iPhone界面如何实现下拉列表

代码如下：
    #import <UIKit/UIKit.h>
    @interface DropDownList : UIView<UITableViewDelegate,UITableViewDataSource> {
    UITextField* textField;   //文本输入框
    NSArray* list;            //下拉列表数据
    BOOL showList;            //是否弹出下拉列表
    UITableView* listView;    //下拉列表
    CGRect oldFrame,newFrame;   //整个控件（包括下拉前和下拉后）的矩形
    UIColor *lineColor,*listBgColor;//下拉框的边框色、背景色
    CGFloat lineWidth;               //下拉框边框粗细
    UITextBorderStyle borderStyle;   //文本框边框style
    }
    @property (nonatomic,retain)UITextField *textField;
    @property(nonatomic,retain)NSArray* list;
    @property (nonatomic,retain)UITableView* listView;
    @property (nonatomic,retain)UIColor *lineColor,*listBgColor;
    @property (nonatomic,assign)UITextBorderStyle borderStyle;
    -(void)drawView;
    -(void)setShowList:(BOOL)b;
    @end
    #import "DropDownList.h"
    @implementationDropDownList
    @synthesize textField,list,listView,lineColor,listBgColor,borderStyle;
    - (id)initWithFrame:(CGRect)frame {
    
    if(self=[super initWithFrame:frame]){
    //默认的下拉列表中的数据
    list=[[NSArray alloc]initWithObjects:@"1",@"2",@"3",@"4",nil];
    
    borderStyle=UITextBorderStyleRoundedRect;
    
    showList=NO;//默认不显示下拉框
    oldFrame=frame; //未下拉时控件初始大小
    //当下拉框显示时，计算出控件的大小。
    newFrame=CGRectMake(frame.origin.x, frame.origin.y, frame.size.width, frame.size.height*5);
    
    lineColor=[UIColor lightGrayColor];//默认列表边框线为灰色
    listBgColor=[UIColor whiteColor];//默认列表框背景色为白色
    lineWidth=1;    //默认列表边框粗细为1
    
    //把背景色设置为透明色，否则会有一个黑色的边
    self.backgroundColor=[UIColor clearColor];
    [self drawView];//调用方法，绘制控件
    
    }
    returnself;
    }
    -(void)drawView{
    //文本框
    textField=[[UITextField alloc]
      initWithFrame:CGRectMake(0,0,
    oldFrame.size.width, 
    oldFrame.size.height)];
    textField.borderStyle=borderStyle;//设置文本框的边框风格
    [self addSubview:textField];
        [textField addTarget:self action:@selector(dropdown) forControlEvents:UIControlEventAllTouchEvents]; 
    
    //下拉列表
    listView=[[UITableView alloc]initWithFrame:
      CGRectMake(lineWidth,oldFrame.size.height+lineWidth, 
    oldFrame.size.width-lineWidth*2,
    oldFrame.size.height*4-lineWidth*2)];
    listView.dataSource=self;
    listView.delegate=self;
    listView.backgroundColor=listBgColor;
    listView.separatorColor=lineColor;
    listView.hidden=!showList;//一开始listView是隐藏的，此后根据showList的值显示或隐藏
    
    [self addSubview:listView]; 
    [listView release];
    }
    -(void)dropdown{
    [textField resignFirstResponder];
    if(showList) {//如果下拉框已显示，什么都不做
    return;
    }else{//如果下拉框尚未显示，则进行显示
    //把dropdownList放到前面，防止下拉框被别的控件遮住
    
    [self.superview bringSubviewToFront:self];
    [self setShowList:YES];//显示下拉框
    }
    }
    #pragma mark listViewdataSource method and delegate method
    -(NSInteger)tableView:(UITableView *)table numberOfRowsInSection:(NSInteger)section{
    return list.count;
    }
    -(UITableViewCell*)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
    static NSString *cellid=@"listviewid";
    UITableViewCell* cell=[tableView dequeueReusableCellWithIdentifier:cellid];
    if(cell==nil){
    cell=[[[UITableViewCell alloc]initWithStyle:UITableViewCellStyleDefault
      reuseIdentifier:cellid]autorelease];
    }
    //文本标签
    cell.textLabel.text=(NSString*)[list objectAtIndex:indexPath.row];
    cell.textLabel.font=textField.font;
    cell.selectionStyle=UITableViewCellSelectionStyleGray;
    return cell;
    }
    -(CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath{
    return oldFrame.size.height;
    }
    //当选择下拉列表中的一行时，设置文本框中的值，隐藏下拉列表
    -(void)tableView:(UITableView *)tableViewdidSelectRowAtIndexPath:(NSIndexPath *)indexPath{
    //NSLog(@"select");
    textField.text=(NSString*)[list objectAtIndex:indexPath.row];
    //NSLog(@"textField.text=%@",textField.text);
    [self setShowList:NO];
    }
    -(BOOL)showList{//setShowList:No为隐藏，setShowList:Yes为显示
    return showList;
    }
    -(void)setShowList:(BOOL)b{
    showList=b;
    NSLog(@"showlist is set ");
    if(showList){
    self.frame=newFrame;
    }else {
    self.frame=oldFrame;
    }
    listView.hidden=!b;
    }
    /*
    // Only override drawRect: if you perform custom drawing.
    // An empty implementation adversely affects performance during animation.
    - (void)drawRect:(CGRect)rect {
        // Drawing code.
    }
    */
    - (void)dealloc {
        [super dealloc];
    }
    @end