Capture the space key in a window containing several NSTextFields - objective-c

Question

I have a large window with a number of NSTextField fields on it, and I would like to intercept the space bar even while any of the text fields have focus, so that pressing space at any time will cause a separate action.
Subclassing NSTextField and overriding -keyUp: allows me to detect the space key, but a blank is still inserted in the text field. -keyDown: does not fire at all.
For other keys like Return and the arrow keys, I could use the control:textView:doCommandBySelector: delegate method, but it does not fire with the space bar.
There's a lot of advice out there for NSTextView, but I have found none for NSTextField.

Answer 1

Have you experimented with adding an event monitor? You create them via the NSEvent class method addLocalMonitorForEventsMatchingMask(_:handler:)
and they give you first dibs on the events you specify. In your case you'd add a monitor for keyDown events, then in the associated block you decide what to do with them. At the end of the block you can return the event just as it arrived, in which case it will behave normally, you can swallow the event (return nil), or you can modify the event by creating a new event object and returning that instead. Here's an example:
// You can add monitors anywhere - I've chosen the AppDelegate for
// this simple example.
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
[NSEvent addLocalMonitorForEventsMatchingMask:NSKeyDownMask
handler:^NSEvent * (NSEvent * theEvent) {
if (theEvent.keyCode == 49) {
NSLog(#"swallowing spacebar");
theEvent = nil;
}
return theEvent;
}];
}

Answer 2

Since the behavior you're trying to implement is at the level of the window (or perhaps a view that's an ancestor of all of the text fields), you should consider implementing -performKeyEquivalent: at that level instead of trying to capture the space key in each individual text field.
In your implementation, check the event's modifierFlags to make sure none of Control, Option, or Command are pressed (assuming you only want to handle unmodified space key presses) and if its charactersIgnoringModifiers equals #" ". If so, do what you want and then return YES to indicate you've handled the key event. Otherwise, return NO to allow it to be handled normally.

Answer 3

If you know how to do it for NSTextView, then you know how to do it for NSTextField, because NSTextField uses an NSTextView called the "field editor" for editing. You can get the window's default field editor like so:
NSTextView* fieldEd = [myWindow fieldEditor: YES forObject: nil];
You can also use a window delegate to provide a custom field editor.

Answer 4

So you might want to look into ReactiveCocoa
Go to the video on this page and look at 5:14 to see something similar to what you might like.
ReactiveCocoa can be hooked into all your text fields and then any time a space is hit a signal handler could pick it up. (Probably, however, more than you want to get into).