如何制作带有初始文本“选择一个"的Android微调器? [英] How to make an Android Spinner with initial text "Select One"?

问题描述

我想使用一个微调器，该微调器最初(当用户尚未进行选择时)显示文本选择一个".当用户单击微调器时，将显示项目列表，并且用户选择选项之一.用户做出选择后，所选项目将显示在微调器中，而不是选择一个".

I want to use a Spinner that initially (when the user has not made a selection yet) displays the text "Select One". When the user clicks the spinner, the list of items is displayed and the user selects one of the options. After the user has made a selection, the selected item is displayed in the Spinner instead of "Select One".

我具有以下代码来创建Spinner:

I have the following code to create a Spinner:

String[] items = new String[] {"One", "Two", "Three"};
Spinner spinner = (Spinner) findViewById(R.id.mySpinner);
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_spinner_item, items);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adapter);

使用此代码，最初显示项目一个".我可以在项目中添加一个新项目选择一个"，但是选择一个"也将作为第一项显示在下拉列表中.

With this code, initially the item "One" is displayed. I could just add a new item "Select One" to the items, but then "Select One" would also be displayed in the dropdown list as first item, which is not what I want.

如何解决此问题?

推荐答案

以下是覆盖Spinner视图的常规解决方案.它会覆盖setAdapter()以将初始位置设置为-1，并代理提供的SpinnerAdapter以显示位置小于0的提示字符串.

Here's a general solution that overrides the Spinner view. It overrides setAdapter() to set the initial position to -1, and proxies the supplied SpinnerAdapter to display the prompt string for position less than 0.

此功能已在Android 1.5到4.2上进行了测试，但请注意！由于此解决方案依赖于反射来调用专用的AdapterView.setNextSelectedPositionInt()和AdapterView.setSelectedPositionInt()，因此不能保证在将来的OS更新中也可以使用该解决方案.似乎有可能，但绝不能保证.

This has been tested on Android 1.5 through 4.2, but buyer beware! Because this solution relies on reflection to call the private AdapterView.setNextSelectedPositionInt() and AdapterView.setSelectedPositionInt(), it's not guaranteed to work in future OS updates. It seems likely that it will, but it is by no means guaranteed.

通常我不会容忍这样的事情，但是这个问题已经被问了足够多次了，而且似乎是一个足够合理的请求，我以为我会发布我的解决方案.

Normally I wouldn't condone something like this, but this question has been asked enough times and it seems like a reasonable enough request that I thought I would post my solution.

/**
 * A modified Spinner that doesn't automatically select the first entry in the list.
 *
 * Shows the prompt if nothing is selected.
 *
 * Limitations: does not display prompt if the entry list is empty.
 */
public class NoDefaultSpinner extends Spinner {

    public NoDefaultSpinner(Context context) {
        super(context);
    }

    public NoDefaultSpinner(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public NoDefaultSpinner(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    public void setAdapter(SpinnerAdapter orig ) {
        final SpinnerAdapter adapter = newProxy(orig);

        super.setAdapter(adapter);

        try {
            final Method m = AdapterView.class.getDeclaredMethod(
                               "setNextSelectedPositionInt",int.class);
            m.setAccessible(true);
            m.invoke(this,-1);

            final Method n = AdapterView.class.getDeclaredMethod(
                               "setSelectedPositionInt",int.class);
            n.setAccessible(true);
            n.invoke(this,-1);
        } 
        catch( Exception e ) {
            throw new RuntimeException(e);
        }
    }

    protected SpinnerAdapter newProxy(SpinnerAdapter obj) {
        return (SpinnerAdapter) java.lang.reflect.Proxy.newProxyInstance(
                obj.getClass().getClassLoader(),
                new Class[]{SpinnerAdapter.class},
                new SpinnerAdapterProxy(obj));
    }



    /**
     * Intercepts getView() to display the prompt if position < 0
     */
    protected class SpinnerAdapterProxy implements InvocationHandler {

        protected SpinnerAdapter obj;
        protected Method getView;


        protected SpinnerAdapterProxy(SpinnerAdapter obj) {
            this.obj = obj;
            try {
                this.getView = SpinnerAdapter.class.getMethod(
                                 "getView",int.class,View.class,ViewGroup.class);
            } 
            catch( Exception e ) {
                throw new RuntimeException(e);
            }
        }

        public Object invoke(Object proxy, Method m, Object[] args) throws Throwable {
            try {
                return m.equals(getView) && 
                       (Integer)(args[0])<0 ? 
                         getView((Integer)args[0],(View)args[1],(ViewGroup)args[2]) : 
                         m.invoke(obj, args);
            } 
            catch (InvocationTargetException e) {
                throw e.getTargetException();
            } 
            catch (Exception e) {
                throw new RuntimeException(e);
            }
        }

        protected View getView(int position, View convertView, ViewGroup parent) 
          throws IllegalAccessException {

            if( position<0 ) {
                final TextView v = 
                  (TextView) ((LayoutInflater)getContext().getSystemService(
                    Context.LAYOUT_INFLATER_SERVICE)).inflate(
                      android.R.layout.simple_spinner_item,parent,false);
                v.setText(getPrompt());
                return v;
            }
            return obj.getView(position,convertView,parent);
        }
    }
}

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