回调中使用的正确上下文 [英] Correct context to use within callbacks

问题描述

标题几乎说明了一切.如果您有一个从一个类到另一个类的回调，并且需要从需要上下文的回调中调用某个方法，那么要使用的正确上下文是什么?一个常见的例子是带有对使用它的 Activity 或 Fragment 的回调的 AsyncTask.

The title pretty much says it all. If you have a callback from one class to another and need to call some method from within the callback that requires a context what is the correct context to use? A common example would be an AsyncTask with a callback to the Activity or Fragment that used it.

我通常尽量避免使用 getApplicationContext() 但我不能使用 this 作为回调中的上下文.在这种情况下，使用更广泛的上下文是否合适?

I generally try to avoid using getApplicationContext() but I cannot use this as the context from within a callback. Is this a case where using a broader context is appropriate?

为了进一步澄清，我正在考虑使用 AsyncTask 和活动之间的接口的回调.一旦我进入覆盖的接口方法，我就无法从那里获取活动上下文.

To clarify further I'm thinking of a callback with an interface between an AsyncTask and an activity. Once I'm inside the overridden interface method I can't get the activities context from within there.

推荐答案

使用活动的上下文.示例:

Use the Activity's context. Example:

MyAsyncTask mat = new MyAsyncTask(this);

MyAsyncTask 构造函数:

MyAsyncTask contructor:

public MyAsyncTask(MyActivity context) {
    mContext = context;
}

从 MyAsyncTask 中调用 MyActivity 的方法 methodToCall():

To call MyActivity's method methodToCall() from within MyAsyncTask:

((MyActivity)mContext).methodToCall();

编辑 1:

我猜你的问题是这样的:

I am guessing your problem is this:

public class MyActivity extends Activity {

    Button b;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.some_layout);

        b = (Button) findViewById(...);

        b.setOnClickListener(new OnClickListener() {    
            @Override
            public void onClick(View v) {
                Button newButton = new Button(this);        // Won't work!!
            }
        });
    }
}

解决方法:

• 在 MyActivity 中声明一个方法:getContext()

public Context getContext() {
    return (Context)this;
}

b.setOnClickListener(new OnClickListener() {    
    @Override
    public void onClick(View v) {
        Button newButton = new Button(getContext());    // Will work
    }
});

使用MyActivity.this代替this.

另一种方式是声明 MyActivity 实现了接口:

Another way is to state that MyActivity implements the interface:

public class MyActivity extends Activity implements View.OnClickListener {

    ....
    ....

    @Override
    public void onClick(View v) {
        Button newButton = Button (this)                // Will Work
    }
}

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