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這篇文章主要介紹了Android中View繪制的三大流程是什么,具有一定借鑒價(jià)值,感興趣的朋友可以參考下,希望大家閱讀完這篇文章之后大有收獲,下面讓小編帶著大家一起了解一下。
最近對(duì)Android中View的繪制機(jī)制有了一些新的認(rèn)識(shí),所以想記錄下來并分享給大家。View的工作流程主要是指measure、layout、draw這三大流程,即測量、布局和繪制,其中measure確定View的測量寬高,layout根據(jù)測量的寬高確定View在其父View中的四個(gè)頂點(diǎn)的位置,而draw則將View繪制到屏幕上,這樣通過ViewGroup的遞歸遍歷,一個(gè)View樹就展現(xiàn)在屏幕上了。
說的簡單,下面帶大家一步一步從源碼中分析:
Android的View是樹形結(jié)構(gòu)的:
基本概念
在介紹View的三大流程之前,我們必須先介紹一些基本的概念,才能更好地理解這整個(gè)過程。
Window的概念
Window表示的是一個(gè)窗口的概念,它是站在WindowManagerService角度上的一個(gè)抽象的概念,Android中所有的視圖都是通過Window來呈現(xiàn)的,不管是Activity、Dialog還是Toast,只要有View的地方就一定有Window。
這里需要注意的是,這個(gè)抽象的Window概念和PhoneWindow這個(gè)類并不是同一個(gè)東西,PhoneWindow表示的是手機(jī)屏幕的抽象,它充當(dāng)Activity和DecorView之間的媒介,就算沒有PhoneWindow也是可以展示View的。
拋開一切,僅站在WindowManagerService的角度上,Android的界面就是由一個(gè)個(gè)Window層疊展現(xiàn)的,而Window又是一個(gè)抽象的概念,它并不是實(shí)際存在的,它是以View的形式存在,這個(gè)View就是DecorView。
關(guān)于Window這方面的內(nèi)容,我們這里先了解一個(gè)大概
DecorView的概念
DecorView是整個(gè)Window界面的最頂層View,View的測量、布局、繪制、事件分發(fā)都是由DecorView往下遍歷這個(gè)View樹。DecorView作為頂級(jí)View,一般情況下它內(nèi)部會(huì)包含一個(gè)豎直方向的LinearLayout,在這個(gè)LinearLayout里面有上下兩個(gè)部分(具體情況和Android的版本及主題有關(guān)),上面是【標(biāo)題欄】,下面是【內(nèi)容欄】。在Activity中我們通過setContentView所設(shè)置的布局文件其實(shí)就是被加載到【內(nèi)容欄】中的,而內(nèi)容欄的id是content,因此指定布局的方法叫setContent().
ViewRoot的概念
ViewRoot對(duì)應(yīng)于ViewRootImpl類,它是連接WindowManager和DecorView的紐帶,View的三大流程均是通過ViewRoot來完成的。在ActivityThread中,當(dāng)Activity對(duì)象被創(chuàng)建完之后,會(huì)講DecorView添加到Window中,同時(shí)會(huì)創(chuàng)建對(duì)應(yīng)的ViewRootImpl,并將ViewRootImpl和DecorView建立關(guān)聯(lián),并保存到WindowManagerGlobal對(duì)象中。
WindowManagerGlobal.java root = new ViewRootImpl(view.getContext(), display); root.setView(view, wparams, panelParentView);
View的繪制流程是從ViewRoot的performTraversals方法開始的,它經(jīng)過measure、layout和draw三個(gè)過程才能最終將一個(gè)View繪制出來,大致流程如下圖:
Measure測量
為了更好地理解View的測量過程,我們還需要理解MeasureSpec,它是View的一個(gè)內(nèi)部類,它表示對(duì)View的測量規(guī)格。MeasureSpec代表一個(gè)32位int值,高2位代表SpecMode(測量模式),低30位代表SpecSize(測量大小),我們可以看看它的具體實(shí)現(xiàn):
MeasureSpec.java public static class MeasureSpec { private static final int MODE_SHIFT = 30; private static final int MODE_MASK = 0x3 << MODE_SHIFT; /** * UNSPECIFIED 模式: * 父View不對(duì)子View有任何限制,子View需要多大就多大 */ public static final int UNSPECIFIED = 0 << MODE_SHIFT; /** * EXACTYLY 模式: * 父View已經(jīng)測量出子Viwe所需要的精確大小,這時(shí)候View的最終大小 * 就是SpecSize所指定的值。對(duì)應(yīng)于match_parent和精確數(shù)值這兩種模式 */ public static final int EXACTLY = 1 << MODE_SHIFT; /** * AT_MOST 模式: * 子View的最終大小是父View指定的SpecSize值,并且子View的大小不能大于這個(gè)值, * 即對(duì)應(yīng)wrap_content這種模式 */ public static final int AT_MOST = 2 << MODE_SHIFT; //將size和mode打包成一個(gè)32位的int型數(shù)值 //高2位表示SpecMode,測量模式,低30位表示SpecSize,某種測量模式下的規(guī)格大小 public static int makeMeasureSpec(int size, int mode) { if (sUseBrokenMakeMeasureSpec) { return size + mode; } else { return (size & ~MODE_MASK) | (mode & MODE_MASK); } } //將32位的MeasureSpec解包,返回SpecMode,測量模式 public static int getMode(int measureSpec) { return (measureSpec & MODE_MASK); } //將32位的MeasureSpec解包,返回SpecSize,某種測量模式下的規(guī)格大小 public static int getSize(int measureSpec) { return (measureSpec & ~MODE_MASK); } //... }
MeasureSpec通過將SpecMode和SpecSize打包成一個(gè)int值來避免過多的對(duì)象內(nèi)存分配,并提供了打包和解包的方法。
SpecMode有三種類型,每一類都表示特殊的含義:
UNSPECIFIED
父容器不對(duì)View有任何限制,要多大就給多大,這種情況一般用于系統(tǒng)內(nèi)部,表示一種測量的狀態(tài);
EXACTLY
父容器已經(jīng)檢測出View所需的精確大小,這個(gè)時(shí)候View的最終打消就是SpecSize所指定的值。它對(duì)應(yīng)于LayoutParams中的match_parent和具體數(shù)值這兩種模式。
AT_MOST
父容器指定了一個(gè)可用大小即SpecSize,View的大小不能大于這個(gè)值,具體是什么值要看不同View的具體實(shí)現(xiàn)。它對(duì)應(yīng)于LayoutParams中wrap_content。
View的MeasureSpec是由父容器的MeasureSpec和自己的LayoutParams決定的,但是對(duì)于DecorView來說有點(diǎn)不同,因?yàn)樗鼪]有父類。在ViewRootImpl中的measureHierarchy方法中有如下一段代碼展示了DecorView的MeasureSpec的創(chuàng)建過程,其中desiredWindowWidth和desireWindowHeight是屏幕的尺寸大?。?/p>
ViewGroup的measure
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width); childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
再看看getRootMeasureSpec方法:
private static int getRootMeasureSpec(int windowSize, int rootDimension) { int measureSpec; switch (rootDimension) { case ViewGroup.LayoutParams.MATCH_PARENT: // Window can't resize. Force root view to be windowSize. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: // Window can resize. Set max size for root view. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST); break; default: // Window wants to be an exact size. Force root view to be that size. measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); break; } return measureSpec; }
通過以上代碼,DecorView的MeasureSpec的產(chǎn)生過程就很明確了,因?yàn)镈ecorView是FrameLyaout的子類,屬于ViewGroup,對(duì)于ViewGroup來說,除了完成自己的measure過程外,還會(huì)遍歷去調(diào)用所有子元素的measure方法,各個(gè)子元素再遞歸去執(zhí)行這個(gè)過程。和View不同的是,ViewGroup是一個(gè)抽象類,他沒有重寫View的onMeasure方法,這里很好理解,因?yàn)槊總€(gè)具體的ViewGroup實(shí)現(xiàn)類的功能是不同的,如何測量應(yīng)該讓它自己決定,比如LinearLayout和RelativeLayout。
因此在具體的ViewGroup中需要遍歷去測量子View,這里我們看看ViewGroup中提供的測量子View的measureChildWithMargins方法:
protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) { final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin + widthUsed, lp.width); final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin + heightUsed, lp.height); child.measure(childWidthMeasureSpec, childHeightMeasureSpec); }
上述方法會(huì)對(duì)子元素進(jìn)行measure,在調(diào)用子元素的measure方法之前會(huì)先通過getChildMeasureSpec方法來得到子元素的MeasureSpec。從代碼上看,子元素的MeasureSpec的創(chuàng)建與父容器的MeasureSpec和本身的LayoutParams有關(guān),此外和View的margin和父類的padding有關(guān),現(xiàn)在看看getChildMeasureSpec的具體實(shí)現(xiàn):
ViewGroup.java public static int getChildMeasureSpec(int spec, int padding, int childDimension) { int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); int size = Math.max(0, specSize - padding); int resultSize = 0; int resultMode = 0; switch (specMode) { // Parent has imposed an exact size on us case MeasureSpec.EXACTLY: if (childDimension >= 0) { resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size. So be it. resultSize = size; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent has imposed a maximum size on us case MeasureSpec.AT_MOST: if (childDimension >= 0) { // Child wants a specific size... so be it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size, but our size is not fixed. // Constrain child to not be bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent asked to see how big we want to be case MeasureSpec.UNSPECIFIED: if (childDimension >= 0) { // Child wants a specific size... let him have it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size... find out how big it should // be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size.... find out how // big it should be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } break; } //noinspection ResourceType return MeasureSpec.makeMeasureSpec(resultSize, resultMode); }
上述代碼根據(jù)父類的MeasureSpec和自身的LayoutParams創(chuàng)建子元素的MeasureSpec,具體過程同學(xué)們自行分析,最終的創(chuàng)建規(guī)則如下表:
ViewGroup在遍歷完子View后,需要根據(jù)子元素的測量結(jié)果來決定自己最終的測量大小,并調(diào)用setMeasuredDimension方法保存測量寬高值。
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),heightSizeAndState);
這里調(diào)用了resolveSizeAndState來確定最終的大小,主要是保證測量的大小不能超過父容器的最大剩余空間maxWidth,這里我們看看它里面的實(shí)現(xiàn):
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) { final int specMode = MeasureSpec.getMode(measureSpec); final int specSize = MeasureSpec.getSize(measureSpec); final int result; switch (specMode) { case MeasureSpec.AT_MOST: if (specSize < size) { result = specSize | MEASURED_STATE_TOO_SMALL; } else { result = size; } break; case MeasureSpec.EXACTLY: result = specSize; break; case MeasureSpec.UNSPECIFIED: default: result = size; } return result | (childMeasuredState & MEASURED_STATE_MASK); }
關(guān)于具體ViewGroup的onMeasure過程這里不做分析,由于每種布局的測量方式不一樣,不可能逐個(gè)分析,但在它們的onMeasure里面的步驟是有一定規(guī)律的:
1.根據(jù)各自的測量規(guī)則遍歷Children元素,調(diào)用getChildMeasureSpec方法得到Child的measureSpec;
2.調(diào)用Child的measure方法;
3.調(diào)用setMeasuredDimension確定最終的大小。
View的measure
View的measure過程由其measure方法來完成,measure方法是一個(gè)final類型的方法,這意味著子類不能重寫此方法,在View的measure方法里面會(huì)去調(diào)用onMeasure方法,我們這里只要看onMeasure的實(shí)現(xiàn)即可,如下:
View.java protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); }
代碼很簡單,我們繼續(xù)看看getDefaultSize方法的實(shí)現(xiàn):
View.java public static int getDefaultSize(int size, int measureSpec) { int result = size; int specMode = MeasureSpec.getMode(measureSpec); int specSize = MeasureSpec.getSize(measureSpec); switch (specMode) { case MeasureSpec.UNSPECIFIED: result = size; break; case MeasureSpec.AT_MOST: case MeasureSpec.EXACTLY: result = specSize; break; } return result; }
從上述代碼可以得出,View的寬/高由specSize決定,直接繼承View的自定義控件需要重寫onMeasure方法并設(shè)置wrap_content時(shí)的自身大小,否則在布局中使用wrap_content就相當(dāng)于使用match_parent。
上述就是View的measure大致過程,在measure完成之后,通過getMeasuredWidth/Height方法就可以獲得測量后的寬高,這個(gè)寬高一般情況下就等于View的最終寬高了,因?yàn)閂iew的layout布局的時(shí)候就是根據(jù)measureWidth/Height來設(shè)置寬高的,除非在layout中修改了measure值。
Layout布局
Layout的作用是ViewGroup用來確定子元素的位置,當(dāng)ViewGroup的位置被確定后,它在onLayout中會(huì)遍歷所有的子元素并調(diào)用其layout方法。簡單的來說就是,layout方法確定View本身的位置,而onLayout方法則會(huì)確定所有子元素的位置。
先看看View的layout方法:
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; boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) { onLayout(changed, l, t, r, b); if (shouldDrawRoundScrollbar()) { if(mRoundScrollbarRenderer == null) { mRoundScrollbarRenderer = new RoundScrollbarRenderer(this); } } else { mRoundScrollbarRenderer = null; } 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); } } } mPrivateFlags &= ~PFLAG_FORCE_LAYOUT; mPrivateFlags3 |= PFLAG3_IS_LAID_OUT; }
主要看到這里:
boolean changed = isLayoutModeOptical(mParent) ?setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
isLayoutModeOptical方法判斷是否顯示邊界布局(這個(gè)東西不知道是啥,暫時(shí)不理會(huì)),setOpticalFrame方法內(nèi)部最終也是調(diào)用setFrame方法,這里我們看setFrame方法就可以了:
protected boolean setFrame(int left, int top, int right, int bottom) { boolean changed = false; if (DBG) { Log.d("View", this + " View.setFrame(" + left + "," + top + "," + right + "," + bottom + ")"); } //1、如果有一個(gè)值發(fā)生了改變,那么就需要重新調(diào)用onLayout方法了,后面會(huì)分析到 if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) { changed = true; // Remember our drawn bit int drawn = mPrivateFlags & PFLAG_DRAWN; //2、保存舊的寬和高 int oldWidth = mRight - mLeft; int oldHeight = mBottom - mTop; //計(jì)算新的寬和高 int newWidth = right - left; int newHeight = bottom - top; //3、判斷寬高是否有分生變化 boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight); //Invalidate our old position //4、如果大小變化了,在已繪制了的情況下就請(qǐng)求重新繪制 invalidate(sizeChanged); //5、存儲(chǔ)新的值 mLeft = left; mTop = top; mRight = right; mBottom = bottom; mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom); mPrivateFlags |= PFLAG_HAS_BOUNDS; if (sizeChanged) { //6、大小變化時(shí)進(jìn)行處理 sizeChange(newWidth, newHeight, oldWidth, oldHeight); } if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) { //7、如果此時(shí)View是可見狀態(tài)下,立即執(zhí)行繪制操作 invalidate(sizeChanged); } mPrivateFlags |= drawn; mBackgroundSizeChanged = true; if (mForegroundInfo != null) { mForegroundInfo.mBoundsChanged = true; } notifySubtreeAccessibilityStateChangedIfNeeded(); } return changed; }
首先判斷四個(gè)頂點(diǎn)的位置是否有變化;
判斷寬高是否有變化,如果變化了則請(qǐng)求重新繪制;
保存新的值TOP、LEFT、BOTTOM、RIGHT。
可以看到changed的值只與四個(gè)點(diǎn)是否發(fā)生了變化有關(guān)。同時(shí),我們還發(fā)現(xiàn),在setframe方法后,就可以獲得某個(gè)view的top、left、right、bottom的值了。
回到layout方法中,繼續(xù)執(zhí)行會(huì)調(diào)用onLayout方法,我們看看其代碼:
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {}
可以看到這是一個(gè)空實(shí)現(xiàn),和onMeasure方法類似,onLayout的實(shí)現(xiàn)和具體的布局有關(guān),具體ViewGroup的子類需要重寫onLayout方法,并根據(jù)具體布局規(guī)則遍歷調(diào)用Children的layout方法。
通過上面的分析,可以得到兩個(gè)結(jié)論:
View通過layout方法來確認(rèn)自己在父容器中的位置
ViewGroup通過onLayout 方法來確定View在容器中的位置
接下來我們看看FrameLayout的onLayout方法是怎么實(shí)現(xiàn)的:
@Override protected void onLayout(boolean changed, int left, int top, int right, int bottom) { layoutChildren(left, top, right, bottom, false /* no force left gravity */); } void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) { final int count = getChildCount(); 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; 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; 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; } 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); } } }
1、獲取父View的內(nèi)邊距padding的值
2、遍歷子View,處理子View的layout_gravity屬性、根據(jù)View測量后的寬和高、父View的padding值、來確定子View的布局參數(shù),
3、調(diào)用child.layout方法,對(duì)子View進(jìn)行布局
draw繪制
Draw過程就比較簡單了,它的作用是將View繪制到屏幕上面。View的繪制過程遵循如下幾部:
繪制背景background.draw(canvas);
繪制自己onDraw;
繪制children:dispatchDraw;
繪制裝飾onDrawForeground;
這里我們看看draw方法:
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; } ... ... }
View的繪制過程的傳遞是通過dispatchDraw來實(shí)現(xiàn)的,dispatchDraw會(huì)遍歷調(diào)用所有子元素的draw方法,如此draw事件就一層層地傳遞了下去。
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