[英]Dependency Injection with Dagger2, how to get context
I'm learning dependency injection with Dagger2. 我正在学习Dagger2的依赖注入。
I created a HttpRequester class that has a get method and returns some data from a server. 我创建了一个具有get方法的HttpRequester类,并从服务器返回了一些数据。
This is my code: 这是我的代码:
MainActivity 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 HttpRequesterModule
@Module
public class HttpRequesterModule {
@Provides @Singleton
HttpRequester provideHttpRequester(Context context){
return new HttpRequester(context);
}
}
HttpRequesterComponent HttpRequesterComponent
@Singleton
@Component(modules = {HttpRequesterModule.class})
public interface HttpRequesterComponent {
HttpRequester provideHttpRequester();
}
HttpRequester 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);
}
}
The problem is that when I call httpRequester.get() I get a NullPointerException since the context is null. 问题是,当我调用httpRequester.get()时,由于上下文为null,所以会收到NullPointerException。 I want to know how to inject the context. 我想知道如何注入上下文。 Thanks for your help! 谢谢你的帮助!
You have to notice that you never initialize the context attribute con HttpRequester. 您必须注意,您永远不会初始化上下文属性con HttpRequester。 Receive a Context
in your constructor so it has a value: 在构造函数中接收Context
,因此它具有一个值:
public class HttpRequester {
Context context;
public HttpRequester(Context context) {
this.context = context
}
...
}
After that, you just have to provide it in your HttpRequesterModule
: 之后,您只需要在HttpRequesterModule
提供它即可:
@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;
}
}
Now you just have to pass a Context to your module 现在,您只需将Context传递给模块
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