如何使用livedata进行数据绑定？

Question

如何使用livedata进行数据绑定？

activity_user_detail.xml:

<data>
    <variable
        name="viewModel"
        type="com.test.viewmodel.UserViewModel" />
</data>
<TextView
        android:id="@+id/tv_amount"
        android:layout_width="match_parent"
        android:text="@{viewModel.age}"
 ....

UserViewModel.java:

public class UserViewModel extends ViewModel {
    public LiveData<User> user;
    public void getUserById(UserDao userDao, String userId){
        transaction = UserDao .load(userId);
    }
}

UserDao.java:

@Query("SELECT * FROM `user` WHERE id = :userId")
LiveData<User> load(String userId);

UserDetailActivity.java:

private ActivityUserDetailBinding binding;
binding = DataBindingUtil.setContentView(this, R.layout.activity_user_detail);
viewModel = ViewModelProviders.of(this).get(UserViewModel.class);
viewModel.getUserById(userDao, userId);
viewModel.user.observe(this, user -> binding.setViewModel(user)); // How to bind livedata?

我也尝试了这个:

binding.setViewModel(viewModel);

Answer 1

这是一个了解LiveData和ObservableField如何工作的示例.您需要使用LiveData更改T object和setValue(),或使用ObservableField设置(T).更改对象T的属性不会更新UI.

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <data>

        <variable
            name="viewModel"
            type="com.example.tutorial3livedata_databinding2objects.UserViewModel" />
    </data>

    <android.support.constraint.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <TextView
            android:id="@+id/user_info"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text='@{viewModel.userMutableLiveData.name+ ", email " + viewModel.userMutableLiveData.email}'
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintHorizontal_bias="0.501"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.1" />

        <EditText
            android:id="@+id/et_name"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginEnd="8dp"
            android:layout_marginStart="8dp"
            android:layout_marginTop="48dp"
            android:text="@={viewModel.userMutableLiveData.name}"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.501"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/user_info" />

        <EditText
            android:id="@+id/et_email"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginEnd="8dp"
            android:layout_marginStart="8dp"
            android:layout_marginTop="28dp"
            android:text="@={viewModel.userMutableLiveData.email}"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.501"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/et_name" />

        <Button
            android:id="@+id/button_change_name"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="32dp"
            android:onClick="@{() -> viewModel.changeUserName()}"
            android:text="Change Name"
            app:layout_constraintEnd_toStartOf="@+id/button_change_user"
            app:layout_constraintHorizontal_chainStyle="spread"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/et_email" />

        <Button
            android:id="@+id/button_change_user"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="@{() -> viewModel.changeUser()}"
            android:text="Change User"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toEndOf="@+id/button_change_name"
            app:layout_constraintTop_toTopOf="@+id/button_change_name" />

        <Button
            android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="56dp"
            android:text="Display User"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/button_change_name" />

    </android.support.constraint.ConstraintLayout>
</layout>

视图模型

public class UserViewModel extends ViewModel {

    public MutableLiveData<User> userMutableLiveData = new MutableLiveData<>();
    private User mUser;

    public UserViewModel() {
        mUser = new User("User", "asd@abc.com");
        userMutableLiveData.setValue(mUser);

    }

    public void changeUserName() {

        // Both updates LiveData but does not update UI
        mUser.setName("Name is Changed");
        // userMutableLiveData.getValue().setName("Updated Name");

        // This one Updates UI
        //  userMutableLiveData.setValue(userMutableLiveData.getValue());
    }

    public void changeUser() {
        mUser = new User("New User Name", "myemail@mail.com");
        // Without setting new value UI is not updated and observe is not called
        userMutableLiveData.setValue(mUser);
    }
}

主要活动

/*
    Without binding.setLifecycleOwner(this), liveData.setValue() does not update UI
    liveData.setValue() updates UI 
    EditText changes LiveData but does not update UI
 */
public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        final ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        UserViewModel userViewModel = ViewModelProviders.of(this).get(UserViewModel.class);
        // LiveData should call setValue() to update UI via binding
        binding.setViewModel(userViewModel);

        // Required to update UI with LiveData
        binding.setLifecycleOwner(this);
    }
}

此代码用于学习目的.

您可以从此代码获得的结果:

1-使用changeUserName()更改用户名会更新现有LiveData用户的名称,但不会更新UI.旋转设备时,UI会更新.

2-当您更改LiveData的用户和setValue()数据绑定时.

3-使用EditText更改用户属性双向绑定会 android:text="@={viewModel.userMutableLiveData.name}"更改LiveData的用户名​​,但在设备旋转之前不会更新UI,因为用户仍然相同.