使用 Dagger2 进行依赖注入，如何获取上下文

Question

我正在学习使用 Dagger2 进行依赖注入。

我创建了一个 HttpRequester class，它有一个 get 方法和 returns 来自服务器的一些数据。

这是我的代码：

主要活动

public class MainActivity extends AppCompatActivity {

@Inject
HttpRequester httpRequester;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    getHttpRequester();

    httpRequester.get("http://url.to.get.the.data", new HttpRequester.Listener() {
        @Override
        public void onDataRetrieved(String result) {

            Log.d("App", "Result: " + result);
        }
    });
}

@Override
protected void onStop () {
    super.onStop();
    if (httpRequester != null) {

        httpRequester.cancelAll();
    }
}

void getHttpRequester() {

    HttpRequesterComponent httpRequesterComponent = DaggerHttpRequesterComponent.builder().httpRequesterModule(new HttpRequesterModule()).build();

    httpRequester = httpRequesterComponent.provideHttpRequester();
}
}

HttpRequesterModule

@Module
public class HttpRequesterModule {

    @Provides @Singleton
    HttpRequester provideHttpRequester(Context context){

        return new HttpRequester(context);
    }
}

HttpRequesterComponent

@Singleton
@Component(modules = {HttpRequesterModule.class})
public interface HttpRequesterComponent {

    HttpRequester provideHttpRequester();
}

HttpRequester

public class HttpRequester {

    Context context;

    public HttpRequester(Context context) {

        this.context = context;
    }

    public interface Listener {

        void onDataRetrieved(String result);
    }

    private RequestQueue queue = Volley.newRequestQueue(context);

    public static final String TAG = "TAG";

    public void get(String url, final Listener listener){

        StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String res) {

                        listener.onDataRetrieved(res);
                    }
                }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {

            }
        });

        stringRequest.setTag(TAG);

        queue.add(stringRequest);
    }

    public void cancelAll() {

        queue.cancelAll(TAG);
    }
}

问题是，当我调用 httpRequester.get() 时，我得到一个 NullPointerException，因为上下文为空。我想知道如何注入上下文。感谢您的帮助！

Answer 1

您必须注意，您从未初始化 con HttpRequester 的上下文属性。在你的构造函数中接收一个 Context 所以它有一个值：

public class HttpRequester {

    Context context;

    public HttpRequester(Context context) {
        this.context = context
    }

    ...
}

之后，您只需在 HttpRequesterModule:

中提供

@Module
public class HttpRequesterModule {

    private final Context context;

    public HttpRequesterModule(Context context) {
        this.context = context;
    }

    @Provides @Singleton
    HttpRequester provideHttpRequester(Context context){
        return new HttpRequester(context);
    }

    @Provides
    Context provideContext() {
        return context;
    }
}

现在您只需将上下文传递给您的模块

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