广播 | Xi Chen

Android

Publish Date: 2021-09-13

Update Date: 2021-12-11

Word Count: 1k

Read Times: 4 Min

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广播机制简介

标准广播（Normal broadcasts）

完全异步执行的广播，广播发出后所有的广播接收器几乎在统一时刻收到这条广播信息，无法被截断。

有序广播（Ordered broadcasts)

同步执行的广播，广播发出后，同一时刻只会有一个广播接收器能收到这条广播消息，当广播接收器中的逻辑执行完毕后，广播才会继续传递。

接收系统广播

Android内置很多系统级别的广播，我们可以在应用程序中监听这些广播。这就需要广播接收器，下面将介绍其用法：

广播接受器可以对自己感兴趣的广播进行注册，这样当有相应广播发出时，接收器就能够收到。

注册广播的方式一般有两种，在代码中注册和在AndroidManifest.xml中注册，前者被称为动态注册，后者被称为静态注册

动态注册监听网络变化

public class MainActivity extends AppCompatActivity {

    private IntentFilter intentFiler;

    private NetworkChangeReceiver networkchangeReceiver;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        intentFiler = new IntentFilter();
        intentFiler.addAction("android.net.conn.CONNECTIVITY_CHANGE");
        networkchangeReceiver = new NetworkChangeReceiver();
        registerReceiver(networkchangeReceiver,intentFiler);
    }

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

    class NetworkChangeReceiver extends BroadcastReceiver{
        @Override
        public void onReceive(Context context, Intent intent) {
            Toast.makeText(context,"network changes",Toast.LENGTH_SHORT).show();
        }
    }
}

定义类NetworkChangeReceiver继承自BroadcastReceiver，重写onReceive（）方法得到信息
在onCreat（）方法中创建IntentFiler的实例。添加监听的广播
创建NetworkChangeReceiver的实例
调用registerReceiver（）方法注册，将NetworkChangeReceive和InterFilter的实例传进去。
此时，NetworkChangeReceive就可以收到指定值的广播。
重写onDestroy（）方法，加入
```
unregisterReceiver(networkchangeReceiver);
```
取消注册

优化代码

重写NetworkChangeReceiver类

class NetworkChangeReceiver extends BroadcastReceiver{
    @Override
    public void onReceive(Context context, Intent intent) {
        ConnectivityManager connectivityManager = (ConnectivityManager)
                getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = connectivityManager.getActiveNetworkInfo();
        if(networkInfo != null && networkInfo.isAvailable()){
            Toast.makeText(context,"network is available",Toast.LENGTH_SHORT).show();
        }else{
            Toast.makeText(context,"network is not available",Toast.LENGTH_SHORT).show();
        }
    }
}

通过getSystemService（）方法得到了ConnectivityManager的实例，这是一个系统服务类，专门用于管理网络连接。
调用ConnectivityManager的getActiveNetworkInfo（）方法得到NetworkInfo实例
接着调用NetworkInfo的isAvailable（）方法，判断是否有网络

在AndroidManifest,xml文件中加入权限：

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>

静态注册实现开机启动

创建广播接收器

Exported：是否允许广播接收器接受程序以外信息

Enabled：是否启用这个广播接收器

修改BootCompleteReceiver中的代码

public class BootCompleteReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        // TODO: This method is called when the BroadcastReceiver is receiving
        // an Intent broadcast.
        Toast.makeText(context,"Boot Complete",Toast.LENGTH_SHORT).show();
    }
}

添加权限

<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>

修改AndroidManifest.xml

<receiver
    android:name=".BootCompleteReceiver"
    android:enabled="true"
    android:exported="true">
    <intent-filter>
        <action android:name="android.intent.action.BOOT_COMPLETED"/>
    </intent-filter>
</receiver>

发送自定义广播

新建广播接收器
在AndroidManifest.xml中

<receiver
    android:name=".MyBroadcastReceiver"
    android:enabled="true"
    android:exported="true">
    <intent-filter>
        <action android:name="com.example.broadcasttest.MY_BROADCAST"/>
    </intent-filter>
</receiver>

自定义广播名

发出对应的广播”com.example.broadcasttest.MY_BROADCAST”时接收器可以收到该广播

对于无法接收静态广播的情况（Android 8.0后）：

加入

intent.setComponent(new ComponentName(getPackageName(), "包名"));

发送有序广播

将sendBroadcast()方法改为sendOrderBroadcast()方法，接收intent和null
设置优先级

<receiver
    android:name=".MyBroadcastReceiver"
    android:enabled="true"
    android:exported="true">
    <intent-filter android:priority="100">
        <action android:name="com.example.broadcasttest01.MY_BROADCAST"/>
    </intent-filter>
</receiver>

截断代码，在OnReceive()方法中添加abortBroadcast（）方法。

使用本地广播

public class MainActivity extends AppCompatActivity {

    private IntentFilter intentFilter;

    private LocalReceiver localReceiver;

    private LocalBroadcastManager localBroadcastManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button button = findViewById(R.id.button);
        localBroadcastManager = LocalBroadcastManager.getInstance(this);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent("com.example.LOCAL_BROADCAST");
                localBroadcastManager.sendBroadcast(intent);
            }
        });
        intentFilter = new IntentFilter();
        intentFilter.addAction("com.example.LOCAL_BROADCAST");
        localReceiver = new LocalReceiver();
        localBroadcastManager.registerReceiver(localReceiver,intentFilter);
    }

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

    class LocalReceiver extends BroadcastReceiver{
        @Override
        public void onReceive(Context context, Intent intent) {
            Toast.makeText(context,"received in local broadcast",Toast.LENGTH_SHORT).show();
        }
    }
}