MVVM LiveData 和数据绑定
MVVM LiveData and DataBinding
我正在研究架构组件，现在正在学习 LiveData 和数据绑定。我可以将 LiveData 与整数值的布局绑定，但我无法找到用户列表。
这是具有数据绑定的 LiveData 的工作示例
<?xml version="1.0" encoding="utf-8"?>
<layout>
<data>
<variable
name="viewModel"
type="com.example.tutorial3livedataanddatabinding.viewmodel.CounterViewModel" />
</data>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{String.valueOf(viewModel.counter)}"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>
</layout>
ViewModel class 和 Activity
public class CounterViewModel extends ViewModel {
private MutableLiveData<Integer> counter = new MutableLiveData<>();
public MutableLiveData<Integer> getCounter() {
return counter;
}
public void setCounter(MutableLiveData<Integer> counter) {
this.counter = counter;
}
public void counterValue(int val) {
this.counter.setValue(val);
}
}
public class MainActivity extends AppCompatActivity {
private int mCounter = 0;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
/*
LifeCycleOwner should be set for LiveData changes to be propagated to UI for this binding
*/
binding.setLifecycleOwner(this);
final CounterViewModel counterViewModel = ViewModelProviders.of(this).get(CounterViewModel.class);
binding.setViewModel(counterViewModel);
increaseCounter(counterViewModel);
}
private void increaseCounter(final CounterViewModel counterViewModel) {
final Handler handler = new Handler();
handler.postDelayed(new Runnable() {
@Override
public void run() {
mCounter ++;
counterViewModel.counterValue(mCounter);
handler.postDelayed(this, 1000);
}
}, 1000);
}
}
对于上面的示例，数据与数据的绑定工作正常。计数器更新，但与文档所述不同，它不会在应用程序停止或 LiveData
时停止计数
Note: LiveData objects only send updates when the activity, or the
LifecycleOwner is active. If you navigate to a different app, the log
messages pause until you return. LiveData objects only consider
subscriptions as active when their respective lifecycle owner is
either STARTED or RESUMED.
我真正的问题是如何使用 LiveData<List<User>> class.
实现数据绑定
<?xml version="1.0" encoding="utf-8"?>
<layout>
<data>
<variable
name="viewModel"
type="com.example.tutorial3livedataanddatabinding2.viewmodel.MyViewModel"/>
</data>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.tutorial3livedataanddatabinding2.MainActivity">
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{viewModel.getUserRecords()}"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.349" />
<Button
android:id="@+id/button_shufle"
android:layout_width="88dp"
android:layout_height="wrap_content"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="20dp"
android:onClick="@{() -> viewModel.shuffleUsers()}"
android:text="Shuffle"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/textView" />
<Button
android:id="@+id/button_add"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:onClick="@{() -> viewModel.addUser()}"
android:text="Add"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/button_shufle" />
</android.support.constraint.ConstraintLayout>
</layout>
android:text="@{viewModel.getUserRecords() 在开始时将用户数据绑定为字符串，但在新用户加入时不会更新除非旋转设备，否则添加或随机排列列表。
public class MyViewModel extends ViewModel {
private MutableLiveData<List<User>> users;
public LiveData<List<User>> getUsers() {
if (users == null) {
users = new MutableLiveData<>();
loadUsers();
}
return users;
}
private void loadUsers() {
// do async operation to fetch users
List<User> userList = new ArrayList<>();
User user = new User();
user.setFirstName("Jack");
user.setLastName("Daniels");
userList.add(user);
user = new User();
user.setFirstName("Johnny");
user.setLastName("Walker");
userList.add(user);
user = new User();
user.setFirstName("James");
user.setLastName("Jameson");
userList.add(user);
user = new User();
user.setFirstName("Arthur");
user.setLastName("Guinness");
userList.add(user);
users.setValue(userList);
}
public void shuffleUsers() {
if (users != null && users.getValue() != null && users.getValue().size() > 0) {
Collections.shuffle(users.getValue());
// Needed to update Live Data observers
users.setValue(users.getValue());
}
}
public void addUser() {
if (users != null && users.getValue() != null) {
User user = new User();
user.setFirstName("Winston");
user.setLastName("Whiskey");
users.getValue().add(user);
// Needed to update Live Data observers
users.setValue(users.getValue());
}
}
public String getUserRecords() {
if (users != null && users.getValue() != null && users.getValue().size() > 0) {
StringBuilder sb = new StringBuilder();
for (User user : users.getValue()) {
sb.append( user.getFirstName() + " " + user.getLastName() + "\n");
}
return sb.toString();
}
return "empty list";
}
}
Activity
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MyViewModel myViewModel = ViewModelProviders.of(MainActivity.this).get(MyViewModel.class);
myViewModel.getUsers();
ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
binding.setViewModel(myViewModel);
// Required for binding with LiveData
binding.setLifecycleOwner(this);
}
}
编辑： 我在 Blackbelt 的回答后编辑了 MyViewModel class
现在我得到 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observeForever(android.arch.lifecycle.Observer)' on a null object reference
apply() 中的 System.out.println() 未被调用，其他 2 个被调用并且不会 return null for users 和 users.getValue()
import android.arch.core.util.Function;
import android.arch.lifecycle.LiveData;
import android.arch.lifecycle.MutableLiveData;
import android.arch.lifecycle.Transformations;
import android.arch.lifecycle.ViewModel;
import com.example.tutorial3livedataanddatabinding2.model.User;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class MyViewModel extends ViewModel {
private MutableLiveData<List<User>> users;
public LiveData<String> userStringLiveData = Transformations.map(users, new Function<List<User>, String>() {
@Override
public String apply(List<User> input) {
System.out.println("userStringLiveData users: " + users);
return getUserRecords();
}
});
public LiveData<List<User>> getUsers() {
if (users == null) {
users = new MutableLiveData<>();
loadUsers();
}
return users;
}
private void loadUsers() {
// do async operation to fetch users
List<User> userList = new ArrayList<>();
User user = new User();
user.setFirstName("Jack");
user.setLastName("Daniels");
userList.add(user);
user = new User();
user.setFirstName("Johnny");
user.setLastName("Walker");
userList.add(user);
user = new User();
user.setFirstName("James");
user.setLastName("Jameson");
userList.add(user);
user = new User();
user.setFirstName("Arthur");
user.setLastName("Guinness");
userList.add(user);
users.setValue(userList);
System.out.println("MyViewModel users " + users);
System.out.println("MyViewModel users List" + users.getValue());
}
public void shuffleUsers() {
if (users != null && users.getValue() != null && users.getValue().size() > 0) {
Collections.shuffle(users.getValue());
// Needed to update Live Data observers
users.setValue(users.getValue());
}
}
public void addUser() {
if (users != null && users.getValue() != null) {
User user = new User();
user.setFirstName("Winston");
user.setLastName("Whiskey");
users.getValue().add(user);
// Needed to update Live Data observers
users.setValue(users.getValue());
}
}
public String getUserRecords() {
if (users != null && users.getValue() != null && users.getValue().size() > 0) {
StringBuilder sb = new StringBuilder();
for (User user : users.getValue()) {
sb.append(user.getFirstName() + " " + user.getLastName() + "\n");
}
return sb.toString();
}
return "empty list";
}
}
在 android:text="@{viewModel.getUserRecords()} getUserRecords() returns 中是一个 String，不是 LiveData 对象。您应该改为绑定到 LiveData<String> 对象。
例如
public LiveData<String> userStringLiveData = Transformations.map(getUsers(), new Function<List<User>, String>() {
@Override
public String apply(List<User> input) {
return getUserRecords();
}
});
并在您的布局中 android:text="@{viewModel.userStringLiveData}。这样，当您 post 用户的新值时，将调用地图并更新 UI。
看起来 users 在创建对象时很可能为 null，这是第一次评估 userStringLiveData 时。所以 Transformations.map 将传递一个 null 作为它的第一个参数，而不是你希望它稍后操作的 LiveData。
这是我的建议：
public class r_RealTulipCoin {
private LiveData<List<User>> users;
public LiveData<String> userStringLiveData;
public void onCreate() {
getUsersLiveDataFromRoomOrSomething();
prepareUserStringLiveData();
}
private void prepareUserStringLiveData() {
userStringLiveData = Transformations.map(users, new android.arch.core.util.Function<List<User>, String>() {
@Override
public String apply(List<User> input) {
System.out.println("userStringLiveData users: " + users);
return getUserRecordsAsSingleStringFromThisParticularList(input);
}
});
}
private String getUserRecordsAsSingleStringFromThisParticularList(List<User> a_list_of_users) {
return "i concatenated them or something";
}
private class User {
String a_name_or_something;
}
}
这是一个模型，但我希望它能说明我的观点。你会注意到我做了一些事情......
在我尝试将其传递给 Transformations.map 之前，我确保用户实际上是非空的。
我使用地图提供的输入参数。感谢您已经知道它指的是什么，这就是为什么您的代码依赖于使用 users 字段，但是考虑它的来源以及转换 class 试图做的事情可能会对您有所帮助。
对于你的情况，我建议你这样做：
public LiveData<List<User>> getUsers() {
if (users == null) {
users = new MutableLiveData<>();
// this is the first time the users pointer is not null
loadUsers();
prepareUserStringLiveData();
}
return users;
}
最后一个替代方案是返回到您的原始代码并在您首次构建转换时使用 getUsers() 而不是用户：
public LiveData<String> userStringLiveData = Transformations.map(getUsers(), blah...)
这不是最简洁的解决方案，但再次说明了为什么我一直坚持要将 null 传递给 Transformations.map。
我正在研究架构组件，现在正在学习 LiveData 和数据绑定。我可以将 LiveData 与整数值的布局绑定，但我无法找到用户列表。
这是具有数据绑定的 LiveData 的工作示例
<?xml version="1.0" encoding="utf-8"?>
<layout>
<data>
<variable
name="viewModel"
type="com.example.tutorial3livedataanddatabinding.viewmodel.CounterViewModel" />
</data>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{String.valueOf(viewModel.counter)}"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>
</layout>
ViewModel class 和 Activity
public class CounterViewModel extends ViewModel {
private MutableLiveData<Integer> counter = new MutableLiveData<>();
public MutableLiveData<Integer> getCounter() {
return counter;
}
public void setCounter(MutableLiveData<Integer> counter) {
this.counter = counter;
}
public void counterValue(int val) {
this.counter.setValue(val);
}
}
public class MainActivity extends AppCompatActivity {
private int mCounter = 0;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
/*
LifeCycleOwner should be set for LiveData changes to be propagated to UI for this binding
*/
binding.setLifecycleOwner(this);
final CounterViewModel counterViewModel = ViewModelProviders.of(this).get(CounterViewModel.class);
binding.setViewModel(counterViewModel);
increaseCounter(counterViewModel);
}
private void increaseCounter(final CounterViewModel counterViewModel) {
final Handler handler = new Handler();
handler.postDelayed(new Runnable() {
@Override
public void run() {
mCounter ++;
counterViewModel.counterValue(mCounter);
handler.postDelayed(this, 1000);
}
}, 1000);
}
}
对于上面的示例，数据与数据的绑定工作正常。计数器更新，但与文档所述不同，它不会在应用程序停止或 LiveData
时停止计数Note: LiveData objects only send updates when the activity, or the LifecycleOwner is active. If you navigate to a different app, the log messages pause until you return. LiveData objects only consider subscriptions as active when their respective lifecycle owner is either STARTED or RESUMED.
我真正的问题是如何使用 LiveData<List<User>> class.
<?xml version="1.0" encoding="utf-8"?>
<layout>
<data>
<variable
name="viewModel"
type="com.example.tutorial3livedataanddatabinding2.viewmodel.MyViewModel"/>
</data>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.tutorial3livedataanddatabinding2.MainActivity">
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{viewModel.getUserRecords()}"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.349" />
<Button
android:id="@+id/button_shufle"
android:layout_width="88dp"
android:layout_height="wrap_content"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="20dp"
android:onClick="@{() -> viewModel.shuffleUsers()}"
android:text="Shuffle"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/textView" />
<Button
android:id="@+id/button_add"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:onClick="@{() -> viewModel.addUser()}"
android:text="Add"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/button_shufle" />
</android.support.constraint.ConstraintLayout>
</layout>
android:text="@{viewModel.getUserRecords() 在开始时将用户数据绑定为字符串，但在新用户加入时不会更新除非旋转设备，否则添加或随机排列列表。
public class MyViewModel extends ViewModel {
private MutableLiveData<List<User>> users;
public LiveData<List<User>> getUsers() {
if (users == null) {
users = new MutableLiveData<>();
loadUsers();
}
return users;
}
private void loadUsers() {
// do async operation to fetch users
List<User> userList = new ArrayList<>();
User user = new User();
user.setFirstName("Jack");
user.setLastName("Daniels");
userList.add(user);
user = new User();
user.setFirstName("Johnny");
user.setLastName("Walker");
userList.add(user);
user = new User();
user.setFirstName("James");
user.setLastName("Jameson");
userList.add(user);
user = new User();
user.setFirstName("Arthur");
user.setLastName("Guinness");
userList.add(user);
users.setValue(userList);
}
public void shuffleUsers() {
if (users != null && users.getValue() != null && users.getValue().size() > 0) {
Collections.shuffle(users.getValue());
// Needed to update Live Data observers
users.setValue(users.getValue());
}
}
public void addUser() {
if (users != null && users.getValue() != null) {
User user = new User();
user.setFirstName("Winston");
user.setLastName("Whiskey");
users.getValue().add(user);
// Needed to update Live Data observers
users.setValue(users.getValue());
}
}
public String getUserRecords() {
if (users != null && users.getValue() != null && users.getValue().size() > 0) {
StringBuilder sb = new StringBuilder();
for (User user : users.getValue()) {
sb.append( user.getFirstName() + " " + user.getLastName() + "\n");
}
return sb.toString();
}
return "empty list";
}
}
Activity
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MyViewModel myViewModel = ViewModelProviders.of(MainActivity.this).get(MyViewModel.class);
myViewModel.getUsers();
ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
binding.setViewModel(myViewModel);
// Required for binding with LiveData
binding.setLifecycleOwner(this);
}
}
编辑： 我在 Blackbelt 的回答后编辑了 MyViewModel class
现在我得到 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observeForever(android.arch.lifecycle.Observer)' on a null object reference
apply() 中的 System.out.println() 未被调用，其他 2 个被调用并且不会 return null for users 和 users.getValue()
import android.arch.core.util.Function;
import android.arch.lifecycle.LiveData;
import android.arch.lifecycle.MutableLiveData;
import android.arch.lifecycle.Transformations;
import android.arch.lifecycle.ViewModel;
import com.example.tutorial3livedataanddatabinding2.model.User;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class MyViewModel extends ViewModel {
private MutableLiveData<List<User>> users;
public LiveData<String> userStringLiveData = Transformations.map(users, new Function<List<User>, String>() {
@Override
public String apply(List<User> input) {
System.out.println("userStringLiveData users: " + users);
return getUserRecords();
}
});
public LiveData<List<User>> getUsers() {
if (users == null) {
users = new MutableLiveData<>();
loadUsers();
}
return users;
}
private void loadUsers() {
// do async operation to fetch users
List<User> userList = new ArrayList<>();
User user = new User();
user.setFirstName("Jack");
user.setLastName("Daniels");
userList.add(user);
user = new User();
user.setFirstName("Johnny");
user.setLastName("Walker");
userList.add(user);
user = new User();
user.setFirstName("James");
user.setLastName("Jameson");
userList.add(user);
user = new User();
user.setFirstName("Arthur");
user.setLastName("Guinness");
userList.add(user);
users.setValue(userList);
System.out.println("MyViewModel users " + users);
System.out.println("MyViewModel users List" + users.getValue());
}
public void shuffleUsers() {
if (users != null && users.getValue() != null && users.getValue().size() > 0) {
Collections.shuffle(users.getValue());
// Needed to update Live Data observers
users.setValue(users.getValue());
}
}
public void addUser() {
if (users != null && users.getValue() != null) {
User user = new User();
user.setFirstName("Winston");
user.setLastName("Whiskey");
users.getValue().add(user);
// Needed to update Live Data observers
users.setValue(users.getValue());
}
}
public String getUserRecords() {
if (users != null && users.getValue() != null && users.getValue().size() > 0) {
StringBuilder sb = new StringBuilder();
for (User user : users.getValue()) {
sb.append(user.getFirstName() + " " + user.getLastName() + "\n");
}
return sb.toString();
}
return "empty list";
}
}
在 android:text="@{viewModel.getUserRecords()} getUserRecords() returns 中是一个 String，不是 LiveData 对象。您应该改为绑定到 LiveData<String> 对象。
例如
public LiveData<String> userStringLiveData = Transformations.map(getUsers(), new Function<List<User>, String>() {
@Override
public String apply(List<User> input) {
return getUserRecords();
}
});
并在您的布局中 android:text="@{viewModel.userStringLiveData}。这样，当您 post 用户的新值时，将调用地图并更新 UI。
看起来 users 在创建对象时很可能为 null，这是第一次评估 userStringLiveData 时。所以 Transformations.map 将传递一个 null 作为它的第一个参数，而不是你希望它稍后操作的 LiveData。
这是我的建议：
public class r_RealTulipCoin {
private LiveData<List<User>> users;
public LiveData<String> userStringLiveData;
public void onCreate() {
getUsersLiveDataFromRoomOrSomething();
prepareUserStringLiveData();
}
private void prepareUserStringLiveData() {
userStringLiveData = Transformations.map(users, new android.arch.core.util.Function<List<User>, String>() {
@Override
public String apply(List<User> input) {
System.out.println("userStringLiveData users: " + users);
return getUserRecordsAsSingleStringFromThisParticularList(input);
}
});
}
private String getUserRecordsAsSingleStringFromThisParticularList(List<User> a_list_of_users) {
return "i concatenated them or something";
}
private class User {
String a_name_or_something;
}
}
这是一个模型，但我希望它能说明我的观点。你会注意到我做了一些事情...... 在我尝试将其传递给 Transformations.map 之前，我确保用户实际上是非空的。 我使用地图提供的输入参数。感谢您已经知道它指的是什么，这就是为什么您的代码依赖于使用 users 字段，但是考虑它的来源以及转换 class 试图做的事情可能会对您有所帮助。
对于你的情况，我建议你这样做：
public LiveData<List<User>> getUsers() {
if (users == null) {
users = new MutableLiveData<>();
// this is the first time the users pointer is not null
loadUsers();
prepareUserStringLiveData();
}
return users;
}
最后一个替代方案是返回到您的原始代码并在您首次构建转换时使用 getUsers() 而不是用户：
public LiveData<String> userStringLiveData = Transformations.map(getUsers(), blah...)
这不是最简洁的解决方案，但再次说明了为什么我一直坚持要将 null 传递给 Transformations.map。