如何检查下载的PNG图像是否已损坏？

Question

我从下面的代码下载多个图像并保存到数据库.但对于一些图像,我得到低于错误.

错误:ImageIO:PNG无效距离太远错误:ImageIO:PNG错误数据检查

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);                               
dispatch_async(queue, ^{
    NSString *imgStr = [dict  objectForKey:@"image"];                  
    imgStr = [imgStr stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
    NSData *imgData = [NSData dataWithContentsOfURL:[NSURL URLWithString:imgStr]];
    UIImage *image = [UIImage imageWithData:imgData];

    dispatch_sync(dispatch_get_main_queue(), ^{
       mYImageView.image = image;
    });
});

如何检查存储的图像是否有效,所以我可以再次下载图像？

Answer 1

对于PNG图像,检查它们的前两个字节和最后两个字节.以下是方法,希望它有所帮助.

谢谢.:)

- (BOOL)isImageValid:(NSData *)data
{
    BOOL val = YES;

    if ([data length] < 4) 
        val = NO;

    const char * bytes = (const char *)[data bytes];

    if (bytes[0] != 0x89 || bytes[1] != 0x50) 
        val = NO;
    if (bytes[[data length] - 2] != 0x60 || 
        bytes[[data length] - 1] != 0x82) 
        val = NO;

    return val;
}