iOS7中自定义UITextField的Placeholder属性

通过重写UITextField中的drawPlaceholderInRect方法自定义Placeholder的属性，包括颜色，字体，及在UITextField中呈现的位置。
1. 创建继承自UITextField的类CustomTextField
2. 在CustomTextField中重写drawPlaceholderInRect方法，代码如下

- (void)drawPlaceholderInRect:(CGRect)rect

{
　　UIColor *placeholderColor = [UIColor whiteColor];//设置颜色
　　[placeholderColor setFill];

　　CGRect placeholderRect = CGRectMake(rect.origin.x, (rect.size.height- self.font.pointSize)/2-2, rect.size.width, self.font.pointSize+1);　　//设置距离


　　NSMutableParagraphStyle *style = [[NSMutableParagraphStyle alloc] init];
　　style.lineBreakMode = NSLineBreakByTruncatingTail;
　　style.alignment = self.textAlignment;
　　NSDictionary *attr = [NSDictionary dictionaryWithObjectsAndKeys:style,NSParagraphStyleAttributeName, self.font, NSFontAttributeName, placeholderColor, NSForegroundColorAttributeName, nil];

　　[self.placeholder drawInRect:placeholderRect withAttributes:attr];
}