1、Toast的基本使用
Toast在Android中屬於系統消息通知，用來提示用戶完成了什么操作、或者給用戶一個必要的提醒。Toast的官方定義是這樣的：
A toast provides simple feedback about an operation in a small popup. It only fills the amount of space required for the message and the current activity remains visible and interactive.
它僅僅用作一個簡單的反饋機制。使用也比較簡單:
Context context = getApplicationContext(); CharSequence text = "Hello toast!"; int duration = Toast.LENGTH_SHORT; Toast toast = Toast.makeText(context, text, duration); toast.show();
一般情況下，我們傳入一個String就基本上滿足大多數的需求。但要想自定義一個View，然后通過Toast進行顯示，也僅僅多了設置View的操作。
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/toast_layout_root" android:orientation="horizontal" android:layout_width="fill_parent" android:layout_height="fill_parent" android:padding="8dp" android:background="#DAAA" > <ImageView android:src="@drawable/droid" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_marginRight="8dp" /> <TextView android:id="@+id/text" android:layout_width="wrap_content" android:layout_height="wrap_content" android:textColor="#FFF" /> </LinearLayout>
我們把這個文件命名為toast_layout.xml，然后在代碼中加載它。
LayoutInflater inflater = getLayoutInflater(); View layout = inflater.inflate(R.layout.toast_layout, (ViewGroup) findViewById(R.id.toast_layout_root)); TextView text = (TextView) layout.findViewById(R.id.text); text.setText("This is a custom toast"); Toast toast = new Toast(getApplicationContext()); toast.setGravity(Gravity.CENTER_VERTICAL, 0, 0); toast.setDuration(Toast.LENGTH_LONG); toast.setView(layout); toast.show();
其實就是這么簡單。
2、Toast原理解剖
但現實是，產品需求說你給我控制Toast顯示的時間。咋一看好像也不難嘛。
不是有個setDuration方法么？當你翻看源碼的時候，你會發現它的描述參數只有以下兩種：
LENGTH_SHORT
LENGTH_LONG
這兩個常量對應着2秒和3.5秒，你傳個其它數字進入，效果並不是你所預料。其實這兩個常量僅僅是個flag，並不是我們想的多少秒。官方API文檔告訴我們：
This time could be user-definable.
但，它又不提供一個公開的方法讓你設置。抓狂！先看一下Toast的顯示和隱藏在代碼層面做了什么事情。
/** * Show the view for the specified duration. */ public void show() { if (mNextView == null) { throw new RuntimeException("setView must have been called"); } INotificationManager service = getService(); String pkg = mContext.getOpPackageName(); TN tn = mTN; tn.mNextView = mNextView; try { service.enqueueToast(pkg, tn, mDuration); } catch (RemoteException e) { // Empty } }
/** * Close the view if it's showing, or don't show it if it isn't showing yet. * You do not normally have to call this. Normally view will disappear on its own * after the appropriate duration. */ public void cancel() { mTN.hide(); try { getService().cancelToast(mContext.getPackageName(), mTN); } catch (RemoteException e) { // Empty } }
理解這兩個方法，需要深挖getService()到底調用了那個類做enqueueToast的操作？TN類是干什么的？繼續跟蹤代碼。
static private INotificationManager getService() { if (sService != null) { return sService; } sService = INotificationManager.Stub.asInterface(ServiceManager.getService("notification")); return sService; }
看到Stub.asInterface，我們知道這是利用Binder進行跨進程調用了。而TN類就是遵循AIDL的實現。
private static class TN extends ITransientNotification.Stub
TN類內部使用Handler機制：post一個mShow和mHide：
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; } };
再來看handleShow()方法的實現：
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(); String packageName = mView.getContext().getOpPackageName(); 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; mParams.packageName = packageName; 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(); } }
大概意思就是通過WindowManager的addView方法實現Toast的顯示。其中trySendAccessibilityEvent()方法會把當前的類名、應用的包名通過AccessibilityManager來做進一步的分發，以供后續的處理。
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); }
先回到前面的enqueueToast方法，看它做了什么事情。前面的INotificationManager service = getService()返回的就是NotificationManagerService，所以enqueueToast方法的最終實現在NotificationManagerService類中。
@Override public void enqueueToast(String pkg, ITransientNotification callback, int duration) { if (DBG) { Slog.i(TAG, "enqueueToast pkg=" + pkg + " callback=" + callback + " duration=" + duration); } if (pkg == null || callback == null) { Slog.e(TAG, "Not doing toast. pkg=" + pkg + " callback=" + callback); return ; } final boolean isSystemToast = isCallerSystem() || ("android".equals(pkg)); if (ENABLE_BLOCKED_TOASTS && !noteNotificationOp(pkg, Binder.getCallingUid())) { if (!isSystemToast) { Slog.e(TAG, "Suppressing toast from package " + pkg + " by user request."); return; } } synchronized (mToastQueue) { int callingPid = Binder.getCallingPid(); long callingId = Binder.clearCallingIdentity(); try { ToastRecord record; int index = indexOfToastLocked(pkg, callback); // If it's already in the queue, we update it in place, we don't // move it to the end of the queue. if (index >= 0) { record = mToastQueue.get(index); record.update(duration); } else { // Limit the number of toasts that any given package except the android // package can enqueue. Prevents DOS attacks and deals with leaks. 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; } } } } record = new ToastRecord(callingPid, pkg, callback, duration); mToastQueue.add(record); index = mToastQueue.size() - 1; keepProcessAliveLocked(callingPid); } // If it's at index 0, it's the current toast. It doesn't matter if it's // new or just been updated. Call back and tell it to show itself. // If the callback fails, this will remove it from the list, so don't // assume that it's valid after this. if (index == 0) { showNextToastLocked(); } } finally { Binder.restoreCallingIdentity(callingId); } } }
static final int MAX_PACKAGE_NOTIFICATIONS = 50;
static final int LONG_DELAY = 3500; // 3.5 seconds static final int SHORT_DELAY = 2000; // 2 seconds
這段代碼主要做了以下幾件事情：
- 獲取當前進程的Id。
- 查看這個Toast是否在隊列中，有的話直接返回，並更新顯示時間。
- 如果是非系統的Toast（通過應用包名進行判斷），且Toast的總數大於等於50，不再把新的Toast放入隊列。
- 最后通過keepProcessAliveLocked(callingPid)方法來設置對應的進程為前台進程，保證不被銷毀。
- 如果index = 0，說明Toast就處於隊列的頭部，直接進行顯示。
- 我們在NotificationManagerService類中確認了前面提到的LENGTH_SHORT和LENGTH_LONG的顯示時長。
關於上述的第四點，我們通過Toast類型的定義來印證代碼：
/** * Window type: transient notifications. * In multiuser systems shows only on the owning user's window. */ public static final int TYPE_TOAST = FIRST_SYSTEM_WINDOW+5;
所以一旦應用被銷毀，它對應的Toast也將不會再顯示：shows only on the owning user's window. 再來看這個keepProcessAliveLocked方法：
// lock on mToastQueue 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. } }
其中mAm是一個ActivityManagerService實例，所以調用最終進入到ActivityManagerService的setProcessForeground方法進行再次處理。下面我用一張序列圖展示整個調用流程：
其中第八步的scheduleTimeoutLocked()實質上就是利用Handler延時發送一個Message，回調TN類的hide()方法，最終通過WindowManager的removeView()來隱藏之前顯示的Toast。
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); }
至此，Toast的顯示和隱藏已經分析完畢。原理搞清楚了，讓我們回到一開始提到的問題，如何控制Toast的顯示時長？
思路1：通過反射的方式調用TN類中的show和hide方法。
代碼大概像這樣：
Object obj = message.obj; Method method = obj.getClass().getDeclaredMethod("hide", null); method.invoke(obj, null);
但是很可惜，Method method = obj.getClass().getDeclaredMethod("hide", null); 這種方法在4.0之上已經不適用了。
思路2：不讓Toast進入系統隊列，我們自己維護一個隊列。
這種方式其實仿照一下TN類中的實現，結合LinkedBlockingQueue和WindowManager就可以了。關於如何實現，后面有相應的源碼鏈接。
3、Toast在某些系統無法顯示問題
此問題常見於小米系統。MIUI上可能是出於“綠化”的考慮，在維護Toast隊列的時候，Toast只能在自己進程運行在頂端的時候才能彈出來，否則就“invisible to user”。亂改系統行為，簡直喪心病狂有木有，最終苦的是廣大Android開發人員。不過有了上面的理論准備，要解決也是沒有問題的，參照思路2。
對於這個問題，已經有人給出了源碼實現，請參考問題描述：解決小米MIUI系統上后台應用沒法彈Toast的問題，Github源碼地址：https://github.com/zhitaocai/ToastCompat
本來到這里就可以結束了，但筆者在實際開發中遭遇了一個小小的坑。
mWindowManager = (WindowManager) mContext.getSystemService(Context.WINDOW_SERVICE);
這個坑就是上面的mContext，它必須是ApplicationContext，不然在小米3或小米Note（Android 4.4.4）無法起作用！
以上。
參考：
Toast相關源碼