有时候为了将异步回调改为同步操作，会用到wait和notify，如果notify发生在wait之后，将出现“锁死”的现象

Executors.getInstance().background(new Runnable() {
    public void run() {
        getPermissionRequestDelegate().requestPermissions(permissions, new PermissionResultReceiver() {
            public void onGranted(String[] permissions) {
                notifySignal();
            }

            public void onDenied(String[] permissions) {
                notifySignal();
            }

            public void onCanceled() {
                notifySignal();
            }
    });
}
});

waitSignal();

如果background内的方法先于waitSignal执行，就会出现“锁死”

简答的解决办法就是判断是否已经调用了noitfy方法

final int[] invoked = new int[]{0};
Executors.getInstance().background(new Runnable() {
    public void run() {
        getPermissionRequestDelegate().requestPermissions(permissions, new PermissionResultReceiver() {
            public void onGranted(String[] permissions) {
                invoked[0] = 1;
                notifySignal();
            }

            public void onDenied(String[] permissions) {
                invoked[0] = 1;
                notifySignal();
            }

            public void onCanceled() {
                invoked[0] = 1;
                notifySignal();
            }
    });
}
});

synchronized (signal) {
    if (invoked[0] == 0) {
        waitSignal();
    }
}

参考：Memory Visibility