在開發過程中，我們經常需要通過網絡請求加載圖片。有時，我們需要在創建UIImageView時就知道請求圖片的尺寸，根據圖片尺寸來設置控件的Frame。如微信朋友圈，新浪微博，在圖片未加載出來之前，控件的Frame就已經設置好。（圖片寬高不一致100?200或者 400 ? 150）。
- 通過服務器返回圖片尺寸設置
這種方法是體驗最好的方法。在后臺返回url的同時，返回圖片寬高，再設置控件frame； - 通過獲取image文件頭方法
//類方法 傳入圖片url 返回圖片size
+(CGSize)downloadImageSizeWithURL:(id)imageURL{
NSURL* URL = nil;
//判斷url類型
if([imageURL isKindOfClass:[NSURL class]]){
URL = imageURL;
}
if([imageURL isKindOfClass:[NSString class]]){
URL = [NSURL URLWithString:imageURL];
}
if(URL == nil)
return CGSizeZero;
NSString* absoluteString = URL.absoluteString;
//通過SDWebimage 查看圖片是否已有緩存，如果有直接獲取，如果沒有，請求文件頭
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;
}
}
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:URL];
NSString* pathExtendsion = [URL.pathExtension lowercaseString];
CGSize size = CGSizeZero;
//區分加載圖片的類型
if([pathExtendsion isEqualToString:@"png"]){
size = [self downloadPNGImageSizeWithRequest:request];
}else if([pathExtendsion isEqual:@"gif"]){
size = [self downloadGIFImageSizeWithRequest:request];
}else{
size = [self downloadJPGImageSizeWithRequest:request];
}
if(CGSizeEqualToSize(CGSizeZero, size)){
NSData* data = [NSURLConnection sendSynchronousRequest:[NSURLRequest requestWithURL:URL] returningResponse:nil error:nil];
UIImage* image = [UIImage imageWithData:data];
if(image){
[[SDImageCache sharedImageCache] storeImage:image recalculateFromImage:YES imageData:data forKey:URL.absoluteString toDisk:YES];
size = image.size;
}
}
return size;
}
//PNG格式的圖片
+(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;
}
//GIF格式
+(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;
}
//JPG格式
+(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;
}
}
}
該方法通過請求少量字節的網絡請求，獲取到圖片的文件頭中的尺寸信息，效果還是比較理想的。
但是，如果在網絡環境較差的情況下，這種方法又會有一個致命的弱點--同步請求。若自定義cell中用此方法獲取圖片尺寸， 較差的網絡環境，TableView的不斷滾動，可以明顯感受到主線程阻塞帶來的卡頓。
總結
如果在自定義cell中想預先獲取加載圖片的尺寸，最理想的方法還是與后臺協商返回吧！
當然，還可以通過SDWebImage獲取
[imageView sd_setImageWithURL: imageUrl completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
//圖片的寬
CGFloat width = image.size.width;
//圖片的高
CGFloat height = image.size.height;
}];
異步加載完畢后，重新布局cell，設置高度，reload當前cell。
