为什么我的片段中的上下文为空? [英] Why is my context in my Fragment null?

问题描述

我有一个关于片段中上下文使用的问题.我的问题是我总是得到一个 NullpointerException.这是我所做的:

I have got a question regarding the usage of context in a fragment. My problem is that I always get a NullpointerException. Here is what i do:

创建一个扩展 SherlockFragment 的类.在那个类中，我有另一个 Helper 类的实例:

Create a class that extends the SherlockFragment. In that class I have an instance of another Helper class:

public class Fragment extends SherlockFragment { 
    private Helper helper = new Helper(this.getActivity());

    // More code ...
}

这是另一个 Helper 类的摘录:

Here is an extract of the other Helper class:

public class Helper {
    public Helper(Context context) {
        this.context = context;
    }
    // More code ...
}

每次我调用 context.someMethod(例如 context.getResources() )时，我都会收到 NullPointerException.这是为什么?

Everytime I call context.someMethod (e.g. context.getResources() ) I get a NullPointerException. Why is that?

推荐答案

当 Fragment 首次实例化时，您正试图获得一个 Context.那时，它没有附加到Activity，所以没有有效的Context.

You're attempting to get a Context when the Fragment is first instantiated. At that time, it is NOT attached to an Activity, so there is no valid Context.

查看片段生命周期.onAttach() 到 onDetach() 之间的所有内容都包含对有效 Context 实例的引用.这个 Context 实例通常通过 getActivity()

Have a look at the Fragment Lifecycle. Everything between onAttach() to onDetach() contain a reference to a valid Context instance. This Context instance is usually retrieved via getActivity()

代码示例:

private Helper mHelper;

@Override
public void onAttach(Activity activity){
   super.onAttach (activity);
   mHelper = new Helper (activity);
}

我在示例中使用了 onAttach()，@LaurenceDawson 使用了 onActivityCreated().注意差异.由于 onAttach() 已经将 Activity 传递给它，所以我没有使用 getActivity().相反，我使用了传递的参数.对于生命周期中的所有其他方法，您必须使用getActivity().

I used onAttach() in my example, @LaurenceDawson used onActivityCreated(). Note the differences. Since onAttach() gets an Activity passed to it already, I didn't use getActivity(). Instead I used the argument passed. For all other methods in the lifecycle, you will have to use getActivity().

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