一丶原理

修改placeholder的方法:

1.

myTextField.placeholder = @"请输入用户名!";  
[myTextField setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];  
[myTextField setValue:[UIFont boldSystemFontOfSize:16] forKeyPath:@"_placeholderLabel.font"];

2.建议使用:

NSMutableAttributedString *placeholderAtbString = [[NSMutableAttributedString alloc] initWithString:placeholder];
[placeholderAtbString addAttribute:NSForegroundColorAttributeName
                    value:kMianColor_8997B0
                    range:NSMakeRange(0, placeholder.length)];
self.attributedPlaceholder = placeholderAtbString;

修改全局:

runtime替换系统placeholder方法;

二丶代码

#import "UITextField+HYBPlaceHolder.h"

@implementation UITextField (HYBPlaceHolder)

+ (void)load{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        
        Class selfClass = [self class];
        
        SEL oriSEL = @selector(setPlaceholder:);
        Method oriMethod = class_getInstanceMethod(selfClass, oriSEL);
        
        SEL cusSEL = @selector(ZB_setPlaceholder:);
        Method cusMethod = class_getInstanceMethod(selfClass, cusSEL);
        
        BOOL addSucc = class_addMethod(selfClass, oriSEL, method_getImplementation(cusMethod), method_getTypeEncoding(cusMethod));
        if (addSucc) {
            class_replaceMethod(selfClass, cusSEL, method_getImplementation(oriMethod), method_getTypeEncoding(oriMethod));
        }else {
            method_exchangeImplementations(oriMethod, cusMethod);
        }
        
    });
    
}

- (void)ZB_setPlaceholder:(NSString *)placeholder{
    
    NSMutableAttributedString *placeholderAtbString = [[NSMutableAttributedString alloc] initWithString:placeholder];
    [placeholderAtbString addAttribute:NSForegroundColorAttributeName
                        value:kMianColor_8997B0
                        range:NSMakeRange(0, placeholder.length)];
    self.attributedPlaceholder = placeholderAtbString;
    
    [self ZB_setPlaceholder:placeholder];
}


@end

注意事项:

项目中的Placeholder必须用代码去设置,xib的并不走setPlaceholder方法;
xib实现代码:

+ (void)load{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        
        Class selfClass = [self class];
        {
            SEL oriSEL = @selector(initWithCoder:);
            Method oriMethod = class_getInstanceMethod(selfClass, oriSEL);
            
            SEL cusSEL = @selector(ZB_initWithCoder:);
            Method cusMethod = class_getInstanceMethod(selfClass, cusSEL);
            
            BOOL addSucc = class_addMethod(selfClass, oriSEL, method_getImplementation(cusMethod), method_getTypeEncoding(cusMethod));
            if (addSucc) {
                class_replaceMethod(selfClass, cusSEL, method_getImplementation(oriMethod), method_getTypeEncoding(oriMethod));
            }else {
                method_exchangeImplementations(oriMethod, cusMethod);
            }
        }
      
    });
    
}
- (instancetype)ZB_initWithCoder:(NSCoder *)coder
{
    id obj = [self ZB_initWithCoder:coder];
    if (obj) {
         [self setValue:kColor_8997B0 forKeyPath:@"_placeholderLabel.textColor"];
    }
    return  obj;
}