1.选择器(UIPickerView)

UIPickerView是一个选择器，可以生成单列的选择器，也可生成多列的选择器，可以自定义选择器的外观。UIPickerView直接继承了UIView，没有继承UIControl，UIPickerView的事件由委托对象完成。

UIPickerVIew常用的属性和方法如下：

1. numberOfComponents 只读，获取指定列包含的列表项的数量。
2. showSelectionIndicator 控制是否显示UIPickerView中的选中标记(高亮背景)
3. -numberOfRowsInComponent: 获取包含的列数量
4. -rorSizeForComponent: 获取包含的指定列中列表项的大小。CGSize
5. -selectRow:inComponent:animated: 设置选中指定列的特定列表项。
6. -selectRowInComponent: 返回控件指定列中被选中的列表项
7. viewForRow:forComponent: 返回指定列的列表项所使用的UiView控件

UIPickerView包含的列及列包含的列表项由UIPickerViewDataSource对象设置，需要实现该对象的两个方法：

• -numberOfComponentsInPickerView: 该UIPickerView将通过该方法来判断包含多少列
• -pickerView:numberOfRowsInComponent: 通过该方法判断指定列包含多少列表项

控制UIPickerView中的列的宽度、列表项的大小和外观，以及控件的事件响应，需要设置UIPickerViewDeletegate委托对象，根据需要，实现委托对象的如下方法：

• -pickerView:rowHeightForComponent:该方法返回的CGFloat将作为该UIPickerView控件中指定列表项的高度
• -pickerView:widthForComponent: 返回的CGFloat作为指定列表项的宽度
• - pickerView:titleForRow:forComponent NSString 作为指定列表项的文本标题
• -pickerView:viewForRow:forComponent:reusingView:返回的UIView控件直接作为UIPickerView指定列的指定列表项
• -pickerView:didSelectRow:inComponent: 选中指定列的指定列表项是触发

案例：单列选择器/多列选择器/级联选择器

定义三个UIPickerView分别用作单列、多列、级联选择器，并关联IBOutlet，控制器分别实现UIPickerViewDataSource,UIPickerViewDelegate

@property (weak, nonatomic) IBOutlet UIPickerView *picker;@property (weak, nonatomic) IBOutlet UIPickerView *multiPicker;@property (weak, nonatomic) IBOutlet UIPickerView *relPicker;

在控制器实现类中实现协议的方法：

NSArray* province;NSArray* drink;NSArray* size;NSDictionary* city;NSArray* prov;NSString* selectedProv ;-(void) viewDidLoad{    [super viewDidLoad];    //初始化NSArray对象    province = [NSArray arrayWithObjects:@"广东",@"湖北",@"北京",@"四川",@"浙江",nil];        size = [NSArray arrayWithObjects:@"大杯",@"中杯",@"小杯", nil];    drink = [NSArray arrayWithObjects:@"红茶",@"绿茶",@"咖啡",@"拿铁",@"芒果冰沙", nil];        city = [NSDictionary dictionaryWithObjectsAndKeys:            [NSArray arrayWithObjects:@"朝阳区",@"海淀区",@"大兴区",@"房山区", nil],@"北京",            [NSArray arrayWithObjects:@"广州",@"深圳",@"珠海", nil],@"广东",            [NSArray arrayWithObjects:@"武汉",@"荆州",@"宜昌",@"荆门",@"黄冈", nil],@"湖北",nil];        prov = [[city allKeys] sortedArrayUsingSelector:@selector(compare:)];    //为UIPickerView设置dataSrouce和delegate    self.picker.dataSource = self;    self.picker.delegate = self;        self.multiPicker.dataSource = self;    self.multiPicker.delegate = self;        selectedProv = [prov objectAtIndex:0];    self.relPicker.dataSource = self;    self.relPicker.delegate = self;}//UIPickerViewDataSource中定义的方法，该方法返回值决定控件包含多少列-(NSInteger) numberOfComponentsInPickerView:(UIPickerView *)pickerView{    if(pickerView == self.multiPicker)    {        return 2;    }    else if(pickerView == self.relPicker)    {        return 2;    }    return 1;//返回1 表明只包含1列}//UIPickerViewDataSrouce定义，该方法返回值决定控件指定列包含多少列表项-(NSInteger) pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component{    if(pickerView == self.multiPicker)    {        if(component == 0)//如果是第一列        {            return size.count;        }        return drink.count; //其它列    }    else if(pickerView == self.relPicker)    {        if(component == 0)        {            return prov.count;        }        NSInteger c = [[city objectForKey:selectedProv] count];        return c;    }    return province.count;}//UiPickerViewDelegate定义方法返回值作为指定列的标题文本-(NSString*) pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component{    if(pickerView == self.multiPicker)    {        if(component == 0)        {            return  [size objectAtIndex:row];        }        return [drink objectAtIndex:row];    }    else if(pickerView == self.relPicker)    {        if(component ==0)        {            return [prov objectAtIndex:row];        }        return [[city objectForKey:selectedProv] objectAtIndex:row];    }    return [province objectAtIndex:row];}//选中指定列和列表项时触发-(void) pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component{    NSString* msg ;    if(pickerView == self.picker)    {        msg =[NSString stringWithFormat:@"选中的省份是%@",[province objectAtIndex:row]];    }    else if(pickerView == self.multiPicker)    {        NSArray* tmp = component == 0? size : drink;        msg =[NSString stringWithFormat:@"选中的是%@",[tmp objectAtIndex:row]];    }    else if(pickerView == self.relPicker)    {        if(component ==0)        {        selectedProv = [prov objectAtIndex:row];        //控制重写加载第二个列表        [self.relPicker reloadComponent:1];        }        NSArray* tmp = component ==0?prov:[city objectForKey:selectedProv];        msg = [NSString stringWithFormat:@"选中的是%@",[tmp objectAtIndex:row]];    }    UIAlertView* alert = [[UIAlertView alloc]                          initWithTitle:@"提示"                          message:msg                          delegate:nil cancelButtonTitle:@"确定" otherButtonTitles: nil];    [alert show];}-(CGFloat)pickerView:(UIPickerView *)pickerView widthForComponent:(NSInteger)component{    if(pickerView == self.multiPicker || pickerView == self.relPicker)    {        if(component==0)        {            return 90;        }        return 210;    }    return 390;}