Create closure type; allow async event handlers in props; allow short hand event handlers (#2437)

* create closure type; allow async event handlers in props; allow shorthand event handlers

* test forwarding event handlers with the shorthand syntax

* fix clippy

* fix imports in spawn async doctest

packages/core-macro/tests/event_handler.rs

@@ -0,0 +1,65 @@
+use dioxus::prelude::*;
+
+// This test just checks that event handlers compile without explicit type annotations
+// It will not actually run any code
+#[test]
+#[allow(unused)]
+fn event_handlers_compile() {
+    fn app() -> Element {
+        let mut todos = use_signal(String::new);
+        rsx! {
+            input {
+                // Normal event handlers work without explicit type annotations
+                oninput: move |evt| todos.set(evt.value()),
+            }
+            button {
+                // async event handlers work without explicit type annotations
+                onclick: |event| async move {
+                    println!("{event:?}");
+                },
+            }
+
+            // New! You can now use async closures for custom event handlers!
+            // This shouldn't require an explicit type annotation
+            TakesEventHandler { onclick: |event| async move {
+                println!("{event:?}");
+            } }
+            // Or you can accept a callback that returns a value
+            // This shouldn't require an explicit type annotation
+            TakesEventHandlerWithArg { double: move |value| (value * 2) as i32 }
+        }
+    }
+
+    #[component]
+    fn TakesEventHandler(onclick: EventHandler<MouseEvent>) -> Element {
+        rsx! {
+            button {
+                // You can pass in EventHandlers directly to events
+                onclick: onclick,
+                "Click!"
+            }
+            button {
+                // Or use the shorthand syntax
+                onclick,
+                "Click!"
+            }
+
+            // You should also be able to forward event handlers to other components with the shorthand syntax
+            TakesEventHandler {
+                onclick
+            }
+        }
+    }
+
+    #[component]
+    fn TakesEventHandlerWithArg(double: Callback<u32, i32>) -> Element {
+        let mut count = use_signal(|| 2);
+        rsx! {
+            button {
+                // Callbacks let you easily inject custom logic into your components
+                onclick: move |_| count.set(double(count()) as u32),
+                "{count}"
+            }
+        }
+    }
+}