简单一个例子 UITableViewCell的异步绘制

//异步绘制
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        CGRect rect = [_data[@"frame"] CGRectValue];
        UIGraphicsBeginImageContextWithOptions(rect.size, YES, 0);
        CGContextRef context = UIGraphicsGetCurrentContext();
	//整个内容的背景
        [[UIColor colorWithRed:250/255.0 green:250/255.0 blue:250/255.0 alpha:1] set];
        CGContextFillRect(context, rect);
	//转发内容的背景
        if ([_data valueForKey:@"subData"]) {
            [[UIColor colorWithRed:243/255.0 green:243/255.0 blue:243/255.0 alpha:1] set];
            CGRect subFrame = [_data[@"subData"][@"frame"] CGRectValue];
            CGContextFillRect(context, subFrame);
            [[UIColor colorWithRed:200/255.0 green:200/255.0 blue:200/255.0 alpha:1] set];
            CGContextFillRect(context, CGRectMake(0, subFrame.origin.y, rect.size.width, .5));
        }
        
        {
	    //名字
            float leftX = SIZE_GAP_LEFT+SIZE_AVATAR+SIZE_GAP_BIG;
            float x = leftX;
            float y = (SIZE_AVATAR-(SIZE_FONT_NAME+SIZE_FONT_SUBTITLE+6))/2-2+SIZE_GAP_TOP+SIZE_GAP_SMALL-5;
            [_data[@"name"] drawInContext:context withPosition:CGPointMake(x, y) andFont:FontWithSize(SIZE_FONT_NAME)
                             andTextColor:[UIColor colorWithRed:106/255.0 green:140/255.0 blue:181/255.0 alpha:1]
                                andHeight:rect.size.height];
	    //时间+设备
            y += SIZE_FONT_NAME+5;
            float fromX = leftX;
            float size = [UIScreen screenWidth]-leftX;
            NSString *from = [NSString stringWithFormat:@"%@  %@", _data[@"time"], _data[@"from"]];
            [from drawInContext:context withPosition:CGPointMake(fromX, y) andFont:FontWithSize(SIZE_FONT_SUBTITLE)
                   andTextColor:[UIColor colorWithRed:178/255.0 green:178/255.0 blue:178/255.0 alpha:1]
                      andHeight:rect.size.height andWidth:size];
        }
	//将绘制的内容以图片的形式返回，并调主线程显示
	UIImage *temp = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        dispatch_async(dispatch_get_main_queue(), ^{
            if (flag==drawColorFlag) {
                postBGView.frame = rect;
                postBGView.image = nil;
                postBGView.image = temp;
            }
	}
	//内容如果是图文混排，就添加View，用CoreText绘制
	[self drawText];
}}

 上面代码放在自定义 draw方法里    或者  去掉GCD多线程，直接放到 drawRect方法里，重写它，因为drawRect本来就是异步绘制