iOS 用keypath设置一些属性

button.contentHorizontalAlignment = UIControlContentHorizontalAlignmentLeft;

textField.placeholder = @"this is placeholder";  
[textFieldsetValue:[UIColor redColor]forKeyPath:@"_placeholderLabel.textColor"];  
[textFieldsetValue:[UIFontboldSystemFontOfSize:16]forKeyPath:@"_placeholderLabel.font"]; 

NSMutableAttributedString *placeholder = [[NSMutableAttributedString alloc]initWithString:@"this is placeholder"];

[placeholderaddAttribute:NSForegroundColorAttributeName
                  value:[UIColor redColor]
                  range:NSMakeRange(0, placeholder.length)];

[placeholderaddAttribute:NSFontAttributeName
                  value:[UIFontboldSystemFontOfSize:14]
                  range:NSMakeRange(0, placeholder.length)];


textField.attributedPlaceholder = placeholder;

[self.tabBar setValue:@(YES) forKeyPath:@"_hidesShadow"];