為UITextField 添加分類屬性

.h

@property (nonatomic,strong) UIColor * placeholderColor;

.m

// 靜態(tài) 底層屬性變量
static NSString * const  PlaceholderColorKey = @"placeholderLabel.textColor";

// set 方法賦值
- (void)setPlaceholderColor:(UIColor *)placeholderColor{
// 提前設(shè)置占位文字 ,目的:使它提前創(chuàng)建placeholderLabel.
      NSString * oldPlaceholder = self.placeholder;
      self.placeholder = @" ";
      self.placeholder = oldPlaceholder;

// 恢復(fù)到默認的占位文字顏色
      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];
}