详解Android中的Toast源码

Toast源码实现
Toast入口
    我们在应用中使用Toast提示的时候，一般都是一行简单的代码调用，如下所示：
[java] view plaincopyprint?在CODE上查看代码片派生到我的代码片

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

makeText就是Toast的入口，我们从makeText的源码来深入理解Toast的实现。源码如下（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;
  }

从makeText的源码里，我们可以看出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>

系统Toast的布局文件非常简单，就是在垂直布局的LinearLayout里放置了一个TextView。接下来，我们继续跟到show()方法，研究一下布局形成之后的展示代码实现：

 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方法中有两点是需要我们注意的。（1）TN是什么东东？（2）INotificationManager服务的作用。带着这两个问题，继续我们Toast源码的探索。
TN源码
    很多问题都能通过阅读源码找到答案，关键在与你是否有与之匹配的耐心和坚持。mTN的实现在Toast的构造函数中，源码如下：

  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);
  }

接下来，我们就从TN类的源码出发，探寻TN的作用。TN源码如下：

  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;
      }
    }
  }

通过源码，我们能很明显的看到继承关系，TN类继承自ITransientNotification.Stub，用于进程间通信。这里假设读者都有Android进程间通信的基础（不太熟的建议学习罗升阳关于Binder进程通信的一系列博客）。既然TN是用于进程间通信，那么我们很容易想到TN类的具体作用应该是Toast类的回调对象，其他进程通过调用TN类的具体对象来操作Toast的显示和消失。
    TN类继承自ITransientNotification.Stub，ITransientNotification.aidl位于frameworks/base/core/java/android/app/ITransientNotification.aidl，源码如下：

  package android.app; 

  /** @hide */
  oneway interface ITransientNotification {
    void show();
    void hide();
  }

ITransientNotification定义了两个方法show()和hide()，它们的具体实现就在TN类当中。TN类的实现为：

  /**
   * 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);
  }

这里我们就能知道，Toast的show和hide方法实现是基于Handler机制。而TN类中的Handler实现是：

  final Handler mHandler = new Handler();

而且，我们在TN类中没有发现任何Looper.perpare()和Looper.loop()方法。说明，mHandler调用的是当前所在线程的Looper对象。所以，当我们在主线程（也就是UI线程中）可以随意调用Toast.makeText方法，因为Android系统帮我们实现了主线程的Looper初始化。但是，如果你想在子线程中调用Toast.makeText方法，就必须先进行Looper初始化了，不然就会报出java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare() 。Handler机制的学习可以参考我之前写过的一篇博客：http://blog.csdn.net/wzy_1988/article/details/38346637。
    接下来，继续跟一下mShow和mHide的实现，它俩的类型都是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的真正实现分别是调用了handleShow()和handleHide()方法。我们先来看handleShow()的具体实现：

 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();
    }
  }

从源码中，我们知道Toast是通过WindowManager调用addView加载进来的。因此，hide方法自然是WindowManager调用removeView方法来将Toast视图移除。
    总结一下，通过对TN类的源码分析，我们知道了TN类是回调对象，其他进程调用tn类的show和hide方法来控制这个Toast的显示和消失。
NotificationManagerService
    回到Toast类的show方法中，我们可以看到，这里调用了getService得到INotificationManager服务，源码如下：

  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服务后，调用了enqueueToast方法将当前的Toast放入到系统的Toast队列中。传的参数分别是pkg、tn和mDuration。也就是说，我们通过Toast.makeText(context, msg, Toast.LENGTH_SHOW).show()去呈现一个Toast，这个Toast并不是立刻显示在当前的window上，而是先进入系统的Toast队列中，然后系统调用回调对象tn的show和hide方法进行Toast的显示和隐藏。
    这里INofiticationManager接口的具体实现类是NotificationManagerService类，位于frameworks/base/services/java/com/android/server/NotificationManagerService.java。
    首先，我们来分析一下Toast入队的函数实现enqueueToast，源码如下：

  public void enqueueToast(String pkg, ITransientNotification callback, int duration)
  {
    // packageName为null或者tn类为null，直接返回，不进队列
    if (pkg == null || callback == null) {
      return ;
    } 

    // (1) 判断是否为系统Toast
    final boolean isSystemToast = isCallerSystem() || ("android".equals(pkg)); 

    // 判断当前toast所属的pkg是否为系统不允许发生Toast的pkg.NotificationManagerService有一个HashSet数据结构，存储了不允许发生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) 查看该Toast是否已经在队列当中
        int index = indexOfToastLocked(pkg, callback);
        // 如果Toast已经在队列中，我们只需要更新显示时间即可
        if (index >= 0) {
          record = mToastQueue.get(index);
          record.update(duration);
        } else {
          // 非系统Toast，每个pkg在当前mToastQueue中Toast有总数限制，不能超过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;
                 }
               }
            }
          } 

          // 将Toast封装成ToastRecord对象，放入mToastQueue中
          record = new ToastRecord(callingPid, pkg, callback, duration);
          mToastQueue.add(record);
          index = mToastQueue.size() - 1;
          // (3) 将当前Toast所在的进程设置为前台进程
          keepProcessAliveLocked(callingPid);
        }
        // (4) 如果index为0,说明当前入队的Toast在队头，需要调用showNextToastLocked方法直接显示
        if (index == 0) {
          showNextToastLocked();
        }
      } finally {
        Binder.restoreCallingIdentity(callingId);
      }
    }
  }

可以看到，我对上述代码做了简要的注释。代码相对简单，但是还有4点标注代码需要我们来进一步探讨。
    (1) 判断是否为系统Toast。如果当前Toast所属的进程的包名为“android”，则为系统Toast，否则还可以调用isCallerSystem()方法来判断。该方法的实现源码为：

  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的源码也比较简单，就是判断当前Toast所属进程的uid是否为SYSTEM_UID、0、PHONE_UID中的一个，如果是，则为系统Toast;如果不是，则不为系统Toast。
    是否为系统Toast，通过下面的源码阅读可知，主要有两点优势：

系统Toast一定可以进入到系统Toast队列中，不会被黑名单阻止。
    系统Toast在系统Toast队列中没有数量限制，而普通pkg所发送的Toast在系统Toast队列中有数量限制。

(2) 查看将要入队的Toast是否已经在系统Toast队列中。这是通过比对pkg和callback来实现的，具体源码如下所示：

  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;
  }

通过上述代码，我们可以得出一个结论，只要Toast的pkg名称和tn对象是一致的，则系统把这些Toast认为是同一个Toast。
    (3) 将当前Toast所在进程设置为前台进程。源码如下所示：

  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.
    }
  }

这里的mAm=ActivityManagerNative.getDefault()，调用了setProcessForeground方法将当前pid的进程置为前台进程，保证不会系统杀死。这也就解释了为什么当我们finish当前Activity时，Toast还可以显示，因为当前进程还在执行。
    (4) index为0时，对队列头的Toast进行显示。源码如下：

  private void showNextToastLocked() {
    // 获取队列头的ToastRecord
    ToastRecord record = mToastQueue.get(0);
    while (record != null) {
      try {
        // 调用Toast的回调对象中的show方法对Toast进行展示
        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;
        }
      }
    }
  }

这里Toast的回调对象callback就是tn对象。接下来，我们看一下，为什么系统Toast的显示时间只能是2s或者3.5s，关键在于scheduleTimeoutLocked方法的实现。原理是，调用tn的show方法展示完Toast之后，需要调用scheduleTimeoutLocked方法来将Toast消失。（如果大家有疑问：不是说tn对象的hide方法来将Toast消失，为什么要在这里调用scheduleTimeoutLocked方法将Toast消失呢？是因为tn类的hide方法一执行，Toast立刻就消失了，而平时我们所使用的Toast都会在当前Activity停留几秒。如何实现停留几秒呢？原理就是scheduleTimeoutLocked发送MESSAGE_TIMEOUT消息去调用tn对象的hide方法，但是这个消息会有一个delay延迟，这里也是用了Handler消息机制）。

  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，我们看到这里并不是直接发送了MESSAGE_TIMEOUT消息，而是有个delay的延迟。而delay的时间从代码中“long delay = r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY;”看出只能为2s或者3.5s，这也就解释了为什么系统Toast的呈现时间只能是2s或者3.5s。自己在Toast.makeText方法中随意传入一个duration是无作用的。
    接下来，我们来看一下WorkerHandler中是如何处理MESSAGE_TIMEOUT消息的。mHandler对象的类型为WorkerHandler，源码如下：

  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对MESSAGE_TIMEOUT类型的消息处理是调用了handlerTimeout方法，那我们继续跟踪handleTimeout源码：

  private void handleTimeout(ToastRecord record)
  {
    synchronized (mToastQueue) {
      int index = indexOfToastLocked(record.pkg, record.callback);
      if (index >= 0) {
        cancelToastLocked(index);
      }
    }
  }

handleTimeout代码中，首先判断当前需要消失的Toast所属ToastRecord对象是否在队列中，如果在队列中，则调用cancelToastLocked(index)方法。真相就要浮现在我们眼前了，继续跟踪源码：

  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，看到这里，我们回调对象的hide方法也被调用了，同时也将该ToastRecord对象从mToastQueue中移除了。到这里，一个Toast的完整显示和消失就讲解结束了。