I had a problem where I was using performSelectorOnMainThread:… to update UI but was getting exceptions when an update occurred whilst the mouse was down for a user. Using the modes parameter of performSelectorOnMainThread:… doesn’t work since even though NSTextView’s mouseDown function runs the event loop in NSEventTrackingRunLoopMode, the queuePeriodicEvent framework function gets called which runs the loop in NSDefaultRunLoopMode.

To workaround the problem, I’ve used the following code, i.e. basically sub-classed NSApplication and overridden sendEvent.

@interface NSObject (CSSafePerform)

• (void)safelyPerformSelectorOnMainThread:(SEL)selector withObject:(id)arg; @end

@interface NSApplication (CSSafePerform)

• (void)safelyPerformSelectorOnMainThread:(SEL)selector target:(id)self withObject:(id)arg; @end

@interface CSApplication : NSApplication { BOOL processingEvent; NSMutableArray *performQueue; } @end

@implementation NSObject (CSSafePerform)

• (void)safelyPerformSelectorOnMainThread:(SEL)selector withObject:(id)arg { NSAssert ([NSApp isKindOfClass:[CSApplication class]], @”safelyPerformSelectorOnMainThread not supported”);

  [NSApp safelyPerformSelectorOnMainThread:selector target:self withObject:arg]; } @end

@implementation CSApplication

typedef struct { SEL selector; id target; id arg; } PerformInfo;

• (void)performOnMainThread:(NSValue *)info { if (processingEvent) { if (!performQueue) performQueue = [[NSMutableArray alloc] init]; [performQueue addObject:info]; } else { PerformInfo pi;

  [info getValue:&pi];

  [pi.target performSelector:pi.selector withObject:pi.arg];

  [pi.target release]; [pi.arg release]; } }

• (void)safelyPerformSelectorOnMainThread:(SEL)selector target:(id)target withObject:(id)arg { PerformInfo pi = (PerformInfo) { selector, [target retain], [arg retain] };

  NSValue *v = [NSValue valueWithBytes:&pi objCType:@encode (PerformInfo)];

  [self performSelectorOnMainThread:@selector (performOnMainThread:) withObject:v waitUntilDone:NO]; }

• (void)sendEvent:(NSEvent *)anEvent { BOOL oldProcessingEvent = processingEvent;

  processingEvent = YES; @try { [super sendEvent:anEvent]; } @finally { if (!oldProcessingEvent) { processingEvent = NO; if (performQueue) { NSEnumerator *e = [performQueue objectEnumerator]; NSValue *v;

  [performQueue release]; performQueue = nil;

  while ((v = [e nextObject])) [self performOnMainThread:v]; } } } }

@end

Chris Suter, Coriolis Systems