valueForKeyPath

- (id)valueForKeyPath:(NSString *)keyPath
这个方法非常的强大，举个例子:

NSArray *array = @[@"name", @"age", @"height", @"weight"];
NSLog(@"%@", [array valueForKeyPath:@"uppercaseString"]);

打印

(
NAME,
AGE,
HEIGHT,
WEIGHT
)

相当于数组中的每个成员执行了uppercaseString方法，然后把返回的对象组成一个新数组返回。既然可以用uppercaseString方法，那么NSString的其他方法也可以.

对NSNumber数组快速计算数组求和、平均数、最大值、最小值

NSArray *array = @[@1, @2, @3, @4, @10];  
NSNumber *sum = [array valueForKeyPath:@"@sum.self"];
NSNumber *avg = [array valueForKeyPath:@"@avg.self"];
NSNumber *max = [array valueForKeyPath:@"@max.self"];
NSNumber *min = [array valueForKeyPath:@"@min.self"];

或者指定输出类型

 NSNumber *sum = [array valueForKeyPath:@"@sum.floatValue"];
 NSNumber *avg = [array valueForKeyPath:@"@avg.floatValue"];
 NSNumber *max = [array valueForKeyPath:@"@max.floatValue"];
 NSNumber *min = [array valueForKeyPath:@"@min.floatValue"];

剔除重复数据

 NSArray *array = @[@"name", @"w", @"aa", @"jimsa", @"aa"];
 NSLog(@"%@", [array valueForKeyPath:@"@distinctUnionOfObjects.self"]);

注意，这种方法得到的数组是无序的。

对NSDictionary数组快速找出相应key对的值

NSArray *array = @[
 @{@"name": @"cookeee",@"code": @1},
 @{@"name": @"jim",@"code": @2},
 @{@"name": @"jim",@"code": @1},
 @{@"name": @"jbos",@"code": @1}];
NSLog(@"%@", [array valueForKeyPath:@"name"]);

直接得到字典中namekey对应的值组成的数组，显然比循环取值再加入到新数组中方便快捷

(
cookeee,
jim,
jim,
jbos
)

同样可以嵌套使用，先剔除name对应值的重复数据再取值

NSArray *array = @[
 @{@"name": @"cookeee",@"code": @1},
 @{@"name": @"jim",@"code": @2},
 @{@"name": @"jim",@"code": @1},
 @{@"name": @"jbos",@"code": @1}];

 NSLog(@"%@", [array valueForKeyPath:@"@distinctUnionOfObjects.name"]);

打印

(
cookeee,
jim,
jbos
)

改变UITextfiedl的placeholder的颜色

[searchField setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];

比起重写- (void)drawPlaceholderInRect:(CGRect)rect;要方便很多