[摘要]本文是對iOS獲取網絡圖片大小的講解，對學習IOS蘋果軟件開發有所幫助,與大家分享。

　　在iOS開發過程中經常需要通過網絡請求加載圖片，有時，需要在創建UIImageView或UIButton來顯示圖片之前需要提前知道圖片的尺寸，根據圖片尺寸創建對應大小的控件。但是對於網絡圖片來說，要想通過最優的方法獲得尺寸就略微有點困難，大體思路就是下面這種：

如果有使用SDWebImage，則首先檢查是否緩存過該圖片，如果沒有，先通過文件頭獲取圖片大小(針對格式為png、gif、jpg文件獲取其尺寸大小)，如果獲取失敗，則下載完整的圖片data，然後計算大小，如果有使用SDWebImage，則使用SDWebImage緩存該圖片。

+(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，則忽略；緩存該圖片

#ifdef dispatch_main_sync_safe

[[SDImageCache sharedImageCache] storeImage:image recalculateFromImage:YES imageData:data forKey:URL.absoluteString toDisk:YES];

#endif

size = image.size;

}

}

//過濾掉不符合大小的圖片，大圖太大浪費流量，用戶體驗不好

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) {// 肯定只有一個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) {// 兩個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 {// 一個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;

}

}

}