Android中View繪制流程詳細介紹

2019-10-22 18:22:31

字體：大中小

來源：轉載

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創建Window

Window即窗口，這個概念在AndroidFramework中的實現為android/54360.html">android/276553.html">android/269641.html">android.view.Window這個抽象類，這個抽象類是對Android系統中的窗口的抽象。在介紹這個類之前，我們先來看看究竟什么是窗口呢？

實際上，窗口是一個宏觀的思想，它是屏幕上用于繪制各種UI元素及響應用戶輸入事件的一個矩形區域。通常具備以下兩個特點：

獨立繪制，不與其它界面相互影響；

不會觸發其它界面的輸入事件；

在Android系統中，窗口是獨占一個Surface實例的顯示區域，每個窗口的Surface由WindowManagerService分配。我們可以把Surface看作一塊畫布，應用可以通過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這個類是Framework為我們提供的Android窗口概念的具體實現。接下來我們先來介紹一下android.view.Window這個抽象類。

這個抽象類包含了三個核心組件：

WindowManager.LayoutParams:窗口的布局參數；

Callback:窗口的回調接口，通常由Activity實現；

ViewTree:窗口所承載的控件樹。

在Activity的attach方法中通過調用PolicyManager.makeNewWindo創建Window,將一個View add到WindowManager時，WindowManagerImpl創建一個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);

創建DecorView

DecorView為整個Window界面的最頂層View。
Activity中的Window對象幫我們創建了一個PhoneWindow內部類DecorView（父類為FrameLayout）窗口頂層視圖，然后通過LayoutInflater將xml內容布局解析成View樹形結構添加到DecorView頂層視圖中id為content的FrameLayout父容器上面。Activity的content內容布局最終會添加到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; }

創建ViewRoot并關聯View

WindowManagerImpl保存DecorView到mViews，創建對應的ViewRoot；
ViewRoot用于管理窗口的根View，并和global window manger進行交互。ViewRoot中有一個nested class： W，W是一個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W類型的成員mWindow，ViewRoot在Constructor中創建一個W的instance并賦值給mWindow。 ViewRoot是Handler的子類， W會通過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管理系統與GUI呈現系統之間的橋梁，需要注意它并不是一個View類型，。
它的主要作用如下：

1、向DecorView分發收到的用戶發起的event事件，如按鍵，觸屏，軌跡球等事件；
2、與WindowManagerService交互，完成整個Activity的GUI的繪制。
View繪制基本流程

這里先給出Android系統View的繪制流程：依次執行View類里面的如下三個方法：

measure(int ,int) :測量View的大小
layout(int ,int ,int ,int) ：設置子View的位置
draw(Canvas) ：繪制View內容到Canvas畫布上

android,view繪制流程,view,繪制,view繪制過程,view的繪制

整個View樹的繪圖流程是在ViewRoot.java類的performTraversals()函數展開的，該函數做的執行過程可簡單概況為根據之前設置的狀態，判斷是否需要重新計算視圖大小(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View樹計算實際的大小，即設置實際的高(mMeasuredHeight)和寬(mMeasureWidth)，每個View的控件的實際寬高都是由父視圖和本身視圖決定的。

具體的調用如下：

ViewRootImpl 的performTraversals方法中，調用measureHierarchy，然后調用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)調用measure()方法去計算View樹的大小，回調

View/ViewGroup對象的onMeasure()方法，該方法實現的功能如下：

1、設置本View視圖的最終大小，該功能的實現通過調用setMeasuredDimension()方法去設置實際的高(mMeasuredHeight)和寬(mMeasureWidth)

2、如果該View對象是個ViewGroup類型，需要重寫onMeasure()方法，對其子視圖進行遍歷的measure()過程。

對每個子視圖的measure()過程，是通過調用父類ViewGroup.java類里的measureChildWithMargins()方法去實現，該方法內部只是簡單地調用了View對象的measure()方法。

整個measure調用流程就是個樹形的遞歸過程

measure()方法兩個參數都是父View傳遞過來的，也就是代表了父view的規格。他由兩部分組成，高2位表示MODE，定義在MeasureSpec類（View的內部類）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Window類的DecorVIew對象Mode一般都為MeasureSpec.EXACTLY，而size分別對應屏幕寬高。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：為將整個根據子視圖的大小以及布局參數將View樹放到合適的位置上。
具體的調用如下：
ViewRootImpl 的performTraversals方法中，調用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，繼而回調給View/ViewGroup類中的layout()方法。具體流程如下

1 、layout方法會設置該View視圖位于父視圖的坐標軸，即mLeft，mTop，mLeft，mBottom(調用setFrame()函數去實現)，接下來回調onLayout()方法(如果該View是ViewGroup對象，需要實現該方法，對每個子視圖進行布局)。
2、如果該View是個ViewGroup類型，需要遍歷每個子視圖chiildView，調用該子視圖的layout()方法去設置它的坐標值。

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，調用了mView的draw方法

mView.draw()開始繪制，draw()方法實現的功能如下：

1、繪制該View的背景

2、為顯示漸變框做一些準備操作

3、調用onDraw()方法繪制視圖本身(每個View都需要重載該方法，ViewGroup不需要實現該方法)

4、調用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類已經為我們重寫了dispatchDraw()的功能實現，應用程序一般不需要重寫該方法，但可以重載父類函數實現具體的功能。

dispatchDraw()方法內部會遍歷每個子視圖，調用drawChild()去重新回調每個子視圖的draw()方法。

5、繪制滾動條

刷新視圖

Android中實現view的更新有兩個方法，一個是invalidate，另一個是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。