一、简介
LiveData：LiveData是一种持有可被观察数据的类。LiveData有生命周期感知能力的，可以在activities, fragments, 或者 services生命周期是活跃状态（STARTED和RESUMED）时会通知数据变化，更新这些组件。
特点：
- 感知生命周期，实时获取数据变化；组件销毁后，会清除数据，避免内存泄漏。
- 旋转屏幕，组件被系统回收后可以恢复数据。
- 共享数据：使用单例模式扩展LiveData对象以包装系统服务，以便可以在应用程序中共享它们。 LiveData对象连接到系统服务一次，然后任何需要该资源的观察者都可以只观看LiveData对象。
二、使用
2.1 基本使用
使用MutableLiveData，通常在onCreate中使用执行
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MutableLiveData mutableLiveData = new MutableLiveData<>();
mutableLiveData.observe(this, new Observer() {//1
@Override
public void onChanged(@Nullable final String s) {
Log.d(TAG, "onChanged:"+s);
}
});
mutableLiveData.postValue("LiveData");
}
同步：接收端数据回调与发送端同一个线程。
userLiveData.setValue(user);
异步：接收端在主线程回调数据。
userLiveData.postValue(user);
2.2 自定义LiveData
数据共享，保持LiveData为单例，在不同的地方调用，达到数据共享
重要方法：
//处于活跃状态
void onActive ()
//有活跃状态变为不活跃状态
void onInactive ()
实现监听网络，网络发生变化时通过setValue()通知
public class NetworkLiveData extends LiveData {
private final Context mContext;
static NetworkLiveData mNetworkLiveData;
private NetworkReceiver mNetworkReceiver;
private final IntentFilter mIntentFilter;
private static final String TAG = "NetworkLiveData";
public NetworkLiveData(Context context) {
mContext = context.getApplicationContext();
mNetworkReceiver = new NetworkReceiver();
mIntentFilter = new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION);
}
public static NetworkLiveData getInstance(Context context) {
if (mNetworkLiveData == null) {
mNetworkLiveData = new NetworkLiveData(context);
}
return mNetworkLiveData;
}
@Override
protected void onActive() {
super.onActive();
Log.d(TAG, "onActive:");
mContext.registerReceiver(mNetworkReceiver, mIntentFilter);
}
@Override
protected void onInactive() {
super.onInactive();
Log.d(TAG, "onInactive: ");
mContext.unregisterReceiver(mNetworkReceiver);
}
private static class NetworkReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
ConnectivityManager manager = (ConnectivityManager) context
.getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo activeNetwork = manager.getActiveNetworkInfo();
getInstance(context).setValue(activeNetwork);
}
}
}
NetworkLiveData.getInstance(this).observe(this, new Observer() {
@Override
public void onChanged(@Nullable NetworkInfo networkInfo) {
Log.d(TAG, "onChanged: networkInfo=" +networkInfo);
}
});
三、修改合并LiveData
3.1、修改LiveData数据
修改LiveData数据是在LiveData对象分发给观察者之前可以使用Transformations.map()和Transformations.switchMap()对其中存储的值进行更改
3.1.1、Transformations.map
Transformations.map()：在LiveData对象分发给观察者之前对其中存储的值进行更改
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MutableLiveData mutableLiveData = new MutableLiveData<>();
mutableLiveData.observe(this, new Observer() {//1
@Override
public void onChanged(@Nullable final String s) {
Log.i(TAG, "onChanged: " + s);
}
});
Transformations.map(mutableLiveData, new Function() {
@Override
public String apply(String input) {
return input + "Transformations.map";
}
}).observe(this, new Observer() {
@Override
public void onChanged(String s) {
Log.i(TAG, "onChanged: " + s);
}
});
输出：
2019-10-31 23:06:48.525 21243-21243/com.xxxx.xxxx I/MainActivity: onChanged: LiveData:
2019-10-31 23:06:48.526 21243-21243/com.xxxx.xxxx I/MainActivity: onChanged: LiveData:Transformations.map
3.1.2、Transformations.switchMap
转化为新的LiveData，有点类似于RxJava的Map方法
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final MutableLiveData mutableLiveData = new MutableLiveData<>();
MutableLiveData booleanMutableLiveData = new MutableLiveData<>();
Transformations.switchMap(booleanMutableLiveData, new Function>() {
@Override
public LiveData apply(Boolean input) {
return mutableLiveData;
}
}).observe(this, new Observer() {
@Override
public void onChanged(@Nullable final String s) {
Log.d(TAG, "onChanged:" + s);
}
});
booleanMutableLiveData.postValue(false);
mutableLiveData.postValue("mutableLiveData");
}
输出：
2019-10-31 23:14:15.570 22036-22036/com.xxxx.xxxx D/MainActivity: onChanged:mutableLiveData
3.1、合并LiveData数据-MediatorLiveData
MediatorLiveData可以将多个LiveData数据源集合起来，可以达到一个组件监听多个LiveData数据变化的目的。
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MutableLiveData mutableLiveData1 = new MutableLiveData<>();
MutableLiveData mutableLiveData2 = new MutableLiveData<>();
MediatorLiveData liveDataMerger = new MediatorLiveData();
liveDataMerger.addSource(mutableLiveData1, new Observer() {
@Override
public void onChanged(Object o) {
Log.i(TAG, "onChanged mutableLiveData1: "+Objects.requireNonNull(o).toString());
}
});
liveDataMerger.addSource(mutableLiveData2, new Observer() {
@Override
public void onChanged(@Nullable Object o) {
Log.i(TAG, "onChanged mutableLiveData2: "+Objects.requireNonNull(o).toString());
}
});
liveDataMerger.observe(this, new Observer() {
@Override
public void onChanged(@Nullable Object o) {
Log.d(TAG, "onChanged:"+ Objects.requireNonNull(o).toString());
}
});
mutableLiveData1.postValue("MediatorLiveData");
mutableLiveData2.postValue("MediatorLiveData");
}
输出：
2019-10-31 23:23:56.704 23685-23685/? I/MainActivity: onChanged mutableLiveData1: MediatorLiveData
2019-10-31 23:23:56.705 23685-23685/? I/MainActivity: onChanged mutableLiveData2: MediatorLiveData