iOS开发中用imageIO渐进加载图片及获取exif的方法

    NSURLRequest    *_request;
    NSURLConnection *_conn;
    
    CGImageSourceRef _incrementallyImgSource;
    
    NSMutableData   *_recieveData;
    long long       _expectedLeght;
    bool            _isLoadFinished;
}

@property (nonatomic, retain) UIImage *image;
@property (nonatomic, retain) UIImage *thumbImage;

@end

@implementation SvIncrementallyImage

@synthesize imageURL = _imageURL;
@synthesize image    = _image;
@synthesize thumbImage = _thumbImage;

- (id)initWithURL:(NSURL *)imageURL
{
    self = [super init];
    if (self) {
        _imageURL = [imageURL retain];
        
        _request = [[NSURLRequest alloc] initWithURL:_imageURL];
        _conn    = [[NSURLConnection alloc] initWithRequest:_request delegate:self];
        
        _incrementallyImgSource = CGImageSourceCreateIncremental(NULL);
        
        _recieveData = [[NSMutableData alloc] init];
        _isLoadFinished = false;
    }
    
    return self;
}

#pragma mark -
#pragma mark NSURLConnectionDataDelegate

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    _expectedLeght = response.expectedContentLength;
    NSLog(@"expected Length: %lld", _expectedLeght);
    
    NSString *mimeType = response.MIMEType;
    NSLog(@"MIME TYPE %@", mimeType);
    
    NSArray *arr = [mimeType componentsSeparatedByString:@"/"];
    if (arr.count < 1 || ![[arr objectAtIndex:0] isEqual:@"image"]) {
        NSLog(@"not a image url");