LiveData

原来LiveData只要调用setValue就会回调onChange(),不关心数据值是否真的发生变化了。不过会关心observer的state

from:https://developer.android.com/topic/libraries/architecture/livedata

LiveData是一个可被监听其中数据变化的Observer
LiveData is lifecycle-aware,知道组件的生命周期变化
LiveData只会在active lifecycle state状态才会触发数据变化的回调
activite lifecycle state是指在lifecycle的STARTED和RESUMED状态之间
LiveData会在LifeCycle状态变化为DESTORYED时，removed掉observer

LiveData优点

观察者模式确保数据和UI显示一致
Lifecycle机制自动注销observer,确保不会内存泄漏
UI不可见的时候不会更新界面，减少由此触发的程序崩溃
不用在自己处理生命周期相关的业务了(no more manual lifecycle handling)
UI总会在回到前台时获取到最新的数据（always up to date data）
配置变化导致UI重建时，UI能收到最新的可用数据，恢复界面
Fragment,Activity,Servie间共享数据

LiveData使用

创建LiveData对象
创建Observer对象，实现onChanged()方法
通过LiveData.observer()方法注册监听

创建LiveData对象

public class NameViewModel extends ViewModel {

// Create a LiveData with a String
private MutableLiveData<String> mCurrentName;

    public MutableLiveData<String> getCurrentName() {
        if (mCurrentName == null) {
            mCurrentName = new MutableLiveData<String>();
        }
        return mCurrentName;
    }

// Rest of the ViewModel...
}

LiveData放在ViewModel中，不放在Activity,Frgment中的原因

避免和activities,fragments产生依赖，ui只需要负责展示数据，而不必关心数据状态
和activities,fragments脱钩，让LiveData可以在配置发生变化时依然可用

Observe LiveData objects

回调时机

LiveData中的数据发生变化
Observer的状态由inactive变化成了activie state时,仅仅只在第一次的时候。后面状态变化不会再触发了
如果LiveData中没有设置数据，不会触发回调

public class NameActivity extends AppCompatActivity {

    private NameViewModel mModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Other code to setup the activity...

        // Get the ViewModel.
        mModel = ViewModelProviders.of(this).get(NameViewModel.class);


        // Create the observer which updates the UI.
        final Observer<String> nameObserver = new Observer<String>() {
            @Override
            public void onChanged(@Nullable final String newName) {
                // Update the UI, in this case, a TextView.
                mNameTextView.setText(newName);
            }
        };

        // Observe the LiveData, passing in this activity as the LifecycleOwner and the observer.
        mModel.getCurrentName().observe(this, nameObserver);
    }
}

Update LiveData objects

MutableLiveData使用setValue(T),postValue(T)方法更新数据

mButton.setOnClickListener(new OnClickListener() {
    @Override
    public void onClick(View v) {
        String anotherName = "John Doe";
        mModel.getCurrentName().setValue(anotherName);
    }
});

Use LiveData with Room

Extend LiveData

public class StockLiveData extends LiveData<BigDecimal> {
    private StockManager mStockManager;

    private SimplePriceListener mListener = new SimplePriceListener() {
        @Override
        public void onPriceChanged(BigDecimal price) {
            setValue(price);
        }
    };

    public StockLiveData(String symbol) {
        mStockManager = new StockManager(symbol);
    }

    @Override
    protected void onActive() {
        mStockManager.requestPriceUpdates(mListener);
    }

    @Override
    protected void onInactive() {
        mStockManager.removeUpdates(mListener);
    }
}

LiveData监听回调关系

If the Lifecycle object is not in an activity state. then the observer isn’t called even if the value changes.
After the Lifecycle object is destroyed, the observer is auotmatically removed;

在多个Activity,Fragment,Service之间共享LiveData数据

public class StockLiveData extends LiveData<BigDecimal> {
    private static StockLiveData sInstance;
    private StockManager mStockManager;

    private SimplePriceListener mListener = new SimplePriceListener() {
        @Override
        public void onPriceChanged(BigDecimal price) {
            setValue(price);
        }
    };

    @MainThread
    public static StockLiveData get(String symbol) {
        if (sInstance == null) {
            sInstance = new StockLiveData(symbol);
        }
        return sInstance;
    }

    private StockLiveData(String symbol) {
        mStockManager = new StockManager(symbol);
    }

    @Override
    protected void onActive() {
        mStockManager.requestPriceUpdates(mListener);
    }

    @Override
    protected void onInactive() {
        mStockManager.removeUpdates(mListener);
    }
}

use is as follow

public class MyFragment extends Fragment {
    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        StockLiveData.get(getActivity()).observe(this, price -> {
            // Update the UI.
        });
    }
}

Transform LiveData

将LiveData中的数据转换为observer中想要的数据类型

LiveData<User> userLiveData = ...;
LiveData<String> userName = Transformations.map(userLiveData, user -> {
    user.name + " " + user.lastName
});

private LiveData<User> getUser(String id) {
  ...;
}

LiveData<String> userId = ...;
LiveData<User> user = Transformations.switchMap(userId, id -> getUser(id) );

Merge multiple LiveData sources

使用MediatorLiveData聚合多个LiveData

yeshu blog

LiveData笔记