Android LiveData-如何在不同活动中重用相同的ViewModel? [英] Android LiveData - how to reuse the same ViewModel on different activities?

问题描述

ViewModel示例:

Example ViewModel:

public class NameViewModel extends ViewModel {
    // Create a LiveData with a String
    private MutableLiveData<String> mCurrentName;

    public MutableLiveData<String> getCurrentName() {
        if (mCurrentName == null) {
            mCurrentName = new MutableLiveData<>();
        }
        return mCurrentName;
    }

}

主要活动:

mModel = ViewModelProviders.of(this).get(NameViewModel.class);

// Create the observer which updates the UI.
final Observer<String> nameObserver = textView::setText;

// Observe the LiveData, passing in this activity as the LifecycleOwner and the observer.
mModel.getCurrentName().observe(this, nameObserver);

我想在第二个活动中调用mModel.getCurrentName().setValue(anotherName);，并使MainActivity接收更改.有可能吗?

I want to call mModel.getCurrentName().setValue(anotherName); in second activity and make MainActivity receive changes. Is that possible?

推荐答案

调用ViewModelProviders.of(this)时，实际上创建/保留了绑定到this的ViewModelStore，因此不同的活动具有不同的ViewModelStore和每个ViewModelStore使用给定的工厂创建ViewModel的不同实例，因此您不能在不同的ViewModelStore中具有相同的ViewModel实例.

When you call ViewModelProviders.of(this), you actually create/retain a ViewModelStore which is bound to this, so different Activities have different ViewModelStore and each ViewModelStore creates a different instance of a ViewModel using a given factory, so you can not have the same instance of a ViewModel in different ViewModelStores.

但是您可以通过传递自定义ViewModel工厂的单个实例(充当单例工厂)来实现此目的，因此它将始终在不同活动之间传递您ViewModel的相同实例.

But you can achieve this by passing a single instance of a custom ViewModel factory which acts as a singleton factory, so it will always pass the same instance of your ViewModel among different activities.

例如:

public class SingletonNameViewModelFactory extends ViewModelProvider.NewInstanceFactory {


    NameViewModel t;

    public SingletonNameViewModelFactory() {
      //  t = provideNameViewModelSomeHowUsingDependencyInjection
    }

    @Override
    public NameViewModel create(Class<NameViewModel> modelClass) {
        return t;
    }
}

因此，您需要制作SingletonNameViewModelFactory单身人士(例如，使用Dagger)并像这样使用它:

So what you need is to make SingletonNameViewModelFactory singleton (e.g. using Dagger) and use it like this:

mModel = ViewModelProviders.of(this,myFactory).get(NameViewModel.class);

注意:

在不同范围之间保留ViewModel是一种反模式.强烈建议保留您的数据层对象(例如，使您的DataSource或Repository单例)并在不同范围(活动)之间保留数据.

Preserving ViewModels among different scopes is an anti-pattern. It's highly recommended to preserve your data-layer objects (e.g. make your DataSource or Repository singleton) and retain your data between different scopes (Activities).

有关详细信息，请阅读这篇文章.

Read this article for details.

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