Add dispose to FutureProvider

Author: RossComputerGuy

packages/provider/lib/src/async_provider.dart

@@ -181,10 +181,15 @@ class FutureProvider<T> extends DeferredInheritedProvider<Future<T>?, T> {
   /// Creates a [Future] from `create` and subscribes to it.
   ///
   /// `create` must not be `null`.
+  ///
+  /// The optional [dispose] callback is invoked with the last exposed value
+  /// when the provider is removed from the widget tree, allowing cleanup of
+  /// resources (closing streams, cancelling timers, etc.).
   FutureProvider({
     Key? key,
     required Create<Future<T>?> create,
     required T initialData,
+    Dispose<T>? dispose,
     ErrorBuilder<T>? catchError,
     UpdateShouldNotify<T>? updateShouldNotify,
     bool? lazy,
@@ -195,6 +200,7 @@ class FutureProvider<T> extends DeferredInheritedProvider<Future<T>?, T> {
           lazy: lazy,
           builder: builder,
           create: create,
+          dispose: dispose,
           updateShouldNotify: updateShouldNotify,
           startListening: _futureStartListening(
             catchError: catchError,

packages/provider/lib/src/deferred_inherited_provider.dart

@@ -35,7 +35,7 @@ class DeferredInheritedProvider<T, R> extends InheritedProvider<R> {
   DeferredInheritedProvider({
     Key? key,
     required Create<T> create,
-    Dispose<T>? dispose,
+    Dispose<R>? dispose,
     required DeferredStartListening<T, R> startListening,
     UpdateShouldNotify<R>? updateShouldNotify,
     bool? lazy,
@@ -165,7 +165,7 @@ class _CreateDeferredInheritedProvider<T, R> extends _DeferredDelegate<T, R> {
   }) : super(updateShouldNotify, startListening);
 
   final Create<T> create;
-  final Dispose<T>? dispose;
+  final Dispose<R>? dispose;
 
   @override
   _CreateDeferredInheritedProviderElement<T, R> createState() {
@@ -220,7 +220,13 @@ class _CreateDeferredInheritedProviderElement<T, R>
   void dispose() {
     super.dispose();
     if (_didBuild) {
-      delegate.dispose?.call(element!, _controller as T);
+      if (T is Future<R>) {
+        (_controller as Future<R>).then((value) => delegate.dispose?.call(element!, value));
+      } else if (isLoaded) {
+        delegate.dispose?.call(element!, value);
+      } else {
+        delegate.dispose?.call(element!, _controller as R);
+      }
     }
   }