Why is NSOpenPanel/NSSavePanel showing memory leak?

Question

Not sure why, but making a simple [[NSOpenPanel openPanel] runModal]; creates a memory leak - seen in Leaks Instrument.

Seems off.

It's an auto-released object, shouldn't it be automatically released after ARpool is drained?

Is there a way to fix this?

Answer 1

NSOpenPanel is a singleton, which means you always get the same instance of the object every time you use it. This means that the first time you call [NSOpenPanel openPanel], an instance of NSOpenPanel is created and not released.

This is not a leak, it's an optimisation. However, sometimes the Leaks instrument picks up such once-only instantiations as leaks because the instances are (by design) never released.

NSOpenPanel is such a widely-used and tested class that any leaks in its standard implementation are unlikely to exist.

Answer 2

NSOpenPanel is not a singleton. It may have been at one time but looking at the latest NSOpenPanel.h file makes it explicitly clear it is not a singleton or at the very least Apple doesn't want you taking advantage of this implementation detail.

As for the leak, I was confused on when I should release my open panel and was retaining it. From the Using the Open and Save Panels section of the File System Programming Guide your life is a lot easier in 10.7 and above:

Important: In OS X 10.6 and earlier, you must retain an open panel prior to displaying it and release it when you are done with it. Because the openPanel method returns an autoreleased object, the panel is normally released shortly after it appears on screen. Retaining the panel prevents it from being deallocated and dismissed prematurely. You do not need to retain the panel if it is attached to a window and you do not need to retain the panel in OS X 10.7 and when using ARC.

Once I stopped retaining it things got easier and became a lot easier :)

Answer 3

Instruments is not perfect at detecting leaks - particularly for autoreleased objects, and has a tendency to have false positives. You might try creating a new NSAutoreleasePool, then draining it when you are finished with the NSOpenPanel to force release early - but I suspect you don't actually have a leak. If you are confident that the code looks good and it is autoreleased, then its probably fine.

Answer 4

I was seeing "leaks" reported in Xcode's memory graph tool when using NSOpenPanel in an unsandboxed app built on OS X 10.11.6 using the 10.12 SDK and Swift 3.0.1. The "leaks" were reported in PlugInKit classes (PKHostPlugin, PKDiscoveryDriver, etc.) and would show up even if the only line of code was let openPanel = NSOpenPanel().

NSOpenPanel's documentation states

In a sandboxed environment, Open panels are drawn in a separate process by the powerbox, not by AppKit itself. When the user chooses a file to open, macOS adds that file to the app’s sandbox.

After I sandboxed the application, the "leaks" did not show up in Xcode's memory graph as the NSOpenPanel implementation code was no longer in the application's address space, so I no longer had to worry about it.