fix: prevents the shortcut's callback from being executing on registering of duplicated shortcuts

When registering a duplicated shortcut, the `addShortcut` and
`addSequenceShortcut` methods return `of(null)` which immediately emits on
subscribe. In case of duplicated shortcuts, we instead return EMPTY which
doesn't emit and completes immediately on subscribe.

Author: Ghislain Thau

README.md

@@ -300,6 +300,8 @@ That's all for now! Make sure to check out the `playground` inside the `src` [fo
 
 No. It's not possible to define a hotkey multiple times. Each hotkey has a description and a group, so it doesn't make sense assigning a hotkey to different actions.
 
+In case of registering duplicated hotkeys, the `addShortcut` and `addSequenceShortcut` methods return an `EMPTY` observable, allowing to detect duplicated hotkeys registration and execute custom logic.
+
 **Why am I not receiving any event?**
 
 If you've added a hotkey to a particular element of your DOM, make sure it's focusable. Otherwise, hotkeys cannot capture any keyboard event.

projects/ngneat/hotkeys/src/lib/hotkeys.service.ts

@@ -137,7 +137,7 @@ export class HotkeysService {
 
         if (sequenceSummary.hotkeyMap.has(normalizedKeys)) {
           console.error('Duplicated shortcut');
-          return of(null);
+          return EMPTY;
         }
 
         sequenceSummary.hotkeyMap.set(normalizedKeys, hotkeySummary);
@@ -170,7 +170,7 @@ export class HotkeysService {
 
     if (this.hotkeys.has(normalizedKeys)) {
       console.error('Duplicated shortcut');
-      return of(null);
+      return EMPTY;
     }
 
     this.hotkeys.set(normalizedKeys, mergedOptions);

projects/ngneat/hotkeys/src/lib/tests/hotkeys.service.spec.ts

@@ -27,6 +27,16 @@ describe('Service: Hotkeys', () => {
     expect(spectator.service.getHotkeys().length).toBe(0);
   });
 
+  it('should not emit after registering duplicated shortcuts', () => {
+    const spyFcn = createSpy('subscribe', (...args) => {});
+    spectator.service.addShortcut({ keys: 'a' }).subscribe(spyFcn);
+    spectator.service.addShortcut({ keys: 'a' }).subscribe(spyFcn);
+    spectator.service.addSequenceShortcut({ keys: 'g>a' }).subscribe(spyFcn);
+    spectator.service.addSequenceShortcut({ keys: 'g>a' }).subscribe(spyFcn);
+
+    expect(spyFcn).not.toHaveBeenCalled();
+  });
+
   it('should unsubscribe shortcuts when removed', () => {
     const subscription = spectator.service.addShortcut({ keys: 'meta.a' }).subscribe();
     spectator.service.removeShortcuts('meta.a');