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ViewModel

ViewModel 類旨在以注重生命周期的方式存儲和管理界面相關(guān)的數(shù)據(jù)。ViewModel 類讓數(shù)據(jù)可在發(fā)生屏幕旋轉(zhuǎn)等配置更改后繼續(xù)留存

界面控制器雷點

• 系統(tǒng)銷毀或重新創(chuàng)建界面控制器，則存儲在其中的任何瞬態(tài)界面相關(guān)數(shù)據(jù)都會丟失
• 界面控制器經(jīng)常需要進行可能需要一些時間才能返回的異步調(diào)用。界面控制器需要管理這些調(diào)用，并確保系統(tǒng)在其銷毀后清理這些調(diào)用以避免潛在的內(nèi)存泄漏

實現(xiàn) ViewModel

public class MyViewModel extends ViewModel {
    private MutableLiveData<List<User>> users;
    public LiveData<List<User>> getUsers() {
        if (users == null) {
            users = new MutableLiveData<List<User>>();
            loadUsers();
        }
        return users;
    }

    private void loadUsers() {
        // Do an asynchronous operation to fetch users.
    }
}

public class MyActivity extends AppCompatActivity {
    public void onCreate(Bundle savedInstanceState) {
        // Create a ViewModel the first time the system calls an activity's onCreate() method.
        // Re-created activities receive the same MyViewModel instance created by the first activity.

        MyViewModel model = new ViewModelProvider(this).get(MyViewModel.class);
        model.getUsers().observe(this, users -> {
            // update UI
        });
    }
}

如果重新創(chuàng)建了該 Activity，它接收的 MyViewModel 實例與第一個 Activity 創(chuàng)建的實例相同。當所有者 Activity 完成時，框架會調(diào)用 ViewModel 對象的 onCleared() 方法，以便它可以清理資源。

ViewModel 的生命周期

viewmodel-lifecycle.png

在 Fragment 之間共享數(shù)據(jù)

public class SharedViewModel extends ViewModel {
    private final MutableLiveData<Item> selected = new MutableLiveData<Item>();

    public void select(Item item) {
        selected.setValue(item);
    }

    public LiveData<Item> getSelected() {
        return selected;
    }
}

public class MasterFragment extends Fragment {
    private SharedViewModel model;

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        model = new ViewModelProvider(requireActivity()).get(SharedViewModel.class);
        itemSelector.setOnClickListener(item -> {
            model.select(item);
        });
    }
}

public class DetailFragment extends Fragment {

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        SharedViewModel model = new ViewModelProvider(requireActivity()).get(SharedViewModel.class);
        model.getSelected().observe(getViewLifecycleOwner(), item -> {
           // Update the UI.
        });
    }
}