四大组件，就剩下最后一个Service ，他比较重要，相当于后台服务，基本上大部分的app，都会有一两个这样的服务Service 。
Service用处非常的多，可以根据后台的特性来决定Service的用法。
Service 的使用也非常的简单，简单的建立和绑定，就能完成Service的动作。
建立Service
这里我们创建一个Service，用它来发送消息服务，这里从服务的建立和用Binder 来绑定服务，这样可以建立起Service 和Activity之间的通讯问题。
建立一个
internal class MyBinder(private val service: NotificationService) : Binder() {
fun getService() : NotificationService{
return service
}
}MyBinder 是我们的中间人，我们需要通过它来找到真正的Service。
NotificationService 如下：
class NotificationService : Service() {
private lateinit var mNotification: Notification
private val mNotificationId: Int = 1000
private var mBinder = MyBinder(this@NotificationService)
companion object {
const val CHANNEL_ID = "com.kotlin.kotlin_start_ch18.CHANNEL_ID"
const val CHANNEL_NAME = "Sample Notification"
}
override fun onBind(intent: Intent): IBinder {
return mBinder
}这里NotificationService 是一个空的，什么任务也没有，为他加一个简单的任务，就是消息推送通知。
@SuppressLint("NewApi")
private fun createChannel() {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
// Create the NotificationChannel, but only on API 26+ because
// the NotificationChannel class is new and not in the support library
val context = this.applicationContext
val notificationManager = context.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
val importance = NotificationManager.IMPORTANCE_HIGH
val notificationChannel = NotificationChannel(CHANNEL_ID, CHANNEL_NAME, importance)
notificationChannel.enableVibration(true)
notificationChannel.setShowBadge(true)
notificationChannel.enableLights(true)
notificationChannel.lightColor = Color.parseColor("#e8334a")
notificationChannel.description = getString(R.string.notification_channel_description)
notificationChannel.lockscreenVisibility = Notification.VISIBILITY_PUBLIC
notificationManager.createNotificationChannel(notificationChannel)
}
}通过上面的代码，NotificationService 就有了自己的事情做了，可以通过notifyMessage()
public fun notifyMessage(){
//Create Channel
createChannel()
val context = this.applicationContext
var notificationManager: NotificationManager =
context.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
val notifyIntent = Intent(this, ResultActivity::class.java)
val title = "Sample Notification"
val message =
"You have received a sample notification. This notification will take you to the details page."
notifyIntent.putExtra("title", title)
notifyIntent.putExtra("message", message)
notifyIntent.putExtra("notification", true)
notifyIntent.flags = Intent.FLAG_ACTIVITY_NEW_TASK
val pendingIntent =
PendingIntent.getActivity(context, 0, notifyIntent, PendingIntent.FLAG_IMMUTABLE)
val res = this.resources
val uri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
mNotification = Notification.Builder(this, CHANNEL_ID)
// Set the intent that will fire when the user taps the notification
.setContentIntent(pendingIntent)
.setSmallIcon(R.drawable.ic_stat_name)
.setLargeIcon(BitmapFactory.decodeResource(res, R.mipmap.ic_launcher))
.setAutoCancel(true)
.setContentTitle(title)
.setStyle(
Notification.BigTextStyle()
.bigText(message)
)
.setContentText(message).build()
} else {
mNotification = Notification.Builder(this)
// Set the intent that will fire when the user taps the notification
.setContentIntent(pendingIntent)
.setSmallIcon(R.drawable.ic_stat_name)
.setLargeIcon(BitmapFactory.decodeResource(res, R.mipmap.ic_launcher))
.setAutoCancel(true)
.setPriority(Notification.PRIORITY_MAX)
.setContentTitle(title)
.setStyle(
Notification.BigTextStyle()
.bigText(message)
)
.setSound(uri)
.setContentText(message).build()
}
notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
// mNotificationId is a unique int for each notification that you must define
notificationManager.notify(mNotificationId, mNotification)
}当我们发送的通知消息被点击以后，会回到我们app的ResultActivity 中，只要在程序中把ResultActivity 实现为自己的逻辑，就能调整到ResultActivity 页面中。
绑定服务
启动服务可以有两种方法，一种是直接启动，一种还要进行相应的绑定。
val service = Intent(this@MainActivity, NotificationService::class.java)
service.putExtra("reason", intent.getStringExtra("reason"))
service.putExtra("timestamp", intent.getLongExtra("timestamp", 0))
service.data = Uri.parse("custom://" + System.currentTimeMillis())
startService(service)我们需要和Service 进行通讯，所以我们采用绑定的方式。
private fun bindService() {
connection = object : ServiceConnection {
override fun onServiceConnected(name: ComponentName, service: IBinder) {
binder = service as NotificationService.MyBinder
}
override fun onServiceDisconnected(name: ComponentName) {}
}
val intent = Intent(this, NotificationService::class.java)
startService(intent)
bindService(intent, connection as ServiceConnection, Context.BIND_AUTO_CREATE)
}
如上，我们可以通过服务，发送通知消息了。
小结
四大组件，我们已经一个一个的进行了简单的介绍，你会慢慢的了解到安卓开发中主要的组件形式和使用的方法，后面还会慢慢的安卓的其他的特性进行介绍。这四大组件非常的重要，可以在其他的demo中注意这四个组件的用法，对开发程序会有很大的帮助。
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