Android中View繪制流程詳細(xì)介紹
創(chuàng)建Window
Window即窗口,這個(gè)概念在AndroidFramework中的實(shí)現(xiàn)為android.view.Window這個(gè)抽象類,這個(gè)抽象類是對Android系統(tǒng)中的窗口的抽象。在介紹這個(gè)類之前,我們先來看看究竟什么是窗口呢?
實(shí)際上,窗口是一個(gè)宏觀的思想,它是屏幕上用于繪制各種UI元素及響應(yīng)用戶輸入事件的一個(gè)矩形區(qū)域。通常具備以下兩個(gè)特點(diǎn):
獨(dú)立繪制,不與其它界面相互影響;
不會(huì)觸發(fā)其它界面的輸入事件;
在Android系統(tǒng)中,窗口是獨(dú)占一個(gè)Surface實(shí)例的顯示區(qū)域,每個(gè)窗口的Surface由WindowManagerService分配。我們可以把Surface看作一塊畫布,應(yīng)用可以通過Canvas或OpenGL在其上面作畫。畫好之后,通過SurfaceFlinger將多塊Surface按照特定的順序(即Z-order)進(jìn)行混合,而后輸出到FrameBuffer中,這樣用戶界面就得以顯示。
android.view.Window這個(gè)抽象類可以看做Android中對窗口這一宏觀概念所做的約定,而PhoneWindow這個(gè)類是Framework為我們提供的Android窗口概念的具體實(shí)現(xiàn)。接下來我們先來介紹一下android.view.Window這個(gè)抽象類。
這個(gè)抽象類包含了三個(gè)核心組件:
WindowManager.LayoutParams:窗口的布局參數(shù);
Callback:窗口的回調(diào)接口,通常由Activity實(shí)現(xiàn);
ViewTree:窗口所承載的控件樹。
在Activity的attach方法中通過調(diào)用PolicyManager.makeNewWindo創(chuàng)建Window,將一個(gè)View add到WindowManager時(shí),WindowManagerImpl創(chuàng)建一個(gè)ViewRoot來管理該窗口的根View。并通過ViewRoot.setView方法把該View傳給ViewRoot。
final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config) {
attachBaseContext(context);
mFragments.attachActivity(this, mContainer, null);
mWindow = PolicyManager.makeNewWindow(this);
mWindow.setCallback(this);
mWindow.getLayoutInflater().setPrivateFactory(this);
創(chuàng)建DecorView
DecorView為整個(gè)Window界面的最頂層View。
Activity中的Window對象幫我們創(chuàng)建了一個(gè)PhoneWindow內(nèi)部類DecorView(父類為FrameLayout)窗口頂層視圖,然后通過LayoutInflater將xml內(nèi)容布局解析成View樹形結(jié)構(gòu)添加到DecorView頂層視圖中id為content的FrameLayout父容器上面。Activity的content內(nèi)容布局最終會(huì)添加到DecorView窗口頂層視圖上面。
protected boolean initializePanelDecor(PanelFeatureState st) {
st.decorView = new DecorView(getContext(), st.featureId);
st.gravity = Gravity.CENTER | Gravity.BOTTOM;
st.setStyle(getContext());
return true;
}
創(chuàng)建ViewRoot并關(guān)聯(lián)View
WindowManagerImpl保存DecorView到mViews,創(chuàng)建對應(yīng)的ViewRoot;
ViewRoot用于管理窗口的根View,并和global window manger進(jìn)行交互。ViewRoot中有一個(gè)nested class: W,W是一個(gè)Binder子類,用于接收global window manager的各種消息, 如按鍵消息, 觸摸消息等。 ViewRoot有一個(gè)W類型的成員mWindow,ViewRoot在Constructor中創(chuàng)建一個(gè)W的instance并賦值給mWindow。 ViewRoot是Handler的子類, W會(huì)通過Looper把消息傳遞給ViewRoot。 ViewRoot在setView方法中把mWindow傳給sWindowSession。
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
if (view == null) {
throw new IllegalArgumentException("view must not be null");
}
if (display == null) {
throw new IllegalArgumentException("display must not be null");
}
if (!(params instanceof WindowManager.LayoutParams)) {
throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
}
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;
if (parentWindow != null) {
parentWindow.adjustLayoutParamsForSubWindow(wparams);
}
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
// Start watching for system property changes.
if (mSystemPropertyUpdater == null) {
mSystemPropertyUpdater = new Runnable() {
@Override public void run() {
synchronized (mLock) {
for (ViewRootImpl viewRoot : mRoots) {
viewRoot.loadSystemProperties();
}
}
}
};
SystemProperties.addChangeCallback(mSystemPropertyUpdater);
}
int index = findViewLocked(view, false);
if (index >= 0) {
throw new IllegalStateException("View " + view
+ " has already been added to the window manager.");
}
// If this is a panel window, then find the window it is being
// attached to for future reference.
if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
final int count = mViews != null ? mViews.length : 0;
for (int i=0; i<count; i++) {
if (mRoots[i].mWindow.asBinder() == wparams.token) {
panelParentView = mViews[i];
}
}
}
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
if (mViews == null) {
index = 1;
mViews = new View[1];
mRoots = new ViewRootImpl[1];
mParams = new WindowManager.LayoutParams[1];
} else {
index = mViews.length + 1;
Object[] old = mViews;
mViews = new View[index];
System.arraycopy(old, 0, mViews, 0, index-1);
old = mRoots;
mRoots = new ViewRootImpl[index];
System.arraycopy(old, 0, mRoots, 0, index-1);
old = mParams;
mParams = new WindowManager.LayoutParams[index];
System.arraycopy(old, 0, mParams, 0, index-1);
}
index--;
mViews[index] = view;
mRoots[index] = root;
mParams[index] = wparams;
}
// do this last because it fires off messages to start doing things
try {
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
synchronized (mLock) {
final int index = findViewLocked(view, false);
if (index >= 0) {
removeViewLocked(index, true);
}
}
throw e;
}
}
ViewRoot是GUI管理系統(tǒng)與GUI呈現(xiàn)系統(tǒng)之間的橋梁,需要注意它并不是一個(gè)View類型,。
它的主要作用如下:
1、向DecorView分發(fā)收到的用戶發(fā)起的event事件,如按鍵,觸屏,軌跡球等事件;
2、與WindowManagerService交互,完成整個(gè)Activity的GUI的繪制。
View繪制基本流程
這里先給出Android系統(tǒng)View的繪制流程:依次執(zhí)行View類里面的如下三個(gè)方法:
measure(int ,int) :測量View的大小
layout(int ,int ,int ,int) :設(shè)置子View的位置
draw(Canvas) :繪制View內(nèi)容到Canvas畫布上

整個(gè)View樹的繪圖流程是在ViewRoot.java類的performTraversals()函數(shù)展開的,該函數(shù)做的執(zhí)行過程可簡單概況為根據(jù)之前設(shè)置的狀態(tài),判斷是否需要重新計(jì)算視圖大小(measure)、是否重新需要安置視圖的位置(layout)、以及是否需要重繪 (draw)
mesarue()測量過程
主要作用:為整個(gè)View樹計(jì)算實(shí)際的大小,即設(shè)置實(shí)際的高(mMeasuredHeight)和寬(mMeasureWidth),每個(gè)View的控件的實(shí)際寬高都是由父視圖和本身視圖決定的。
具體的調(diào)用如下:
ViewRootImpl 的performTraversals方法中,調(diào)用measureHierarchy,然后調(diào)用performMeasure
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
ViewRoot根對象地屬性mView(其類型一般為ViewGroup類型)調(diào)用measure()方法去計(jì)算View樹的大小,回調(diào)
View/ViewGroup對象的onMeasure()方法,該方法實(shí)現(xiàn)的功能如下:
1、設(shè)置本View視圖的最終大小,該功能的實(shí)現(xiàn)通過調(diào)用setMeasuredDimension()方法去設(shè)置實(shí)際的高(mMeasuredHeight)和寬(mMeasureWidth)
2、如果該View對象是個(gè)ViewGroup類型,需要重寫onMeasure()方法,對其子視圖進(jìn)行遍歷的measure()過程。
對每個(gè)子視圖的measure()過程,是通過調(diào)用父類ViewGroup.java類里的measureChildWithMargins()方法去實(shí)現(xiàn),該方法內(nèi)部只是簡單地調(diào)用了View對象的measure()方法。
整個(gè)measure調(diào)用流程就是個(gè)樹形的遞歸過程
measure()方法兩個(gè)參數(shù)都是父View傳遞過來的,也就是代表了父view的規(guī)格。他由兩部分組成,高2位表示MODE,定義在MeasureSpec類(View的內(nèi)部類)中,有三種類型,MeasureSpec.EXACTLY表示確定大小,MeasureSpec.AT_MOST表示最大大小,MeasureSpec.UNSPECIFIED不確定。低30位表示size,也就是父View的大小。對于系統(tǒng)Window類的DecorVIew對象Mode一般都為MeasureSpec.EXACTLY,而size分別對應(yīng)屏幕寬高。對于子View來說大小是由父View和子View共同決定的。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
layout布局過程
主要作用 :為將整個(gè)根據(jù)子視圖的大小以及布局參數(shù)將View樹放到合適的位置上。
具體的調(diào)用如下:
ViewRootImpl 的performTraversals方法中,調(diào)用performLayout
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
Log.v(TAG, "Laying out " + host + " to (" +
host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
try {
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;
}
host.layout()開始View樹的布局,繼而回調(diào)給View/ViewGroup類中的layout()方法。具體流程如下
1 、layout方法會(huì)設(shè)置該View視圖位于父視圖的坐標(biāo)軸,即mLeft,mTop,mLeft,mBottom(調(diào)用setFrame()函數(shù)去實(shí)現(xiàn)),接下來回調(diào)onLayout()方法(如果該View是ViewGroup對象,需要實(shí)現(xiàn)該方法,對每個(gè)子視圖進(jìn)行布局)。
2、如果該View是個(gè)ViewGroup類型,需要遍歷每個(gè)子視圖chiildView,調(diào)用該子視圖的layout()方法去設(shè)置它的坐標(biāo)值。
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
public void layout(int l, int t, int r, int b) {
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);
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;
}
draw()繪圖過程
ViewRootImpl的performTraversals方法中,調(diào)用了mView的draw方法
mView.draw()開始繪制,draw()方法實(shí)現(xiàn)的功能如下:
1、繪制該View的背景
2、為顯示漸變框做一些準(zhǔn)備操作
3、調(diào)用onDraw()方法繪制視圖本身(每個(gè)View都需要重載該方法,ViewGroup不需要實(shí)現(xiàn)該方法)
4、調(diào)用dispatchDraw()方法繪制子視圖(如果該View類型不為ViewGroup,即不包含子視圖,不需要重載該方法)
值得說明的是,ViewGroup類已經(jīng)為我們重寫了dispatchDraw()的功能實(shí)現(xiàn),應(yīng)用程序一般不需要重寫該方法,但可以重載父類函數(shù)實(shí)現(xiàn)具體的功能。
dispatchDraw()方法內(nèi)部會(huì)遍歷每個(gè)子視圖,調(diào)用drawChild()去重新回調(diào)每個(gè)子視圖的draw()方法。
5、繪制滾動(dòng)條
刷新視圖
Android中實(shí)現(xiàn)view的更新有兩個(gè)方法,一個(gè)是invalidate,另一個(gè)是postInvalidate,其中前者是在UI線程自身中使用,而后者在非UI線程中使用。
requestLayout()方法:會(huì)導(dǎo)致調(diào)用measure()過程和layout()過程。
說明:只是對View樹重新布局layout過程包括measure()和layout()過程,不會(huì)調(diào)用draw()過程,但不會(huì)重新繪制
任何視圖包括該調(diào)用者本身。
一般引起invalidate()操作的函數(shù)如下:
1、直接調(diào)用invalidate()方法,請求重新draw(),但只會(huì)繪制調(diào)用者本身。
2、setSelection()方法:請求重新draw(),但只會(huì)繪制調(diào)用者本身。
3、setVisibility()方法:當(dāng)View可視狀態(tài)在INVISIBLE轉(zhuǎn)換VISIBLE時(shí),會(huì)間接調(diào)用invalidate()方法,繼而繪制該View。
4、setEnabled()方法:請求重新draw(),但不會(huì)重新繪制任何視圖包括該調(diào)用者本身。
總結(jié)
以上就是本文關(guān)于Android中View繪制流程詳細(xì)介紹的全部內(nèi)容,希望對大家有所幫助。如有不足之處,歡迎留言指出。
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