Removing object from NSMutableArray after upgrading to SDK 5 for iOS causes my app to crash - objective-c

Question

[Edit] I rearranged how I remove obj from the list. That's originally how it was still causing the same error.
This is the code in question. I believe I have the error narrowed down to the line where it removes the object from the _blocks NSMutableArray. I have this exact same code in other parts of my code removing similar object from the same array. For some reason when this function is called it causes the game to crash. It didn't have this problem before I upgraded to the latest XCode which supports iOS SDK 5 and armv7. Before I upgraded XCode it worked fine. I would walk into an item and the item would disappear from the screen. Now it just crashes when I get an item. Any help would be greatly appreciated.
-(void)itemCollision:(Collidable *)obj :(int)itemID :(int)objID: (bool)withPlayer{
[background removeChild:[obj getSprite] cleanup:YES];
[background removeChild:[obj getArrow] cleanup:YES];
[_blocks removeObject:obj];
//[obj release];
if(withPlayer){
if(itemID == 1){
[[SimpleAudioEngine sharedEngine] playEffect:#"Item2.mp3" pitch:1.0f pan:0.0f gain:0.3f];
points += (10 + level * 22);
}
}
}
#import <UIKit/UIKit.h>
int main(int argc, char *argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
int retVal = UIApplicationMain(argc, argv, nil, #"AppDelegate");
[pool release];
return retVal;
}
When the app crashes it highlights this line:
int retVal = UIApplicationMain(argc, argv, nil, #"AppDelegate");
The error message is: "Thread 1: Program received signal: "SIGARBRT".
Here is the code where I call itemCollision:
-(bool) isCollision {
CCSprite *playerSprite = mob;//[mob getSprite];
CGRect playerRect = CGRectMake(playerSprite.position.x - (playerSprite.contentSize.width/2),
playerSprite.position.y - (playerSprite.contentSize.height/2),
playerSprite.contentSize.width,
playerSprite.contentSize.height);
//Player Collision
BOOL collision = FALSE;
for (Block *block in _blocks) {
CCSprite *blockSprite = [block getSprite];
CGRect blockRect = CGRectMake(blockSprite.position.x - (blockSprite.contentSize.width/2),
blockSprite.position.y - (blockSprite.contentSize.height/2),
blockSprite.contentSize.width,
blockSprite.contentSize.height + 1 + [mob getSpeed]);
if (CGRectIntersectsRect(blockRect, playerRect)) {
if ([block getItemID] != 0) {
[self itemCollision:block :[block getItemID] :[block getID] :TRUE];
continue;
}
if(playerSprite.position.y > blockSprite.position.y){
collision = TRUE;
}
if(collision) {
[[SimpleAudioEngine sharedEngine] playEffect:#"squish.caf"];
[self die];
return TRUE;
}
}
}
return FALSE;
}
Here is how I remove objects from _blocks elsewhere in the code. This is in the gravity timer function:
NSMutableArray *blocksToDelete = [[NSMutableArray alloc] init];
for (Block *block in _blocks) {
CCSprite *blockSprite = [block getSprite];
//[block addSpeed:gravity];
[block setPreX:blockSprite.position.x];
[block setPreY:blockSprite.position.y];
blockSprite.position = ccp(blockSprite.position.x, blockSprite.position.y - [block getSpeed]);
if (blockSprite.position.y < -30 + blockSprite.contentSize.height / 2) {
[blocksToDelete addObject:block];
}
if(blockSprite.position.y - blockSprite.contentSize.height < [block getArrow].position.y){
[block getArrow].visible = FALSE;
}
}
for (Block *b in blocksToDelete) {
[_blocks removeObject:b];
[background removeChild:[b getSprite] cleanup:YES];
[background removeChild:[b getArrow] cleanup:YES];
[b release];
}
I get to the items before they reach the point where they get deleted by this function. Again, this code has been working for months until I upgraded my XCode.

Answer 1

If the array had the last retain on the object, it will be dealloced when you remove it. You go on to use the object for the rest of the method, so it needs to stay alive. Certainly checking the retainCount of obj to see if the NSMutableArray has the last retain is the first place to start.
If we assume, for the moment, that that's the case, then try this alternate approach:
-(void)itemCollision:(Collidable *)obj :(int)itemID :(int)objID: (bool)withPlayer
{
[[obj retain] autorelease];
[_blocks removeObject:obj];
[background removeChild:[obj getSprite] cleanup:YES];
[background removeChild:[obj getArrow] cleanup:YES];
if(withPlayer){
if(itemID == 1){
[[SimpleAudioEngine sharedEngine] playEffect:#"Item2.mp3" pitch:1.0f pan:0.0f gain:0.3f];
points += (10 + level * 22);
}
}
}
When the debugger goes to main.m, it's often because it's dying while popping an autorelease pool -- a task you don't have debug symbols for, so it goes to the deepest stack frame for which it has source to show you, which in the case of autorelease pools on the main thread, will be your main() function.
You should also try running this case with the Zombies instrument in Instruments. It can be very helpful for tracking this stuff down.

Answer 2

It's hard to know exactly what's happening beyond the scope of the code sample you've shared, but a wild guess would be that your 'obj' is being over-released. It's released once when you remove it from the array, then you're releasing it again. This may or may not be appropriate considering the retain count of that object as its passed in.
To begin troubleshooting, try using the Instrument Zombie tool - that'll help you observe if it is indeed being overreleased.
You could additionally comment out your:
[obj release]
To see if that stops the error (which may or may not be the solution but should be insightful in any case).