DIsplaying Asynchronous stillImageOutput from AVVideoCapture Session using AVFoundation - ios

Question

I am capturing video in preview mode and would like to display a still image captured by the camera.
I currently save the image and capture output to ivars defined in the interface as:
UIImage *snapshot
AVCaptureStillImageOutput* stillImageOutput;
The video displays fine. However, when I try to capture and display a still image, nothing is appearing and, in fact, the debugger shows the stillImageOutput and image are nil. I think this may be a timing issue with the asynchronous capture and that I need to use a completion handler, but I am weak on completion handlers.
What is the proper way to display a still image immediately after capturing it without tying up UI:
Code to capture still:
- (void)takeSnapshot {
AVCaptureConnection *videoConnection = nil;
for (AVCaptureConnection *connection in stillImageOutput.connections) {
for (AVCaptureInputPort *port in [connection inputPorts]) {
if ([[port mediaType] isEqual:AVMediaTypeVideo]) {
videoConnection = connection;
break;
}
}
if (videoConnection) {
break;
}
}
[stillImageOutput captureStillImageAsynchronouslyFromConnection:videoConnection
completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error) {
if (imageDataSampleBuffer != NULL) {
NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageDataSampleBuffer];
snapshot = [UIImage imageWithData:imageData];
}
}];
}
Code to display still. Note absence of completion handler which may be issue, however, I'm not sure how to write that...
[self takeSnapshot];
self.imageView.image = snapshot;

Answer 1

I would change the takeSnapshot method to take in a completion block and then call that completion block within the completion block of your other async method:
captureStillImageAsynchronouslyFromConnection:completionHandler
Here's an example of a method taking a completion block and then calling back to it in the completion block of a method called internally:
// this correlates to your takeSnapshot method
// you want to add a completion portion to this method
- (void)doSomethingAsynchronouslyWithCompletion:(void (^)(NSData *completionData))completion {
// call your other async method
[self anotherAsyncMethodWithItsOwnCompletion:^(NSData *completionDataFromSecondMethod) {
if (completionDataFromSecondMethod.length > 0) {
// this is where you would receive the CMSampleBufferRef from the completion handler of captureStillImageAsynchronouslyFromConnection:completionHandler
// and convert it over to to data
// make sure the completion block isn't nil if it's nullable
if (completion) {
// you would want to pass back the NSData imageData in the completion block here
completion(completionDataFromSecondMethod);
}
}
}];
}
// this method would simulate the captureStillImageAsynchronouslyFromConnection:completionHandler: method
- (void)anotherAsyncMethodWithItsOwnCompletion:(void (^)(NSData * completionDataFromSecondMethod))anotherCompletion {
// this is just to simulate some time waiting for the asnyc task to complete
// never call sleep in your own code
sleep(3);
if (anotherCompletion) {
// this simulates the fake CFSampleBufferRef passed back by the captureStillImage...
NSData *fakeCompletionData = [#"FakeCompletionString" dataUsingEncoding:NSUTF8StringEncoding];
anotherCompletion(fakeCompletionData);
}
}
And an example of how you would call it:
[self doSomethingAsynchronouslyWithCompletion:^(NSData *completionData) {
if (completionData.length > 0) {
// come back on the main queue to modify any UI Elements
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
// this is where you want want to set your self.imageView.image
// self.imageView.image = [UIImage imageWithData:{{dataFromCompletion}}]
NSLog(#"The completionString result = %#", [[NSString alloc] initWithData:completionData encoding:NSUTF8StringEncoding]);
}];
}
}];
This link may be helpful for getting you started with block syntax: http://goshdarnblocksyntax.com