Runtime实现设置placeholder的文本颜色

1.基础办法:(在扩展中使用KVC获取属性的值)(先设置文本颜色，后设置placeholder, placeholder颜色无效)

NS_ASSUME_NONNULL_BEGIN

@interface UITextField (Placeholder)
@property UIColor *placeholderColor;
@end

NS_ASSUME_NONNULL_END


#import "UITextField+Placeholder.h"
#import <objc/message.h>
// 设置占位文字颜色
//_textField.placeholderColor = [UIColor greenColor];
// 设置占位文字:每次设置占位文字的后,在拿到之前保存占位文字颜色,重新设置
// [_textField setHk_Placeholder:@"123"];

@implementation UITextField (Placeholder)
 - (void)setPlaceholderColor:(UIColor *)placeholderColor
 {
 // 设置占位文字颜色
 UILabel *placeholderLabel = [self valueForKey:@"placeholderLabel"];
 placeholderLabel.textColor = placeholderColor;
 }
 
 - (UIColor *)placeholderColor
 {
 return nil;
 }
@end
// 基础方法 
//设置占位文字:每次设置占位文字的后,在拿到之前保存占位文字颜色,重新设置
// 分析:为什么先设置占位文字颜色,就没有效果 => 占位文字label拿不到

2.使用Runtime实现

NS_ASSUME_NONNULL_BEGIN

@interface UITextField (Placeholder)
@property UIColor *placeholderColor;
@end

NS_ASSUME_NONNULL_END


#import "UITextField+Placeholder.h"
#import <objc/message.h>
// 设置占位文字颜色
//_textField.placeholderColor = [UIColor greenColor];
// 设置占位文字:每次设置占位文字的后,在拿到之前保存占位文字颜色,重新设置
// [_textField setHk_Placeholder:@"123"];

@implementation UITextField (Placeholder)
+ (void)load
{
    // setPlaceholder
    Method setPlaceholderMethod = class_getInstanceMethod(self, @selector(setPlaceholder:));
    Method setHk_PlaceholderMethod = class_getInstanceMethod(self, @selector(setHk_Placeholder:));
    method_exchangeImplementations(setPlaceholderMethod, setHk_PlaceholderMethod);
}
- (void)setPlaceholderColor:(UIColor *)placeholderColor
{
    
    // 给成员属性赋值 runtime给系统的类添加成员属性
    // 添加成员属性
    objc_setAssociatedObject(self, @"placeholderColor", placeholderColor, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    // 获取占位文字label控件
    UILabel *placeholderLabel = [self valueForKey:@"placeholderLabel"];
    
    // 设置占位文字颜色
    placeholderLabel.textColor = placeholderColor;
}

- (UIColor *)placeholderColor
{
    return objc_getAssociatedObject(self, @"placeholderColor");
}

// 设置占位文字
// 设置占位文字颜色
- (void)setHk_Placeholder:(NSString *)placeholder
{
    [self setHk_Placeholder:placeholder];
    
    self.placeholderColor = self.placeholderColor;
}

@end