Android Spinner 的 onItemSelected 在返回 Fragment 时执行了两次

Question

我为我的 Fragment 创建了一个微调器，它使用从 HTTP 标注检索到的数据填充它。首次创建 Fragment 时，我使用其选择选项填充微调器，设置其 setOnItemSelectedListener 并在 onCreateView() 中设置其初始选择。

    stateSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {

        public void onItemSelected(AdapterView<?> parent, View view, int position, long id)
        {


            if (spinnerPosition != position)
            {
                spinnerPosition = position;
                TextView stateSelected = (TextView) view;
                String stateSelectedStr = stateSelected.getText().toString();


                LinearLayout ballotsDisplay = (LinearLayout) getActivity().findViewById(R.id.ballotsDisplay);
                ballotsDisplay.removeAllViews();

                Map<String, String> calloutParams = new HashMap<String, String>();
                calloutParams.put("state", stateSelectedStr);

                // Create and execute AsyncTask to retrieve ballots
                new RetrieveBallots().execute(calloutParams);
            }
        }

        public void onNothingSelected(AdapterView<?> parent) {
            return;
        }
    });

    // Set default selection for spinner
    int defaultState = adapter.getPosition(userState);

    if (defaultState == -1)
    {
        defaultState = 0;
    }

    stateSpinner.setSelection(defaultState);

创建 Fragment 后，一切正常，微调器位置设置为其默认值，并且微调器项被选中一次，如下面的日志所示：

5009-5009/com.project.test D/TEST﹕ onCreateView called
5009-5009/com.project.test D/TEST﹕ stateSpinner.setSelection
5009-5009/com.project.test D/TEST﹕ onActivityCreated called
5009-5009/com.project.test D/TEST﹕ onResume called
5009-5009/com.project.test D/TEST﹕ spinner item selected

当我从 Fragment 导航到另一个 Fragment 时会出现问题（我将 Fragment 存储到 backstack 上）。当我点击返回回到我原来的片段时，微调器似乎选择了它的项目两次：

5009-5009/com.project.test D/TEST﹕ onCreateView called
5009-5009/com.project.test D/TEST﹕ stateSpinner.setSelection
5009-5009/com.project.test D/TEST﹕ onActivityCreated called
5009-5009/com.project.test D/TEST﹕ onResume called
5009-5009/com.project.test D/TEST﹕ spinner item selected
5009-5009/com.project.test D/TEST﹕ spinner item selected

所以我有两个问题：

1) 为什么微调器在从“后退”按钮返回时会记录 2 个项目选择事件。

2) 是否有防止发生 2 个项目选择的修复程序？现在该片段正在填充重复数据，因为它检索了两次数据。

** 编辑 ** 更改为 stateSpinner.setSelection(defaultState, false) 后，我会在 ballotsDisplay.removeAllViews(); 处得到一个空指针异常，似乎 ballotsDisplay 由于某种原因被设置为 null

堆栈跟踪：

05-15 07:25:48.303 6153-6153/com.poliseewriters.polisee E/AndroidRuntime: 致命异常: main java.lang.NullPointerException 在 com.polisee.ballotmeasures.BallotMeasuresFragment$1.onItemSelected(BallotMeasuresFragment.java:287) 在 android.widget.AdapterView.fireOnSelected(AdapterView.java:882) 在 android.widget.AdapterView.selectionChanged(AdapterView.java:865) 在 android.widget.AdapterView.checkSelectionChanged(AdapterView.java:1017) 在 android.widget.Spinner.layout(Spinner.java:363) 在 android.widget.AbsSpinner.setSelectionInt(AbsSpinner.java:292) 在 android.widget.AbsSpinner.setSelection(AbsSpinner.java:269) 在 com.polisee.ballotmeasures.BallotMeasuresFragment.setStateSpinner(BallotMeasuresFragment.java:314) 在 com.polisee.ballotmeasures.BallotMeasuresFragment.onCreateView(BallotMeasuresFragment.java:201) 在 android.support.v4.app.Fragment.performCreateView(Fragment.java:1786) 在 android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:953) 在 android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1136) 在 android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739) 在 android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1499) 在 android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:456) 在 android.os.Handler.handleCallback(Handler.java:605) 在 android.os.Handler.dispatchMessage(Handler.java:92) 在 android.os.Looper.loop(Looper.java:137) 在 android.app.ActivityThread.main(ActivityThread.java:4441) 在 java.lang.reflect.Method.invokeNative(Native Method) 在 java.lang.reflect.Method.invoke(Method.java:511) 在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 在 dalvik.system.NativeStart.main(Native Method)

** 编辑：更新代码以防止 onItemSelected 执行两次，需要为 ballotsDisplay 添加空检查 **

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
    if (savedInstanceState != null) {
        Log.d("TEST", "bundle = " + savedInstanceState.toString());
    }

    Log.d("TEST", "onCreateView called");

    View view = (View) inflater.inflate(R.layout.fragment_ballot_measures, container, false);

    setStateSpinner(view);

    return view;
}


private void setStateSpinner(View view) {

    try {
        states = Utilities.getAllStateNames();
    }
    catch (Exception e) {
        Log.e("Error", "Error retrieving names: " + e.getMessage());
    }

    Spinner stateSpinner = (Spinner) view.findViewById(R.id.stateSpinner);

    final ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity(), R.layout.ballotmeasures_state_spinner, states);
    adapter.setDropDownViewResource(R.layout.ballotmeasures_state_spinner_dropdown);

    stateSpinner.setAdapter(adapter);

    stateSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {

        // Callback method to invoke when a state has been selected
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id)
        {
            Log.d("TEST", "spinner item selected");

                TextView stateSelected = (TextView) view;
                String stateSelectedStr = stateSelected.getText().toString();

                // Remove all currently displayed views in the layout
                LinearLayout ballotsDisplay = (LinearLayout) getActivity().findViewById(R.id.ballotsDisplay);
                if (ballotsDisplay != null)
                {
                    ballotsDisplay.removeAllViews();
                }

                Map<String, String> calloutParams = new HashMap<String, String>();
                calloutParams.put("state", stateSelectedStr);

                // AsyncTask to execute data retrieval 
                new RetrieveBallots().execute(calloutParams);
            }

        public void onNothingSelected(AdapterView<?> parent) {
            return;
        }
    });

    // Set default selection for spinner
    int defaultState = adapter.getPosition(userState);

    if (defaultState == -1)
    {
        defaultState = 0;
    }

    Log.d("TEST", "stateSpinner.setSelection");
    stateSpinner.setSelection(defaultState, false);
}

Answer 1

使用stateSpinner.setSelection(defaultState, false); 代替stateSpinner.setSelection(defaultState);

Answer 2

问题是onItemSelected() 回调被Android 框架调用了两次（可能是设计使然），第一次view 参数是null，第二次被实例化。

您无法阻止 2 个项目选择的发生，但您可以检查 view 变量是否为空，如果不是，则执行其余操作。

Answer 3

像这样在 ItemSelectedListener 中添加条件视图！=null：

spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
                    @Override
                    public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {

                        if(view!=null)
                        {//write your code
                        }
                    }

                    @Override
                    public void onNothingSelected(AdapterView<?> parent) {

                    }
                });