-(UIImage *)screenShot
{
    CGRect originalFrame = self.frame;
    CGPoint originalOffset = self.scrollView.contentOffset;
    CGSize entireSize = [self sizeThatFits:CGSizeZero];
    [self setFrame: CGRectMake(0, 0, entireSize.width, entireSize.height)];
    
    CGRect rect = [self positionOfElementWithId:@"post1"];
    //如果没有找到这个元素，就取整个页面
    if (rect.origin.y != 0) {
        entireSize.height = rect.origin.y;
    }
    entireSize.height += 40;
    
    //第三个参数为0就是取系统默认的scale，支持retina。网上带if的只是兼容3.X以下的系统。
    UIGraphicsBeginImageContextWithOptions(entireSize, NO, 0);
    
    CGContextRef context = UIGraphicsGetCurrentContext();
    [self.layer renderInContext:context];
    
    //多出来的部分画个白色背景色以放置我们尾部的文字
    CGContextSetRGBStrokeColor(context, 1.0, 1.0, 1.0, 1.0);
    CGContextSetRGBFillColor(context, 1.0, 1.0, 1.0, 1.0);
    CGContextFillRect(context, CGRectMake(0, entireSize.height-30, entireSize.width, 30));
    
    CGContextSetRGBStrokeColor(context, 0.0, 0.0, 0.0, 1.0);
    CGContextSetRGBFillColor(context, 0.0, 0.0, 0.0, 1.0);
    NSString *contentString = @“这里是你需要加上的文字
    UIFont *font = [UIFont systemFontOfSize:12];
    [contentString drawInRect:CGRectMake(12, entireSize.height-27, entireSize.width, 25) withFont:font lineBreakMode:UILineBreakModeWordWrap];
    
    UIImage *screenshot = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    [self setFrame:originalFrame];
    self.scrollView.contentOffset = originalOffset;
    return screenshot;
}

- (CGRect)positionOfElementWithId:(NSString *)elementID {
    NSString *js = @"function f(){ var r = document.getElementById('%@').getBoundingClientRect(); return '{{'+r.left+','+r.top+'},{'+r.width+','+r.height+'}}'; } f();";
    NSString *result = [self stringByEvaluatingJavaScriptFromString:[NSString stringWithFormat:js, elementID]];
    CGRect rect = CGRectFromString(result);
    return rect;
}