osx - Animating and removing an NSImageView object failed? -

my app situation:
having app displaying dynamic picture. , want capture current image , save disc.

my goal:
working on animated visual effect: add image view on top, , animates origin frame cgrecrzero. finally, remove image view vie hierarchy after animation done.

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now problem:
first time triggerring action, seems work. when trigger @ second time, image view not animate , stay atop of subviews.

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codes of view controller(with arc):

firstly, ibaction trigger snapshot:

- (ibaction)actionsavesnapshot:(id)sender {     ...    // save image file. works everytime.      nsthread *tempthread = [[nsthread alloc] initwithtarget:self                                                    selector:@selector(threadsimulatescreencapture:)                                                       object:nil];     if (tempthread)     {         [tempthread start];         tempthread = nil;     // release      } } 

and here thread:

- (void)threadsimulatescreencapture:(id)arg {@autoreleasepool {     nsimageview __strong *subimageview;     subimageview = [[nsimageview alloc] init];     nsrect subrect = [_imageview frame];    // dynamic image view want snapshot     const cgfloat animationtime = 0.4;      [subimageview setwantslayer:yes];     [subimageview setimagescaling:nsimagescaleaxesindependently];     [subimageview setimage:[_imageview image]];      dispatch_async(dispatch_get_main_queue(), ^{         [catransaction begin];         [self.view addsubview:subimageview];         [subimageview setframe:subrect];         [subimageview setneedsdisplay:yes];         [catransaction commit];     });      [nsthread sleepfortimeinterval:0.01];     dispatch_async(dispatch_get_main_queue(), ^{         [catransaction begin];         [catransaction setanimationduration:animationtime];         [subimageview.layer setframe:cgrectzero];         [catransaction commit];     });      [nsthread sleepfortimeinterval:(nstimeinterval)animationtime];    // wait end of animation      dispatch_async(dispatch_get_main_queue(), ^{         [subimageview removefromsuperview];     });      nslog(@"subview count: %@\n%@", self.view.subviews, subimageview);  // mark. described below     subimageview = nil;    // release image }} // ends: thread , aotureleasepool 

every time program runs "mark" line , print subview array, clear thar subimageview not removed superview then. , every time thread ends, "deleted thread uncommitted catransaction ...." raised.

what wrong did do?

i may solved own problem: @ line [subimageview setframe:subrect];, insert line with: [subimageview.layer setframe:subrect];

meanwhile, add line before "addsubview:" method: [self.view setwantslayer:yes];.

it seemed work. not know why. why should set both view , layer's frame?