Showing progress percentage while downloading a file from iCloud?

I'd like to show a progress bar while a UIDocument is being downloaded from iCloud.

I tried keeping a reference to the NSMetadataItem from the query asking it for its attributes every second, but the key NSMetadataUbiquitousItemPercentDownloadedKey either does not exist, or it suddenly changes to 100 once the document was downloaded.

Relevant code:

 // get meta data item from query

 NSNumber *downloadProgress = [self.documentMetadataItem valueForAttribute:NSMetadataUbiquitousItemPercentDownloadedKey];
    NSLog(@"download status: %@", downloadProgress);

Solution

Instead of polling the NSMetadataItem for changes, you should just listen for updates from the NSMetadataQuery and get NSMetadataUbiquitousItemPercentDownloadedKey once (and update the UI) each time you receive an update.

(I think existing NSMetadataItem objects are not updated as the download progresses, except to transition from downloading to fully downloaded).

Here is a way of listening to NSMetadataQuery updates using KVO:

// To register for KVO updates:
[query addObserver:self
         forKeyPath:@"results"
            options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
            context:nil];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    // check that "object" is your metadata query then:
    NSUInteger changeKind = [[change objectForKey:NSKeyValueChangeKindKey] unsignedIntegerValue];

    switch (changeKind) {
        case NSKeyValueChangeSetting:
            // The whole array of results changed, update your model accordingly
            // ...
            break;
        case NSKeyValueChangeReplacement:
            // This is where you update your progress bar in most cases
            // Enumerate [change objectForKey:NSKeyValueChangeNewKey], this is a list of NSMetadataItems that will have all the desired properties
            break;
        case NSKeyValueChangeInsertion:
            // Handle a new document
            // ...
            break;
        case NSKeyValueChangeRemoval:
            // Handle a removed document
            // ...
            break;
    }
}

If you still see the progress not updating, please file a bug with Apple.