Checking if MPMediaItem exists by URL

If you are usings AVPlayer with Asset URLs, you might want to check asset's existence on the device.
If the asset is a file in local folder - no problem, you can use NSFileManager to check for existance? But what if it's inside the iPod library? The following trick wasn't easy to find, but here it is:

   NSURL* furl = [f trackUrl];
//        ipod-library://item/item.mp3?id=
        if([[furl scheme] isEqualToString:@"ipod-library"]){
            NSNumber* pid = [NSNumber numberWithLongLong: [[f.name substringFromIndex:32] longLongValue] ];;
            MPMediaPropertyPredicate *predicate = [MPMediaPropertyPredicate predicateWithValue:pid forProperty:MPMediaItemPropertyPersistentID];
            MPMediaQuery *songQuery = [[[MPMediaQuery alloc] init] autorelease];
            [songQuery addFilterPredicate: predicate];
            if (songQuery.items.count == 0) {
                return NO; // NOT FOUND!
            }

Tracing Objective-C Allocation

So, let's say you have a problem tracking down some pesky retain/autorelease issue in your code. You can use NSZombieEnabled to catch double-releases, but sometimes those happen inside somebody else's code, and it's hard to track it down.

Based on a code I've found in this blog post, I've wrote a template for subclassing somebody else's class to print out the allocation history.

Here's how it looks:

	#define SYNTESIZE_TRACE(X) \
	@interface Trace##X : X { \
	} \
	@end \
	\
	@implementation Trace##X\
	\
	- (id)retain {\
	NSUInteger oldRetainCount = [super retainCount];\
	id result = [super retain];\
	NSUInteger newRetainCount = [super retainCount];\
	NSLog(@"%@<%@> ++retainCount: %d => %d\n", [self class] , self, oldRetainCount, newRetainCount);\
	NSLog(@"%@\n", [NSThread callStackSymbols] );\
	return result;\
	}\
	\
	- (void)release {\
	NSUInteger oldRetainCount = [super retainCount];\
	BOOL gonnaDealloc = oldRetainCount == 1;\
	if (gonnaDealloc) {\
	NSLog(@"%@<%@> --retainCount: 1 => 0 (gonna dealloc)\n", [self class] , self);\
	NSLog(@"%@\n", [NSThread callStackSymbols] );\
	}\
	[super release];\
	if (!gonnaDealloc) {\
	NSUInteger newRetainCount = [super retainCount];\
	NSLog(@"%@<%@> --retainCount: %d => %d\n", [self class] , self, oldRetainCount, newRetainCount);\
	NSLog(@"%@\n", [NSThread callStackSymbols] );\
	}\
	}\
	\
	@end

view raw trace.m hosted with ❤ by GitHub

And here's how you use it (in this example, I want to track AVPlayer):

SYNTESIZE_TRACE(AVPlayer)

AVPlayer* p = [[TraceAVPlayer alloc] init];


All done. Now, while running, you'll get a record of all retainCount modifications, with their stack trace.