为UITextField 添加分类属性

.h

@property (nonatomic,strong) UIColor * placeholderColor;

.m

// 静态 底层属性变量
static NSString * const  PlaceholderColorKey = @"placeholderLabel.textColor";

// set 方法赋值
- (void)setPlaceholderColor:(UIColor *)placeholderColor{
// 提前设置占位文字 ,目的:使它提前创建placeholderLabel.
      NSString * oldPlaceholder = self.placeholder;
      self.placeholder = @" ";
      self.placeholder = oldPlaceholder;

// 恢复到默认的占位文字颜色
      if (placeholderColor == nil){
            placeholderColor = [UIColor colorWithRed:0 green:0 bule:0.0980392 alpha:0.22];
      }
     [self setValue:placeholderColor forKeyPath: PlaceholderColorKey];
}

// get方法取值
- (UIColor *)placeholderColor{
       return [self valueForKeyPath: PlaceholderColorKey];
}