「onreceive」广播onReceive()方法的context类型探究

onreceive

1. 铺垫

1). 非Activity类型的context并没有所谓的任务栈；

2). 出于安全原因的考虑，Android是不允许Activity或Dialog凭空出现的，一个Activity的启动必须要建立在另一个Activity的基础之上，也就是以此形成的返回栈。而Dialog则必须在一个Activity上面弹出（除非是System alert类型的Dialog），因此在这种场景下，我们只能使用Activity类型的context，否则将会出错。

2. 动态、静态注册的广播，其onreceive()方法里的context类型

1). 静态注册广播，如果没声明android:process，那么BroadcastReceiver和Activity是在同一个进程的；onReceive()里传进来的context的类型最初以为是APPlication，但打印发现是android.app.ReceiverrestrictedContext,。此context非Activity类型，不可直接用来构造alertdialog。

    @Override
    protected void onCreate(Bundle savedinstanceState) {
        super.onCreate(savedInstanceState);
        setcontentView(R.layout.activity_main);

        Log.e("main", "pid = " + Process.myPid() + ", tid = "+ Process.myTid());
        Thread thread = Thread.currentThread();
        Log.e("main", "tid = " + thread.getId() + ", name = "+ thread.getName());
    }

    //static receiver
    public static class MyReceiver extends BroadcastReceiver{
        @Override
        public void onReceive(Context context, intent intent) {
            Log.e("static receiver", "pid = " + Process.myPid() + ", tid = "+ Process.myTid());
            Thread thread = Thread.currentThread();
            Log.e("static receiver", "tid = " + thread.getId() + ", name = "+ thread.getName());

            Log.e("static receiver", "Context class = " + context.getClass().getName());
            Log.e("static receiver", "Application Context class = "
                    + context.getApplicationContext().getClass().getName());
        }
    }

输出结果：

2). 动态注册中BroadcastReceiver和Activity也是运行在同一个进程中的，因此调用sendBraodcast()时，传入onReceive()方法里的Context对象context其实就是调用sendBroadcast()的Activty对象，这里的context可以直接用来构造AlertDialog。

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        IntentFilter filter = new IntentFilter("my.action_LYL");
        registerreceiver(receiver, filter);

        Log.e("main", "pid = " + Process.myPid() + ", tid = "+ Process.myTid());
        Thread thread = Thread.currentThread();
        Log.e("main", "tid = " + thread.getId() + ", name = "+ thread.getName());
    }

   //dynamic receiver
    private BroadcastReceiver receiver = new BroadcastReceiver(){
        @Override
        public void onReceive(Context context, Intent intent) {
            Log.e("dynamic receiver", "pid = " + Process.myPid() + ", tid = "+ Process.myTid());
            Thread thread = Thread.currentThread();
            Log.e("dynamic receiver", "tid = " + thread.getId() + ", name = "+ thread.getName());

            Log.e("dynamic receiver", "Activity Context class  = "
                    + MainActivity.this.getClass().getName());
            Log.e("dynamic receiver", "Context class  = " + context.getClass().getName());

            Log.e("dynamic receiver", "Application Context class  = "
                    + context.getApplicationContext().getClass().getName());
        }
    };

输出结果：

故：

静态注册时，onRecice()方法的context不是Activity类型，故此时不能在接收器中弹出对话框。

动态注册时，BroadcastReceiver的onRecice()方法的context来自Activity，可弹框提示。

同时，在源码中，我们也可以看到，在onReceive()方法前有这么一句注释：* @param context The Context in which the receiver is running.


    /**
     * This method is called when the BroadcastReceiver is receiving an Intent
     * broadcast.  During this time you can use the other methods on
     * BroadcastReceiver to view/modify the current result values.  This method
     * is always called within the main thread of its process, unless you
     * explicitly asked for it to be scheduled on a different thread using
     * {@link android.content.Context#registerReceiver(BroadcastReceiver,
     * IntentFilter, String, android.os.handler)}. When it runs on the main
     * thread you should
     * never perform long-running operations in it (there is a timeout of
     * 10 seconds that the system allows before considering the receiver to
     * be blocked and a candidate to be killed). You cannot launch a popup dialog
     * in your implementation of onReceive().
     *
     * @param context The Context in which the receiver is running.
     * @param intent The Intent being received.
     */
    public abstract void onReceive(Context context, Intent intent);

在onReceive()方法的解释中，不要在广播的onReceive()方法中执行耗时操作，也不要添加过多的逻辑。因为广播接收器中不允许开启线程，当onReceive()运行较长时间而没有结束，程序就会报错。广播接收器的作用是用于打开其他组件。

在Android四大组件：BroadcastReceiver史上最全面解析一文中，作者介绍了OnReceive()的context类型：

对于不同注册方式的广播接收器回调OnReceive(Context context，Intent intent)中的context返回值是不一样的：
    对于静态注册（全局+应用内广播），回调onReceive(context, intent)中的context返回值是：ReceiverRestrictedContext；
    对于全局广播的动态注册，回调onReceive(context, intent)中的context返回值是：Activity Context；
    对于应用内广播的动态注册（localBroadcastManager方式），回调onReceive(context, intent)中的context返回值是：Application Context。
    对于应用内广播的动态注册（非LocalBroadcastManager方式），回调onReceive(context, intent)中的context返回值是：Activity Context；

3. 在打印线程id的时候，使用到了Process.myTid() 和Thread.currentThread().getId()

在Android: Process.myTid() VS Thread.currentThread().getId()一文中，作者对二者做出了解释：

Process.myTid()返回linux内核给出的线程ID。
Thread.getId()，对每个线程来说，只是“java层”静态自增长的long型数据

4. 代码

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        IntentFilter filter = new IntentFilter("my.action_LYL");
        registerReceiver(receiver, filter);

        Log.e("main", "pid = " + Process.myPid() + ", tid = "+ Process.myTid());
        Thread thread = Thread.currentThread();
        Log.e("main", "tid = " + thread.getId() + ", name = "+ thread.getName());
    }

    @Override
    protected void onDestroy() {
        unregisterReceiver(receiver);
        super.onDestroy();
    }

    public void send_bc(View v){
        Intent intent = new Intent("my.action_LYL");
        intent.setPackage("test.com.testforbc_20180718");//static register must set package name
        sendBroadcast(intent);
    }

    //dynamic receiver
    private BroadcastReceiver receiver = new BroadcastReceiver(){
        @Override
        public void onReceive(Context context, Intent intent) {
            Log.e("dynamic receiver", "pid = " + Process.myPid() + ", tid = "+ Process.myTid());
            Thread thread = Thread.currentThread();
            Log.e("dynamic receiver", "tid = " + thread.getId() + ", name = "+ thread.getName());

            Log.e("dynamic receiver", "Activity Context class  = "
                    + MainActivity.this.getClass().getName());
            Log.e("dynamic receiver", "Context class  = " + context.getClass().getName());

            Log.e("dynamic receiver", "Application Context class  = "
                    + context.getApplicationContext().getClass().getName());
        }
    };

    //static receiver
    public static class MyReceiver extends BroadcastReceiver{
        @Override
        public void onReceive(Context context, Intent intent) {
            Log.e("static receiver", "pid = " + Process.myPid() + ", tid = "+ Process.myTid());
            Thread thread = Thread.currentThread();
            Log.e("static receiver", "tid = " + thread.getId() + ", name = "+ thread.getName());

            Log.e("static receiver", "Context class = " + context.getClass().getName());
            Log.e("static receiver", "Application Context class = "
                    + context.getApplicationContext().getClass().getName());
        }
    }
}

输出结果：

5. 参考

Context是什么，怎么用

Android中，Context，什么是Context？

Android四大组件：BroadcastReceiver史上最全面解析

What is the Context passed into onReceive() of a BroadcastReceiver?

能否在BroadcastReceiver里创建AlertDialog并显示？

Context都没弄明白，还怎么做Android开发？

Broadcast的onReceive方法中弹出AlertDialog

Android: Process.myTid() VS Thread.currentThread().getId()

广播onReceive()方法的context类型探究

onreceive

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