java - onAttach 中无法初始化接口(interface)对象？

Question

我创建了一个接口(interface) DataPasserInterface 将数据从 fragment 传递到其托管 Activity 。我在 onAttach 中初始化了界面，如下代码所示，问题是，在运行时，应用程序崩溃，并且我收到下面发布的 logCat 错误。

我不知道为什么 this.dataPasserObj = (DataPasserInterface) Activity; 会导致错误？ logcat 说，接口(interface)对象无法转换？!

界面:

private DataPasserInterface dataPasserObj;
...
...
...
 public interface DataPasserInterface {

void onTopicsListChosen(ArrayList<String> chosenTopicsList);

}

onAttach:

@Override
public void onAttach(Activity activity) {
    // TODO Auto-generated method stub
    super.onAttach(activity);
    Log.w(TAG, "@onAttach().");

    this.dataPasserObj = (DataPasserInterface) activity;
    this.designList = new ArrayList<ItemDesign>();
}

logcat:

 E/AndroidRuntime(7828): FATAL EXCEPTION: main
 E/AndroidRuntime(7828): Process: com.example.mqtt_designlayout_02, PID: 7828
 E/AndroidRuntime(7828): java.lang.ClassCastException: com.example.mqtt_designlayout_02.SettingsActivity cannot be cast to com.example.mqtt_designlayout_02.DataPasserInterface
 E/AndroidRuntime(7828):    at com.example.mqtt_designlayout_02.Eco_Settings_Frag.onAttach(Eco_Settings_Frag.java:95)

更新

托管 fragment 的 Activity 的 onCreate() 且该 Activity 实现该接口(interface)

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.w(TAG, "@onCreate().");

    this.mqttSettingsDB = new MQTT_Settings_DB(this);
    this.sqliteDB = this.mqttSettingsDB.getWritableDatabase();

    if (this.fragList == null) {
        this.fragList = new ArrayList<Fragment>();

        this.fragList.add(new MQTT_Settings_Frag());
        this.fragList.add(new Eco_Settings_Frag());
        this.fragList.add(new Logger_Settings_Frag());
    }

    initviews(R.layout.settings_activity);
    setUpActionBar();
    this.viewPager.setOnPageChangeListener(viewPagerListener);
}

Answer 1

ClassCastException: com.example.mqtt_designlayout_02.SettingsActivity

可能没有在 SettingsActivity 中实现 DataPasserInterface 接口(interface)

建议在Eco_Settings_Frag类中创建构造函数来获取DataPasserInterface接口(interface)并初始化其他值:

public Eco_Settings_Frag(Activity activity){
    this.dataPasserObj = (DataPasserInterface) activity;
    this.designList = new ArrayList<ItemDesign>();
}

并在创建Eco_Settings_Frag类的对象时传递SettingsActivity上下文:

 .....
 this.fragList.add(new Eco_Settings_Frag(SettingsActivity.this));
 this.fragList.add(new Logger_Settings_Frag());
 .....

还在SettingsActivity Activity中实现DataPasserInterface接口(interface)