// 开辟一条子线程dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// 因为来回滑动, 都会去重新下载图片, 那么我们下载过的图片, 就直接缓存到本地, 然后下载直接从本地取(肯定比现下快)
NSString *cachePath = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES).lastObject;
[cachePath stringByAppendingPathComponent:@"imageCache"];// 获取图片网址转换的文件名字
NSString *imagePath = [NSString stringWithFormat:@"%@/%d", cachePath, [self.theExhib.worksPic hash]];NSFileManager *fileManager = [NSFileManager defaultManager];NSData *da = nil;// 判断一下图片在本地在不在
if ([fileManager fileExistsAtPath:imagePath]) {// 如果在, 直接就取
da = [NSData dataWithContentsOfFile:imagePath];}
else {// 如果不在, 就重新下载(self.theExhib.worksPic是网址)
da = [NSData dataWithContentsOfURL:[NSURL URLWithString:self.theExhib.worksPic]];// 把图片流写入本地
[da writeToFile:imagePath atomically:YES];
}
// 把NSData流转化成UIImage对象
UIImage *ima = [UIImage imageWithData:da];

// 调用自己的方法imageWithImageSimple scaldToSize: (Size后面填写的你要缩小成的图片分辨率)
ima = [self imageWithImageSimple:ima scaledToSize:CGSizeMake(100, 200)];// 回到主线程刷新UI
dispatch_async(dispatch_get_main_queue(), ^{[self.bacImageV setImage:ima];
});
});- ( UIImage *)imageWithImageSimple:( UIImage *)image scaledToSize:( CGSize )newSize{// Create a graphics image context

UIGraphicsBeginImageContext (newSize);// Tell the old image to draw in this new context, with the desired// new size

[image drawInRect : CGRectMake ( 0 , 0 ,newSize. width ,newSize. height )];// Get the new image from the context

UIImage * newImage = UIGraphicsGetImageFromCurrentImageContext ();// End the context

UIGraphicsEndImageContext ();// Return the new image.return newImage;}