本文会深入解析LiveData的原理及其应用

1、背景

之前的业务中有一个问题，我一个异步数据，供一个Activity使用，如果用强引用的callback，一是会造成内存泄漏，二是如果Activity destroy了，回调会有问题。另外一种方案是采用weakreference的callback，但这样一来就不能使用匿名内部类，使用起来不是很方便，所以研究一下能不能用LiveData来实现。

2、实现

首先是异步的DataLoader：

public class DataLoader {

    public Observable<UserInfoBean> getUserInfoQuickly(String name) {
        return Observable.just(name)
                .map(new Func1<String, UserInfoBean>() {
                    @Override
                    public UserInfoBean call(String s) {
                        int id = new Random().nextInt(1000);
                        return new UserInfoBean(s, id, id + "");
                    }
                })
                .delay(1, TimeUnit.SECONDS);
    }

    public Observable<UserInfoBean> getUserInfoSlowly(String name) {
        return Observable.just(name)
                .map(new Func1<String, UserInfoBean>() {
                    @Override
                    public UserInfoBean call(String s) {
                        int id = new Random().nextInt(1000);
                        return new UserInfoBean(s, id, id + "");
                    }
                })
                .delay(5, TimeUnit.SECONDS);
    }
}

使用LiveData的LiveDataLoader:

public class LiveDataLoader {

    private DataLoader loader = new DataLoader();
//    private MutableLiveData<UserInfoBean> userInfo = new MutableLiveData<>();

    public LiveData<UserInfoBean> getUserInfoQuickly(String name) {
        final MutableLiveData<UserInfoBean> userInfo = new MutableLiveData<>();
        loader.getUserInfoQuickly(name)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Action1<UserInfoBean>() {
                    @Override
                    public void call(UserInfoBean userInfoBean) {
                        userInfo.setValue(userInfoBean);
                    }
                });
        return userInfo;
    }

    public LiveData<UserInfoBean> getUserInfoSlowly(String name) {
        final MutableLiveData<UserInfoBean> userInfo = new MutableLiveData<>();
        loader.getUserInfoSlowly(name)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Action1<UserInfoBean>() {
                    @Override
                    public void call(UserInfoBean userInfoBean) {
                        userInfo.setValue(userInfoBean);
                    }
                });
        return userInfo;
    }
}

在Activity中使用：

public class ViewModelDemo2 extends AppCompatActivity {

    private ActivityViewModelDemo2Binding binding;
    private UserInfoViewModel viewModel;
    private LiveDataLoader liveDataLoader = new LiveDataLoader();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = DataBindingUtil.setContentView(this, R.layout.activity_view_model_demo2);
        viewModel = ViewModelProviders.of(this).get(UserInfoViewModel.class);
        binding.setUserinfo(viewModel);
        binding.setHandler(this);
        binding.setLifecycleOwner(this);
//        liveDataLoader = ViewModelProviders.of(this).get(LiveDataLoader.class);
    }

    public void doLoadUserInfo1() {
        liveDataLoader.getUserInfoQuickly(viewModel.editName.getValue())
                .observe(this, new Observer<UserInfoBean>() {
                    @Override
                    public void onChanged(@Nullable UserInfoBean userInfoBean) {
                        Toast.makeText(ViewModelDemo2.this, userInfoBean.toString(), Toast.LENGTH_SHORT).show();
                    }
                });
    }

    public void doLoadUserInfo2() {
        liveDataLoader.getUserInfoSlowly(viewModel.editName.getValue())
                .observe(this, new Observer<UserInfoBean>() {
                    @Override
                    public void onChanged(@Nullable UserInfoBean userInfoBean) {
                        Toast.makeText(ViewModelDemo2.this, userInfoBean.toString(), Toast.LENGTH_SHORT).show();
                    }
                });
    }
}

LiveDataLoader并没有继承ViewModel，关于LiveData是否需要结合ViewModel使用，官网文档中有这样一段描述：

Note: Make sure to store LiveData objects that update the UI in ViewModel objects, as opposed to an activity or fragment, for the following reasons:

To avoid bloated activities and fragments. Now these UI controllers are responsible for displaying data but not holding data state.
To decouple LiveData instances from specific activity or fragment instances and allow LiveData objects to survive configuration changes.

大概意思就是说LiveData要结合ViewModel使用

其实只要不是跟界面结合很紧，单独使用也是可以的，而且这样一来可以在使用时才注册回调，非常方便

LiveData可以理解成带生命周期的callback