如何通过Android中的ViewModel类在Activity和Fragment之间共享数据? [英] How to Share Data between Activity and Fragment via ViewModel Class in Android?

问题描述

我想知道是否可以传递在Activity类中声明的String数据，然后将String数据传递给ViewModel类，然后将数据传递给Fragment类.

I wondered if it's possible to pass a String data which has declared in Activity class and pass the String data to ViewModel class then pass the data to Fragment class.

ViewModel类

class TimeTableViewModel extends ViewModel {

private MutableLiveData<String> start_time_str = new MutableLiveData<>();

void send_StartTime(String start_Time){
    start_time_str.setValue(start_Time);
}

LiveData<String> get_StartTime(){
    return start_time_str;
}}

在ViewModel类中，我有 MutableLiveData< String>start_time_str ，它已被初始化为 new MutableLiveData<>();

我想在Activity类中使用 void send_StartTime(String start_Time)函数来设置参数 String start_Time 的值并调用 start_time_str 在Fragment类中.

I would like to use void send_StartTime(String start_Time) function in Activity class to set value of argument String start_Time and call the start_time_str in Fragment class.

活动分类

@Override
public boolean onOptionsItemSelected(MenuItem item){
    switch (item.getItemId()){
        case android.R.id.home:
            finish();
            break;
        case R.id.add_schedule_save:
            String start_time_str = startTime.getText().toString();
            Intent intent_restart0 = new Intent(TimeTable_Add_New_Schedule.this, MainActivity.class);
            startActivity(intent_restart0);
            TimeTableViewModel timeTableViewModel = new TimeTableViewModel();
            timeTableViewModel.send_StartTime(start_time_str);
            Toast.makeText(this,""+start_time_str,Toast.LENGTH_LONG).show();
            break;
    }
    return super.onOptionsItemSelected(item);
}

片段类

 @Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    TimeTableViewModel timeTableViewModel = new TimeTableViewModel();
    timeTableViewModel.get_StartTime().observe(getViewLifecycleOwner(), new Observer<String>() {
        @Override
        public void onChanged(String s) {
            mon_textView_11.setText(s);
        }
    });
}

在Fragment类中，我调用 get_StartTime()函数以获取 start_time_str 并将String值设置为TextView.我认为 start_time_str 已成功通过Activity类中的 timeTableViewModel.send_StartTime(start_time_str); 的功能成功设置，因为Toast.maketext的工作原理很吸引人.但是，TextView没有显示任何内容.我测试过文本颜色"不是白色，因此如果正确调用了字符串值，则应该在屏幕上显示它.如果您有任何建议，我很想听听您的建议.非常感谢

In the Fragment class I call get_StartTime() function to get start_time_str and set the String value to my TextView. I think the start_time_str has been successfully set by function of timeTableViewModel.send_StartTime(start_time_str); in the Activity Class because of Toast.maketext is worked like a charm. However the TextView is not shown anything. I have tested Text Color is not white so that if the string value is correctly called, it should be appear on screen. If you have any suggestions, I would love to hear your advice. Thank you very much

推荐答案

这实际上取决于您如何创建ViewModel实例.现在，您要通过其构造函数创建ViewModel，但这不是正确的方法.您应使用由Google团队创建的ViewModelProvider或扩展方法.

It really depends on how do you create your ViewModel instance. Now you are creating ViewModel by its constructor, but that is not a proper way. You should use ViewModelProvider or extension methods that were created by Google team.

如果您使用ViewmodelProvider，则应该这样:

If you go with ViewmodelProvider you should do it like this:

TimeTableViewModel viewModel = new ViewModelProvider(this).get(TimeTableViewModel.class);

将正确的上下文传递给 of 方法调用非常重要.如果您处于片段状态，而只使用 getContext()而不是 getActivity()，则将不会获得与在Activity中创建的实例相同的实例.您将创建一个ViewModel的新实例，该实例的作用域仅在片段生命周期内.因此，在两个部分的活动上下文中使用它来获取相同的实例很重要.

It is important to pass the correct context to of method call. If you are in fragment and you will just use getContext() instead of getActivity(), you will not get the same instance as it was created in Activity. You will create a new instance of ViewModel, that will be scoped only inside of fragment lifecycle. So it is important to use in both parts activity context to get the same instance.

活动部分:

TimeTableViewModel viewModel = new ViewModelProvider(this).get(TimeTableViewModel.class);

片段部分:

TimeTableViewModel viewModel = new ViewModelProvider(getActivity()).get(TimeTableViewModel.class);

重要的是，您的片段必须位于与使用此ViewModel相同的活动中.

Is important that your fragment is located inside the same activity that is using this ViewModel.

但是Google的家伙通过一些扩展方法使我们更容易了.但据我所知，它们仅在科特林班上工作.因此，如果您有kotlin代码，则可以像这样声明您的viewModel:

But guys at Google has make it easier for us with some extension methods. But as far as I know, they are working only in kotlin classes. So if you have kotlin code, you can declare your viewModel simply like this:

private val quizViewModel: TimeTableViewModel by activityViewModels()

对于Fragment范围的ViewModel，您需要编写如下内容:

For Fragment scoped ViewModel you need to write something like this:

private val quizViewModel: TimeTableViewModel by viewModels()

但是您必须将kotlin ktx依赖项添加到您的项目build.gradle文件中.例如这样的

But you have to add kotlin ktx dependency to your project build.gradle file. For example like this:

implementation 'androidx.fragment:fragment-ktx:1.1.0'

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