xcode ios ARC (automatic reference counting) regarding dispatch_async and nsdata - objective-c

Question

I have a beginner question about xcode ARC. The following code works without memory issue because the memory is freed by the ARC.
- (void)viewDidLoad
{
[super viewDidLoad];
// test nsmutabledata
dispatch_queue_t testQueue = dispatch_queue_create("testQueue", NULL);
dispatch_async(testQueue, ^{
while (1) {
NSMutableData *testData = [[NSMutableData alloc]initWithCapacity:1024*1024*5];
NSLog(#"testData size: %d", testData.length);
}
});
}
However, the following does not, and gives me memory allocation error after a few seconds.
+ (NSMutableData *) testDataMethod
{
NSMutableData *testDataLocal = [[NSMutableData alloc]initWithCapacity:1024*1024*5];
return testDataLocal;
}
- (void)viewDidLoad
{
[super viewDidLoad];
// test nsmutabledata
dispatch_queue_t testQueue = dispatch_queue_create("testQueue", NULL);
dispatch_async(testQueue, ^{
while (1) {
NSMutableData *testData = [RootViewController testDataMethod];
NSLog(#"testData size: %d", testData.length);
}
});
}
Do I have the wrong understanding of ARC? I though the testDataLocal is counted once but goes out of the scope when the method exits. testData is another count but at the next iteration of the loop testData should have no count, and be freed by the system.

Answer 1

In the first bit of code, the NSMutableData object is released at the end of each loop iteration which avoids any memory issues.
In the second bit of code, the return value of the testDataMethod is most likely being autoreleased. Since your app in running in a tight loop, the autorelease pool in never given a chance to be flushed so you quickly run out of memory.
Try changing your second bit of code to this:
while (1) {
#autoreleasepool {
NSMutableData *testData = [RootViewController testDataMethod];
NSLog(#"testData size: %d", testData.length);
}
}