Android Toast源碼實作

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：

Toast.makeText(context, msg, Toast.LENGTH_SHORT).show();

makeText就是Toast的入口，我們從makeText的源碼來深入了解Toast的實作。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的源碼裡，我們可以看出Toast的布局檔案是transient_notification.xml，位于frameworks/base/core/res/res/layout/transient_notification.xml：

轉：http://www.2cto.com/kf/201502/374764.html

<!--?xml version="1.0" encoding="utf-8"?-->

<linearlayout android:background="?android:attr/toastFrameBackground" android:orientation="vertical" android:layout_height="match_parent" android:layout_width="match_parent" xmlns:android="http://schemas.android.com/apk/res/android">

    <textview android:id="@android:id/message" android:layout_height="wrap_content" android:layout_width="wrap_content" android:shadowradius="2.75" android:shadowcolor="#BB000000" android:textcolor="@color/bright_foreground_dark" android:textappearance="@style/TextAppearance.Toast" android:layout_gravity="center_horizontal" android:layout_weight="1">

</textview></linearlayout>

系統Toast的布局檔案非常簡單，就是在垂直布局的LinearLayout裡放置了一個TextView。接下來，我們繼續跟到show()方法，研究一下布局形成之後的展示代碼實作：

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是什麼東東？（2）INotificationManager服務的作用。帶着這兩個問題，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比對的耐心和堅持。mTN的實作在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;

    }

    @Override

    public void show() {

        if (localLOGV) Log.v(TAG, "SHOW: " + this);

        mHandler.post(mShow);

    }

    @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，用于程序間通信。這裡假設讀者都有Android程序間通信的基礎（不太熟的建議學習羅升陽關于Binder程序通信的一系列部落格）。既然TN是用于程序間通信，那麼我們很容易想到TN類的具體作用應該是Toast類的回調對象，其他程序通過調用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;

oneway interface ITransientNotification {

    void show();

    void hide();

}

ITransientNotification定義了兩個方法show()和hide()，它們的具體實作就在TN類當中。TN類的實作為：

@Override

public void show() {

    if (localLOGV) Log.v(TAG, "SHOW: " + this);

    mHandler.post(mShow);

}

@Override

public void hide() {

    if (localLOGV) Log.v(TAG, "HIDE: " + this);

    mHandler.post(mHide);

}

這裡我們就能知道，Toast的show和hide方法實作是基于Handler機制。而TN類中的Handler實作是：

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調用的是目前所線上程的Looper對象。是以，當我們在主線程（也就是UI線程中）可以随意調用Toast.makeText方法，因為Android系統幫我們實作了主線程的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部落格：http://blog.csdn.net/wzy_1988/article/details/38346637。 接下來，繼續跟一下mShow和mHide的實作，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的真正實作分别是調用了handleShow()和handleHide()方法。我們先來看handleShow()的具體實作：

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。是以，hide方法自然是WindowManager調用removeView方法來将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，其他程序調用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，這裡調用了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服務後，調用了enqueueToast方法将目前的Toast放入到系統的Toast隊列中。傳的參數分别是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并不是立刻顯示在目前的window上，而是先進入系統的Toast隊列中，然後系統調用回調對象tn的show和hide方法進行Toast的顯示和隐藏。 這裡INofiticationManager接口的具體實作類是NotificationManagerService類，位于frameworks/base/services/java/com/android/server/NotificationManagerService.java。 首先，我們來分析一下Toast入隊的函數實作enqueueToast，源碼如下：

public void enqueueToast(String pkg, ITransientNotification callback, int duration)

{

    // packageName為null或者tn類為null，直接傳回，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));

    // 判斷目前toast所屬的pkg是否為系統不允許發生Toast的pkg.NotificationManagerService有一個HashSet資料結構，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) 檢視該Toast是否已經在隊列當中

            int index = indexOfToastLocked(pkg, callback);

            // 如果Toast已經在隊列中，我們隻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在目前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; (count="" if="" count++;="" {="" (r.pkg.equals(pkg))="" r="mToastQueue.get(i);" toastrecord="" final="" i++)="">= MAX_PACKAGE_NOTIFICATIONS) {

                                 Slog.e(TAG, "Package has already posted " + count

                                        + " toasts. Not showing more. Package=" + pkg);

                                 return;

                             }

                         }

                    }

                }

                // 将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) 将目前Toast所在的程序設定為前台程序

                keepProcessAliveLocked(callingPid);

            }

            // (4) 如果index為0,說明目前入隊的Toast在隊頭，需要調用showNextToastLocked方法直接顯示

            if (index == 0) {

                showNextToastLocked();

            }

        } finally {

            Binder.restoreCallingIdentity(callingId);

        }

    }

}</n;>

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點标注代碼需要我們來進一步探讨。 (1) 判斷是否為系統Toast。如果目前Toast所屬的程序的包名為“android”，則為系統Toast，否則還可以調用isCallerSystem()方法來判斷。該方法的實作源碼為：

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，就是判斷目前Toast所屬程序的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) 檢視将要入隊的Toast是否已經在系統Toast隊列中。這是通過比對pkg和callback來實作的，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; if="" {="" r="list.get(i);" toastrecord="" i++)="" pre="" }<="" -1;="" return="" }="" i;="" cbak)="" r.callback.asbinder()="=" &&="" (r.pkg.equals(pkg)="">    通過上述代碼，我們可以得出一個結論，隻要Toast的pkg名稱和tn對象是一緻的，則系統把這些Toast認為是同一個Toast。

    (3) 将目前Toast所在程序設定為前台程序。源碼如下所示：

<pre class="brush:java;">    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; if="" {="" r="list.get(i);" toastrecord="" i++)="" }="" toastcount="" pid,="" mam.setprocessforeground(mforegroundtoken,="" try="" toastcount++;="" pid)="" (r.pid="="> 0);

        } catch (RemoteException e) {

            // Shouldn't happen.

        }

    }</n;></toastrecord></pre>    這裡的mAm=ActivityManagerNative.getDefault()，調用了setProcessForeground方法将目前pid的程序置為前台程序，保證不會系統殺死。這也就解釋了為什麼當我們finish目前Activity時，Toast還可以顯示，因為目前程序還在執行。

    (4) index為0時，對隊列頭的Toast進行顯示。源碼如下：

<pre class="brush:java;">    private void showNextToastLocked() {

        // 擷取隊列頭的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;

                }

            }

        }

    }</pre>    這裡Toast的回調對象callback就是tn對象。接下來，我們看一下，為什麼系統Toast的顯示時間隻能是2s或者3.5s，關鍵在于scheduleTimeoutLocked方法的實作。原理是，調用tn的show方法展示完Toast之後，需要調用scheduleTimeoutLocked方法來将Toast消失。（<strong>如果大家有疑問：不是說tn對象的hide方法來将Toast消失，為什麼要在這裡調用scheduleTimeoutLocked方法将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都會在目前Activity停留幾秒。如何實作停留幾秒呢？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，這裡也是用了Handler消息機制</strong>）。<br>

<pre class="brush:java;">    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);

    }</pre>    首先，我們看到這裡并不是直接發送了MESSAGE_TIMEOUT消息，而是有個delay的延遲。<strong>而delay的時間從代碼中“long delay = r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY;”看出隻能為2s或者3.5s，這也就解釋了為什麼系統Toast的呈現時間隻能是2s或者3.5s。自己在Toast.makeText方法中随意傳入一個duration是無作用的。</strong>

    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：

<pre class="brush:java;">    private final class WorkerHandler extends Handler

    {

        @Override

        public void handleMessage(Message msg)

        {

            switch (msg.what)

            {

                case MESSAGE_TIMEOUT:

                    handleTimeout((ToastRecord)msg.obj);

                    break;

            }

        }

    }</pre>    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：

<pre class="brush:java;">    private void handleTimeout(ToastRecord record)

    {

        synchronized (mToastQueue) {

            int index = indexOfToastLocked(record.pkg, record.callback);

            if (index >= 0) {

                cancelToastLocked(index);

            }

        }

    }</pre>    handleTimeout代碼中，首先判斷目前需要消失的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：

<pre class="brush:java;">    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();

        }

    }</pre>    哈哈，看到這裡，我們回調對象的hide方法也被調用了，同時也将該ToastRecord