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Android必知必会的四大组件 -- Service篇
前言
写着一篇文章的原因,主要是因为在面试中,服务这个关键词的出现频率非常高。很多时候,面试官会问你,Service中能否进行耗时操作? 我们稍后就会揭晓那么这个答案。
思维导图
生命周期
由图中可以直观的看出几点。
启动方式 | 存在方式 |
---|---|
startService() | 独立于Activity运行,自行结束或被叫停 |
bindService() | 绑定于Activity运行,Activity结束时,会被叫停 |
涉及方法 | 用途 |
---|---|
onCreate() | |
onDestroy() | |
onStartCommand() | 用于计数,服务被调用的次数 |
onBind() | 与Activity组件绑定 |
onUnbind() | 与Activity组件解绑 |
使用方法
Service方法需要在AndroidManifest.xml中进行注册
// 第一步:在AndroidManifest.xml中进行注册
<service android:name=".LocalService"/>
// 第二步:启动
① startService(Intent);
② bindService(Intent, ServiceConnection, Int);
// 第三步:解绑(使用方法② 启动时操作)
unBindService(ServiceConnection);
// 第四步:暂停
stopService(Intent);
Activity和Service的通信
Activity和Service的通信其实就是基于IBinder来进行实现的。但是IBinder其实是一个接口,对我们而言一般使用他的实现类Binder并通过强制转换来完成操作。
/**
* Service方法继承
* onBind()是一个抽象方法。
*/ public class LocalService extends Service { private final IBinder binder = new ServiceBinder(); @Nullable @Override public IBinder onBind(Intent intent) { return binder;
} public class ServiceBinder extends Binder { LocalService getLocalService(){ return LocalService.this;
}
}
} 复制代码
以上代码,是一个用于通信的基础版本。
既然需要通信,那我们总需要知道对方是谁,如果使用的是startService(),上文已经提到他是独立于Activity的,所以势必使用的是bindService()。
在上文的使用方法中已经提到了bindService()使用到的参数,Intent、ServiceConnection、Int。
ServiceConnection
/**
* bindService()方法中的参数之一。
* 用于对service进行操作
*/ ServiceConnection connection = new ServiceConnection() { // Activity和Service绑定时调用 @Override public void onServiceConnected(ComponentName name, IBinder binder) { // 基于Binder拿到我们要的Service service = ((LocalService.ServiceBinder)binder).getLocalService(); // 干你需要干的事情 } // Activity和Service解绑时调用 @Override public void onServiceDisconnected(ComponentName name) {
service = null;
}
};
Int
- BIND_AUTO_CREATE:收到绑定需求,如果Service尚未创建,则立即创建。
- BIND_DEBUG_UNBIND:用于测试使用,对unbind调用不匹配的调试帮助。
- BIND_NOT_FOREGROUND:不允许此绑定将目标服务的进程提升到前台调度优先级
这是一个已经存在于Service类中的值,这里并不全部例举,一般来说都是使用BIND_AUTO_CREATE。
必须要调用的unbindService(ServiceConnection)
为什么我们一定要调用这个方法,如果我们不解绑会出现什么样的问题?
经过测试,Logcat中爆出了这样的错误Activity has leaked ServiceConnection that was originally bound here。也就是说ServiceConnection内存泄漏了。这也是为什么我们一直说需要解绑的原因。
IntentService
public class LocalIntentService extends IntentService { /**
* Creates an IntentService. Invoked by your subclass's constructor.
*
* @param name Used to name the worker thread, important only for debugging.
*/ public LocalIntentService(String name) { super(name);
} @Override protected void onHandleIntent(@Nullable Intent intent) {
}
}
先看一段我们的继承代码,和Service不同的地方就是,必须重写的方法是onHandleIntent(Intent intent)。 那我们也和之前一样做一个源码导读好了。
IntentService源码导读
public abstract class IntentService extends Service { private volatile Looper mServiceLooper; @UnsupportedAppUsage private volatile ServiceHandler mServiceHandler; private String mName; private boolean mRedelivery; private final class ServiceHandler extends Handler { public ServiceHandler(Looper looper) { super(looper);
} @Override public void handleMessage(Message msg) {
onHandleIntent((Intent)msg.obj);
stopSelf(msg.arg1);
}
}
}
其实从整个代码的变量我们已经可以做一个猜测了。Looper+Handler+Service的组成成分。那它的处理过程势必依赖于一个Handler的通信机制。另外看到了ServiceHandler中的stopSelf()方法,我们也就清楚了一个问题为什么我们不需要去控制IntentService的暂停。
接下来从生命周期的角度来看看这个IntentService,因为Binder机制上是一致的,所以分析主线就是onCreate() --> onStartCommand() --> onDestroy()。
onCreate()
public void onCreate() { // TODO: It would be nice to have an option to hold a partial wakelock // during processing, and to have a static startService(Context, Intent) // method that would launch the service & hand off a wakelock. super.onCreate();
HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
thread.start();
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
创建了一个HandlerThread,去初始化了Looper和Handler,也就说明服务在内部处理。
onStartCommand()
public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
onStart(intent, startId); // 1 --> return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY; // 2 --> } 复制代码
onStart()
public void onStart(@Nullable Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
其他都是和Handler一致的,整体流程也就是Message封装,再通过Handler进行一个发送。
mRedelivery
这个变量是干什么的?
/**
* Constant to return from {@link #onStartCommand}: if this service's
* process is killed while it is started (after returning from
* {@link #onStartCommand}), and there are no new start intents to
* deliver to it, then take the service out of the started state and
* don't recreate until a future explicit call to
* {@link Context#startService Context.startService(Intent)}. The
* service will not receive a {@link #onStartCommand(Intent, int, int)}
* call with a null Intent because it will not be restarted if there
* are no pending Intents to deliver.
*
* <p>This mode makes sense for things that want to do some work as a
* result of being started, but can be stopped when under memory pressure
* and will explicit start themselves again later to do more work. An
* example of such a service would be one that polls for data from
* a server: it could schedule an alarm to poll every N minutes by having
* the alarm start its service. When its {@link #onStartCommand} is
* called from the alarm, it schedules a new alarm for N minutes later,
* and spawns a thread to do its networking. If its process is killed
* while doing that check, the service will not be restarted until the
* alarm goes off.
*/ public static final int START_NOT_STICKY = 2; /**
* Constant to return from {@link #onStartCommand}: if this service's
* process is killed while it is started (after returning from
* {@link #onStartCommand}), then it will be scheduled for a restart
* and the last delivered Intent re-delivered to it again via
* {@link #onStartCommand}. This Intent will remain scheduled for
* redelivery until the service calls {@link #stopSelf(int)} with the
* start ID provided to {@link #onStartCommand}. The
* service will not receive a {@link #onStartCommand(Intent, int, int)}
* call with a null Intent because it will only be restarted if
* it is not finished processing all Intents sent to it (and any such
* pending events will be delivered at the point of restart).
*/ public static final int START_REDELIVER_INTENT = 3;
一大段冗长的英文很烦,更何况我也就低分飘过6级的水平呢,哈哈哈哈!!
就不折磨你们了,直接做出一个解释吧。
- START_NOT_STICKY:默认模式,这是一个容许被杀的模式,随时允许被叫停
- START_REDELIVER_INTENT:告诉系统在崩溃后重新启动服务,并重新传递在崩溃时存在的意图
好了,以上基本就是整个IntentService的介绍了。
总结
- 在ANR机制中,Service的响应时长不能超过20s,其实也可以比较直观的看出,Service其实并不能进行所谓耗时操作。但是如果加上了Thread进行异步处理,那么其实他还是可以进行耗时操作的。(具体看你怎么进行回答,主要还是一个知识点,Service运行在主线程)
- Service存在的原因是Activity是一个经常会被销毁的组件,虽然我们同样可以通过Thread进行异步操作,但是当Activity实例被销毁时,相应的捆绑在Activity生命周期内的Thread实例我们也没有能力再去寻找了。
原文链接:https://juejin.im/post/5e53e1986fb9a07cd443b7eb
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