关于UIPickerView

• UIPickerView是一个选择器控件，它比UIDatePicker更加通用，它可以生成单列的选择器，也可生成多列的选择器，而且开发者完全可以自定义选择项的外观，因此用法非常灵活。UIPickerView直接继承了UIView，没有继承UIControl，因此，它不能像UIControl那样绑定事件处理方法，UIPickerView的事件处理由其委托对象完成。

• UIDatePicker控件只是负责该控件的通用行为，而该控件包含多少列，各列包含多少个列表项则由UIPickerViewDataSource对象负责。开发者必须为UIPickerView设置UIPickerViewDataSource对象，并实现如下两个方法。

  - numberOfComponentsInPickerView:

• 该UIPickerView将通过该方法来判断应该包含多少列。

  - pickerView:numberOfRowsInComponent:

• 该UIPickerView将通过该方法判断指定列应该包含多少个列表项。

• 如果程序需要控制UIPickerView中各列的宽度，以及各列中列表项的大小和外观，或程序需要为UIPickerView的选中事件提供响应，都需要为UIPickerView设置UIPickerViewDelegate委托对象，并根据需要实现该委托对象中的如下方法。

  - pickerView:rowHeightForComponent:

• 该方法返回的CGFloat值将作为该UIPickerView控件中指定列中列表项的高度。

  - pickerView:widthForComponent:

• 该方法返回的CGFloat值将作为该UIPickerView控件中指定列的宽度。

  - pickerView:titleForRow:forComponent:

• 该方法返回的NSString值将作为该UIPickerView控件中指定列的列表项的文本标题。

  - pickerView:viewForRow:forComponent:reusingView:

• 该方法返回的UIView控件将直接作为该UIPickerView控件中指定列的指定列表项。

  - pickerView:didSelectRow:inComponent:

• 当用户单击选中该UIPickerView控件的指定列的指定列表项时将会激发该方法。

－ example：此处为关联选择的选择器

#import "ViewController.h"

#define kStateComponent 0
#define kZipcomponent 1

@interface ViewController ()<UIPickerViewDataSource,UIPickerViewDelegate>

// firstArr为第一列的数据，secondArr为第二列的数据
@property (strong, nonatomic) UIPickerView *pick;
//dataDic用于存储两列的数据
@property (strong, nonatomic) NSDictionary *dataDic;
@property (nonatomic, strong) NSArray *firstArr;
@property (nonatomic,strong) NSArray *secondArr;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.view.backgroundColor = [UIColor blackColor];
    // 创建一个UIPickerView对象，并设置代理
    _pick = [[UIPickerView alloc] initWithFrame:CGRectMake(0, 210, 320, 80)];
    _pick.dataSource = self;
    _pick.delegate = self;
    _pick.backgroundColor = [UIColor whiteColor];
    [self.view addSubview:_pick];
    
    // 获取数据
    NSBundle *bundle = [NSBundle mainBundle];
    NSURL *plistURL = [bundle URLForResource:@"statedictionary" withExtension:@"plist"];
    
    self.dataDic = [NSDictionary dictionaryWithContentsOfURL:plistURL];
    NSArray *allData = [self.dataDic allKeys];
    NSArray *sortedStates = [allData sortedArrayUsingSelector:@selector(compare:)];
    self.firstArr = sortedStates;
    
    // 获取字典里的第一个元素的key对应的value（即一个数组）作为第二列的默认数据
    NSString *selectedState = self.firstArr[0];
    self.secondArr = self.dataDic[selectedState];
}

#pragma mark -Picker Datasource
// 列数
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView {
    return 2;
}

// 每列的个数
- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component {
    if (component == kStateComponent) {
        return [self.firstArr count];
    } else {
        return [self.secondArr count];
    }
}


#pragma mark - Picker Delegate
// 返回当前行的内容，确定每个轮子的每一项显示的内容
- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component {
    if (component == kStateComponent) {
        return self.firstArr[row];
    } else {
        return self.secondArr[row];
    }
}

// 监听轮子的滚动
- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component {
    if (component == kStateComponent) {
        NSString * selectedState = self.firstArr[row];
        self.secondArr = self.dataDic[selectedState];
       
       // !!! 更新第二个轮子的数据
        [_pick reloadComponent:kZipcomponent];
        [_pick selectRow:0 inComponent:kZipcomponent animated:YES];
    }
    
    // 获取UIPickerView的不同的component选中的那行
    NSInteger stateRow = [self.pick selectedRowInComponent:kStateComponent];
    NSInteger zipRow = [self.pick selectedRowInComponent:kZipcomponent];
    
    NSString *state = self.firstArr[stateRow];
    NSString *zip = self.secondArr[zipRow];
    
    NSString * title = [[NSString alloc] initWithFormat:@"You selected zip code %@",zip];
    NSString * message = [[NSString alloc] initWithFormat:@"%@ is in %@",zip,state];
    
    //官方解释：UIAlertView is deprecated in iOS 8. (Note that UIAlertViewDelegate is also deprecated.) To create and manage alerts in iOS 8 and later, instead use UIAlertController with a preferredStyle of UIAlertControllerStyleAlert. (主要是说UIAlertView在iOS 8中被废弃了，取而代之的是UIAlertController)

    UIAlertController *alert = [UIAlertController alertControllerWithTitle:title message:message preferredStyle:UIAlertControllerStyleAlert];
    
    UIAlertAction *action = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:nil];
    [alert addAction:action];
    
    [self presentViewController:alert animated:YES completion:nil];
}

// 每列的宽度
- (CGFloat)pickerView:(UIPickerView *)pickerView widthForComponent:(NSInteger)component {
    CGFloat pickerWidth = _pick.bounds.size.width;
    if (component == kZipcomponent) {
        return pickerWidth / 3;
    } else {
        return 2 * pickerWidth / 3;
    }
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end