adding `set_fn` and `set_fn_silent` (#529)

* adding change and change_silent

* using name set_fn to more clearly align with set

* Use `f` for function name

Co-authored-by: Luke <37006668+lukechu10@users.noreply.github.com>

* Use `f` for function name

Co-authored-by: Luke <37006668+lukechu10@users.noreply.github.com>

Co-authored-by: Luke <37006668+lukechu10@users.noreply.github.com>

packages/sycamore-reactive/src/signal.rs

@@ -228,6 +228,25 @@ impl<T> Signal<T> {
         self.trigger_subscribers();
     }
 
+    /// Set the value of the state using a function that receives the current value.
+    ///
+    /// This will notify and update any effects and memos that depend on this value.
+    ///
+    /// # Example
+    /// ```
+    /// # use sycamore_reactive::*;
+    /// # create_scope_immediate(|cx| {
+    /// let state = create_signal(cx, 0);
+    /// assert_eq!(*state.get(), 0);
+    ///
+    /// state.set_fn(|n| n + 1);
+    /// assert_eq!(*state.get(), 1);
+    /// # });
+    /// ```
+    pub fn set_fn<F: Fn(&T) -> T>(&self, f: F) {
+        self.set(f(&self.get_untracked()));
+    }
+
     /// Set the current value of the state wrapped in a [`Rc`]. Unlike [`Signal::set()`], this
     /// method accepts the value wrapped in a [`Rc`] because the underlying storage is already using
     /// [`Rc`], thus preventing an unnecessary clone.
@@ -258,6 +277,13 @@ impl<T> Signal<T> {
         self.set_rc_silent(Rc::new(value));
     }
 
+    /// Set the value of the state using a function that receives the current value _without_ triggering subscribers.
+    ///
+    /// Make sure you know what you are doing because this can make state inconsistent.
+    pub fn set_fn_silent<F: Fn(&T) -> T>(&self, f: F) {
+        self.set_silent(f(&self.get_untracked()));
+    }
+
     /// Set the current value of the state wrapped in a [`Rc`] _without_ triggering subscribers.
     ///
     /// See the documentation for [`Signal::set_rc()`] for more information.
@@ -636,6 +662,9 @@ mod tests {
 
             state.set(1);
             assert_eq!(*state.get(), 1);
+
+            state.set_fn(|n| n + 1);
+            assert_eq!(*state.get(), 2);
         });
     }
 
@@ -660,6 +689,9 @@ mod tests {
             assert_eq!(*double.get(), 0);
             state.set_silent(1);
             assert_eq!(*double.get(), 0); // double value is unchanged.
+
+            state.set_fn_silent(|n| n + 1);
+            assert_eq!(*double.get(), 0); // double value is unchanged.
         });
     }