1、将数组中的单词做大小写转换：

  NSArray *arr = @[@"Hello",@"Word",@"Objective",@"c"];
  NSArray *arrUpper = [arr valueForKeyPath:@"uppercaseString"];
  NSLog(@"%@",arrUpper);
        
 NSArray *arrLower = [arr valueForKeyPath:@"lowercaseString"];
 NSLog(@"%@",arrLower);

运行结果:

2019-04-20 15:49:06.968723+0800 Demo[4177:143487] (
HELLO,
WORD,
OBJECTIVE,
C
)
2019-04-20 15:49:06.969021+0800 Demo[4177:143487] (
hello,
word,
objective,
c
)

2、获取数组中所有单词的长度：

NSArray *arrLength = [arr valueForKeyPath:@"length"];
NSLog(@"%@",arrLength);

运行结果：

2019-04-20 15:49:06.969107+0800 Demo[4177:143487] (
5,
4,
9,
1
)

3、计算数组中所有对象某个属性的平均值：

NSArray *arrRate = @[@{@"orderId":@"0001",@"overtimeRate":@"0.6"},
            @{@"orderId":@"0002",@"overtimeRate":@"0.2"},
            @{@"orderId":@"0003",@"overtimeRate":@"0.0"},
            @{@"orderId":@"0004",@"overtimeRate":@"0.5"}];
        
NSNumber *avg = [arrRate valueForKeyPath:@"@avg.overtimeRate.floatValue"];
NSLog(@"avg = %.2f %%",[avg floatValue] *100);

运行结果：

2019-04-20 15:49:09.644375+0800 Demo[4177:143487] avg = 32.50 %

备注：

和计算key：@"@sum.floatValue"
取最大值key：@"@max.floatValue"
取最小值key：@"@min.floatValue"

4、设置UITextField 的placeholder 的文本颜色：

[passwordTextField setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];

placeholderLabel.png

5、获取数组对象某个key的所有内容

NSArray *arrRate = @[@{@"orderId":@"0001",@"overtimeRate":@"0.6"},
            @{@"orderId":@"0002",@"overtimeRate":@"0.2"},
            @{@"orderId":@"0003",@"overtimeRate":@"0.0"},
            @{@"orderId":@"0004",@"overtimeRate":@"0.5"}];
 NSArray *arrRates = [arrRate valueForKeyPath:@"overtimeRate"];
 NSLog(@"%@",arrRates);

运行结果：

2019-04-20 18:08:26.595803+0800 Demo[8980:223529] (
"0.6",
"0.2",
"0.0",
"0.5"
)