GridView中的ItemSelected事件无法触发的疑点

GridView中的ItemSelected事件无法触发的疑问
实在想不通，为什么Item被选中后，TextView中没有任何反应？


这是java代码部分：
package com.example.t1209_gridview;

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.ArrayAdapter;
import android.widget.GridView;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends ActionBarActivity implements OnItemSelectedListener{

/*
 * 定义控件
 */
    private TextView selection;
    private GridView grid;
    String[] items ={"one","two","three","four","five","six","seven","eight","ten",
     "eleven","twelve","thirteen","fourteen","fifteen","sixteen","seventeen",
     "eighteen","twenty","one","two","three","four","five","six","seven","eight","ten",
     "eleven","twelve","thirteen","fourteen","fifteen","sixteen","seventeen",
     "eighteen","twenty"};
@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        /*
         * 初始化控件
         */
        selection = (TextView) findViewById(R.id.selection);
        grid = (GridView) findViewById(R.id.grid);
        /*
         * 创建适配器
         */
        ArrayAdapter<String> aa = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1, items);
        /*
         * 设置适配器和监听事件
         */
        grid.setAdapter(aa);
        grid.setOnItemSelectedListener(this);
    }

@Override
public void onItemSelected(AdapterView<?> parent, View view, int position,
long id) { 
selection.setText(items[position]);
Toast.makeText(this, items[position], Toast.LENGTH_SHORT).show();
}


@Override
public void onNothingSelected(AdapterView<?> parent) {
selection.setText("");

}
}
这是XML布局部分：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.t1209_gridview.MainActivity" >
    <TextView
        android:id="@+id/selection"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Text"/>

    <GridView
        android:id="@+id/grid"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:numColumns="auto_fit"
        android:horizontalSpacing="20px"
        android:verticalSpacing="5px"
        android:stretchMode="columnWidth" 
        android:gravity="center">
    </GridView>   

</LinearLayout>


------解决思路----------------------

//让Activity实现OnItemClickListener而不是 OnItemSelectedListener

//		grid.setOnItemSelectedListener(this);

		grid.setOnItemClickListener(this);

	@Override

	public void onItemClick(AdapterView<?> parent, View view, int position,

			long id) {

		selection.setText(items[position]);

	}

------解决思路----------------------