从适配器回调 [英] Callback from Adapter

问题描述

所以我有一个自定义列表视图。这就像Instagram布局，每个列表项中有1个图像和一堆按钮。所以这就是问题：

我想实现分享按钮。为此，我尝试创建从适配器到活动的回调。但它似乎没有用。这是我到目前为止（我裁掉了不相关的部分）：

MainActivity

 公共类MainActivity扩展ActionBarActivity实现ListAdapter.OnShareClickedListener {
 
 ListView main_list; 
 List< String> URL_LIST; 
 
 @Override 
 protected void onCreate（Bundle savedInstanceState）{
 super.onCreate（savedInstanceState）; 
 setContentView（R.layout.activity_main）; 
 main_list =（ListView）findViewById（R.id.listView）; 
 ListAdapter nListAdapter = new ListAdapter（context，url_list）; 
 main_list.setAdapter（nListAdapter）; 
} 
 
 @Override 
 public void ShareClicked（String url）{
 Log.e（Test，url）; 
} 
 
} 
  

ListAdapter

 公共类ListAdapter扩展BaseAdapter实现View.OnClickListener {
 
 OnShareClickedListener mCallback; 
上下文上下文; 
 public static List< String> URL_LIST; 
 
 public ListAdapter（Context c，List< String> list）{
 this.context = c; 
 url_list = list; 
} 
 
公共接口OnShareClickedListener {
 public void ShareClicked（String url）; 
} 
 
 
 @Override 
 public void onClick（查看v）{
 mCallback.ShareClicked（分享此文字。）; 
} 
} 
 
} 
  

错误日志：

 尝试调用接口方法'void com.packagename .ListAdapter $ OnShareClickedListener.ShareClicked（java.lang.String）'对空对象引用
  

解决方案

您需要告诉适配器要使用的OnShareClickedListener（）实现。现在在你的适配器中永远不会分配字段mCallback，你需要在你的适配器中有一个setOnSharedClickedListener（）方法，然后从mainActivity调用它并使用main活动的实现设置它，或者你需要接受构造函数。

我的建议是使用setter而不是构造函数。所以你需要做的就是这个。

你的ListAdapter

 公共类ListAdapter扩展BaseAdapter实现View.OnClickListener {
 
 OnShareClickedListener mCallback; 
上下文上下文; 
 public static List< String> URL_LIST; 
 
 public ListAdapter（Context c，List< String> list）{
 this.context = c; 
 url_list = list; 
} 
 
 public void setOnShareClickedListener（OnShareClickedListener mCallback）{
 this.mCallback = mCallback; 
} 
 
公共接口OnShareClickedListener {
 public void ShareClicked（String url）; 
} 
 
 
 @Override 
 public void onClick（查看v）{
 mCallback.ShareClicked（分享此文字。）; 
} 
} 
  

您的MainActivty

 公共类MainActivity扩展ActionBarActivity实现ListAdapter.OnShareClickedListener {
 
 ListView main_list; 
 List< String> URL_LIST; 
 
 @Override 
 protected void onCreate（Bundle savedInstanceState）{
 super.onCreate（savedInstanceState）; 
 setContentView（R.layout.activity_main）; 
 main_list =（ListView）findViewById（R.id.listView）; 
 ListAdapter nListAdapter = new ListAdapter（this，url_list）; 
 nListAdapter.setOnShareClickedListener（this）; 
 main_list.setAdapter（nListAdapter）; 
} 
 
 @Override 
 public void ShareClicked（String url）{
 Log.e（Test，url）; 
} 
 
} 
  

So I have a custom listview. It's like Instagram layout with 1 image and bunch of buttons in each list items. So here's the problem:

I want to implement the share button. To do this, I tried to create a callback from adapter to activity. But it didn't seem to work. Here is what I have so far(I cropped out the unrelated parts):

MainActivity

public class MainActivity extends ActionBarActivity implements ListAdapter.OnShareClickedListener{

    ListView main_list;
    List<String> url_list; 

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        main_list = (ListView) findViewById(R.id.listView);
        ListAdapter nListAdapter = new ListAdapter(context, url_list);
        main_list.setAdapter(nListAdapter);
    }

    @Override
    public void ShareClicked(String url) {
        Log.e("Test",url);
    }

}

ListAdapter

public class ListAdapter extends BaseAdapter implements View.OnClickListener {

    OnShareClickedListener mCallback;
    Context context;
    public static List<String> url_list;

    public ListAdapter(Context c, List<String> list) {
        this.context = c;
        url_list = list;
    }

    public interface OnShareClickedListener {
        public void ShareClicked(String url);
    }


    @Override
    public void onClick(View v) {
                mCallback.ShareClicked("Share this text.");
        }
    }

}

Error Log:

Attempt to invoke interface method 'void com.packagename.ListAdapter$OnShareClickedListener.ShareClicked(java.lang.String)' on a null object reference

解决方案

You need to tell the adapter which implementation of the OnShareClickedListener() to use. Right now in your adapter the field mCallback is never assigned to, either you need to have a setOnSharedClickedListener() method in your adapter which you then call from your mainActivity and set it with the main activity's implementation or you need to take in the constructor.

My suggestion would be to use a setter instead of constructor. So what you need to do is this.

Your ListAdapter

public class ListAdapter extends BaseAdapter implements View.OnClickListener {

    OnShareClickedListener mCallback;
    Context context;
    public static List<String> url_list;

    public ListAdapter(Context c, List<String> list) {
        this.context = c;
        url_list = list;
    }

    public void setOnShareClickedListener(OnShareClickedListener mCallback) {
        this.mCallback = mCallback;
    }

    public interface OnShareClickedListener {
        public void ShareClicked(String url);
    }


    @Override
    public void onClick(View v) {
        mCallback.ShareClicked("Share this text.");
    }
}

Your MainActivty

public class MainActivity extends ActionBarActivity implements ListAdapter.OnShareClickedListener{

    ListView main_list;
    List<String> url_list;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        main_list = (ListView) findViewById(R.id.listView);
        ListAdapter nListAdapter = new ListAdapter(this, url_list);
        nListAdapter.setOnShareClickedListener(this);
        main_list.setAdapter(nListAdapter);
    }

    @Override
    public void ShareClicked(String url) {
        Log.e("Test", url);
    }

}

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