Android(安卓)Jetpack系列之Lifecycle组件(一篇文章掌握Lifecycle)
Lifecycle是什么?
Lifecycle是一个Android生命周期管理的组件,在Android中,activity和fragment都具有它们自己的生命周期,对于android开发来说,界面的生命周期对我们来说是很重要的,处理不好的话就会出现内存泄漏的问题。在android开发中,很多功能的实现都需要在不同的生命周期中进行相应操作的调用,比如说地图,定位需要在onStart中执行start操作,在onStop中执行stop操作;还有播放器需要在onStart中的进行连接,在onStop中进行中断连接的操作。如果我们忘记了在onStop或者onDestory中释放资源,那么就会导致内存泄漏的问题。
为了更加清楚的了解Lifecycle和传统生命周期管理的区别,我把Google中的示例代码放上,让大家更好的理解Lifecycles
Kotlin代码:
internal class MyLocationListener( private val context: Context, private val callback: (Location) -> Unit) { fun start() { // connect to system location service } fun stop() { // disconnect from system location service }}class MyActivity : AppCompatActivity() { private lateinit var myLocationListener: MyLocationListener override fun onCreate(...) { myLocationListener = MyLocationListener(this) { location -> // update UI } } public override fun onStart() { super.onStart() myLocationListener.start() // manage other components that need to respond // to the activity lifecycle } public override fun onStop() { super.onStop() myLocationListener.stop() // manage other components that need to respond // to the activity lifecycle }}
java代码:
class MyLocationListener { public MyLocationListener(Context context, Callback callback) { // ... } void start() { // connect to system location service } void stop() { // disconnect from system location service }}class MyActivity extends AppCompatActivity { private MyLocationListener myLocationListener; @Override public void onCreate(...) { myLocationListener = new MyLocationListener(this, (location) -> { // update UI }); } @Override public void onStart() { super.onStart(); myLocationListener.start(); // manage other components that need to respond // to the activity lifecycle } @Override public void onStop() { super.onStop(); myLocationListener.stop(); // manage other components that need to respond // to the activity lifecycle }}
这段代码在Android开发中是标准的实例,这样的话在各个生命周期的方法中会有大量的代码,例如onStart()和onStop(),这样使他们难以维护。
Lifecycle介绍
Lifecycle组件包括LifecycleOwner、LifecycleObserver。LifeCyclerObserver是我们要实现的具有生命周期感知的类的需要实现的接口,这个接口没有任何方法。在这个类中我们通过注解来表明函数在LifeCycleOwner的哪个生命周期的时候执行。实现了LifecycleObserver 接口的类,可以在方法上添加注解来监视其组件以来的UI界面的生命周期,可以通过调用Lifecycle类的addObserver()方法传递观察者实例来添加观察者。
public class MyObserver implements LifecycleObserver { @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) public void connectListener() { ... } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) public void disconnectListener() { ... }}myLifecycleOwner.getLifecycle().addObserver(new MyObserver());
看到这里,大家没有理解不用急,后面会有一个完整的功能来演示使用Lifecycle。在上面说到了OnLifecycleEvent注解中(Lifecycle.Event的状态,为了更好的理解,我用Google文档中的一个图片来说明
LifeCycleOwner也是一个接口,这个接口只有getLifeCycle一个方法。用于标志它的实现类是具有生命周期的类。在26.0.1版本后的support库中的Activity、Fragment都实现了LifeCycleOwner接口。所以通常的情况下我们不需要自己去实现LifecycleOwner,我们只要去实现lifecycleObserver就可以了。
Lifecycle如何使用
下面我们会用一个获取定位的案例来演示Lifecycle如何使用,首先我们先建立一个BoundLocationManager的类
public class BoundLocationManager { public static void bindLocationListenerIn(LifecycleOwner lifecycleOwner, LocationListener listener, Context context) { new BoundLocationListener(lifecycleOwner, listener, context); } @SuppressWarnings("MissingPermission") static class BoundLocationListener implements LifecycleObserver { private final Context mContext; private LocationManager mLocationManager; private final LocationListener mListener; public BoundLocationListener(LifecycleOwner lifecycleOwner, LocationListener listener, Context context) { mContext = context; mListener = listener; lifecycleOwner.getLifecycle().addObserver(this); } @OnLifecycleEvent(Lifecycle.Event.ON_RESUME) void addLocationListener() { mLocationManager = (LocationManager) mContext.getSystemService(Context.LOCATION_SERVICE); mLocationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, mListener); Log.d("BoundLocationMgr", "Listener added"); // Force an update with the last location, if available. Location lastLocation = mLocationManager.getLastKnownLocation( LocationManager.GPS_PROVIDER); if (lastLocation != null) { mListener.onLocationChanged(lastLocation); } } @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) void removeLocationListener() { if (mLocationManager == null) { return; } mLocationManager.removeUpdates(mListener); mLocationManager = null; Log.d("BoundLocationMgr", "Listener removed"); } }}
BoundLocationListener实现了LifecycleObserver接口,并在构造函数中需要传入LifecycleOwner(调用BoundLocationManager的Activity或者fragment),LocationListener(定位改变的监听),Context(上下文,初始化定位的需要)。
addLocationListener()方法上面添加了注解@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)的意思是addLocationListener()只有在LifecycleOwner(即Activity或者fragment)的生命周期为onResume()的时候才会执行。
removeLocationListener()方法上面添加了注解 @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)的意思是removeLocationListener()只有在LifecycleOwner(即Activity或者fragment)的生命周期为onPause()的时候才会执行。
也就是BoundLocationListener这个类可以监听Activity或者fragment的生命周期并自动执行其生命周期锁对应的方法。
还有一点需要注意的:在BoundLocationListener构造方法中 lifecycleOwner.getLifecycle().addObserver(this),只有加上这句代码,BoundLocationListener才会检测其Activity或者fragmeng的生命周期。
那么我们在Activity中应该如何使用呢?其实很简单,只需要在Activity或者fragmeng中添加一句代码就可以了
private void bindLocationListener() { BoundLocationManager.bindLocationListenerIn(this, mGpsListener, getApplicationContext());}
看到这里,相信大家对Lifecycle的使用方式已经有了自己的了解,使用Lifecyle可以让我们更好的去管理Activity或者fragment的生命周期,而且极大简化了Activity或者fragment中的代码,并且使我们出现内存泄漏的概率大大的降低了。下面我会放出Activity中的完整代码
public class LocationActivity extends AppCompatActivity { private static final int REQUEST_LOCATION_PERMISSION_CODE = 1; private LocationListener mGpsListener = new MyLocationListener(); @Override public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) { super.onRequestPermissionsResult(requestCode, permissions, grantResults); if (grantResults[0] == PackageManager.PERMISSION_GRANTED && grantResults[1] == PackageManager.PERMISSION_GRANTED) { bindLocationListener(); } else { Toast.makeText(this, "This sample requires Location access", Toast.LENGTH_LONG).show(); } } private void bindLocationListener() { BoundLocationManager.bindLocationListenerIn(this, mGpsListener, getApplicationContext()); } @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.location_activity); if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) { ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION, Manifest.permission.ACCESS_COARSE_LOCATION}, REQUEST_LOCATION_PERMISSION_CODE); } else { bindLocationListener(); } } private class MyLocationListener implements LocationListener { @Override public void onLocationChanged(Location location) { TextView textView = findViewById(R.id.location); textView.setText(location.getLatitude() + ", " + location.getLongitude()); } @Override public void onStatusChanged(String provider, int status, Bundle extras) { } @Override public void onProviderEnabled(String provider) { Toast.makeText(LocationActivity.this, "Provider enabled: " + provider, Toast.LENGTH_SHORT).show(); } @Override public void onProviderDisabled(String provider) { } }}
Activity的布局文件里面只有一个TextView,具体的代码就省略了。
如何自定义LifecycleOwner?
在上文中说过在26.0.1版本后的support库中的Activity、Fragment都实现了LifeCycleOwner接口,那么我们之前版本的Activity、Fragment也想使用Lifecycle应该怎么做?这样的话就需要我们自定义LifecycleOwner,其实自定义LifecycleOwner很简单,只需要让你的类实现LifecycleOwner 接口并在其相应的生命周期中添加几句代码就可以了。
public class MyActivity extends Activity implements LifecycleOwner { private LifecycleRegistry lifecycleRegistry; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); lifecycleRegistry = new LifecycleRegistry(this); lifecycleRegistry.markState(Lifecycle.State.CREATED); } @Override public void onStart() { super.onStart(); lifecycleRegistry.markState(Lifecycle.State.STARTED); } @NonNull @Override public Lifecycle getLifecycle() { return lifecycleRegistry; }}
以上就是Lifecycle的使用方式,Lifecycle的使用方式很简单,相信大家看到这里也基本上掌握了Lifecycle,后面将会讲解ViewModel和LiveData的使用,最后希望大家都能一起进步。
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