objective c - Should NSPurgeableData be reusable after its contents have been discarded? -

i have code using nspurgeabledata working, until machine's memory usage started running high, @ point started exception thrown. message in exception:

*** terminating app due uncaught exception 'nsgenericexception', reason: '*** -[nspurgeabledata length]: accessed outside successful -begincontentaccess , -endcontentaccess calls.'

this essence of code:

- (nsimage *)imageproperty {     if (!self.purgeableimage || ![self.purgeableimage begincontentaccess]) {         if (!self.purgeableimage) {             self.purgeableimage = [nspurgeabledata data];         }          nsimage *image = // image          // line causes exception         [self.purgeableimage setdata: image.tiffrepresentation];     } }  - (void)clearimage {     [self.purgeableimage endcontentaccess];      // putting in line works, shouldn't necessary, should it?     //self.purgeableimage = nil } 

clearimage balanced calls imageproperty. i'm able reproduce exception test case:

- (void)testimageproperty_calltwicewithmemorypressure {     myclass *instance = // initialize sucker      nsimage *image = instance.imageproperty;     nspurgeabledata *purgeabledata = image.purgeableimage;      xctassertnotnil(image, @"image not loaded");     xctassertequal(image.size.width, (cgfloat)988, @"image loaded incorrect width");     xctassertequal(image.size.height, (cgfloat)1500, @"image loaded incorrect height");      [instance clearimage];      // discard, if there memory pressure     [purgeabledata discardcontentifpossible];     xctasserttrue(purgeabledata.iscontentdiscarded, @"failed simulate memory pressure");      // triggers exception     image = instance.imageproperty;     xctassertnotnil(image, @"image not loaded on second attempt"); } 

i can eliminate exception uncommenting second line in -clearimage, didn't think necessary. shouldn't able continue using nspurgeabledata object after original data has been discarded?

update: additional workaround

the problem nilling out after calling -endcontentaccess class provides no caching. solution found update leading if statements so:

    if (!self.purgeableimage || self.purgeableimage.iscontentdiscarded || ![self.purgeableimage begincontentaccess]) {         if (!self.purgeableimage || self.purgeableimage.iscontentdiscarded) {