AndroidX-Jetpack-LiveData

LiveData，当数据改变时可被观察到，同时具有生命感知能力，该能力是由于其内部类LifecycleBoundObserver继承了GenericLifecycleObserver->LifecycleEventObserver->LifecycleObserver。
关键代码：

observe(LifecycleOwner owner, Observer observer) {
  LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
  ...
  owner.getLifecycle().addObserver(wrapper);
}

当组件（如：Activity）生命周期状态改变时，会移除observer，所以也就不会发生内存泄漏。也不用手动处理生命周期。

        public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
            if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
                removeObserver(mObserver);
                return;
            }
            activeStateChanged(shouldBeActive());
        }

使用：
LiveData通常放在ViewModel中，由ViewModel管理LiveData。

import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

public class NameViewModel extends ViewModel {
    MutableLiveData name;

    public MutableLiveData getName() {
        if (name == null) {
            name = new MutableLiveData<>();
        }
        return name;
    }
}

在Activity中，观察LiveData的改变

import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;

public class LiveDataActivity extends AppCompatActivity {
    NameViewModel nameViewModel;
    private TextView tvName;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_live_data);
        nameViewModel = new ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory())
                .get(NameViewModel.class);
        nameViewModel.getName().observe(this, new Observer() {
            @Override
            public void onChanged(String s) {
                //改变LiveData的值后，会被观察到，从而改变tvName的值
                tvName.setText(s);
            }
        });
        nameViewModel.getName().setValue("Android");
        tvName = findViewById(R.id.tvName);
        findViewById(R.id.btnChangeName).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //改变LiveData的值
                nameViewModel.getName().setValue("Google");
            }
        });
    }
}

当LiveData值改变时，TextView的内容也随之改变。