Toast的显示流程

• 从Toast的调用开始，静态方法makeText 构造了Toast实例

public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
            @NonNull CharSequence text, @Duration int duration) {
}

• 接着调用了show(),注意看 service.enqueueToast 方法，是把Toast作为一种通知交给NotificationManagerService 处理（因为它可能很多，必须要做个队列处理）
  这里的tn回调也作为参数传了过去

   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;
        final int displayId = mContext.getDisplayId();
        try {
            service.enqueueToast(pkg, tn, mDuration, displayId);
        } catch (RemoteException e) {
            // Empty
        }
    }
@VisibleForTesting
    final IBinder mService = new INotificationManager.Stub() {
        // Toasts
        // =======================入队列操作=====================================================

        @Override
        public void enqueueToast(String pkg, ITransientNotification callback, int duration,
                int displayId)
        {
                record = new ToastRecord(callingPid, pkg, callback, duration, token,
                                displayId);
                        mToastQueue.add(record);
      }

• 取出消息showNextToastLocked ，回调Toast的内部类show()方法
  record.callback.show

    void showNextToastLocked() {
        ToastRecord record = mToastQueue.get(0);
        while (record != null) {
            if (DBG) Slog.d(TAG, "Show pkg=" + record.pkg + " callback=" + record.callback);
            try {
                record.callback.show(record.token);
                scheduleDurationReachedLocked(record);

                }
               }
        }
//Toast->TN
  public void show(IBinder windowToken) {
            if (localLOGV) Log.v(TAG, "SHOW: " + this);
            mHandler.obtainMessage(SHOW, windowToken).sendToTarget();
        }

• 最后调用到Toast的 public void handleShow(IBinder windowToken) 方法
  其内部实现跟dialog类似，都是创建view 通过WM.addView 显示出来。

• 问：Toast可以在子线程弹出吗
  答：肯定可以
  看他另外一个构造方法，就是需要你自己构建looper

public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
            @NonNull CharSequence text, @Duration int duration)
if (looper == null) {
                // Use Looper.myLooper() if looper is not specified.
                looper = Looper.myLooper();
                if (looper == null) {
                    throw new RuntimeException(
                            "Can't toast on a thread that has not called Looper.prepare()");
                }
            }
//使用方法
new Thread(new Runnable() {
 
                    @Override
                    public void run() {
                        Looper.prepare();
 
                        Toast.makeText(MainActivity.this, "show", Toast.LENGTH_SHORT).show();
                        Looper.loop();
                    }
                }).start();