Re: MLMediaLibrary sometimes does not call my KVO

[Gabriel Zachmann]

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Re: MLMediaLibrary sometimes does not call my KVO

[Gabriel Zachmann]

> It’s very hard for a sandboxed app to get access to the internals of Photos 
> without using MLMediaLibrary, though not too difficult for non-sandboxed. 
> MLMediaLibrary was added because of this need.

I was wondering whether this API (Photos Framework aka PhotoKit) would be 
better suited :
  https://developer.apple.com/reference/photos?language=objc

According to the doc, it should be available on macOS 10.11+

Best regards, 
Gabriel.


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Re: MLMediaLibrary sometimes does not call my KVO

[Graham Cox]

> On 22 Sep 2016, at 10:45 PM, Gabriel Zachmann  wrote:
> 
>> I’ve run into some unreliability in MLMediaLibrary. It appears to be a bug 
>> because it sometimes can be seen in places that use MLMediaLibrary within 
>> general standard components, such as NSOpenPanel (this adds a media browsing 
>> section if it is set up to allow image types, for example). 
> 
> Thanks a lot for your insights.
> 
> I am wondering: how does Microsoft Word , for instance, do it?
> I've never seen that application behave funny when it shows the image browser 
> (it faithfully shows all 10,000 images from my Photos library).
> 
> Does it use a different API?

It’s very hard for a sandboxed app to get access to the internals of Photos 
without using MLMediaLibrary, though not too difficult for non-sandboxed. 
MLMediaLibrary was added because of this need.

There is a standard component - NSMediaLibraryBrowserController 
(https://developer.apple.com/reference/appkit/nsmedialibrarybrowsercontroller?language=objc)
 that gives you a standardised panel for browsing. Unfortunately it’s a 
standalone panel rather than a view that you can embed within a larger UI, but 
it’s sufficient for many needs, and is easy to use. You could look at that and 
see if its behaviour is more reliable than your own code. I have tried it and 
it isn’t for me - I get the same problems with it that I see in NSOpenPanel, or 
when using KVO on MLMediaLibrary elements, which is why the problem seems to be 
in the lower-level layers of MLMediaLibrary.

—Graham



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Re: MLMediaLibrary sometimes does not call my KVO

[Gabriel Zachmann]

> I’ve run into some unreliability in MLMediaLibrary. It appears to be a bug 
> because it sometimes can be seen in places that use MLMediaLibrary within 
> general standard components, such as NSOpenPanel (this adds a media browsing 
> section if it is set up to allow image types, for example). 

Thanks a lot for your insights.

I am wondering: how does Microsoft Word , for instance, do it?
I've never seen that application behave funny when it shows the image browser 
(it faithfully shows all 10,000 images from my Photos library).

Does it use a different API?

Best regards, 
Gabriel.





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Re: MLMediaLibrary sometimes does not call my KVO

[Graham Cox]

> On 22 Sep 2016, at 8:59 AM, Gabriel Zachmann  wrote:
> 
> I am a bit at a loss about the MLMediaLibrary API.
> Maybe, I haven't quite understood it yet, maybe something else is wrong.
> 
> I am writing a screen saver that accesses Photos' albums and the images 
> referenced by them.
> (Code excerpts follow at the end of this email.)
> 
> I create an array of the top level albums at startup time of the screensaver 
> , using the KVO.
> At runtime, I occasionally extract the images referenced by one of those 
> albums.
> Most of the time it works fine, except sometimes, my KVO never gets invoked, 
> at which point my screensaver hangs,
> because I had to stop the animation during that phase.
> 
> I have not found a pattern as to when this happens.
> (it is not a deterministic album, nor a deterministic n-th time.)
> 
> Any suggestions, hints, insights, and pointers will be highly appreciated.


I’ve run into some unreliability in MLMediaLibrary. It appears to be a bug 
because it sometimes can be seen in places that use MLMediaLibrary within 
general standard components, such as NSOpenPanel (this adds a media browsing 
section if it is set up to allow image types, for example). Sometimes it only 
shows the top level of the library, and all of the subgroups are simply 
missing, and never show up no matter how long you wait. Opening the panel a 
second time will finally deliver the subgroups.

My guess is that as it’s an asynchronous API, on the first pass the system has 
to build a lot of internal cached state, and it may fail for any reason. On the 
second pass, it has already primed much of that cache and the failure doesn’t 
occur. While this happens quite often, it’s not every time, so I haven’t 
spotted an absolute pattern.

But I think your code needs to be designed in such a way that if the KVO it is 
expecting (for the contents of a given group, say) is never delivered, it 
doesn’t fail. I’m not sure that is really possible for a photos screensaver (in 
that case you have no photos to display), but there’s no reason it needs to 
hang.

Hopefully 10.12 will fix the issue, but I haven’t had a chance to look into 
that yet.

—Graham



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MLMediaLibrary sometimes does not call my KVO

[Gabriel Zachmann]

I am a bit at a loss about the MLMediaLibrary API.
Maybe, I haven't quite understood it yet, maybe something else is wrong.

I am writing a screen saver that accesses Photos' albums and the images 
referenced by them.
(Code excerpts follow at the end of this email.)

I create an array of the top level albums at startup time of the screensaver , 
using the KVO.
At runtime, I occasionally extract the images referenced by one of those albums.
Most of the time it works fine, except sometimes, my KVO never gets invoked, at 
which point my screensaver hangs,
because I had to stop the animation during that phase.

I have not found a pattern as to when this happens.
(it is not a deterministic album, nor a deterministic n-th time.)

Any suggestions, hints, insights, and pointers will be highly appreciated.

Best regards, 
Gabriel.


The code snippets.  (variables ending with underscore are instance variables.)


This is what I do at startup time (in -initwithFrame:isPreview:)


   albums_ = [[NSMutableArray alloc] init];
   NSDictionary *options = @{
 MLMediaLoadSourceTypesKey: 
@(MLMediaSourceTypeImage),
 MLMediaLoadIncludeSourcesKey: 
@[MLMediaSourcePhotosIdentifier]
 };
   mediaLibrary_ = [[MLMediaLibrary alloc] initWithOptions: options];
   [mediaLibrary_ addObserver: self
   forKeyPath: @"mediaSources"
  options: 0
  context: (__bridge void *) @"mediaLibraryLoaded"];
   [mediaLibrary_.mediaSources objectForKey: MLMediaSourcePhotosIdentifier ];  
// starts asynchronous loading


During runtime, this gets executed occasionally:


   album_ = albums_[album_id];
   [album_ addObserver: self
forKeyPath: @"mediaObjects"
   options: 0
   context: @"mediaObjects"];
   [album_ mediaObjects];


I have checked using log message that addObserver: does actually get executed.


And this is my key value observer:


- (void) observeValueForKeyPath: (NSString *) keyPath   ofObject: (id) object
change: (NSDictionary *) change context: (void *) 
context
{
   MLMediaSource * mediaSource = [mediaLibrary_.mediaSources objectForKey: 
MLMediaSourcePhotosIdentifier];
   if ( context == (__bridge void *) @"mediaLibraryLoaded" )
   {
   [mediaSource addObserver: self
 forKeyPath: @"rootMediaGroup"
options: 0
context: (__bridge void *) @"rootMediaGroupLoaded"];
   [mediaSource rootMediaGroup];   
// start next phase: load groups
   }
   else if ( context == (__bridge void *) @"rootMediaGroupLoaded" )
   {
   MLMediaGroup *albums = [mediaSource mediaGroupForIdentifier: 
@"TopLevelAlbums"];
   for ( MLMediaGroup * album in albums.childGroups )
   {
   NSString * albumName = [album.attributes objectForKey: @"name"];
   [self logMessage: [NSString stringWithFormat: @"album name = %@", 
albumName] asError: NO];
   if ( albumName )
   [albums_ addObject: album ];
   }
   }
   else if ( context == (__bridge void *) @"mediaObjects" )
   {
   NSArray * mediaObjects = album_.mediaObjects;   
   for ( MLMediaObject * mediaObject in mediaObjects )
   {
   if ( mediaObject.mediaType != MLMediaTypeImage )
   // we still get movies as mediaObjects, which might be contained 
in a Photos album
   continue;

   NSURL * url  = mediaObject.URL;
   [photoPaths_ addObject: url.path];
   }
   }
}


Again, I checked with a log message that the KVO never gets invoked in the case 
when the screen saver hangs
(i.e., I am positive the hanging is not caused by an infinite loop or similar 
in the KVO.)




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