How can I determine file size on disk of a video PHAsset in iOS8

Edit

As for iOS 9.3, using requestImageDataForAsset on a video type PHAsset will result in an image, which is the first frame of the video, so it doesn't work anymore. Use the following method instead, for normal video, request option can be nil, but for slow motion video, PHVideoRequestOptionsVersionOriginal needs to be set.

PHVideoRequestOptions *options = [[PHVideoRequestOptions alloc] init];
options.version = PHVideoRequestOptionsVersionOriginal;

[[PHImageManager defaultManager] requestAVAssetForVideo:asset options:options resultHandler:^(AVAsset *asset, AVAudioMix *audioMix, NSDictionary *info) {
    if ([asset isKindOfClass:[AVURLAsset class]]) {
        AVURLAsset* urlAsset = (AVURLAsset*)asset;

        NSNumber *size;

        [urlAsset.URL getResourceValue:&size forKey:NSURLFileSizeKey error:nil];
        NSLog(@"size is %f",[size floatValue]/(1024.0*1024.0)); //size is 43.703005

    }
}];

//original answer

For PHAsset, use this:

[[PHImageManager defaultManager] requestImageDataForAsset:asset options:nil resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {
        float imageSize = imageData.length;
        //convert to Megabytes
        imageSize = imageSize/(1024*1024);
        NSLog(@"%f",imageSize);
 }];

For ALAsset:

ALAssetRepresentation *rep = [asset defaultRepresentation];
float imageSize = rep.size/(1024.0*1024.0);

I tested on one video asset, PHAsset shows the size as 43.703125, ALAsset shows the size as 43.703005.

Edit For PHAsset, another way to get file size. But as @Alfie Hanssen mentioned, it works on normal video, for slow motion video, the following method will return a AVComposition asset in the block, so I added the check for its type. For slow motion video, use the requestImageDataForAsset method.

[[PHImageManager defaultManager] requestAVAssetForVideo:asset options:nil resultHandler:^(AVAsset *asset, AVAudioMix *audioMix, NSDictionary *info) {
    if ([asset isKindOfClass:[AVURLAsset class]]) {
        AVURLAsset* urlAsset = (AVURLAsset*)asset;
        NSNumber *size;

        [urlAsset.URL getResourceValue:&size forKey:NSURLFileSizeKey error:nil];
        NSLog(@"size is %f",[size floatValue]/(1024.0*1024.0)); //size is 43.703005
        NSData *data = [NSData dataWithContentsOfURL:urlAsset.URL];
        NSLog(@"length %f",[data length]/(1024.0*1024.0)); // data size is 43.703005
    }
}];

Swift version with file size formatting:

    let options = PHVideoRequestOptions()
    options.version = .original

    PHImageManager.default().requestAVAsset(forVideo: asset, options: options) { avAsset, _, _ in
        if let urlAsset = avAsset as? AVURLAsset {  // Could be AVComposition class
            if let resourceValues = try? urlAsset.url.resourceValues(forKeys: [.fileSizeKey]),
                let fileSize = resourceValues.fileSize {

                let formatter = ByteCountFormatter()
                formatter.countStyle = .file
                let string = formatter.string(fromByteCount: Int64(fileSize))

                print(string)
            }
        }
    }