Code stuck when requesting size property of ALAssetRepresentation - ios

Question

I have a method that returns the data of an ALAsset. I call this method for one asset after the other to get NSData from them and upload it to a server. After every 4 or 5 calls, the code gets stuck on rep.size call below. When I pause and resume execution in XCode it starts to work again. I am completely stumped, any help would be appreciated.
Clarification: The deadlock/code gets stuck inside the ALAssetsLibrary code, not inside my code.
Additional info: I have just one instance of ALAssetsLibrary and I am making sure it is not being used from any other thread.
+(void)getDataFromAssetURL:(NSString *) myImageURL ofType:(enum ImageType)imageType andPerformBlock:(NSDataBlock)block blocking:(BOOL)blocking
{
NSConditionLock * albumReadLock = nil;
if (blocking) {
albumReadLock = [[NSConditionLock alloc] initWithCondition:PENDING];
}
#autoreleasepool {
#try {
NSURL *str = [[NSURL alloc] initWithString: myImageURL];
ALAsset * asset = [[AppManager sharedInstance].assetObjectCache objectForKey:str];
if (asset && NO) {
block( [Util getDataFromAsset:asset ofType:imageType] );
// notifies the lock that "all tasks are finished"
[albumReadLock lock];
[albumReadLock unlockWithCondition:ALLFINISHED];
}
else {
ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *myasset)
{
#autoreleasepool {
#try {
[[AppManager sharedInstance].assetObjectCache setObject:myasset forKey:str];
NSData * assetData = [Util getDataFromAsset:myasset ofType:imageType];
block(assetData);
// notifies the lock that "all tasks are finished"
[albumReadLock lock];
[albumReadLock unlockWithCondition:ALLFINISHED];
}
#catch (NSException *exception) {
block(nil);
// important: notifies lock that "all tasks finished" (even though they failed)
[albumReadLock lock];
[albumReadLock unlockWithCondition:ALLFINISHED];
}
#finally {
}
}
};
ALAssetsLibraryAccessFailureBlock failureblock = ^(NSError *myerror)
{
block(nil);
// important: notifies lock that "all tasks finished" (even though they failed)
[albumReadLock lock];
[albumReadLock unlockWithCondition:ALLFINISHED];
};
if(str)
{
NSURL *asseturl = str;
[[AppManager sharedInstance].assetslibrary assetForURL:asseturl
resultBlock:resultblock
failureBlock:failureblock];
}
else {
block(nil);
// notifies the lock that "all tasks are finished"
[albumReadLock lock];
[albumReadLock unlockWithCondition:ALLFINISHED];
}
}
}
#catch (NSException *exception) {
block(nil);
// notifies the lock that "all tasks are finished"
[albumReadLock lock];
[albumReadLock unlockWithCondition:ALLFINISHED];
}
#finally {
}
}
if (blocking) {
// non-busy wait for the asset read to finish (specifically until the condition is "all finished")
[albumReadLock lockWhenCondition:ALLFINISHED];
[albumReadLock unlock];
}
}
+(NSData *)getDataFromAsset:(ALAsset *)myasset ofType:(enum ImageType)imageType
{
ALAssetRepresentation *rep = [myasset defaultRepresentation];
CGImageRef iref = nil;
NSData *assetData = nil;
if (imageType == FULL_RESOLUTION) {
Byte *buffer = (Byte*)malloc(rep.size);
NSUInteger buffered = [rep getBytes:buffer fromOffset:0.0 length:rep.size error:nil];
assetData = [NSData dataWithBytesNoCopy:buffer length:buffered freeWhenDone:YES];
}
else if (imageType == LARGE_IMAGE){
iref = [rep fullScreenImage];
UIImage * uiimage = [UIImage imageWithCGImage:iref];
////NSLog(#"%f %f", uiimage.size.width, uiimage.size.height);
assetData = UIImageJPEGRepresentation(uiimage, 1.0f);
}
else if (imageType == SQUARE_THUMB){
iref = [myasset thumbnail];
assetData = UIImageJPEGRepresentation([UIImage imageWithCGImage:iref], 1.0f);
}
else if (imageType == PERSPECTIVE_THUMB){
iref = [myasset aspectRatioThumbnail];
assetData = UIImageJPEGRepresentation([UIImage imageWithCGImage:iref], 1.0f);
}
return assetData;
}
New information: Turns out the problem only occurs on a specific device. As stated above, if I pause and un-pause the debugger in XCode the code moves forward.

Answer 1

You will cause a deadlock in the a ALAsset code if you initiate this block of code from the main thread. You must first drop onto a background thread because the ALAssetsLibrary requires the ability to execute code on the main thread, and if the main thread is blocked in the manner that you have outline above then it will be waiting forever to do so.