Introduce Task::map_with

runtime/src/task.rs

@@ -98,6 +98,50 @@ impl<T> Task<T> {
         self.then(move |output| Task::done(f(output)))
     }
 
+    /// Combines a prefix value with the result of the [`Task`] using
+    /// the provided closure.
+    ///
+    /// Sometimes you will want to identify the source or target
+    /// of some [`Task`] in your UI. This can be achieved through
+    /// normal means by using [`map`]:
+    ///
+    /// ```rust
+    /// # use iced_runtime::Task;
+    /// # let task = Task::none();
+    /// # enum Message { TaskCompleted(u32, ()) }
+    /// let id = 123;
+    ///
+    /// # let _ = {
+    /// task.map(move |result| Message::TaskCompleted(id, result))
+    /// # };
+    /// ```
+    ///
+    /// Quite a mouthful. [`map_with`] lets you write:
+    ///
+    /// ```rust
+    /// # use iced_runtime::Task;
+    /// # let task = Task::none();
+    /// # enum Message { TaskCompleted(u32, ()) }
+    /// # let id = 123;
+    /// # let _ = {
+    /// task.map_with(id, Message::TaskCompleted)
+    /// # };
+    /// ```
+    ///
+    /// Much nicer!
+    pub fn map_with<P, O>(
+        self,
+        prefix: P,
+        mut f: impl FnMut(P, T) -> O + MaybeSend + 'static,
+    ) -> Task<O>
+    where
+        T: MaybeSend + 'static,
+        P: MaybeSend + Clone + 'static,
+        O: MaybeSend + 'static,
+    {
+        self.map(move |result| f(prefix.clone(), result))
+    }
+
     /// Performs a new [`Task`] for every output of the current [`Task`] using the
     /// given closure.
     ///