Substitute keystroke - objective-c

Question

I am trying to intercept a keystroke, and substitute it with a different character. I have been able to intercept the key being pressed, as well as perform some extra operations. Now I need to hold the key being pressed if it matches one of the ones I am watching, and insert a different character. Here is the code I have right now:
#import "AppDelegate.h"
#import <AppKit/AppKit.h>
#import <CoreGraphics/CoreGraphics.h>
#include <ApplicationServices/ApplicationServices.h>
#interface AppDelegate ()
#property (weak) IBOutlet NSWindow *window;
#end
#implementation AppDelegate
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
//Keys that are being watched to be switched out
NSArray *keysToWatch = [[NSArray alloc] initWithObjects:#"c",#".", nil];
// register for keys throughout the device...
[NSEvent addGlobalMonitorForEventsMatchingMask:NSKeyDownMask
handler:^(NSEvent *event){
//Get characters
NSString *chars = [[event characters] lowercaseString];
//Get the actual character being pressed
unichar character = [chars characterAtIndex:0];
//Transform it to a string
NSString *aString = [NSString stringWithCharacters:&character length:1];
//If it is in the list, start looking if Keynote is active
if ([keysToWatch containsObject:[NSString stringWithString:aString]]) {
//DEBUG: Print a message
NSLog(#"Key being watched has been pressed");
//Get a list of all running apps
for (NSRunningApplication *currApp in [[NSWorkspace sharedWorkspace] runningApplications]) {
//Get current active app
if ([currApp isActive]) {
//Check if it is Keynote, if yes perform remap
if ([[currApp localizedName] isEqualToString:#"Keynote"]){
//DEBUG: Print a small message
NSLog(#"Current app is Keynote");
if (character=='.') {
NSLog(#"Pressed a dot");
//I want to post a different character here
PostKeyWithModifiers((CGKeyCode)11, FALSE);
}
else if ([aString isEqualToString:#"c"]) {
NSLog(#"Pressed c");
}
}
else if ([[currApp localizedName] isEqualToString:#"Microsoft PowerPoint"]){
}
}
}
}
}
];
}
- (void)applicationWillTerminate:(NSNotification *)aNotification {
// Insert code here to tear down your application
}
- (BOOL)acceptsFirstResponder {
return YES;
}
void PostKeyWithModifiers(CGKeyCode key, CGEventFlags modifiers)
{
CGEventSourceRef source = CGEventSourceCreate(kCGEventSourceStateCombinedSessionState);
CGEventRef keyDown = CGEventCreateKeyboardEvent(source, key, TRUE);
CGEventSetFlags(keyDown, modifiers);
CGEventRef keyUp = CGEventCreateKeyboardEvent(source, key, FALSE);
CGEventPost(kCGAnnotatedSessionEventTap, keyDown);
CGEventPost(kCGAnnotatedSessionEventTap, keyUp);
CFRelease(keyUp);
CFRelease(keyDown);
CFRelease(source);
}
#end
My problem is that I am not able to stop the original keystroke. Please keep in mind that I am completely new at Obj C, so let me know if there is anything that I can do better. Thanks!

Answer 1

From the docs for +[NSEvent addGlobalMonitorForEventsMatchingMask:handler:]:
Events are delivered asynchronously to your app and you can only
observe the event; you cannot modify or otherwise prevent the event
from being delivered to its original target application.
You would have to use a Quartz Event Tap for that.

Answer 2

So, after a LOT of digging around, I found a way to do this using Quartz Event Taps here. Thanks to #Ken Thomases for pointing me in the correct direction. I then combined my code with the one explained in this article and voilà, it works.