eg:数组的去重，如何不引用新的对象去重数组
NSArray*array=@[@"1234",@"23",@"abc",@"a",@"abc",@"1234"];
array = [array valueForKeyPath:@"@distinctUnionOfObjects.self"];
NSLog(@"%@", array);
eg 数组的符号操作
NSArray *array = @[@"10",@"5",@"6"];
NSNumber *sum = [array valueForKeyPath:@"@sum.floatValue"];
NSNumber *avg = [array valueForKeyPath:@"@avg.floatValue"];
NSNumber *max = [array valueForKeyPath:@"@max.floatValue"];
NSNumber *min = [array valueForKeyPath:@"@min.floatValue"];
eg.数组字符全部大写
NSArray *array = @[@"my", @"Mine", @"name", @"Number"];
NSLog(@"%@", [array valueForKeyPath:@"uppercaseString"]);
eg:改变 UITextfield 的 placeholder 的颜色
[textField setValue:[UIColor whiteColor] forKeyPath:@”_placeholderLabel.textColor”];
更多字符串操作可以自己用到时再查询
/* The following three return the canonical (non-localized) mappings. They are suitable for programming operations that require stable results not depending on the user's locale preference. For locale-aware case mapping for strings presented to users, use the "localized" methods below.
*/
@property (readonly, copy) NSString *uppercaseString;
@property (readonly, copy) NSString *lowercaseString;
@property (readonly, copy) NSString *capitalizedString;
