如何使用Dagger2在Fragment中注入对BroadcastReceiver对象的创建？ [英] How to inject the creation of a BroadcastReceiver object in Fragment using Dagger2?

问题描述

我需要将 NetReceiver 对象的创建注入到片段中，但是出现以下错误：

I need to inject the creation of NetReceiver object into my Fragment but I get the following error:

错误：[Dagger / MissingBinding] com.example.myapp.NetReceiver.OnNetCallback如果没有@Provides注释的方法就无法提供。

error: [Dagger/MissingBinding] com.example.myapp.NetReceiver.OnNetCallback cannot be provided without an @Provides-annotated method.

让我告诉您我所尝试的。这是我的 NetReceiver 类：

Let me show you what I have tried. This is my NetReceiver class:

public class NetReceiver extends BroadcastReceiver {
    private OnNetCallback onNetCallback;

    @Inject
    public NetReceiver(OnNetCallback onNetCallback) {
        this.onNetCallback = onNetCallback;
    }

    @Override
    public void onReceive(Context context, Intent intent) {
        if (someCondition) {
            onNetCallback.enableOperation(true);
        }
    }

    public interface OnNetCallback {
        void enableOperation(boolean isOk);
    }
}

如您所见，我想注入构造函数 NetCallback 对象。我还创建了一个看起来像这样的Module类：

As you can see, I want to inject in the constructor a NetCallback object. I have also created a Module class that looks like this:

@Module
public abstract class NetReceiverModule {
    @ContributesAndroidInjector //Error?!?!?!?!
    abstract NetReceiver.OnNetCallback provideOnNetCallback();

    @Singleton
    @Provides
    static NetReceiver provideNetReceiver(NetReceiver.OnNetCallback onNetCallback) {
        return new NetReceiver(onNetCallback);
    }
}

这是我的片段在工作时的样子：

This is how my fragment look liked when it worked:

public class MapFragment extends Fragment implements NetReceiver.OnNetCallback {
    private NetReceiver netReceiver;

    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        //Inflate View
        netReceiver = new NetReceiver(this); //Worked
    }

    @Override
    public void onResume() {
    super.onResume();
        mainActivity.registerReceiver(netReceiver, intentFilter);
    }

    @Override
    public void onPause() {
    super.onPause();
        mainActivity.unregisterReceiver(netReceiver);
    }
}

这是它不工作时的样子：

And this is how it looks when it is not working:

public class MapFragment extends DaggerFragment implements NetReceiver.OnNetCallback {
    @Inject NetReceiver netReceiver;

    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        //Inflate View
        //netReceiver = new NetReceiver(this); //Worked
    }

    @Override
    public void onResume() {
    super.onResume();
        mainActivity.registerReceiver(netReceiver, intentFilter);
    }

    @Override
    public void onPause() {
    super.onPause();
        mainActivity.unregisterReceiver(netReceiver);
    }
}

推荐答案

问题是 Dagger2 不能做魔术。如何知道从何处提供 OnNetCallback 。您的片段扩展了此回调，但是其构造函数都没有使用 @Inject 注释（因为不建议您创建显式的片段构造器），也没有提供 @Provides 在模块中带有注释的方法， OnNetCallback 也不通过 MapFragment 通过 @Bind 带注释的方法，当您尝试将这样的接口与 @Qualifier 一起注入时，这是必须。

The problem is that Dagger2 cannot do magic. How it should know from where to provide a OnNetCallback. Your fragment extends this callback but neither its constructor is annotated with @Inject annotation(because you are not advised to create explicit fragment constructors) nor it is provided with @Provides annotated method in your module, nor OnNetCallback is bind with MapFragment via @Bind annotated method which is a "must be" when you trying to inject interface like this along with a @Qualifier.

但这不是主要问题。

即使您能够正确地完成所有先前的工作，它仍然不起作用，因为将存在循环依赖关系- MapFragment 取决于 NetReceiver ， NetReceiver 取决于 OnNetCallback ， OnNetCallback 是 MapFragment ，因此 NetReceiver 取决于 MapFragment 。匕首不会允许您这样做。

Even if you will manage to do all the previous stuff correctly it will still not work because there will be a circular dependency - MapFragment depends on NetReceiver, NetReceiver depends on OnNetCallback, OnNetCallback is a MapFragment, thus NetReceiver depends on MapFragment. Dagger won't let you do it.

如果您希望您的 MapFragment 实现 OnNetCallback 我建议如下：

If you want your MapFragment to implement OnNetCallback I would suggest something like:

public class NetReceiver extends BroadcastReceiver {

    public OnNetCallback onNetCallback;

    @Override
    public void onReceive(Context context, Intent intent) {
        if (someCondition && onNetCallback!= null) {
            onNetCallback.enableOperation(true);
        }
    }

    public interface OnNetCallback {
        void enableOperation(boolean isOk);
    }
}


public class MapFragment extends DaggerFragment implements NetReceiver.OnNetCallback {

    @Inject NetReceiver netReceiver;

    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        //Inflate View
        netReceiver.onNetCallback = this
    }

    @Override
    public void onResume() {
    super.onResume();
        mainActivity.registerReceiver(netReceiver, intentFilter);
    }

    @Override
    public void onPause() {
    super.onPause();
        mainActivity.unregisterReceiver(netReceiver);
    }
}

如果要注入，请单独创建一些来自 OnNetCallback 的 Fragment 实现，并@在模块中提供并将其注入片段。

If you want it to be injected - create some separate from Fragment implementation of OnNetCallback and @Provide it in module and inject it into fragment.

也 @ContributesAndroidInjector 应该用于喷射器，但不能用于喷射器-它告诉 Dagger 该类应该注入一些用 @Inject ， @Provides 和 @Bind 批注。

Also @ContributesAndroidInjector should be used for the injeters but not for the injectees - it tells Dagger that this class should be injected with some stuff described with @Inject, @Provides and @Bind annotations.

我也建议您阅读官方指南关于 Dagger2 和其在Android中的用法。

Also I would recommend you to read an official guide regarding Dagger2 and its usage for Android.

希望有帮助。

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