我用LiveData
+ Transformations.map()
:
private final LiveData<List<Task>> observableTasks;
(...)
observableTasks = Transformations.map(tasksRepository.getTasks(), tasks-> filterTasks(tasks));
如何强制LiveData
刷新?我需要Transformations.map()
准备新的列表。当用户改变过滤我需要再次打电话filterTasks(tasks)
并显示新的选择,排序列表。从数据仓库(tasksRepository.getTasks()
)来保持不变。
我想我找到了解决办法。我创造了额外的LiveData
场filterChangeEvent
。每当用户改变滤波顺序,新的值设置为filterChangeEvent
。然后我用switchMap
与filterChangeEvent
作为触发:
observableTasks = Transformations.switchMap(filterChangeEvent, input -> {
final MediatorLiveData<List<Tasks>> result = new MediatorLiveData<>();
result.addSource(tasksRepository.getTasks(), tasks -> result.setValue(filterTasks(tasks)));
return result;
});
根据您所提供的信息,我下面的步骤,我观察到触发得到了预期。我认为你做错事无论是在仓库或在课堂上,你处理的转变。由于没有足够的代码来检查,我创建了自己的虚拟类:
public class Task {
private String id;
public Task(String id) {
this.id = id;
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
@NonNull
@Override
public String toString() {
return "Task id is " +id;
}
}
public class TasksRepository {
private List<Task> taskList = new ArrayList<>();
private MutableLiveData<List<Task>> _tasks = new MutableLiveData<>();
private LiveData<List<Task>> tasks = _tasks;
public TasksRepository() {
for (int i = 0; i < 5; i++) {
taskList.add(new Task(createTaskId()));
}
_tasks.setValue(taskList);
}
private String createTaskId() {
return UUID.randomUUID().toString();
}
public void addTask() {
taskList.add(new Task(createTaskId()));
_tasks.setValue(taskList);
}
public LiveData<List<Task>> getTasks() {
return tasks;
}
}
public class MyViewModel {
private final LiveData<List<Task>> observableTasks;
public MyViewModel(TasksRepository tasksRepository) {
this.observableTasks = Transformations.map(tasksRepository.getTasks(), this::changeId);
}
private List<Task> changeId(List<Task> tasks) {
List<Task> resultTaks = new ArrayList<>();
for (Task task : tasks) {
String newId = "TASK" + task.getId();
task.setId(newId);
resultTaks.add(task);
}
return resultTaks;
}
public LiveData<List<Task>> getObservableTasks() {
return observableTasks;
}
}
TasksRepository tasksRepository = new TasksRepository();
MyViewModel viewModel = new MyViewModel(tasksRepository);
button.setOnClickListener(v -> tasksRepository.addTask());
viewModel.getObservableTasks().observe(this,
tasks -> Log.d(TAG, Arrays.toString(new List[]{tasks})));
Checkout Transformation.switchMap. It explains following case when to use it also.)
场景:
在存储库中包含用户的场景(1,“简”)和用户(2,“约翰”),当userIdLiveData值设置为“1”,则switchMap将调用的getUser(1),将返回一个LiveData包含该值的用户(1,“简”)。所以,现在,该userLiveData会发出用户(1,“简”)。当库的用户被更新到用户(1,“莎拉”),该userLiveData就会自动通知,并会发出用户(1,“莎拉”)。
当setUserId方法被调用用户id =“2”,的userIdLiveData变化,自动的值触发用于从存储库获取与ID“2”的用户的请求。因此,userLiveData发出用户(2, “约翰”)。通过repository.getUserById(1)返回的LiveData作为源被去除。