feat: Add Signal.not (#22842)

Author: Legioth

signals/src/main/java/com/vaadin/signals/Signal.java

@@ -131,6 +131,20 @@ static <T> Signal<T> computed(Supplier<T> computation) {
         return new ComputedSignal<>(computation);
     }
 
+    /**
+     * Crates a new computed signal containing the negation of the provided
+     * boolean-valued signal. <code>null</code> values are preserved as
+     * <code>null</code>.
+     * 
+     * @param signal
+     *            the boolean-valued signal to negate, not <code>null</code>
+     * @return the negated signal, not <code>null</code>
+     */
+    static Signal<Boolean> not(Signal<Boolean> signal) {
+        return Objects.requireNonNull(signal)
+                .map(value -> value == null ? null : !value);
+    }
+
     /**
      * Runs the provided supplier in a transaction. All signal operations
      * performed within the transaction will be staged and atomically committed

signals/src/test/java/com/vaadin/signals/impl/ComputedSignalTest.java

@@ -136,6 +136,20 @@ void map_countCallbackInvocations_invocationsAreNotCached() {
         assertEquals(2, count.get());
     }
 
+    @Test
+    void not_booleanInputs_negatedOutputs() {
+        ValueSignal<Boolean> signal = new ValueSignal<>(Boolean.TRUE);
+        Signal<Boolean> negated = Signal.not(signal);
+
+        assertFalse(negated.value());
+
+        signal.value(false);
+        assertTrue(negated.value());
+
+        signal.value(null);
+        assertNull(negated.value());
+    }
+
     @Test
     void callback_updateOtherSignal_signalUpdated() {
         ValueSignal<String> other = new ValueSignal<>("value");