java - 具有空默认选定项的微调器

Question

我正在尝试创建一个带有默认空选择项的微调器，但它显示了微调器选项中的第一个项目。如果我将空值添加到我的字符串中，这是微调器中选择的来源，那么在打开微调器之后会显示该空行。我应该怎么做？这是我正在使用的代码:

  String[] ch = {"Session1", "Session2", "Session3"};
  Spinner sp = (Spinner)findViewById(R.id.spinner1);
  TextView sess_name = findViewById(R.id.sessname);
  ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item,ch);
  sp.setAdapter(adapter);

  adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

  sp.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener({
      @Override
      public void onItemSelected(AdapterView<?> arg0, View arg1, int arg2, long arg3) {
          int index = arg0.getSelectedItemPosition();
          sess_name.setText(ch[index]);

          Toast.makeText(getBaseContext(), "You have selected item : " + ch[index], Toast.LENGTH_SHORT).show();
      }

Answer 1

Barak 的解决方案有问题。选择第一项时，Spinner 不会调用 OnItemSelectedListener 的 onItemSelected() 刷新空内容，因为前一个位置和选择位置都是 0。

首先在字符串数组的开头放置一个空字符串:

String[] test = {" ", "one", "two", "three"};

第二次构建适配器，不修改getView()，修改getDropDownView()。将空 View 的高度设置为 1px。

public class MyArrayAdapter extends ArrayAdapter<String> {

    private static final int ITEM_HEIGHT = ViewGroup.LayoutParams.WRAP_CONTENT;

    private int textViewResourceId;


    public MyArrayAdapter(Context context,
                          int textViewResourceId,
                          String[] objects) {
        super(context, textViewResourceId, objects);
        this.textViewResourceId = textViewResourceId;
    }

    @Override
    public View getDropDownView(int position, View convertView, @NonNull ViewGroup parent) {
        TextView textView;

        if (convertView == null) {
            textView = (TextView) LayoutInflater.from(getContext())
                   .inflate(textViewResourceId, parent, false);
        } else {
            textView = (TextView) convertView;
        }

        textView.setText(getItem(position));
        if (position == 0) {
            ViewGroup.LayoutParams layoutParams = textView.getLayoutParams();
            layoutParams.height = 1;
            textView.setLayoutParams(layoutParams);
        } else {
            ViewGroup.LayoutParams layoutParams = textView.getLayoutParams();
            layoutParams.height = ITEM_HEIGHT;
            textView.setLayoutParams(layoutParams);
        }

        return textView;
    }
}