Tear Down for async code

teardown.m

@implementation Foo
{
    BOOL _needsTearDown;
}

- (instancetype)init;
{
    self = [super init];
    if (self) {
        _needsTearDown = YES;
        // initialization here
    }
    return self;
}

- (void)tearDown;
{
    if (_needsTearDown) {
        // tear down back pointers and async code here.
        _needsTearDown = NO;
    }
}

- (void)dealloc
{
    if (_needsTearDown) {
        __builtin_trap();
    }
}

@end