iOS UIActivityViewController using UIActivityItemProvider "forgets" items - ios

Question

I have a larger app that should be able to share multiple images.
I implemented this using UIActivityViewController and UIActivityItemProvider to have asynchronous usage of the items (so that i only have to prepare one image at a time and not have to fill the memory with all of them at once to share them).
I was "inspired" by the Apple Airdrop example:
AirdropSample download
However when using my app to share e.g. 9 images (to camera Roll == "Save 9 images") only 4 to 7 images end up being in the camera roll, no error messages whatsoever.
If i repeat it over and over sometimes i get 5 images or 6 seemingly random.
I cannot post my app here, but i modified the above sample in a way that it will also randomly "fail" with delivering all images to the camera Roll...
If you download above sample and replace 4 files with these, it shows the problem:
APLAsyncImageViewController.h:
#import <UIKit/UIKit.h>
#import "APLAsyncImageActivityItemProvider.h"
#interface APLAsyncImageViewController : UIViewController
#end
APLAsyncImageViewController.m:
#import "APLAsyncImageViewController.h"
#import "APLProgressAlertViewController.h"
NSString * const kProgressAlertViewControllerIdentifier = #"APLProgressAlertViewController";
#interface APLAsyncImageViewController ()
#property (strong, nonatomic) UIWindow *alertWindow;
#property (strong, nonatomic) APLProgressAlertViewController *alertViewController;
#property (strong, nonatomic) UIPopoverController *activityPopover;
#property (weak, nonatomic) IBOutlet UIButton *shareImageButton;
- (IBAction)openActivitySheet:(id)sender;
#end
#implementation APLAsyncImageViewController
- (IBAction)openActivitySheet:(id)sender
{
NSMutableArray *itemArray = [[NSMutableArray alloc] init];
for( int i = 0; i < 9;i++)
{
APLAsyncImageActivityItemProvider *aiImageItemProvider = [[APLAsyncImageActivityItemProvider alloc] init];
[itemArray addObject: aiImageItemProvider];
}
//Create an activity view controller with the activity provider item. UIActivityItemProvider (AsyncImageActivityItemProvider's superclass) conforms to the UIActivityItemSource protocol
UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:itemArray applicationActivities:nil];
if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone) {
//iPhone, present activity view controller as is
[self presentViewController:activityViewController animated:YES completion:nil];
}
else
{
//iPad, present the view controller inside a popover
if (![self.activityPopover isPopoverVisible]) {
self.activityPopover = [[UIPopoverController alloc] initWithContentViewController:activityViewController];
[self.activityPopover presentPopoverFromRect:[self.shareImageButton frame] inView:self.view permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES];
}
else
{
//Dismiss if the button is tapped while pop over is visible
[self.activityPopover dismissPopoverAnimated:YES];
}
}
}
#end
APLAsyncImageActivityItemProvider.h:
#import <UIKit/UIKit.h>
#interface APLAsyncImageActivityItemProvider : UIActivityItemProvider
#end
APLAsyncImageActivityItemProvider.m:
#import "APLAsyncImageActivityItemProvider.h"
#import "UIImage+Resize.h"
#implementation APLAsyncImageActivityItemProvider
- (id)activityViewControllerPlaceholderItem:(UIActivityViewController *)activityViewController
{
return [[UIImage alloc] init];
}
- (id)item
{
UIImage *image = [UIImage imageNamed:#"Flower.png"];
//CGSize imageSize;
//imageSize.height = 1000;
//imageSize.width = 1000;
//image = [UIImage imageWithImage:image scaledToFitToSize:imageSize];
return image;
}
- (UIImage *)activityViewController:(UIActivityViewController *)activityViewController thumbnailImageForActivityType:(NSString *)activityType suggestedSize:(CGSize)size
{
//The filtered image is the image to display on the other side.
return [[UIImage alloc] init];
}
#end
If you execute the sample like this (Use menu item "Send Image After Preprocessing", press "SHARE" Button) it will often or mostly fail to deliver all 9 images to the camera roll.
IF you uncomment the 4 lines in "APLAsyncImageActivityItemProvider.m" that basically just scale the output image THEN it will work ALWAYS.
Can you tell me why ? I feel that if i know the answer to that riddle i can also fix my app.
Thank you,
Nils

Answer 1

The reason for this issue is pretty simple.
There are limited number of writer threads available to use by the app, so writing too many images simulteneously may fail due to "Write Busy" error. You can verify that by trying to manually write these images using UIImageWriteToSavedPhotosAlbum (or corresponding Asset/Photos framework counterparts).
So, resizing images just hides the real problem because writing a smaller image takes less time.
The problem can be solved by liming the number of writers and/or retrying in case of error.
Also note that -[UIActivityItemProvider item] is a blocking call, but there are no synchronous ways to write to the gallery out of the box. This can be handled with dispatch_semaphore_wait , NSCondition, etc.
Or, in case you're using ReactiveCocoa, by simply calling waitUntilCompleted.

Answer 2

I would go to say that the [UIImage imageWithImage:image scaledToFitToSize:imageSize] image created is GC before saved to camera roll, but I don't know what is going on in that method without the code. That method is not a normal UIImage method, a category maybe?
EDIT
Replace your for loop code adding to your item array with this code, and then uncomment your resize lines, and see if that works.
NSMutableArray *itemArray = [[NSMutableArray alloc] init];
for( int i = 0; i < 9;i++)
{
UIImage *aiImage = [[[APLAsyncImageActivityItemProvider alloc] init] image];
[itemArray addObject:aiImage];
}