Day 25 | BroadcastReceiver 广播

BroadcastReceiver(广播接收器)是应用程序元件之一，类似於订阅与发布的设计模式，分为Broadcaster(广播器)与Receiver(接收器)，广波器可发送自定义或系统预设的广播事件，让接收器取得讯息，而接受器则注册特定的频道，等待广播器发送讯息并执行对应的处理。

Listener与BroadcastReceiver的差异

建立BroadcastReceiver

1. File→New→Other→Broadcast Receiver

   

2. 修改Receiver名称後点选Finish

   

3. AS会自动产生BroadcastReceiver需要的档案，AndroidManifest.xml也会自动增加Receiver的资讯

   

在Activity中建立BroadcastReceiver物件，就不必在AndroidManifest.xml新增Receiver标签

private val receiver: BroadcastReceiver = object : BroadcastReceiver() {
        override fun onReceive(context: Context, intent: Intent) {
            intent.extras?.let { 
                //...下略
            }
        }
    }

注册与接收BroadcastReceiver

//注册
registerReceiver(receiver, IntentFilter("MyBroadcast"))

//注销
unregisterReceiver(receiver)

Receiver注册时，需使用IntentFilter类别定义要接收的事件，而Broadcaster在发送前，也必须定义事件的「识别标签」，识别标签会决定要接收的事件类型，而广播事件的类型分为系统预设与自行定义。

系统预设事件

• ACTION_BATTERY_LOW：低电量通知
• ACTION_HEADSET_PLUG：耳机插入或拔除
• ACTION_SCREEN_ON：萤幕开启
• ACTION_TIMEZONE_CHANGED：时区改变

class MainActivity : AppCompatActivity() {
    //实作当萤幕开启时显示萤幕开启的Toast
    //Step1:建立BroadcastReceiver物件
    private val receiver = object : BroadcastReceiver() {
        //Step2:在onReceive()中加入接收广播後要执行的动作
        override fun onReceive(context: Context, intent: Intent) {
            //用Toast显示通知讯息
            Toast.makeText(context, "萤幕开启", Toast.LENGTH_SHORT).show()
        }
    }
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        //Step3:建立IntentFilter物件，并指定接收的识别标签为萤幕开启
        val intentFilter = IntentFilter(Intent.ACTION_SCREEN_ON)
        //Step4:注册Receiver
        registerReceiver(receiver,intentFilter)
    }
}

自定义事件

class MainActivity : AppCompatActivity() {
    //Step1:建立BroadcastReceiver物件
    private val receiver = object : BroadcastReceiver() {
        //Step2:在onReceive()中加入接收广播後要执行的动作
        override fun onReceive(context: Context, intent: Intent) {
            intent.extras?.let {
                //解析intent取得字串讯息
                val msg = "资料:${it.getString("msg")}"
                Toast.makeText(context, "萤幕开启", Toast.LENGTH_SHORT).show()
            }
        }
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        //Step3:建立IntentFilter物件，并指定接收的识别标签为MyBroadcast
        val intentFilter = IntentFilter("MyBroadcast")
        //Step4:注册Receiver
        registerReceiver(receiver,intentFilter)
    }
}

由於是自定义的事件，因此需要应用程序执行sendBroadcast()，来发送广播让Receiver接收。通常会撰写在其他程序区块

//建立带有识别标签的Intent
val intent = Intent("MyBroadcast")
//发送自定义的广播事件
sendBroadcast(intent.putExtra("msg","data")