbarRenderer(this); } } else { mRoundScrollbarRenderer = null; } 　　　　　　　// 清除請求 layout 的標記  mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED; ListenerInfo li = mListenerInfo; if (li != null && li.mOnLayoutChangeListeners != null) { ArrayList<OnLayoutChangeListener> listenersCopy = (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone(); int numListeners = listenersCopy.size(); for (int i = 0; i < numListeners; ++i) { listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB); } } } final boolean wasLayoutValid = isLayoutValid(); mPrivateFlags &= ~PFLAG_FORCE_LAYOUT; mPrivateFlags3 |= PFLAG3_IS_LAID_OUT; if (!wasLayoutValid && isFocused()) { mPrivateFlags &= ~PFLAG_WANTS_FOCUS; if (canTakeFocus()) { // We have a robust focus, so parents should no longer be wanting focus.
 clearParentsWantFocus(); } else if (getViewRootImpl() == null || !getViewRootImpl().isInLayout()) { // This is a weird case. Most-likely the user, rather than ViewRootImpl, called // layout. In this case, there's no guarantee that parent layouts will be evaluated // and thus the safest action is to clear focus here.
                clearFocusInternal(null, /* propagate */ true, /* refocus */ false); clearParentsWantFocus(); } else if (!hasParentWantsFocus()) { // original requestFocus was likely on this view directly, so just clear focus
                clearFocusInternal(null, /* propagate */ true, /* refocus */ false); } // otherwise, we let parents handle re-assigning focus during their layout passes.
        } else if ((mPrivateFlags & PFLAG_WANTS_FOCUS) != 0) { mPrivateFlags &= ~PFLAG_WANTS_FOCUS; View focused = findFocus(); if (focused != null) { // Try to restore focus as close as possible to our starting focus.
                if (!restoreDefaultFocus() && !hasParentWantsFocus()) { // Give up and clear focus once we've reached the top-most parent which wants // focus.
                    focused.clearFocusInternal(null, /* propagate */ true, /* refocus */ false); } } } if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) { mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT; notifyEnterOrExitForAutoFillIfNeeded(true); } }

 調用了 setFrame 方法，並把四個位置信息傳遞進去，這個方法用於肯定 View 的四個頂點的位置，看下具體的代碼：ide

protected boolean setFrame(int left, int top, int right, int bottom) { boolean changed = false; if (DBG) { Log.d(VIEW_LOG_TAG, this + " View.setFrame(" + left + "," + top + ","
                    + right + "," + bottom + ")"); } 　　　　　// 有一個不同說明發生了改變 if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) { changed = true; // Remember our drawn bit
            int drawn = mPrivateFlags & PFLAG_DRAWN; int oldWidth = mRight - mLeft; int oldHeight = mBottom - mTop; int newWidth = right - left; int newHeight = bottom - top; boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight); // Invalidate our old position
 invalidate(sizeChanged); mLeft = left; mTop = top; mRight = right; mBottom = bottom; mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom); 　　　　　　　// 作好標記  mPrivateFlags |= PFLAG_HAS_BOUNDS; 　　　　　　　// size 改變的回調 if (sizeChanged) { sizeChange(newWidth, newHeight, oldWidth, oldHeight); } if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) { // If we are visible, force the DRAWN bit to on so that // this invalidate will go through (at least to our parent). // This is because someone may have invalidated this view // before this call to setFrame came in, thereby clearing // the DRAWN bit.
                mPrivateFlags |= PFLAG_DRAWN; invalidate(sizeChanged); // parent display list may need to be recreated based on a change in the bounds // of any child
 invalidateParentCaches(); } // Reset drawn bit to original value (invalidate turns it off)
            mPrivateFlags |= drawn; mBackgroundSizeChanged = true; mDefaultFocusHighlightSizeChanged = true; if (mForegroundInfo != null) { mForegroundInfo.mBoundsChanged = true; } notifySubtreeAccessibilityStateChangedIfNeeded(); } return changed; }
這裏咱們看到它對 mLeft、mTop、mRight、mBottom 這四個值進行了從新賦值，對於每個View，包括 ViewGroup 來講，以上四個值保存了 Viwe 的位置信息，因此這四個值是最終寬高，也便是說，若是要獲得 View 的位置信息， 那麼就應該在 layout 方法完成後調用 getLeft()、getTop() 等方法來取得最終寬高，若是是在此以前調用相應的方法，只能獲得 0 的結果。當初始化完畢後，ViewGroup 的佈局流程也就完成了。

賦值後，先後對比大小，若是發生了改變，就會調用了 sizeChange（）方法，最終會回調 onSizeChanged，標明 view 的尺寸發生了變化，第一次 laout 的時候也會調用。在自定義view 的時候，能夠在這裏獲取控件的寬和高度。oop

private void sizeChange(int newWidth, int newHeight, int oldWidth, int oldHeight) { onSizeChanged(newWidth, newHeight, oldWidth, oldHeight); 　　...... }

子 View Layout 流程

當 DecorView 肯定好了本身的位置以後，開始調用 onLayout 來肯定子 view 的位置。對於 onLayout 方法，View 和 ViewGroup 類是空實現，接下來看 FrameLayout 的實現：佈局

// FrameLayout 
protected void onLayout(boolean changed, int left, int top, int right, int bottom) { layoutChildren(left, top, right, bottom, false /* no force left gravity */); }

 能夠看到，該方法調用了 layoutChildren 來肯定子 view 的位置。post

// FrameLaout 
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) { final int count = getChildCount(); 　　　　　// 獲取 DecoverView 剩餘的空間範圍 final int parentLeft = getPaddingLeftWithForeground(); final int parentRight = right - left - getPaddingRightWithForeground(); final int parentTop = getPaddingTopWithForeground(); final int parentBottom = bottom - top - getPaddingBottomWithForeground(); for (int i = 0; i < count; i++) { final View child = getChildAt(i); if (child.getVisibility() != GONE) { final LayoutParams lp = (LayoutParams) child.getLayoutParams(); final int width = child.getMeasuredWidth(); final int height = child.getMeasuredHeight(); int childLeft; int childTop; 
　　　　　　　　　　// DEFAULT_CHILD_GRAVITY = Gravity.TOP | Gravity.START 默認是在左上角
int gravity = lp.gravity; if (gravity == -1) { gravity = DEFAULT_CHILD_GRAVITY; } final int layoutDirection = getLayoutDirection(); final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection); final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK; 　　　　　　　　　 // 肯定左邊起始點 switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) { case Gravity.CENTER_HORIZONTAL: childLeft = parentLeft + (parentRight - parentLeft - width) / 2 + lp.leftMargin - lp.rightMargin; break; case Gravity.RIGHT: if (!forceLeftGravity) { childLeft = parentRight - width - lp.rightMargin; break; } case Gravity.LEFT: default: childLeft = parentLeft + lp.leftMargin; } 　　　　　　　　　　// 肯定上邊起始點 switch (verticalGravity) { case Gravity.TOP: childTop = parentTop + lp.topMargin; break; case Gravity.CENTER_VERTICAL: childTop = parentTop + (parentBottom - parentTop - height) / 2 + lp.topMargin - lp.bottomMargin; break; case Gravity.BOTTOM: childTop = parentBottom - height - lp.bottomMargin; break; default: childTop = parentTop + lp.topMargin; } child.layout(childLeft, childTop, childLeft + width, childTop + height); } } }

先梳理一下以上邏輯：動畫

  • 首先先獲取父容器的 padding 值，獲得 DecorView 的可用於顯示的空間範圍。

  • 而後遍歷其每個子 View，根據子 View 的 layout_gravity 屬性、子 View 的測量寬高、父容器的 padding 值、來肯定子 View 的左上角的座標位置

  • 而後調用 child.layout 方法，參數是左上角座標和自身寬高結合起來的，這樣就能夠肯定子 View 的位置。

最終調用 layout 是 View#layout，前面已經分析過，就不在分析了。

能夠看到子 View 的佈局流程也很簡單，若是子 View 是一個 ViewGroup，那麼就會重複以上步驟，若是是一個 View，那麼會直接調用 View#layout 方法，根據以上分析，在該方法內部會設置 view 的四個佈局參數，接着調用 onLayout 方法 ：

protected void onLayout(boolean changed, int left, int top, int right, int bottom) {

}

此方法是一個空方法，也就是說須要子類去實現此方法，不一樣的 View 實現方式不一樣，這裏就不分析了。

layout階段的基本思想也是由根View開始，遞歸地完成整個控件樹的佈局（layout）工做。 

對於寬高的獲取這裏在總結下：

 

 

 這裏須要注意一下，在非通常狀況下，也就是經過人爲設置，重寫View的layout()強行設置，這種狀況下，測量的值與最終的值是不同的。

Layout 總體的流程圖

上面分別從 View 和 ViewGroup 的角度講解了佈局流程，這裏再以流程圖的形式概括一下整個 Layout 過程，便於加深記憶：

 

DecorView Draw 流程

Draw 的入口也是在 ViewRootImpl 中，執行 ViewRootImpl#performTraversals 中會執行 ViewRootIml#performDraw：

private void performDraw() { ... //fullRedrawNeeded，它的做用是判斷是否須要從新繪製所有視圖
draw(fullRedrawNeeded); ... }

而後會執行到 ViewRootImpl#draw：

private void draw(boolean fullRedrawNeeded) { ... //獲取mDirty，該值表示須要重繪的區域
 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; } //若是fullRedrawNeeded爲真，則把dirty區域置爲整個屏幕，表示整個視圖都須要繪製 //第一次繪製流程，須要繪製全部視圖
 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; } }

 接着會執行到 ViewRootIml#drawSoftware，而後在 ViewRootIml#drawSoftware 會執行到 mView.draw(canvas)。

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty) { final Canvas canvas; //鎖定canvas區域，由dirty區域決定 //這個canvas就是咱們想在上面繪製東西的畫布
  canvas = mSurface.lockCanvas(dirty); ... //畫布支持位圖的密度，和手機分辨率相關
 canvas.setDensity(mDensity); ... if (!canvas.isOpaque() || yoff != 0 || xoff != 0) { canvas.drawColor(0, PorterDuff.Mode.CLEAR); } ... canvas.translate(-xoff, -yoff); ... //正式開始繪製
 mView.draw(canvas); ... //提交須要繪製的東西
 surface.unlockCanvasAndPost(canvas); }

 mView.draw(canvas) 開始真正的繪製。此處 mView 就是 DecorView，先看 DecorView 中 Draw 的方法：

public void draw(Canvas canvas) { super.draw(canvas); if (mMenuBackground != null) { mMenuBackground.draw(canvas); } }

 裏面會調用 View#draw：

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; //繪製背景
        if (!dirtyOpaque) { drawBackground(canvas); } // 若是能夠跳過2和5步
        final int viewFlags = mViewFlags; //判斷是否有繪製衰退邊緣的標示
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0; boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0; // 若是沒有繪製衰退邊緣只須要3,4,6步
        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); }

能夠看到，draw過程比較複雜，可是邏輯十分清晰，而官方註釋也清楚地說明了每一步的作法。咱們首先來看一開始的標記位 dirtyOpaque，該標記位的做用是判斷當前 View 是不是透明的，若是 View 是透明的，那麼根據下面的邏輯能夠看出，將不會執行一些步驟，好比繪製背景、繪製內容等。這樣很容易理解，由於一個 View 既然是透明的，那就不必繪製它了。接着是繪製流程的六個步驟，這裏先小結這六個步驟分別是什麼，而後再展開來說。繪製流程的六個步驟：

  1. 對 View 的背景進行繪製

  2. 保存當前的圖層信息(可跳過)

  3. 繪製 View 的內容

  4. 對 View 的子 View 進行繪製(若是有子 View )

  5. 繪製 View 的褪色的邊緣，相似於陰影效果(可跳過)

  6. 繪製 View 的裝飾（例如：滾動條）

其中第2步和第5步是能夠跳過的，咱們這裏不作分析，咱們重點來分析其它步驟。

ViewGroup子類默認狀況下就是不執行 onDraw 方法的，在 ViewGroup 源碼中的 initViewGroup() 方法中設置了一個標記，源碼以下：

private void initViewGroup() { // ViewGroup doesn't draw by default
        if (!debugDraw()) { setFlags(WILL_NOT_DRAW, DRAW_MASK); } ...... }

看第二行註釋也知道，ViewGroup 默認狀況下是不會 draw 的。第四行調用 setFlags 方法設置標記 WILL_NOT_DRAW，在回到 View 中 draw 方法看第2行代碼：

1  final int privateFlags = mPrivateFlags; 2  final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
3      (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);

 

setFlags 方法就是對 View中mPrivateFlags 值進行相應改變，咱們設置標記 WILL_NOT_DRAW 那麼 dirtyOpaque 獲得的值就爲 true，從而 if (!dirtyOpaque) 不成立，也就不會執行onDraw 方法。

1. 繪製背景

View#drawBackground

private void drawBackground(Canvas canvas) { //獲取背景的Drawable，沒有就不須要繪製
        final Drawable background = mBackground; if (background == null) { return; } //肯定背景Drawable邊界
 setBackgroundBounds(); ... //若是有偏移量先偏移畫布再將drawable繪製上去
        final int scrollX = mScrollX; final int scrollY = mScrollY; if ((scrollX | scrollY) == 0) { background.draw(canvas); } else { canvas.translate(scrollX, scrollY); //此處會執行各類Drawable對應的draw方法
 background.draw(canvas); //把畫布的原點移回去，drawable在屏幕上的位置不動
            canvas.translate(-scrollX, -scrollY); } }

3. 繪製 View 的內容

先跳過第 2 步，是由於不是全部的 View 都需繪製褪色邊緣。DecorView#onDraw：

　　public void onDraw(Canvas c) { super.onDraw(c); mBackgroundFallback.draw(this, mContentRoot, c, mWindow.mContentParent, mStatusColorViewState.view, mNavigationColorViewState.view); }

View#onDraw

protected void onDraw(Canvas canvas) {
 }

onDraw 是空實現，須要子 View 本身去繪製。對於DecorView 通常也沒啥內容，除了須要背景顏色等，因此自己並須要繪製啥。

4. 繪製子View

DecorView 繪製完成後，開始繪製子 View，因此 ViewGroup 的繪製須要繪製子 View，直接看看 ViewGroup#dispatchDraw：

protected void dispatchDraw(Canvas canvas) { boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode); final int childrenCount = mChildrenCount; final View[] children = mChildren; int flags = mGroupFlags; //ViewGroup是否有設置子View入場動畫，若是有綁定到View // 啓動動畫控制器
 ... //指定修改區域
       int clipSaveCount = 0; final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
　　　　// 不讓子view繪製在pandding裏面，也就是去除padding if (clipToPadding) { clipSaveCount = canvas.save(); canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop, mScrollX + mRight - mLeft - mPaddingRight, mScrollY + mBottom - mTop - mPaddingBottom); } ... for (int i = 0; i < childrenCount; i++) { //先取mTransientViews中的View，mTransientViews中的View經過addTransientView添加，它們只是容器渲染的一個item
           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); } } ... }

ViewGroup#dispatchDraw 的流程是先啓動第一次加到佈局中的動畫，而後肯定繪製區域，遍歷繪製 View，遍歷 View 的時候優先繪製渲染的 mTransientViews，繪製 View 調用到ViewGroup#drawChild：

protected boolean drawChild(Canvas canvas, View child, long drawingTime) { //View中有兩個draw方法 //這個多參數的draw用於view繪製自身內容
        return child.draw(canvas, this, drawingTime); }

 View#draw(canvas, this, drawingTime）

boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) { boolean drawingWithRenderNode = mAttachInfo != null
                && mAttachInfo.mHardwareAccelerated && hardwareAcceleratedCanvas; ... 　　　　 //主要判斷是否有繪製緩存，若是有，直接使用緩存，若是沒有，調用 draw(canvas)方法
        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); } } }

首先判斷是否已經有緩存，即以前是否已經繪製過一次了，若是沒有，則會調用 draw(canvas) 方法，開始正常的繪製，即上面所說的六個步驟，不然利用緩存來顯示。

這一步也能夠概括爲 ViewGroup 繪製過程，它對子 View 進行了繪製，而子 View 又會調用自身的 draw 方法來繪製自身，這樣不斷遍歷子 View 及子 View 的不斷對自身的繪製，從而使得 View 樹完成繪製。

對於自定義 View ，若是須要繪製東西的話，直接從新 onDraw 就能夠了。

6. 繪製裝飾

public void onDrawForeground(Canvas canvas) { 　　　　 //繪製滑動指示
 onDrawScrollIndicators(canvas); 　　　　　//繪製ScrollBar
 onDrawScrollBars(canvas); 　　　　　//獲取前景色的Drawable，繪製到canvas上
        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); } }

 2和5.繪製View的褪色邊緣

當 horizontalEdges 或者 verticalEdges 有一個 true 的時候，表示須要繪製 View 的褪色邊緣：

boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0; boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;

這時候先計算出是否須要繪製上下左右的褪色邊緣和它的參數，而後保存視圖層：

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; if (verticalEdges && (top + length > bottom - length)) { length = (bottom - top) / 2; } 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); }

繪製褪色邊緣，恢復視圖層 ：

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);

所謂的繪製裝飾，就是指 View 除了背景、內容、子 View 的其他部分，例如滾動條等。

最後附上 View 的 draw 流程：

 

 

到此，View 的繪製流程就講完了，下一篇會講自定義 View。

 

原文出處：https://www.cnblogs.com/huansky/p/12000771.html

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