Toast源碼實現
Toast入口
我們在應用中使用Toast提示的時候，一般都是一行簡單的代碼調用，如下所示：
[java] view plaincopyprint?在CODE上查看代碼片派生到我的代碼片
Toast.makeText(context, msg, Toast.LENGTH_SHORT).show();
makeText就是Toast的入口，我們從makeText的源碼來深入理解Toast的實現。源碼如下（frameworks/base/core/java/android/widget/Toast.java）：
public static Toast makeText(Context context, CharSequence text, int duration) { Toast result = new Toast(context); LayoutInflater inflate = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null); TextView tv = (TextView)v.findViewById(com.android.internal.R.id.message); tv.setText(text); result.mNextView = v; result.mDuration = duration; return result; } 從makeText的源碼里，我們可以看出Toast的布局文件是transient_notification.xml，位于frameworks/base/core/res/res/layout/transient_notification.xml：
<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" android:background="?android:attr/toastFrameBackground"> <TextView android:id="@android:id/message" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_weight="1" android:layout_gravity="center_horizontal" android:textAppearance="@style/TextAppearance.Toast" android:textColor="@color/bright_foreground_dark" android:shadowColor="#BB000000" android:shadowRadius="2.75" /> </LinearLayout>
系統Toast的布局文件非常簡單，就是在垂直布局的LinearLayout里放置了一個TextView。接下來，我們繼續跟到show()方法，研究一下布局形成之后的展示代碼實現：
public void show() { if (mNextView == null) { throw new RuntimeException("setView must have been called"); } INotificationManager service = getService(); String pkg = mContext.getPackageName(); TN tn = mTN; tn.mNextView = mNextView; try { service.enqueueToast(pkg, tn, mDuration); } catch (RemoteException e) { // Empty } } show方法中有兩點是需要我們注意的。（1）TN是什么東東？（2）INotificationManager服務的作用。帶著這兩個問題，繼續我們Toast源碼的探索。
TN源碼
很多問題都能通過閱讀源碼找到答案，關鍵在與你是否有與之匹配的耐心和堅持。mTN的實現在Toast的構造函數中，源碼如下：
public Toast(Context context) { mContext = context; mTN = new TN(); mTN.mY = context.getResources().getDimensionPixelSize( com.android.internal.R.dimen.toast_y_offset); mTN.mGravity = context.getResources().getInteger( com.android.internal.R.integer.config_toastDefaultGravity); } 接下來，我們就從TN類的源碼出發，探尋TN的作用。TN源碼如下：
private static class TN extends ITransientNotification.Stub { final Runnable mShow = new Runnable() { @Override public void run() { handleShow(); } }; final Runnable mHide = new Runnable() { @Override public void run() { handleHide(); // Don't do this in handleHide() because it is also invoked by handleShow() mNextView = null; } }; private final WindowManager.LayoutParams mParams = new WindowManager.LayoutParams(); final Handler mHandler = new Handler(); int mGravity; int mX, mY; float mHorizontalMargin; float mVerticalMargin; View mView; View mNextView; WindowManager mWM; TN() { // XXX This should be changed to use a Dialog, with a Theme.Toast // defined that sets up the layout params appropriately. final WindowManager.LayoutParams params = mParams; params.height = WindowManager.LayoutParams.WRAP_CONTENT; params.width = WindowManager.LayoutParams.WRAP_CONTENT; params.format = PixelFormat.TRANSLUCENT; params.windowAnimations = com.android.internal.R.style.Animation_Toast; params.type = WindowManager.LayoutParams.TYPE_TOAST; params.setTitle("Toast"); params.flags = WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE; /// M: [ALPS00517576] Support multi-user params.privateFlags = WindowManager.LayoutParams.PRIVATE_FLAG_SHOW_FOR_ALL_USERS; } /** * schedule handleShow into the right thread */ @Override public void show() { if (localLOGV) Log.v(TAG, "SHOW: " + this); mHandler.post(mShow); } /** * schedule handleHide into the right thread */ @Override public void hide() { if (localLOGV) Log.v(TAG, "HIDE: " + this); mHandler.post(mHide); } public void handleShow() { if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView + " mNextView=" + mNextView); if (mView != mNextView) { // remove the old view if necessary handleHide(); mView = mNextView; Context context = mView.getContext().getApplicationContext(); if (context == null) { context = mView.getContext(); } mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE); // We can resolve the Gravity here by using the Locale for getting // the layout direction final Configuration config = mView.getContext().getResources().getConfiguration(); final int gravity = Gravity.getAbsoluteGravity(mGravity, config.getLayoutDirection()); mParams.gravity = gravity; if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) { mParams.horizontalWeight = 1.0f; } if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) { mParams.verticalWeight = 1.0f; } mParams.x = mX; mParams.y = mY; mParams.verticalMargin = mVerticalMargin; mParams.horizontalMargin = mHorizontalMargin; if (mView.getParent() != null) { if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this); mWM.removeView(mView); } if (localLOGV) Log.v(TAG, "ADD! " + mView + " in " + this); mWM.addView(mView, mParams); trySendAccessibilityEvent(); } } private void trySendAccessibilityEvent() { AccessibilityManager accessibilityManager = AccessibilityManager.getInstance(mView.getContext()); if (!accessibilityManager.isEnabled()) { return; } // treat toasts as notifications since they are used to // announce a transient piece of information to the user AccessibilityEvent event = AccessibilityEvent.obtain( AccessibilityEvent.TYPE_NOTIFICATION_STATE_CHANGED); event.setClassName(getClass().getName()); event.setPackageName(mView.getContext().getPackageName()); mView.dispatchPopulateAccessibilityEvent(event); accessibilityManager.sendAccessibilityEvent(event); } public void handleHide() { if (localLOGV) Log.v(TAG, "HANDLE HIDE: " + this + " mView=" + mView); if (mView != null) { // note: checking parent() just to make sure the view has // been added... i have seen cases where we get here when // the view isn't yet added, so let's try not to crash. if (mView.getParent() != null) { if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this); mWM.removeView(mView); } mView = null; } } } 通過源碼，我們能很明顯的看到繼承關系，TN類繼承自ITransientNotification.Stub，用于進程間通信。這里假設讀者都有Android進程間通信的基礎（不太熟的建議學習羅升陽關于Binder進程通信的一系列博客）。既然TN是用于進程間通信，那么我們很容易想到TN類的具體作用應該是Toast類的回調對象，其他進程通過調用TN類的具體對象來操作Toast的顯示和消失。
TN類繼承自ITransientNotification.Stub，ITransientNotification.aidl位于frameworks/base/core/java/android/app/ITransientNotification.aidl，源碼如下：
package android.app; /** @hide */ oneway interface ITransientNotification { void show(); void hide(); } ITransientNotification定義了兩個方法show()和hide()，它們的具體實現就在TN類當中。TN類的實現為：
/** * schedule handleShow into the right thread */ @Override public void show() { if (localLOGV) Log.v(TAG, "SHOW: " + this); mHandler.post(mShow); } /** * schedule handleHide into the right thread */ @Override public void hide() { if (localLOGV) Log.v(TAG, "HIDE: " + this); mHandler.post(mHide); } 這里我們就能知道，Toast的show和hide方法實現是基于Handler機制。而TN類中的Handler實現是：
final Handler mHandler = new Handler();
而且，我們在TN類中沒有發現任何Looper.perpare()和Looper.loop()方法。說明，mHandler調用的是當前所在線程的Looper對象。所以，當我們在主線程（也就是UI線程中）可以隨意調用Toast.makeText方法，因為Android系統幫我們實現了主線程的Looper初始化。但是，如果你想在子線程中調用Toast.makeText方法，就必須先進行Looper初始化了，不然就會報出java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare() 。Handler機制的學習可以參考我之前寫過的一篇博客：http://blog.csdn.net/wzy_1988/article/details/38346637。
接下來，繼續跟一下mShow和mHide的實現，它倆的類型都是Runnable。
final Runnable mShow = new Runnable() { @Override public void run() { handleShow(); } }; final Runnable mHide = new Runnable() { @Override public void run() { handleHide(); // Don't do this in handleHide() because it is also invoked by handleShow() mNextView = null; } }; 可以看到，show和hide的真正實現分別是調用了handleShow()和handleHide()方法。我們先來看handleShow()的具體實現：
public void handleShow() { if (mView != mNextView) { // remove the old view if necessary handleHide(); mView = mNextView; Context context = mView.getContext().getApplicationContext(); if (context == null) { context = mView.getContext(); } mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE); // We can resolve the Gravity here by using the Locale for getting // the layout direction final Configuration config = mView.getContext().getResources().getConfiguration(); final int gravity = Gravity.getAbsoluteGravity(mGravity, config.getLayoutDirection()); mParams.gravity = gravity; if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) { mParams.horizontalWeight = 1.0f; } if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) { mParams.verticalWeight = 1.0f; } mParams.x = mX; mParams.y = mY; mParams.verticalMargin = mVerticalMargin; mParams.horizontalMargin = mHorizontalMargin; if (mView.getParent() != null) { mWM.removeView(mView); } mWM.addView(mView, mParams); trySendAccessibilityEvent(); } } 從源碼中，我們知道Toast是通過WindowManager調用addView加載進來的。因此，hide方法自然是WindowManager調用removeView方法來將Toast視圖移除。
總結一下，通過對TN類的源碼分析，我們知道了TN類是回調對象，其他進程調用tn類的show和hide方法來控制這個Toast的顯示和消失。
NotificationManagerService
回到Toast類的show方法中，我們可以看到，這里調用了getService得到INotificationManager服務，源碼如下：
private static INotificationManager sService; static private INotificationManager getService() { if (sService != null) { return sService; } sService = INotificationManager.Stub.asInterface(ServiceManager.getService("notification")); return sService; } 得到INotificationManager服務后，調用了enqueueToast方法將當前的Toast放入到系統的Toast隊列中。傳的參數分別是pkg、tn和mDuration。也就是說，我們通過Toast.makeText(context, msg, Toast.LENGTH_SHOW).show()去呈現一個Toast，這個Toast并不是立刻顯示在當前的window上，而是先進入系統的Toast隊列中，然后系統調用回調對象tn的show和hide方法進行Toast的顯示和隱藏。
這里INofiticationManager接口的具體實現類是NotificationManagerService類，位于frameworks/base/services/java/com/android/server/NotificationManagerService.java。
首先，我們來分析一下Toast入隊的函數實現enqueueToast，源碼如下：
public void enqueueToast(String pkg, ITransientNotification callback, int duration) { // packageName為null或者tn類為null，直接返回，不進隊列 if (pkg == null || callback == null) { return ; } // (1) 判斷是否為系統Toast final boolean isSystemToast = isCallerSystem() || ("android".equals(pkg)); // 判斷當前toast所屬的pkg是否為系統不允許發生Toast的pkg.NotificationManagerService有一個HashSet數據結構，存儲了不允許發生Toast的包名 if (ENABLE_BLOCKED_TOASTS && !noteNotificationOp(pkg, Binder.getCallingUid()) && !areNotificationsEnabledForPackageInt(pkg)) { if (!isSystemToast) { return; } } synchronized (mToastQueue) { int callingPid = Binder.getCallingPid(); long callingId = Binder.clearCallingIdentity(); try { ToastRecord record; // (2) 查看該Toast是否已經在隊列當中 int index = indexOfToastLocked(pkg, callback); // 如果Toast已經在隊列中，我們只需要更新顯示時間即可 if (index >= 0) { record = mToastQueue.get(index); record.update(duration); } else { // 非系統Toast，每個pkg在當前mToastQueue中Toast有總數限制，不能超過MAX_PACKAGE_NOTIFICATIONS if (!isSystemToast) { int count = 0; final int N = mToastQueue.size(); for (int i=0; i<N; i++) { final ToastRecord r = mToastQueue.get(i); if (r.pkg.equals(pkg)) { count++; if (count >= MAX_PACKAGE_NOTIFICATIONS) { Slog.e(TAG, "Package has already posted " + count + " toasts. Not showing more. Package=" + pkg); return; } } } } // 將Toast封裝成ToastRecord對象，放入mToastQueue中 record = new ToastRecord(callingPid, pkg, callback, duration); mToastQueue.add(record); index = mToastQueue.size() - 1; // (3) 將當前Toast所在的進程設置為前臺進程 keepProcessAliveLocked(callingPid); } // (4) 如果index為0,說明當前入隊的Toast在隊頭，需要調用showNextToastLocked方法直接顯示 if (index == 0) { showNextToastLocked(); } } finally { Binder.restoreCallingIdentity(callingId); } } } 可以看到，我對上述代碼做了簡要的注釋。代碼相對簡單，但是還有4點標注代碼需要我們來進一步探討。
(1) 判斷是否為系統Toast。如果當前Toast所屬的進程的包名為“android”，則為系統Toast，否則還可以調用isCallerSystem()方法來判斷。該方法的實現源碼為：
boolean isUidSystem(int uid) { final int appid = UserHandle.getAppId(uid); return (appid == Process.SYSTEM_UID || appid == Process.PHONE_UID || uid == 0); } boolean isCallerSystem() { return isUidSystem(Binder.getCallingUid()); } isCallerSystem的源碼也比較簡單，就是判斷當前Toast所屬進程的uid是否為SYSTEM_UID、0、PHONE_UID中的一個，如果是，則為系統Toast;如果不是，則不為系統Toast。
是否為系統Toast，通過下面的源碼閱讀可知，主要有兩點優勢：
系統Toast一定可以進入到系統Toast隊列中，不會被黑名單阻止。
系統Toast在系統Toast隊列中沒有數量限制，而普通pkg所發送的Toast在系統Toast隊列中有數量限制。
(2) 查看將要入隊的Toast是否已經在系統Toast隊列中。這是通過比對pkg和callback來實現的，具體源碼如下所示：
private int indexOfToastLocked(String pkg, ITransientNotification callback) { IBinder cbak = callback.asBinder(); ArrayList<ToastRecord> list = mToastQueue; int len = list.size(); for (int i=0; i<len; i++) { ToastRecord r = list.get(i); if (r.pkg.equals(pkg) && r.callback.asBinder() == cbak) { return i; } } return -1; } 通過上述代碼，我們可以得出一個結論，只要Toast的pkg名稱和tn對象是一致的，則系統把這些Toast認為是同一個Toast。
(3) 將當前Toast所在進程設置為前臺進程。源碼如下所示：
private void keepProcessAliveLocked(int pid) { int toastCount = 0; // toasts from this pid ArrayList<ToastRecord> list = mToastQueue; int N = list.size(); for (int i=0; i<N; i++) { ToastRecord r = list.get(i); if (r.pid == pid) { toastCount++; } } try { mAm.setProcessForeground(mForegroundToken, pid, toastCount > 0); } catch (RemoteException e) { // Shouldn't happen. } } 這里的mAm=ActivityManagerNative.getDefault()，調用了setProcessForeground方法將當前pid的進程置為前臺進程，保證不會系統殺死。這也就解釋了為什么當我們finish當前Activity時，Toast還可以顯示，因為當前進程還在執行。
(4) index為0時，對隊列頭的Toast進行顯示。源碼如下：
private void showNextToastLocked() { // 獲取隊列頭的ToastRecord ToastRecord record = mToastQueue.get(0); while (record != null) { try { // 調用Toast的回調對象中的show方法對Toast進行展示 record.callback.show(); scheduleTimeoutLocked(record); return; } catch (RemoteException e) { Slog.w(TAG, "Object died trying to show notification " + record.callback + " in package " + record.pkg); // remove it from the list and let the process die int index = mToastQueue.indexOf(record); if (index >= 0) { mToastQueue.remove(index); } keepProcessAliveLocked(record.pid); if (mToastQueue.size() > 0) { record = mToastQueue.get(0); } else { record = null; } } } } 這里Toast的回調對象callback就是tn對象。接下來，我們看一下，為什么系統Toast的顯示時間只能是2s或者3.5s，關鍵在于scheduleTimeoutLocked方法的實現。原理是，調用tn的show方法展示完Toast之后，需要調用scheduleTimeoutLocked方法來將Toast消失。（如果大家有疑問：不是說tn對象的hide方法來將Toast消失，為什么要在這里調用scheduleTimeoutLocked方法將Toast消失呢？是因為tn類的hide方法一執行，Toast立刻就消失了，而平時我們所使用的Toast都會在當前Activity停留幾秒。如何實現停留幾秒呢？原理就是scheduleTimeoutLocked發送MESSAGE_TIMEOUT消息去調用tn對象的hide方法，但是這個消息會有一個delay延遲，這里也是用了Handler消息機制）。
private static final int LONG_DELAY = 3500; // 3.5 seconds private static final int SHORT_DELAY = 2000; // 2 seconds private void scheduleTimeoutLocked(ToastRecord r) { mHandler.removeCallbacksAndMessages(r); Message m = Message.obtain(mHandler, MESSAGE_TIMEOUT, r); long delay = r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY; mHandler.sendMessageDelayed(m, delay); } 首先，我們看到這里并不是直接發送了MESSAGE_TIMEOUT消息，而是有個delay的延遲。而delay的時間從代碼中“long delay = r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY;”看出只能為2s或者3.5s，這也就解釋了為什么系統Toast的呈現時間只能是2s或者3.5s。自己在Toast.makeText方法中隨意傳入一個duration是無作用的。
接下來，我們來看一下WorkerHandler中是如何處理MESSAGE_TIMEOUT消息的。mHandler對象的類型為WorkerHandler，源碼如下：
private final class WorkerHandler extends Handler { @Override public void handleMessage(Message msg) { switch (msg.what) { case MESSAGE_TIMEOUT: handleTimeout((ToastRecord)msg.obj); break; } } } 可以看到，WorkerHandler對MESSAGE_TIMEOUT類型的消息處理是調用了handlerTimeout方法，那我們繼續跟蹤handleTimeout源碼：
private void handleTimeout(ToastRecord record) { synchronized (mToastQueue) { int index = indexOfToastLocked(record.pkg, record.callback); if (index >= 0) { cancelToastLocked(index); } } } handleTimeout代碼中，首先判斷當前需要消失的Toast所屬ToastRecord對象是否在隊列中，如果在隊列中，則調用cancelToastLocked(index)方法。真相就要浮現在我們眼前了，繼續跟蹤源碼：
private void cancelToastLocked(int index) { ToastRecord record = mToastQueue.get(index); try { record.callback.hide(); } catch (RemoteException e) { // don't worry about this, we're about to remove it from // the list anyway } mToastQueue.remove(index); keepProcessAliveLocked(record.pid); if (mToastQueue.size() > 0) { // Show the next one. If the callback fails, this will remove // it from the list, so don't assume that the list hasn't changed // after this point. showNextToastLocked(); } } 哈哈，看到這里，我們回調對象的hide方法也被調用了，同時也將該ToastRecord對象從mToastQueue中移除了。到這里，一個Toast的完整顯示和消失就講解結束了。
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