如何等待改造异步Web服务调用？ （call.enqueue）

Question

I am new to java/android and have previously done a lot in C# with the use of async calls where I can use await to force a response before I move onto more code.

From what I can tell call.enqueue is asynchronous, but I want that to be awaited so that my LoginActivity method can handle the next steps based on if the login was successful or not.

I made some Logs that ideally should be displayed as follows:

FIRST
SECOND
THIRD
FOURTH

However because of the async, I instead get:

FIRST
SECOND
FOURTH
THIRD

LoginActivity.java

public boolean tryLoginAttempt(LoginRequest request) {
    // Call login service, login method
    LoginService service = new LoginService();

    Log.d(TAG, "FIRST");

    LoginResponse loginResponse = service.loginMethod(request); // ** want to await this

    Log.d(TAG, "FOURTH");

    // If login works, navigate to tabbed menu
    if (loginResponse.getAuthorized()) {
        Intent intent = new Intent(LoginActivity.this, SelectTeamActivity.class);
        startActivity(intent);
    }

    return loginResponse.getAuthorized();
}

LoginService.java

LoginResponse mLoginResponse = new LoginResponse();

public LoginResponse loginMethod(LoginRequest request) {
    try {
        String baseUrl = "https://myurl.com";
        // Create retrofit object
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(baseUrl)
                .addConverterFactory(GsonConverterFactory.create())
                .build();   //  This instantiates the Retrofit builder we will need for REST calls

        LoginEndpointInterface apiService = retrofit.create(LoginEndpointInterface.class);

        // Call the login method
        Observable<LoginResponse> call = apiService.login(request);

        Log.d(TAG, "SECOND");

        call.enqueue(new Callback<LoginResponse>() { // ** running as async
            @Override
            public void onResponse(Call<LoginResponse> call, Response<LoginResponse> response) {
                Log.d(TAG, "THIRD");

                int statusCode = response.code();

                // Login worked
                if (response.body().getAuthorized()) {
                    Log.d(TAG, "Login worked");
                    mLoginResponse = response.body();
                } else {
                    Log.d(TAG, "Login failed");
                }
            }

            @Override
            public void onFailure(Call<LoginResponse> call, Throwable t) {
                Log.d(TAG, "ERROR in web service call: " + t.toString());
            }
        });
    Log.d(TAG, "returning result of login check: " + result);
    } catch(Exception ex) {
        ex.printStackTrace();
    }

    return mLoginResponse;
}

Retrofit call interface

public interface LoginEndpointInterface {

    @POST("api/login/")
    retrofit2.Call<LoginResponse> login(@Body LoginRequest body);

}

Thanks for the help!

Answer 1

In android, networking or heavy tasks which block the ui can't be done on the main thread and hence all these things are done asynchronously. To achieve what you are trying, you will need to call all your next steps after the network call is done, which can only be done in retrofit callback inside onResponse or onFailure . Its basically chaining of calls. To make your code more readable, you can use your own interfaces for call back mechanisms or you can use libraries like RxJava which makes its really simple.