Android ViewModel与LiveData简单使用

一、基础入门

1.在app的build.gradle内添加依赖

dependencies {
    .........
    implementation "android.arch.lifecycle:extensions:1.1.1"
}

2.创建ViewModel继承AndroidViewModel，

public class MainViewModel extends AndroidViewModel {
    private MutableLiveData users;

    public MainViewModel(@NonNull Application application) {
        super(application);
    }

    public MutableLiveData getUsers() {
        if (users==null){
            users=new MutableLiveData<>();
            //TODO可以从网络获取
            //.......
        }
        return users;
    }

    //setValue():须在主线程中才能使用  postValue():既可在主线程也可在子线程中调用 即在工作线程中调用
    public void setUsers(String name,String city,int age) {
        users.setValue(new User(name,city,age));
    }
}

Activity或Fragment内的简单调用(当Activity内包含多个Fragment时，它们可以共用同一个ViewModel，做到数据共享)。

1.在Activity内使用ViewModel
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view_model);
    final MainViewModel mainViewModel = ViewModelProviders.of(this).get(MainViewModel.class);
    Button button = findViewById(R.id.bt_activity);
    final TextView textView = findViewById(R.id.tv_text_activity);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            mainViewModel.getUsers().postValue(new User("Activity", "hangzhou", 19));
        }
    });

//观察数据是否发生变化，当发生变化时就会执行onChanged内的代码程序
    mainViewModel.getUsers().observe(this, new Observer() {
        @Override
        public void onChanged(User user) {
            textView.setText("我是来自" + user.getName() + "点击事件，现在在" + user.getCity() + "今年" + user.getAge() + "啦");
        }
    });
}

2.Fragment内使用ViewModel
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_one,container,false);
        //此处of内的参数需为getActivity(),不能为this
    final MainViewModel mainViewModel = ViewModelProviders.of(getActivity()).get(MainViewModel.class);
    Button button = view.findViewById(R.id.bt_one_fragment);
    final TextView textView = view.findViewById(R.id.tv_text_one_fragment);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            mainViewModel.getUsers().postValue(new User("OneFragment", "shanghai", 29));
        }
    });

    mainViewModel.getUsers().observe(this, new Observer() {
        @Override
        public void onChanged(User user) {
            textView.setText("我是来自" + user.getName() + "点击事件，现在身处" + user.getCity() + "今年" + user.getAge() + "啦");
        }
    });
    return view;
}

二.与DataBinding的结合使用

与DataBinding的单独使用相同，须在XML内导入该ViewModel

之后须在Activity内将DataBinding与ViewModel绑定

@Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        final MainViewModel mainViewModel = ViewModelProviders.of(this).get(MainViewModel.class);
        final ActivityViewModelBinding activityViewModelBinding= DataBindingUtil.setContentView(this,R.layout.activity_view_model);

        //将ViewModel与DataBinding绑定
        activityViewModelBinding.setData(mainViewModel);
        //让xml内绑定的LiveData和Observer建立连接，数据改变，UI自动会更新
        activityViewModelBinding.setLifecycleOwner(this);

        activityViewModelBinding.btActivity.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) { mainViewModel.setUsers("Activity", "hangzhou", 19);
            }
        });
    }