如何将依赖项注入嵌套的android片段？

Question

对于普通（非嵌套片段）我使用以下方法

1) 创建依赖项(...) 方法来设置片段的依赖项

class MyFragment extends MyFragment {
      void dependencies(Deps deps);
}

2) 在 MyFragment 父活动的 onAttachFragment() 方法中，我只是为片段提供依赖项

class MyActivity{
    void onAttachFragment(Fragment f){
        ((MyFragment)f).dependencies(deps);
    }
}

对于嵌套片段，不再调用 onAttachFragment 片段。 为fragment提供依赖只是为了给嵌套的fragment提供依赖似乎很麻烦。那么我怎样才能为它提供依赖呢？

Answer 1

只需脱离将成为活动的上下文即可。为您的活动的依赖项创建一个吸气剂。无论是否嵌套，片段都可以访问父活动。转换上下文，然后调用 getter 来获取嵌套活动中的依赖关系。

Answer 2

如果MyFragment 依赖于MyNestedFragment，并且MyNestedFragment 依赖于Deps；因此MyFragment 也依赖于Deps。当然，在调用Activity.onAttachFragment() 时不存在MyNestedFragment 的实例，因此您必须等到在MyFragment.onCreateView() 中扩展布局后，才能提供MyNestedFragment 及其依赖项。

public class MyActivity {

    ...

    void onAttachFragment(Fragment f){
        ((MyFragment)f).dependencies(deps);
    }

    public static class MyFragment extends Fragment {

        private Deps deps;

        void dependencies(Deps deps) {
            this.deps = deps;
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);

            // <fragment> element in fragment_main layout has
            //  android:tag set to nested_fragment
            ((MyNestedFragment)getChildFragmentManager()
                .findFragmentByTag("nested_fragment"))
                .dependencies(this.deps);

            return rootView;
        }
    }

    public static class MyNestedFragment extends Fragment {

        void dependencies(Deps deps) {
            ...
        }
    }

    ...
}

如果所有这些看起来有点混乱，那是因为 Fragment 不是 POJO，您可以以任意方式连接起来。它们的生命周期必须由嵌套的 FragmentManager 管理。如果您以编程方式而不是使用 元素来创建片段，您将对它们的生命周期进行更多控制，但代价是更复杂。

如果您想将 Android 视为 IoC 容器，那么RoboGuice 可能就是您要找的：

public class MyActivity extends roboguice.activity.RoboFragmentActivity {

    ...

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        // This only needs to be called once for the whole app, so it could
        // be in the onCreate() method of a custom Application subclass 
        RoboGuice.setUseAnnotationDatabases(false);

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);
    }

    public static class MyNestedFragment extends Fragment {

        @Inject
        private Deps deps;

        @Override
        public void onAttach(Activity activity) {
            super.onAttach(activity);
            // this isn't necessary if you extend RoboFragment
            roboguice.RoboGuice.getInjector(activity).injectMembers(this);
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

            //This would not even be possible in the previous example
            // because onCreateView() is called before dependencies()
            // can be called.
            deps.method();

            View rootView = inflater.inflate(R.layout.fragment_nested, container, false);
            return rootView;
        }
    }
}

@Singleton
public class Deps {
    public void method() {
        System.out.println("Deps.method()");
    }
}

Answer 3

当附加片段时，您尝试设置依赖项。取而代之的是，在需要时尝试从片段中获取依赖项。有一个例子：

public class MyActivity extends Activity {

    public Deps getDepsForFragment(Fragment fragment) {
        if (fragment instanceof MyFragment) {
            return depsForMyFragment;
        } else if (fragment instanceof MyNestedFragment) {
            return depsForMyNestedFragment;
        } else {
            return null;
        }
    }
}

public class MyFragment extends Fragment {

    private Deps deps;

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        try {
            MyActivtiy myActivity = (MyActivtiy) context;
            deps = myActivity.getDepsForFragment(this);
        } catch (ClassCastException e) {
            throw new ClassCastException("This fragment attached to an activity which can't provide the required dependencies.");
        }
    }
}

// this is the same as the MyFragment
public class MyNestedFragment extends Fragment {

    private Deps deps;

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        try {
            MyActivtiy myActivity = (MyActivtiy) context;
            deps = myActivity.getDepsForFragment(this);
        } catch (ClassCastException e) {
            throw new ClassCastException("This fragment attached to an activity which can't provide the required dependencies.");
        }
    }
}

当然，你可以在activity中为get deps做单独的方法（比如getDepsForMyFragment和getDepsForMyNestedFragment）。

Answer 4

只要保持层次逻辑，应该是这样的：

class MyActivity{
    void onAttachFragment(Fragment f){
        ((MyFragment)f).dependencies(deps);
    }
}

class MyFragment extends MyFragment {
      void dependencies(Deps deps) {
          //TODO: do dependencies of my fragment before
          ((MyNestedFragment)childF).nestedDependencies(deps);
          //TODO: do dependencies of my fragment after
      }
}

class MyNestedFragment extends MyNestedFragment {
      void nestedDependencies(Deps deps);
}