1.setValue: forKey
key只能是一个键,不能拼接
示例：
[self setValue:"AA" forKey:@"color"];
2.setValue: forKeyPath
keyPath可以后面加.来拼接键
示例：
[self setValue:[UIColor grayColor] forKeyPath:@"placeholderLabel.textColor"];
错误信息：
setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key placeholderLabel.textColor.'
