feat(Meta): Add castCallback[1,2]ForDirective.

Closes #1489.

PiperOrigin-RevId: 204490128

angular/CHANGELOG.md

@@ -22,6 +22,13 @@
     </div>
     ```
 
+*   Added `package:angular/meta.dart`, a series of utilities for additional
+    static analysis checks and/or functions to retain semantics for migration
+    purposes, starting with `castCallback1ForDirective` and
+    `castCallback2ForDirective`. These methods are _only_ intended to be used
+    as stop-gaps for the lack of generic support in AngularDart directives and
+    components. Closes [#1489][].
+
 ## Bug fixes
 
 *   Fails the build immediately if an element in a component's `pipes` list is
@@ -53,6 +60,7 @@
 [#1295]: https://github.com/dart-lang/angular/issues/1295
 [#1455]: https://github.com/dart-lang/angular/issues/1455
 [#1484]: https://github.com/dart-lang/angular/issues/1484
+[#1849]: https://github.com/dart-lang/angular/issues/1489
 
 ## 5.0.0-beta
 

angular/lib/meta.dart

@@ -0,0 +1,62 @@
+/// Utilities and metadata annotations for static analysis purposes.
+@experimental
+library angular.meta;
+
+// This line is to force that `package:angular/meta.dart` is not used in the VM.
+//
+// ignore: unused_import
+import 'dart:html';
+
+import 'package:meta/meta.dart';
+
+/// Wraps a typed [callback] with a single parameter of type [A].
+///
+/// This function returns an _untyped_ callback with a single parameter
+/// of type `dynamic`, which in turn dynamically is casted/checked to
+/// [A] before invoking [callback].
+///
+/// Common usage is when creating a callback to pass to a `@Component`
+/// that is expecting generic type parameters, but is missing them due
+/// to missing support in Angulardart:
+/// ```dart
+/// typedef ItemRenderer<T> = String Function(T);
+///
+/// abstract class Dog {
+///   String get name;
+/// }
+///
+/// @Component(
+///   selector: 'my-component',
+///   directives: [ItemListComponent],
+/// )
+/// class MyComponent {
+///   final itemRenderer = castCallback1((Dog dog) => dog.name);
+/// }
+///
+/// class ItemListComponent<T> {
+///   @Input()
+///   ItemRenderer<T> itemRenderer;
+/// }
+/// ```
+///
+/// **NOTE**: This method is _only_ intended as a workaround for the lack-of
+/// reified generics for components and directives. Do **not** use as a general
+/// purpose workaround: https://github.com/dart-lang/angular/issues/68
+T Function(dynamic) castCallback1ForDirective<T, A>(
+  T Function(A) callback,
+) {
+  return (element) => callback(element as A);
+}
+
+/// Wraps a typed [callback] with two parameters of types [A] and [B].
+///
+/// See [castCallback2ForDirective] for details.
+///
+/// **NOTE**: This method is _only_ intended as a workaround for the lack-of
+/// reified generics for components and directives. Do **not** use as a general
+/// purpose workaround: https://github.com/dart-lang/angular/issues/68
+T Function(dynamic, dynamic) castCallback2ForDirective<T, A, B>(
+  T Function(A, B) callback,
+) {
+  return (a, b) => callback(a as A, b as B);
+}