实现不选择选项则不可以关闭的对话框之步骤二

本方法在打开对话框后，如果没有选中项则禁用Positive Button，选中了选项后则使Positive Button可用.

import android.app.Activity;
import android.app.AlertDialog;
import android.app.Dialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends Activity {
	private Button btnOpenDialog;
	private String[] books = null;
	private Activity activity;
	private int selectedBookIndex = -1;
	private AlertDialog alertDialog;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		initBooks();
		setContentView(R.layout.activity_main);
		activity = this;
		btnOpenDialog = (Button) findViewById(R.id.btnOpenDialog);
		btnOpenDialog.setOnClickListener(new OnClickListener() {

			@Override
			public void onClick(View v) {
				alertDialog = new AlertDialog.Builder(activity).setTitle(R.string.hello_world).setSingleChoiceItems(books, selectedBookIndex, new DialogInterface.OnClickListener() {

					@Override
					public void onClick(DialogInterface dialog, int which) {
						selectedBookIndex = which;
						allowCloseDialogByTapPositiveButton();
					}

				}).setNegativeButton("取消", new DialogInterface.OnClickListener() {

					@Override
					public void onClick(DialogInterface dialog, int which) {
						selectedBookIndex = -1;
						// distoryDialog(dialog);
						return;
						
					}
				}).setPositiveButton("确定", new DialogInterface.OnClickListener() {

					@Override
					public void onClick(DialogInterface dialog, int which) {
						if(selectedBookIndex == -1){
							alertDialog.getButton(Dialog.BUTTON_POSITIVE).setEnabled(false);
							Toast.makeText(activity, "对不起，你不买书，不能走购物通道啊！", Toast.LENGTH_LONG).show();
							// keepDialog(dialog);
							return;
						}else{
							Toast.makeText(activity, "您选择了图书：" + books[selectedBookIndex] + ".", Toast.LENGTH_LONG).show();
						}
					}
				}).show();
				alertDialog.setCancelable(false);
				alertDialog.setCanceledOnTouchOutside(false);
				allowCloseDialogByTapPositiveButton();
				
			}
		});
	}
	
	private void allowCloseDialogByTapPositiveButton() {
        if(selectedBookIndex > -1) {
			alertDialog.getButton(Dialog.BUTTON_POSITIVE).setEnabled(true);
		}else{
			alertDialog.getButton(Dialog.BUTTON_POSITIVE).setEnabled(false);
		}
    }

	void initBooks() {
		books = new String[] { "飞狐外传", "雪山飞狐", "连城诀", "天龙八部", "射雕英雄传", "白马啸西风", "鹿鼎记", "笑傲江湖", "书剑恩仇录", "神雕侠侣", "侠客行", "倚天屠龙记", "碧血剑", "鸳鸯刀", "越女剑" };
	}
}