iOS: Error When Scrolling Quickly Through Photos? - ios

Question

I'm currently using MWPhotoBrowser in my app and when I scroll quickly through images I get the following error:
Received memory warning.
2014-02-17 16:42:35.117 App[10803:60b] *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSArrayM: 0x156f3160> was mutated while being enumerated.'
*** First throw call stack:
(0x2e71de83 0x38a7a6c7 0x2e71d971 0x151167 0x15139b 0x311643ff 0x3116446f 0x31164665 0x31164805 0x3111ea67 0x38f5f0af 0x38f6072f 0x38f61959 0x2e6e85b1 0x2e6e6e7d 0x2e651471 0x2e651253 0x3338b2eb 0x30f06845 0xff035 0x38f73ab7)
libc++abi.dylib: terminating with uncaught exception of type NSException
I'm currently loading images stored in the app locally.
This is the method throwing the exception:
- (void)releaseAllUnderlyingPhotos:(BOOL)preserveCurrent {
for (id p in _photos) {
if (p != [NSNull null]) {
if (preserveCurrent && p == [self photoAtIndex:self.currentIndex]) {
continue; // skip current
}
[p unloadUnderlyingImage];
}
} // Release photos
}
Any help would be greatly appreciated!

Answer 1

I don't know exactly which version of MWPhotoBrowser you are using, but in the latest one here, the releaseAllUnderlyingPhotos: method reads:
- (void)releaseAllUnderlyingPhotos:(BOOL)preserveCurrent {
// Create a copy in case this array is modified while we are looping through
// Release photos
NSArray *copy = [_photos copy];
for (id p in copy) {
if (p != [NSNull null]) {
if (preserveCurrent && p == [self photoAtIndex:self.currentIndex]) {
continue; // skip current
}
[p unloadUnderlyingImage];
}
}
// Release thumbs
copy = [_thumbPhotos copy];
for (id p in copy) {
if (p != [NSNull null]) {
[p unloadUnderlyingImage];
}
}
}
Please note these two lines:
NSArray *copy = [_photos copy];
for (id p in copy) {
Before iterating through _photos, a new array copy is created to protect this iteration from _photos being modified in other places.
In your code, releaseAllUnderlyingPhotos: is deleting objects straight from the _photos array, but this array could be modified in other parts of the code, for example, didReceiveMemoryWarning, as it is your case.
Once you modify your code to iterate over a copy of _photos in releaseAllUnderlyingPhotos: your problem should go away.