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
// 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);
复制代码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;
}
};
复制代码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) {
}
}
复制代码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);
}
}
}
复制代码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);
}
复制代码创建了一个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);
}
复制代码其他都是和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.
*
*
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;
复制代码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。