[self.purchaseTextFiled setValue:kColor_PlaceHolderColor_888888 forKeyPath:@"_placeholderLabel.textColor"];
[self.purchaseTextFiled setValue:[UIFont boldSystemFontOfSize:15] forKeyPath:@"_placeholderLabel.font"];
一种方法重写 - (void)drawPlaceholderInRect:(CGRect)rect 方法
参考 UITextField and attributedPlaceholder
You should subclass of UITextField, and override the method of：（重写方法）
另外一种写法 :根据 textFiled 的attributedPlaceholder 属性
// 购买金额
self.tfAmount.font = kFont_18;
NSMutableParagraphStyle *style = [self.tfAmount.defaultTextAttributes[NSParagraphStyleAttributeName] mutableCopy];
style.minimumLineHeight = self.tfAmount.font.lineHeight - (self.tfAmount.font.lineHeight - kFont_14.lineHeight) / 2.0;
self.tfAmount.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"100元起投资，单笔最多可购买5万" attributes:@{NSForegroundColorAttributeName: kColor_PlaceHolderColor,NSFontAttributeName : kFont_14,NSParagraphStyleAttributeName : style}];
