Open Radar

 
Area:
AppKit

Summary:
Remove a subview from an NSSplitView while it is being dragged. It throws an exception.

Steps to Reproduce:
1. Install https://developer.apple.com/library/archive/samplecode/SplitViews/Introduction/Intro.html
2. Modify -[AppDelegate doVerticalSplit:] to remove the second subview of the splitview after a delay*
3. Run the app
4. Open the vertical split window
5. Drag the divider and keep dragging until the subview is removed

* Add this to -doVerticalSplit:

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSView *view = _verticalSplitWindController.window.contentView.subviews[0].subviews[1];
        [view removeFromSuperview];
    });

Expected Results:
A graceful exit.

While dragging happens in a tracking runloop, the application still has plenty of opportunities to modify its view hierarchy. In my case, a subview is removed when a child process terminates. It's overly burdensome to expect the application to check if a drag is in progress when it modifies a split view's subviews. This is also hard to work around because once -[NSSplitView mouseDown:] is called the application has way to tell the splitview to cancel an ongoing drag. Catching the exception is risky since it could leave the splitview in an inconsistent state.

Actual Results:
    frame #0: 0x00007fff68543c46 libobjc.A.dylib`objc_exception_throw
    frame #1: 0x00007fff41489b6f CoreFoundation`-[__NSSingleObjectArrayI objectAtIndex:] + 111
    frame #2: 0x00007fff3ea962e5 AppKit`-[NSSplitView _presentDividerDragResult:withParams:] + 1558
    frame #3: 0x00007fff3ec6ab76 AppKit`-[NSSplitView mouseDown:] + 1504
    frame #4: 0x00007fff3f211d6d AppKit`-[NSWindow(NSEventRouting) _handleMouseDownEvent:isDelayedEvent:] + 5891
    frame #5: 0x00007fff3f20e9c4 AppKit`-[NSWindow(NSEventRouting) _reallySendEvent:isDelayedEvent:] + 2359
    frame #6: 0x00007fff3f20dc70 AppKit`-[NSWindow(NSEventRouting) sendEvent:] + 497
    frame #7: 0x00007fff3f06f236 AppKit`-[NSApplication(NSEvent) sendEvent:] + 2462
    frame #8: 0x00007fff3e8cf8b5 AppKit`-[NSApplication run] + 812
    frame #9: 0x00007fff3e89ea72 AppKit`NSApplicationMain + 804
  * frame #10: 0x0000000100001df2 SplitViews`main(argc=3, argv=0x00007ffeefbff5e8) at main.m:13
    frame #11: 0x0000000100001dc4 SplitViews`start + 52


Version/Build:
10.13 SDK
macOS 10.13.5

Configuration: