我应用UIPickerView写了一个城市选择器，可是我发明在省份滚轮滚动的时刻如不雅同时再滚动城市滚轮会崩溃，代码如下：

@interface GLViewController ()<UIPickerViewDataSource, UIPickerViewDelegate>
@property (nonatomic, strong) NSArray *provinces;

@property (nonatomic, weak) UIPickerView *pickerView;

@end

@implementation GLViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
	
    // 1.创建pickerview
    // pickerview有默认的frame
    UIPickerView *pickerView = [[UIPickerView alloc] init];
    pickerView.dataSource = self;
    pickerView.delegate = self;
    [self.view addSubview:pickerView];
    self.pickerView = pickerView;
}

#pragma mark - UIPickerViewDataSource
// 告诉体系有若干列
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView
{
    return 2;
}
// 告诉体系有若干行
- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component
{
    if (0 == component) {
        // 省份列
        return self.provinces.count;
    }else
    {
        // 城市列
        // 获取第0列选中的行
        NSInteger selectIndex = [self.pickerView selectedRowInComponent:0];
        // 1.根据第0列选中的行数获取对应的省
        NJProvince *province = self.provinces[selectIndex];
        // 2.获取对应省份对应的城市
        NSArray *cities = province.cities;
        // 3.返回城市的个数r
        return cities.count;
    }
}
#pragma mark - UIPickerViewDelegate
// 告诉体系每一行显示什么内容
- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component
{
    NSLog(@"titleForRow");
    
    if (0 == component) {
         // 省份
        // 1.获取对应行对应的省份模型
        NJProvince *province = self.provinces[row];
        // 2.返回省份的名称
        return province.name;
    }else
    {
         // 城市列
        // 0.获取第0列选中的行数
        NSInteger selectIndex = [pickerView selectedRowInComponent:0];
        // 1.获取对应的省份
        NJProvince *province = self.provinces[selectIndex];
        // 2.获取对应的城市
        return province.cities[row];
    }
}

// 监听pickerView的选中
- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component
{
    // 断定是否修改了第0列(省份列)
    if (0 == component) {
        // 刷新第1列对应的数据
        [pickerView reloadComponent:1];
        // 让第1列滚动到第0行
        [pickerView selectRow:0 inComponent:1 animated:YES];
    }
}

#pragma mark - 懒加载
- (NSArray *)provinces
{
    if (_provinces == nil) {
        //        加载资本
        NSArray *array = [NSArray arrayWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"cities" ofType:@"plist"]];
        //        转换为模型
        NSMutableArray *models = [NSMutableArray arrayWithCapacity:array.count];
        for (NSDictionary *dict in array) {
            NJProvince *province = [NJProvince provinceWithDictionary:dict];
            [models addObject:province];
        }
        _provinces = [models copy];
    }
    return _provinces;
}
@end

这个崩溃现象是因为省份滚轮在滚动过程中[pickerView selectedRowInComponent]一向在变更，而代劳办法-(void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component只有在滚轮停止滚动式才会调用，所以在滚动城市取数据过程中出现的数组越界，于是我用了另一种写法：

@interface TCityPickerView ()<UIPickerViewDataSource, UIPickerViewDelegate>
@property (nonatomic, weak) UIPickerView *pickerView;
@property (nonatomic, strong) NSArray *citiesArray;
@property (nonatomic, assign) NSInteger provinceIndex;
@end

@implementation TCityPickerView

#pragma mark - public
+ (instancetype)cityPickerView {
    TCityPickerView *pickerView = [[TCityPickerView alloc] init];
    pickerView.backgroundColor = kColorBarBg;
    pickerView.show = NO;
    pickerView.frame = CGRectMake(0, kScreenHeight - 64, kScreenWidth, 244);
    [pickerView hiddenPickerView];
    return pickerView;
}


#pragma mark - lazy
- (NSArray *)citysArray {
    if (!_citiesArray) {
        NSString *filePath = [[NSBundle mainBundle] pathForResource:@"cities.plist" ofType:nil];
        _citiesArray = [NSArray arrayWithContentsOfFile:filePath];
    }
    return _citiesArray;
}    

#pragma mark - init
- (instancetype)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        
        self.provinceIndex = 0;
        [self initSubViews];
    }
    return self;
}

- (void)initSubViews {    
    UIPickerView *picker = [[UIPickerView alloc] init];
    picker.dataSource = self;
    picker.delegate = self;
    [self addSubview:picker];
    self.pickerView = picker;
    }

#pragma mark - UIPickerViewDataSource
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView {
    return 2;
}

- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component {
    if (component == 0) {
        return self.citysArray.count;
    } else {
        return [self.citysArray[self.provinceIndex][@"cities"] count];
    }
}

#pragma mark - UIPickerViewDelegate
- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component {
    NSString *title = nil;
    if (component == 0) {
        title = self.citysArray[row][@"name"];
    } else {
        title = self.citysArray[self.provinceIndex][@"cities"][row];
    }
    return title;
}

- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component {

    if (component == 0) {
        self.provinceIndex = [pickerView selectedRowInComponent:0];
        [self.pickerView reloadComponent:1];
        [self.pickerView selectRow:0 inComponent:1 animated:YES];
    }

}

@end