dispatch_group_notify(group, queue, ^{
// 1.开启新的图形上下文
UIGraphicsBeginImageContext(CGSizeMake(100, 100));
// 2.绘制图片
[self.image1 drawInRect:CGRectMake(0, 0, 50, 100)];
[self.image2 drawInRect:CGRectMake(50, 0, 50, 100)];
// 3.取得上下文中的图片
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
// 4.结束上下文
UIGraphicsEndImageContext();
// 回到主线程显示图片
dispatch_async(dispatch_get_main_queue(), ^{
// 将新图片显示出来
self.imageView.image = image;
});
});