Display contextual NSMenu without blocking main thread

Question

I had some unexpected results when displaying a contextual NSMenu. I'm presenting the menu myself, not using the menu property of an NSResponder. It turned out, presenting an NSMenu is a blocking operation.

In a new template project I only added the following code in applicationDidLaunch:

NSMenu *menu = [[NSMenu alloc] initWithTitle:@"Menu"];
[menu addItemWithTitle:@"Item 1" action:@selector(selected:) keyEquivalent:@""];
[menu addItemWithTitle:@"Item 2" action:@selector(selected:) keyEquivalent:@""];
[menu addItemWithTitle:@"Item 3" action:@selector(selected:) keyEquivalent:@""];
[menu popUpMenuPositioningItem:[menu itemAtIndex:0] 
                    atLocation:CGPointMake(500, 500) 
                        inView:nil];
NSLog(@"Context menu shown.");

The popUpMenuPositioningItem:atLocation:inView: call doesn't return until I select an item or click outside to automatically dismiss the menu. The log is only printed once the menu is closed.

Why does this happen? And how can I prevent it? I found a couple of mentions of this problem, but the "solutions" always consisted of executing the other things in the background, instead of preventing NSMenu from blocking.

EDIT:

If I present the menu in a background thread, the menu becomes unresponsive, doesn't react to clicks inside or outside.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
    [[menu popUpMenuPositioningItem:[menu itemAtIndex:0] 
                         atLocation:CGPointMake(500, 500) 
                             inView:nil];
});

Can you give some more context? What do you want to do while the menu is up that you can't right now? — uliwitness, Apr 21 '13 at 13:51

score 2 · Accepted Answer · answered Apr 21 '13 at 13:57

I don't think what you ask for is possible, however, there are a few things you can do that look darned similar:

1) Use an NSPopover instead of a menu. That is asynchronous and intended to be used in many contexts where a popup menu could be used. If you need to support OS versions that don't have popovers, you can also always create an NSWindow with buttons in it.

2) If you are trying to run timers, or other operations that are scheduled on the run loop (e.g. downloads), you can schedule these in NSEventTrackingRunLoopMode. By default, they're only scheduled for NSDefaultRunLoopMode (which makes sense, because you don't want to cancel the user's menu selection when a timer triggers that shows an alert).

To be honest, what you're trying to do seems kinda pointless, confusing, backwards and an abuse of menus. But that's probably just because I don't know what you're trying to achieve. What's your high-level task? Why does what need to happen on the main thread while a menu is up. Can't you have that other thing (in your example the NSLog()) happening on a second thread and leave the menu on the main thread?

What if I want to redraw a view? `drawRect:` is called on the main thread isn't it? — DrummerB, Apr 21 '13 at 14:01
To display a view, call -display on the view/window. If you're on a non-main thread, you can use performSelectorOnMainThread:withObject:waitUntilDone:modes:, making sure that you pass in NSEventTrackingRunLoopMode, not just the default run loop mode. That queue is checked by the main run loop, which should mean it gets triggered by the event processing code that the menu runs to find out whether you've chosen a menu item or moved the mouse. — uliwitness, Apr 25 '13 at 12:29

Display contextual NSMenu without blocking main thread

1 Answers1