Initial import from SO

http://stackoverflow.com/questions/19646108/modify-keydown-output

.gitignore

@@ -0,0 +1 @@
+/debounce

debounce.m

@@ -0,0 +1,127 @@
+// compile and run from the commandline with:
+//    clang -fobjc-arc -framework Cocoa  ./foo.m  -o foo
+//    sudo ./foo 
+
+#import <Foundation/Foundation.h>
+#import <AppKit/NSEvent.h>
+
+typedef CFMachPortRef EventTap;
+
+@interface KeyChanger : NSObject
+{
+@private
+    EventTap _eventTap;
+    CFRunLoopSourceRef _runLoopSource;
+    CGEventRef _lastEvent;
+}
+@end
+
+CGEventRef _tapCallback(CGEventTapProxy proxy, CGEventType type, CGEventRef event, KeyChanger* listener);
+
+@implementation KeyChanger
+
+- (BOOL)tapEvents
+{
+    if (!_eventTap) {
+        NSLog(@"Initializing an event tap.");
+
+        _eventTap = CGEventTapCreate(kCGSessionEventTap,
+                                     kCGTailAppendEventTap,
+                                     kCGEventTapOptionDefault,
+                                     CGEventMaskBit(kCGEventKeyDown),
+                                     (CGEventTapCallBack)_tapCallback,
+                                     (__bridge void *)(self));
+        if (!_eventTap) {
+            NSLog(@"unable to create event tap. must run as root or add privlidges for assistive devices to this app.");
+            return NO;
+        }
+    }
+    CGEventTapEnable(_eventTap, TRUE);
+
+    return [self isTapActive];
+}
+
+- (BOOL)isTapActive
+{
+    return CGEventTapIsEnabled(_eventTap);
+}
+
+- (void)listen
+{
+    if (!_runLoopSource) {
+        if (_eventTap) {//dont use [self tapActive]
+            _runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault,
+                                                           _eventTap, 0);
+            // Add to the current run loop.
+            CFRunLoopAddSource(CFRunLoopGetCurrent(), _runLoopSource,
+                               kCFRunLoopCommonModes);
+
+            NSLog(@"Registering event tap as run loop source.");
+            CFRunLoopRun();
+        }else{
+            NSLog(@"No Event tap in place! You will need to call listen after tapEvents to get events.");
+        }
+    }
+}
+
+- (CGEventRef)processEvent:(CGEventRef)cgEvent
+{
+    NSEvent* event = [NSEvent eventWithCGEvent:cgEvent];
+
+    // TODO: add other cases and do proper handling of case
+    if ([event.characters caseInsensitiveCompare:@"k"] == NSOrderedSame) {
+        event = [NSEvent keyEventWithType:event.type
+                                 location:NSZeroPoint
+                            modifierFlags:event.modifierFlags
+                                timestamp:event.timestamp
+                             windowNumber:event.windowNumber
+                                  context:event.context
+                               characters:@"к"
+              charactersIgnoringModifiers:@"к"
+                                isARepeat:event.isARepeat
+                                  keyCode:event.keyCode];
+    }
+    _lastEvent = [event CGEvent];
+    CFRetain(_lastEvent); // must retain the event. will be released by the system
+    return _lastEvent;
+}
+
+- (void)dealloc
+{
+    if (_runLoopSource){
+        CFRunLoopRemoveSource(CFRunLoopGetCurrent(), _runLoopSource, kCFRunLoopCommonModes);
+        CFRelease(_runLoopSource);
+    }
+    if (_eventTap){
+        //kill the event tap
+        CGEventTapEnable(_eventTap, FALSE);
+        CFRelease(_eventTap);
+    }
+}
+
+@end
+CGEventRef _tapCallback(CGEventTapProxy proxy, CGEventType type, CGEventRef event, KeyChanger* listener) {
+    //Do not make the NSEvent here.
+    //NSEvent will throw an exception if we try to make an event from the tap timout type
+    @autoreleasepool {
+        if(type == kCGEventTapDisabledByTimeout) {
+            NSLog(@"event tap has timed out, re-enabling tap");
+            [listener tapEvents];
+            return nil;
+        }
+        if (type != kCGEventTapDisabledByUserInput) {
+            return [listener processEvent:event];
+        }
+    }
+    return event;
+}
+
+int main(int argc, const char * argv[])
+{
+    @autoreleasepool {
+        KeyChanger* keyChanger = [KeyChanger new];
+        [keyChanger tapEvents];
+        [keyChanger listen];//blocking call.
+    }
+    return 0;
+}