Headless JS(后台任务)
Headless JS 是一种使用 js 在后台执行任务的方法。它可以用来在后台同步数据、处理推送通知或是播放音乐等等。
JS 端的 API
首先我们要通过AppRegistry
来注册一个异步函数,这个函数我们称之为“任务”。注册方式类似在 index.js 中注册 RN 应用:
import { AppRegistry } from 'react-native';
AppRegistry.registerHeadlessTask('SomeTaskName', () =>
require('SomeTaskName')
);
然后创建 require 中引用的SomeTaskName.js
文件:
module.exports = async taskData => {
// 要做的任务
};
你可以在任务中处理任何事情(网络请求、定时器等等),但唯独不要涉及用户界面!在任务完成后(例如在 promise 中调用 resolve),RN 会进入一个“暂停”模式,直到有新任务需要执行或者是应用回到前台。
原生端的 API
没错,我们还需要一些原生代码,但是请放心并不麻烦。首先需要像下面这样继承HeadlessJsTaskService
,然后覆盖getTaskConfig
方法的实现:
- Java
- Kotlin
package com.your_application_name;
import android.content.Intent;
import android.os.Bundle;
import com.facebook.react.HeadlessJsTaskService;
import com.facebook.react.bridge.Arguments;
import com.facebook.react.jstasks.HeadlessJsTaskConfig;
import javax.annotation.Nullable;
public class MyTaskService extends HeadlessJsTaskService {
@Override
protected @Nullable HeadlessJsTaskConfig getTaskConfig(Intent intent) {
Bundle extras = intent.getExtras();
if (extras != null) {
return new HeadlessJsTaskConfig(
"SomeTaskName",
Arguments.fromBundle(extras),
5000, // timeout in milliseconds for the task
false // optional: defines whether or not the task is allowed in foreground. Default is false
);
}
return null;
}
}
package com.your_application_name;
import android.content.Intent
import com.facebook.react.HeadlessJsTaskService
import com.facebook.react.bridge.Arguments
import com.facebook.react.jstasks.HeadlessJsTaskConfig
class MyTaskService : HeadlessJsTaskService() {
override fun getTaskConfig(intent: Intent): HeadlessJsTaskConfig? {
return intent.extras?.let {
HeadlessJsTaskConfig(
"SomeTaskName",
Arguments.fromBundle(it),
5000, // timeout for the task
false // optional: defines whether or not the task is allowed in foreground.
// Default is false
)
}
}
}
然后记得把服务添加到AndroidManifest
文件里:
<service android:name="com.example.MyTaskService" />
好了,现在当你启动服务时(例如一个周期性的任务或是响应一些系统事件/广播),JS 任务就会开始执行。例如:
- Java
- Kotlin
Intent service = new Intent(getApplicationContext(), MyTaskService.class);
Bundle bundle = new Bundle();
bundle.putString("foo", "bar");
service.putExtras(bundle);
getApplicationContext().startForegroundService(service);
val service = Intent(applicationContext, MyTaskService::class.java)
val bundle = Bundle()
bundle.putString("foo", "bar")
service.putExtras(bundle)
applicationContext.startForegroundService(service)
重试
默认情况下,无头 JS 任务不会执行任何重试。要想进行重试,您需要创建一个HeadlessJsRetryPolicy
并抛出特定的Error
。
LinearCountingRetryPolicy
是HeadlessJsRetryPolicy
的一种实现,它允许您指定最大重试次数,并在每次尝试之间设置固定的延迟。如果您的需求不适合此策略,那么您可以轻松地实现自己的HeadlessJsRetryPolicy
。这些策略只需作为额外的参数传递给HeadlessJsTaskConfig
构造函数,例如,
- Java
- Kotlin
HeadlessJsRetryPolicy retryPolicy = new LinearCountingRetryPolicy(
3, // Max number of retry attempts
1000 // Delay between each retry attempt
);
return new HeadlessJsTaskConfig(
'SomeTaskName',
Arguments.fromBundle(extras),
5000,
false,
retryPolicy
);
val retryPolicy: HeadlessJsTaskRetryPolicy =
LinearCountingRetryPolicy(
3, // Max number of retry attempts
1000 // Delay between each retry attempt
)
return HeadlessJsTaskConfig("SomeTaskName", Arguments.fromBundle(extras), 5000, false, retryPolicy)
仅当抛出特定错误时,才会进行重试尝试。在无头JS任务中,您可以导入错误并在需要重试尝试时抛出。
例如:
import {HeadlessJsTaskError} from 'HeadlessJsTask';
module.exports = async (taskData) => {
const condition = ...;
if (!condition) {
throw new HeadlessJsTaskError();
}
};
如果你想让所有错误都导致重试尝试,你需要捕获它们并抛出上述错误。
注意事项
- 默认情况下,如果您尝试在应用程序处于前台时运行任务,您的应用程序将崩溃。 这是为了防止开发人员在任务中进行大量工作并降低 UI 速度。 您可以通过传递第四个布尔参数来控制此行为。
- 如果您从
BroadcastReceiver
启动服务,请确保在从onReceive()
返回之前调用HeadlessJsTaskService.acquireWakeLockNow()
。
示例
我们可以使用 Java API 来开启一个 service。首先你需要考虑好 Service 启动的时机,并据此实现相关逻辑。下面是一个使用 Service 来处理网络连接变化的简单范例。 接下来的几行代码展示了如何在 Android Manifest 文件中注册一个Broadcast Receiver。
<receiver android:name=".NetworkChangeReceiver" >
<intent-filter>
<action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
</intent-filter>
</receiver>
这个 Broadcast Receiver 主要在 onReceive 函数中处理广播 Intent 。这是一个让你确认 App 是否在前台工作的绝佳时机。如果 App 当前不在前台工作,那么我们就可以开始准备我们用来启动 Service 的 Intent 了。额外提及一点:如果有信息需要传递给Service,可以使用 putExtra 方法把信息打包成 bundle携带。当然也可以不传递任何信息(但是,始终谨记 bundle 只能够承载那些 parcelable 的值)。在最后,Service 将获取到 wakelock 并启动起来。
- Java
- Kotlin
import android.app.ActivityManager;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.net.ConnectivityManager;
import android.net.Network;
import android.net.NetworkCapabilities;
import android.net.NetworkInfo;
import android.os.Build;
import com.facebook.react.HeadlessJsTaskService;
public class NetworkChangeReceiver extends BroadcastReceiver {
@Override
public void onReceive(final Context context, final Intent intent) {
/**
This part will be called every time network connection is changed
e.g. Connected -> Not Connected
**/
if (!isAppOnForeground((context))) {
/**
We will start our service and send extra info about
network connections
**/
boolean hasInternet = isNetworkAvailable(context);
Intent serviceIntent = new Intent(context, MyTaskService.class);
serviceIntent.putExtra("hasInternet", hasInternet);
context.startForegroundService(serviceIntent);
HeadlessJsTaskService.acquireWakeLockNow(context);
}
}
private boolean isAppOnForeground(Context context) {
/**
We need to check if app is in foreground otherwise the app will crash.
https://stackoverflow.com/questions/8489993/check-android-application-is-in-foreground-or-not
**/
ActivityManager activityManager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
List<ActivityManager.RunningAppProcessInfo> appProcesses =
activityManager.getRunningAppProcesses();
if (appProcesses == null) {
return false;
}
final String packageName = context.getPackageName();
for (ActivityManager.RunningAppProcessInfo appProcess : appProcesses) {
if (appProcess.importance ==
ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND &&
appProcess.processName.equals(packageName)) {
return true;
}
}
return false;
}
public static boolean isNetworkAvailable(Context context) {
ConnectivityManager cm = (ConnectivityManager)
context.getSystemService(Context.CONNECTIVITY_SERVICE);
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
Network networkCapabilities = cm.getActiveNetwork();
if(networkCapabilities == null) {
return false;
}
NetworkCapabilities actNw = cm.getNetworkCapabilities(networkCapabilities);
if(actNw == null) {
return false;
}
if(actNw.hasTransport(NetworkCapabilities.TRANSPORT_WIFI) || actNw.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR) || actNw.hasTransport(NetworkCapabilities.TRANSPORT_ETHERNET)) {
return true;
}
return false;
}
// deprecated in API level 29
NetworkInfo netInfo = cm.getActiveNetworkInfo();
return (netInfo != null && netInfo.isConnected());
}
}
import android.app.ActivityManager
import android.app.ActivityManager.RunningAppProcessInfo
import android.content.BroadcastReceiver
import android.content.Context
import android.content.Intent
import android.net.ConnectivityManager
import android.net.NetworkCapabilities
import android.os.Build
import com.facebook.react.HeadlessJsTaskService
class NetworkChangeReceiver : BroadcastReceiver() {
override fun onReceive(context: Context, intent: Intent?) {
/**
* This part will be called every time network connection is changed e.g. Connected -> Not
* Connected
*/
if (!isAppOnForeground(context)) {
/** We will start our service and send extra info about network connections */
val hasInternet = isNetworkAvailable(context)
val serviceIntent = Intent(context, MyTaskService::class.java)
serviceIntent.putExtra("hasInternet", hasInternet)
context.startForegroundService(serviceIntent)
HeadlessJsTaskService.acquireWakeLockNow(context)
}
}
private fun isAppOnForeground(context: Context): Boolean {
/**
* We need to check if app is in foreground otherwise the app will crash.
* https://stackoverflow.com/questions/8489993/check-android-application-is-in-foreground-or-not
*/
val activityManager = context.getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
val appProcesses = activityManager.runningAppProcesses ?: return false
val packageName: String = context.getPackageName()
for (appProcess in appProcesses) {
if (appProcess.importance == RunningAppProcessInfo.IMPORTANCE_FOREGROUND &&
appProcess.processName == packageName
) {
return true
}
}
return false
}
companion object {
fun isNetworkAvailable(context: Context): Boolean {
val cm = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
var result = false
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
val networkCapabilities = cm.activeNetwork ?: return false
val actNw = cm.getNetworkCapabilities(networkCapabilities) ?: return false
result =
when {
actNw.hasTransport(NetworkCapabilities.TRANSPORT_WIFI) -> true
actNw.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR) -> true
actNw.hasTransport(NetworkCapabilities.TRANSPORT_ETHERNET) -> true
else -> false
}
return result
} else {
cm.run {
// deprecated in API level 29
cm.activeNetworkInfo?.run {
result =
when (type) {
ConnectivityManager.TYPE_WIFI -> true
ConnectivityManager.TYPE_MOBILE -> true
ConnectivityManager.TYPE_ETHERNET -> true
else -> false
}
}
}
}
return result
}
}
}