java - MediatorLiveData 多次调用观察者错误

Question

• 我正在开发一个项目，如果登录是下载一些文件成功，然后打开主 Activity 。如果不是这样，则不会登录也不下载文件并提示用户成功登录下载所需的文件。

• 失败后，如果用户写入正确的用户名和密码，则它应该下载文件并打开主 Activity 。然而，在在这种情况下，我的应用程序崩溃了。如果用户正确填写了用户名和第一次尝试密码，应用运行顺利。

• 我有 3 个不同的 LiveData 对象，这样我就可以在 MediatorLiveData 的帮助下观察所有这些对象，并根据条件调用下一个对象。错误输出:

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.cesar.sertificar, PID: 7482 java.lang.IllegalArgumentException: This source was already added with the different observer at android.arch.lifecycle.MediatorLiveData.addSource(MediatorLiveData.java:89) at com.cesar.sertificar.ui.activity.login.LoginViewModel.handleFirstRunProcess(LoginViewModel.java:101) at com.cesar.sertificar.ui.activity.login.LoginActivity.doLogin(LoginActivity.java:68) at com.cesar.sertificar.ui.activity.login.LoginActivity.onClick(LoginActivity.java:88) at android.view.View.performClick(View.java:5214) at android.view.View$PerformClick.run(View.java:20978) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:145) at android.app.ActivityThread.main(ActivityThread.java:6134) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

LoginViewModel.class

void handleFirstRunProcess() {
        mIsNetworkAvailable.setValue(isAnActiveConnection());
        mProcessResult.addSource(mIsNetworkAvailable, isNetworkAvailable -> {
            mProcessResult.removeSource(mIsNetworkAvailable);
            if (isNetworkAvailable == null) return;

            if (!isNetworkAvailable) {
                mProcessResult.setValue(new NetworkState(NetworkState.Status.FAILED,
                        getApplication().getString(R.string.first_run_network_warning)));
                return;

            }

            doLogin(); //First doLogin
        });

        mProcessResult.addSource(mIsLoginSuccessful, isLoginSuccessful -> {
            mProcessResult.removeSource(mIsLoginSuccessful);
            if (isLoginSuccessful == null) return;

            if (isLoginSuccessful.getStatus() == NetworkState.Status.FAILED) {
                mProcessResult.setValue(new NetworkState(NetworkState.Status.FAILED,
                        getApplication().getString(R.string.login_error)));
                return;
            }

            if (preferenceUtil.getBooleanData(Constants.FIRST_RUN_KEY, true)) {
                downloadEmptyRecipientForm(); //Second download form if first run
            } else {
                mProcessResult.setValue(NetworkState.LOADED); //Open an activity
            }
        });

        mProcessResult.addSource(mIsFormDownloadingSuccessful, isFormDownloaded -> {
            mProcessResult.removeSource(mIsFormDownloadingSuccessful);
            if (isFormDownloaded == null) {
                return;
            }

            if (isFormDownloaded.getStatus() == NetworkState.Status.FAILED) {
                mProcessResult.setValue(new NetworkState(NetworkState.Status.FAILED,
                        getApplication().getString(R.string.first_run_empty_form_error)));
                return;
            }

            mProcessResult.setValue(NetworkState.LOADED); //Third open an activity
            preferenceUtil.putBooleanData(Constants.FIRST_RUN_KEY, false);
        });
    }

Answer 1

我在 ViewModel 中遇到了同样的问题。似乎这是 MediatorLiveData 的内部限制。如果它看到添加了相同类型的 LiveData，它希望有相同的监听器来使用它。我将添加我的案例，因为它具有相同类型的错误以及之后的情况(没有错误，工作正常)。错误代码:

networkSum = new MediatorLiveData<>();
    networkSum.addSource(firstData, integer -> {
        if (integer == null) {
            firstIsFinished = false;
            errorInFirst = false;
        } else if (integer == SupportNetworkFunctions.STATUS_IN_PROGRESS) {
            networkSum.postValue(integer);
            firstIsFinished = false;
        } else if (integer == SupportNetworkFunctions.STATUS_FINISHED_WITH_ERROR) {
            firstIsFinished = true;
            errorInFirst = true;
            calculateFinished();
        } else {
            firstIsFinished = false;
            errorInFirst = false;
            calculateFinished();
        }
    });
    networkSum.addSource(secondData, integer -> {
        if (integer == null) {
            secondIsFinished = false;
            errorInSecond = false;
        } else if (integer == SupportNetworkFunctions.STATUS_IN_PROGRESS) {
            networkSum.postValue(integer);
            secondIsFinished = false;
        } else if (integer == SupportNetworkFunctions.STATUS_FINISHED_WITH_ERROR) {
            secondIsFinished = true;
            errorInSecond = true;
            calculateFinished();
        } else {
            secondIsFinished = false;
            errorInSecond = false;
            calculateFinished();
        }
    });

重构后:

networkSum = new MediatorLiveData<>();
    Observer<Integer> statusObserver = new Observer<Integer>() {
        @Override
        public void onChanged(@Nullable Integer integer) {
            if (integer != null) {

                if (integer == SupportNetworkFunctions.STATUS_IN_PROGRESS) {
                    networkSum.postValue(integer);
                } else if (integer == SupportNetworkFunctions.STATUS_FINISHED_WITH_ERROR) {
                    finishedItems++;
                    errorAppeared = true;
                    calculateFinished();
                } else {
                    finishedItems++;
                    calculateFinished();
                }
            }
        }
    };
    networkSum.addSource(firstData, statusObserver);
    networkSum.addSource(secondData, statusObserver);