释放ARC / non-ARC中的对象会使内存处于使用状态

Question

I have a general issue with memory management. I can create an object with the following code, fill it with data, then clean up and release it, but even after the object is released the memory it used is still in use.

- (void)viewDidLoad {
    [super viewDidLoad];
        // Do any additional setup after loading the view, typically from a nib.
    NSBundle *bundle1 = [NSBundle mainBundle];
    NSString *path = [bundle1 pathForResource:@"Oxford Latin Dictionary - Optimized" ofType:@"pdf"];
    NSURL *pathURL = [NSURL fileURLWithPath:path];
    PDFObject* pdfObject = [[PDFObject alloc] initWithURL:pathURL withCachedPages:25 startAtPage:1 withFrame:self.view.frame];
    [pdfObject readPdfAtPage:1];
    [pdfObject generateThumbnails:self.view.frame.size.width/10];
    [pdfObject cleanThumbnailsAndSubviews];
    [pdfObject clearMemory];
    [pdfObject release];
}

The program uses about 9MB before creating the pdfObject ( PDFObject* pdfObject = [[PDFObject alloc] init... ), it uses about 23MB when that object is initialized and set up ( [pdfObject generateThumbnails:self.view.frame.size.width/10]; ), then the program still uses about 23MB after all the objects within pdfObject are released and pdfObject itself is released. I have the same problem with ARC turned on and using NSObject = nil to force a release of the object. This eventually causes a crash when I try to create and destroy too many of these objects.

I must be missing some simple part of objective c memory management, but I thought I was following good practices (ie if you create an object you must eventually destroy it). Coming from a JAVA background doesn't help things.

Answer 1

Use ARC. Using manual reference counting in 2014 is just silly. It makes life harder without good reason.

Given that you are using manual reference counting, your code looks reasonable. You create a number of objects, but they all appear to be autoreleased temporary objects except the PDFObject that you alloc/init, and then release at the end.

My guess is that the PDFObject is doing image caching internally. That would cause your app's memory footprint to rise, but not in a bad way. If memory pressure increases, the system will flush cached images before taking more serious steps like sending you memory warnings or killing your app.

It's also possible that the PDFObject class has memory leaks in it, or that it's doing it's own caching on top of system-based image caching.

You may want to use the memory analysis Instruments to take a look at the objects that are adding to your app's memory footprint. Explaining how to do that is beyond the scope of a forum post however. There have been WWDC session videos in the past about this, and there are also quite a few blog posts and online tutorials out there that explain how to use the instruments tool to figure out why your app's memory use is growing.