Android ViewModel 和 startActivity

Question

我正在学习ViewModel，LiveData并且在此过程中产生了疑问。

如果我需要启动一个，我应该怎么做Activity？

是否可以将上下文作为参数传递给ViewModel（上下文不会存储在 ViewModel 中）？

ActivityAViewModel : ViewModel() {
    // ...

    fun openActivityB(context: Context) {
        context.startActivity(...)
    }

    // ...
}

ActivityA {
    // ...

    fun onSomethingHappened() {
        viewModel.openActivityB(this)
    }

    // ...
}

如果不是，在这种情况下最正确的做法是什么？

Answer 1

您应该从活动中调用 startActivity，而不是从视图模型中调用。如果你想从 viewmodel 打开它，你需要在 viewmodel 中创建带有一些导航参数的 livedata 并观察活动中的 livedata。

Answer 2

我喜欢触发事件。:D

正如大家所说，ViewModel不应包含Context或引用包含Context. 所以startActivity从ViewModel.

我要做的是有一个包含事件数据的 LiveData。此事件将根据您的业务逻辑从您的 ViewModel 触发（也许您正在显示一个倒计时，并在它结束时移动到下一个活动？）。它是一个LiveData，你可以在上面观察。根据此事件的数据，您可以开始您的活动。

你可能想看看SingleLiveEvent

Answer 3

恕我直言，viewmodel 应该对视图以及它如何向用户呈现信息一无所知。

/**
 * Activity (as view) responsible only for gathering actions and intentions from user and
 * show result state.
 * View must know "What user want". View knows meaning its interface.
 * Click on button 'login' means INTENTION to login somewhere.
 * This intention pass to ViewModel to process it and wait some changing state from LiveData.
 * For example implemented as Actions.
 */
public class LoginActivity extends AppCompatActivity {
    private LoginViewModel mLoginViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mLoginViewModel = ViewModelProviders.of(this).get(LoginViewModel.class);
        mLoginViewModel.getAction().observe(this, new Observer<Action>() {
            @Override
            public void onChanged(@Nullable final Action action) {
                if(action != null){
                    handleAction(action);
                }
            }
        });

        //Emulate user intention
        mLoginViewModel.userWantToLogin("0123456789", "admin");
    }

    private void handleAction(@NonNull final Action action) {
        switch (action.getValue()){
            case Action.SHOW_WELCOME:
                //show Activity. 
                break;
            case Action.SHOW_INVALID_PASSWARD_OR_LOGIN:
                //show Toast
                break;
        }
    }
}

    public class LoginViewModel extends ViewModel {
        //Stores actions for view.
        private MutableLiveData<Action> mAction = new MutableLiveData<>();

        public LiveData<Action> getAction() {
            return mAction;
        }

        /**
         * Takes intention to login from user and process it.
         *
         * @param password Dummy password.
         * @param login Dummy login.
         */
        public void userWantToLogin(String password, String login){
            if(validateInfo(password, login)){
                showWelcomeScreen();
            }else {
                showPasswordOrLoginInvalid();
            }
        }

        /*
         * Changes LiveData. Does not act directly with view.
         * View can implement any way to show info
          * to user (show new activity, alert or toast)
         */
        private void showPasswordOrLoginInvalid() {
            mAction.setValue(new Action(Action.SHOW_INVALID_PASSWARD_OR_LOGIN));
        }

        /*
         * Changes LiveData. Does not act directly with view.
         * View can implement any way to show info
         * to user (show new activity, alert or toast)
         */
        private void showWelcomeScreen() {
            mAction.setValue(new Action(Action.SHOW_WELCOME));
        }

        //As example of some logic.
        private boolean validateInfo(String password, String login) {
            return password.equals("0123456789") && login.equals("admin");
        }
    }

public class Action {
    public static final int SHOW_WELCOME = 0;
    public static final int SHOW_INVALID_PASSWARD_OR_LOGIN = 1;
    private final int mAction;

    public Action(int action) {
        mAction = action;
    }

    public int getValue() {
        return mAction;
    }
}