4.1 在@implementation下一行添加代码：
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@synthesize provinceCities;
@synthesize provinces;
@synthesize cities;
4.2 在ViewDidLoad方法中添加代码：
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- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
NSBundle *bundle = [NSBundle mainBundle];
NSURL *plistURL = [bundle URLForResource:@"provinceCities" withExtension:@"plist"];
NSDictionary *dictionary = [NSDictionary dictionaryWithContentsOfURL:plistURL];
self.provinceCities = dictionary;
NSArray *components = [self.provinceCities allKeys];
NSArray *sorted = [components sortedArrayUsingSelector:@selector(compare:)];
self.provinces = sorted;
NSString *selectedState = [self.provinces objectAtIndex:0];
NSArray *array = [provinceCities objectForKey:selectedState];
self.cities = array;
}
代码中
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NSBundle *bundle = [NSBundle mainBundle];
用于获得当前程序的Main Bundle，这个Bundle可以看成是一个文件夹，其中的内容遵循特定的框架。Main Bundle的一种主要用途是使用程序中的资源，如图片、声音等，本例中使用的是plist文件。下面的一行
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NSURL *plistURL = [bundle URLForResource:@"provinceCities" withExtension:@"plist"];
用来获取provinceCities.plist的路径，之后将这个文件中的内容都放在一个NSDictionary对象中，用的是
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NSDictionary *dictionary = [NSDictionary dictionaryWithContentsOfURL:plistURL];
4.3 找到viewDidUnload方法，添加代码：
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- (void)viewDidUnload
{
[super viewDidUnload];
// Release any retained subviews of the main view.
// e.g. self.myOutlet = nil;
self.picker = nil;
self.provinceCities = nil;
self.provinces = nil;
self.cities = nil;
}
4.4 在@end之前添加代码，实现buttonPressed方法：
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- (IBAction)buttonPressed:(id)sender {
NSInteger provinceRow = [picker selectedRowInComponent:kProvinceComponent];
