有时，需要在创建UIImageView或UIButton来显示图片之前需要提前知道图片的尺寸，根据图片尺寸创建对应大小的控件。但是对于网络图片来说，要想通过最优的方法获得尺寸就略微有点困难，大体思路就是下面这种：
如果有使用SDWebImage，则首先检查是否缓存过该图片，如果没有，先通过文件头获取图片大小(针对格式为png、gif、jpg文件获取其尺寸大小)，如果获取失败，则下载完整的图片data，然后计算大小，如果有使用SDWebImage，则使用SDWebImage缓存该图片。
如果有使用SDWebImage，则首先检查是否缓存过该图片，如果没有，先通过文件头获取图片大小(针对格式为png、gif、jpg文件获取其尺寸大小)，如果获取失败，则下载完整的图片data，然后计算大小，如果有使用SDWebImage，则使用SDWebImage缓存该图片。
+(CGSize)downloadImageSizeWithURL:(id)imageURL { NSURL* URL = nil; if([imageURL isKindOfClass:[NSURL class]]){ URL = imageURL; } if([imageURL isKindOfClass:[NSString class]]){ URL = [NSURL URLWithString:imageURL]; } if(URL == nil) return CGSizeZero; #ifdef dispatch_main_sync_safe if([[SDImageCache sharedImageCache] diskImageExistsWithKey:absoluteString]) { UIImage* image = [[SDImageCache sharedImageCache] imageFromMemoryCacheForKey:absoluteString]; if(!image) { NSData* data = [[SDImageCache sharedImageCache] performSelector:@selector(diskImageDataBySearchingAllPathsForKey:) withObject:URL.absoluteString]; image = [UIImage imageWithData:data]; } if(image) { return image.size; } } #endif NSMutableURLRequest *request = [[NSMutableURLRequest alloc]initWithURL:URL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:1]; NSString* pathExtendsion = [URL.pathExtension lowercaseString]; CGSize size = CGSizeZero; if ([pathExtendsion rangeOfString:@"png"].location != NSNotFound) { size = [self downloadPNGImageSizeWithRequest:request]; } else if([pathExtendsion rangeOfString:@"gif"].location != NSNotFound) { size = [self downloadGIFImageSizeWithRequest:request]; } else{ size = [self downloadJPGImageSizeWithRequest:request]; } if(CGSizeEqualToSize(CGSizeZero, size)) { NSData* data = [NSData dataWithContentsOfURL:URL]; UIImage* image = [UIImage imageWithData:data]; if(image) { //如果未使用SDWebImage，则忽略；缓存该图片 #ifdef dispatch_main_sync_safe [[SDImageCache sharedImageCache] storeImage:image recalculateFromImage:YES imageData:data forKey:URL.absoluteString toDisk:YES]; #endif size = image.size; } } //过滤掉不符合大小的图片，大图太大浪费流量，用户体验不好 if (size.height > 2048 || size.height <= 0 || size.width > 2048 || size.width <= 0 ) { return CGSizeZero; } else { return size; } } +(CGSize)downloadPNGImageSizeWithRequest:(NSMutableURLRequest*)request { [request setValue:@"bytes=16-23" forHTTPHeaderField:@"Range"]; NSData* data = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil]; if(data.length == 8) { int w1 = 0, w2 = 0, w3 = 0, w4 = 0; [data getBytes:&w1 range:NSMakeRange(0, 1)]; [data getBytes:&w2 range:NSMakeRange(1, 1)]; [data getBytes:&w3 range:NSMakeRange(2, 1)]; [data getBytes:&w4 range:NSMakeRange(3, 1)]; int w = (w1 << 24) + (w2 << 16) + (w3 << 8) + w4; int h1 = 0, h2 = 0, h3 = 0, h4 = 0; [data getBytes:&h1 range:NSMakeRange(4, 1)]; [data getBytes:&h2 range:NSMakeRange(5, 1)]; [data getBytes:&h3 range:NSMakeRange(6, 1)]; [data getBytes:&h4 range:NSMakeRange(7, 1)]; int h = (h1 << 24) + (h2 << 16) + (h3 << 8) + h4; return CGSizeMake(w, h); } return CGSizeZero; } +(CGSize)downloadGIFImageSizeWithRequest:(NSMutableURLRequest*)request { [request setValue:@"bytes=6-9" forHTTPHeaderField:@"Range"]; NSData* data = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil]; if(data.length == 4) { short w1 = 0, w2 = 0; [data getBytes:&w1 range:NSMakeRange(0, 1)]; [data getBytes:&w2 range:NSMakeRange(1, 1)]; short w = w1 + (w2 << 8); short h1 = 0, h2 = 0; [data getBytes:&h1 range:NSMakeRange(2, 1)]; [data getBytes:&h2 range:NSMakeRange(3, 1)]; short h = h1 + (h2 << 8); return CGSizeMake(w, h); } return CGSizeZero; } +(CGSize)downloadJPGImageSizeWithRequest:(NSMutableURLRequest*)request { [request setValue:@"bytes=0-209" forHTTPHeaderField:@"Range"]; NSData* data = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:nil]; if ([data length] <= 0x58) { return CGSizeZero; } if ([data length] < 210) {// 肯定只有一个DQT字段 short w1 = 0, w2 = 0; [data getBytes:&w1 range:NSMakeRange(0x60, 0x1)]; [data getBytes:&w2 range:NSMakeRange(0x61, 0x1)]; short w = (w1 << 8) + w2; short h1 = 0, h2 = 0; [data getBytes:&h1 range:NSMakeRange(0x5e, 0x1)]; [data getBytes:&h2 range:NSMakeRange(0x5f, 0x1)]; short h = (h1 << 8) + h2; return CGSizeMake(w, h); } else { short word = 0x0; [data getBytes:&word range:NSMakeRange(0x15, 0x1)]; if (word == 0xdb) { [data getBytes:&word range:NSMakeRange(0x5a, 0x1)]; if (word == 0xdb) {// 两个DQT字段 short w1 = 0, w2 = 0; [data getBytes:&w1 range:NSMakeRange(0xa5, 0x1)]; [data getBytes:&w2 range:NSMakeRange(0xa6, 0x1)]; short w = (w1 << 8) + w2; short h1 = 0, h2 = 0; [data getBytes:&h1 range:NSMakeRange(0xa3, 0x1)]; [data getBytes:&h2 range:NSMakeRange(0xa4, 0x1)]; short h = (h1 << 8) + h2; return CGSizeMake(w, h); } else {// 一个DQT字段 short w1 = 0, w2 = 0; [data getBytes:&w1 range:NSMakeRange(0x60, 0x1)]; [data getBytes:&w2 range:NSMakeRange(0x61, 0x1)]; short w = (w1 << 8) + w2; short h1 = 0, h2 = 0; [data getBytes:&h1 range:NSMakeRange(0x5e, 0x1)]; [data getBytes:&h2 range:NSMakeRange(0x5f, 0x1)]; short h = (h1 << 8) + h2; return CGSizeMake(w, h); } } else { return CGSizeZero; } } }