自Android API级别23起，onAttach(Activity Activity)已弃用。你需要使用onAttach(Context Context)。http://developer.android.com/reference/android/app/Fragment.html onAttach (android.app.Activity)

Activity是一个上下文，所以如果你可以简单地检查上下文是否是一个Activity，并在必要时强制转换它。

@Override
public void onAttach(Context context) {
    super.onAttach(context);

    Activity a;

    if (context instanceof Activity){
        a=(Activity) context;
    }

}

编写一个通用方法，确保永远不会得到空Activity。

public class BaseFragment extends Fragment {
    private Context contextNullSafe;

     @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
         /*View creation related to this fragment is finished here. So in case if contextNullSafe is null
         * then we can populate it here.In some discussion in - https://stackoverflow.com/questions/6215239/getactivity-returns-null-in-fragment-function
         * and https://stackoverflow.com/questions/47987649/why-getcontext-in-fragment-sometimes-returns-null,
         * there are some recommendations to call getContext() or getActivity() after onCreateView() and
         * onViewCreated() is called after the onCreateView() call every time */
        if (contextNullSafe == null) getContextNullSafety();
    }


   @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        contextNullSafe = context;
    }

    /**CALL THIS IF YOU NEED CONTEXT*/
    public Context getContextNullSafety() {
                if (getContext() != null) return getContext();
                if (getActivity() != null) return getActivity();
                if (contextNullSafe != null) return contextNullSafe;
                if (getView() != null && getView().getContext() != null) return getView().getContext();
                if (requireContext() != null) return requireContext();
                if (requireActivity() != null) return requireActivity();
                if (requireView() != null && requireView().getContext() != null)
                    return requireView().getContext();
                
                return null;
            
        }

    /**CALL THIS IF YOU NEED ACTIVITY*/
    public FragmentActivity getActivityNullSafety() {
        if (getContextNullSafety() != null && getContextNullSafety() instanceof FragmentActivity) {
            /*It is observed that if context it not null then it will be
             * the related host/container activity always*/
            return (FragmentActivity) getContextNullSafety();
        }
        return null;
    }

那些仍然有问题的onAttach(活动活动)，它只是改变了上下文-

    @Override
public void onAttach(Context context) {
    super.onAttach(context);
    this.context = context;
}

在大多数情况下，保存上下文对您来说就足够了——例如，如果您想执行getResources()，您可以直接从上下文执行。如果你仍然需要将上下文放入你的活动中，那么就这样做吧

 @Override
public void onAttach(Context context) {
    super.onAttach(context);
    mActivity a; //Your activity class - will probably be a global var.
    if (context instanceof mActivity){
        a=(mActivity) context;
    }
}

由user1868713建议。

在哪里调用这个函数?如果在Fragment的构造函数中调用它，它将返回null。

当onCreateView()方法被执行时，只需调用getActivity()。

这发生在您在另一个线程中调用getActivity()时，该线程在片段被删除后结束。典型的情况是在HTTP请求完成时调用getActivity()(例如Toast)(例如onResponse)。

为了避免这种情况，您可以定义一个字段名mActivity，并使用它来代替getActivity()。该字段可以在Fragment的onAttach()方法中初始化，如下所示:

@Override
public void onAttach(Context context) {
    super.onAttach(context);

    if (context instanceof Activity){
        mActivity =(Activity) context;
    }
}

在我的项目中，我通常用这个特性为我所有的Fragments定义一个基类:

public abstract class BaseFragment extends Fragment {

    protected FragmentActivity mActivity;

    @Override
public void onAttach(Context context) {
    super.onAttach(context);

    if (context instanceof Activity){
        mActivity =(Activity) context;
    }
}
}

快乐的编码,

The other answers that suggest keeping a reference to the activity in onAttach are just suggesting a bandaid to the real problem. When getActivity returns null it means that the Fragment is not attached to the Activity. Most commonly this happens when the Activity has gone away due to rotation or the Activity being finished but the Fragment still has some kind of callback listener registered preventing it from being garbage collected. When the listener gets called if you need to do something with the Activity but the Activity is gone there isn't much you can do. In your code you should just check getActivity() != null and if it's not there then don't do anything. If you keep a reference to the Activity that is gone you are preventing the Activity from being garbage collected. Any UI things you might try to do won't be seen by the user. I can imagine some situations where in the callback listener you want to have a Context for something non-UI related, in those cases it probably makes more sense to get the Application context. Note that the only reason that the onAttach trick isn't a big memory leak is because normally after the callback listener executes it won't be needed anymore and can be garbage collected along with the Fragment, all its View's and the Activity context. If you setRetainInstance(true) there is a bigger chance of a memory leak because the Activity field will also be retained but after rotation that could be the previous Activity not the current one.