Facts about AVAsset

• Made up of “Tracks” : Could be audio or video track.
• Provides information about the collection of audio tracks and video tracks as a whole, including title, duration, size, etc.
• Not tied to any file format
• Initializaing an asset doesn’t mean it’s ready to use. Therefore use a block for asynchronous usage.

A. Preparing AVAsset

1. Typical Scenario

// 1. Get File Manager
NSFileManager *filemgr = [NSFileManager defaultManager];

// 2. Get vid.mov inside tmp directory
NSString *fileName = [NSTemporaryDirectory() stringByAppendingPathComponent:@"vid.mov"];

// 3. Get a NSURL version of the filename
NSURL *url = [NSURL fileURLWithPath: fileName];

// 4. Create an asset with the URL
AVURLAsset *anAsset = [[AVURLAsset alloc] initWithURL:url options:nil];

2. Initialize for precise timing

For just playing, probably no need for passing the option. However if I want to add the asset to an AVMutableComposition, I may need to.

NSDictionary *options = @{ AVURLAssetPreferPreciseDurationAndTimingKey : @YES };
AVURLAsset *anAssetToUseInAComposition = [[AVURLAsset alloc] initWithURL:url options:options];

B. Using AVAsset

Running initWithURL:options: does not mean that the asset is prepared to be accessed. Therefore must use loadValuesAsynchronouslyForKeys:completionHandler: to access the asset asynchronously.

1. Accessing the Asset’s duration

// ..
// Continued from above...
AVURLAsset *anAsset = [[AVURLAsset alloc] initWithURL:url options:nil];
NSArray *keys = @[@"duration"];
[asset loadValuesAsynchronouslyForKeys:keys completionHandler:^() {
    // This block is run after the "duration" value is loaded
    NSError *error = nil;
    AVKeyValueStatus tracksStatus = [asset statusOfValueForKey:@"duration" error:&error];
    switch (tracksStatus) {
        case AVKeyValueStatusLoaded:
            [self updateUserInterfaceForDuration];
            break;
        case AVKeyValueStatusFailed:
            [self reportError:error forAsset:asset];
            break;
        case AVKeyValueStatusCancelled:
            // Do whatever is appropriate for cancelation.
            break;
   }
}];

2. Getting an image capture

AVAsset anAsset = <#Get an asset#>;
if ([[anAsset tracksWithMediaType:AVMediaTypeVideo] count] > 0) {
    AVAssetImageGenerator *imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:anAsset];
    // Implementation continues...
}