fix(upgrade): support input signal bindings (#57020)

`@angular/upgrade` writes to inputs when downgrading an Angular 2+ component
into an Angular.JS adapter. Previously, it wrote directly to the input
property, which isn't compatible with input signals. It also handles
`ngOnChanges` directly.

The correct way to support input signals would be to refactor upgrade to use
`ComponentRef.setInput`, which also handles `ngOnChanges` internally.
However, this refactoring might be more breaking since it would change the
timing of certain operations. Instead, this commit updates the code to
recognize `InputSignal` and write it through the `InputSignalNode`. This
avoids the above breaking changes for now, until a bigger refactoring can be
tested.

Fixes #56860.

PR Close #57020

goldens/public-api/core/index.api.md

@@ -1006,7 +1006,7 @@ export interface InputSignalWithTransform<T, TransformT> extends Signal<T> {
     // (undocumented)
     [ɵINPUT_SIGNAL_BRAND_WRITE_TYPE]: TransformT;
     // (undocumented)
-    [SIGNAL]: InputSignalNode<T, TransformT>;
+    [SIGNAL]: ɵInputSignalNode<T, TransformT>;
 }
 
 // @public
@@ -1149,7 +1149,7 @@ export interface ModelOptions {
 // @public
 export interface ModelSignal<T> extends WritableSignal<T>, InputSignal<T>, OutputRef<T> {
     // (undocumented)
-    [SIGNAL]: InputSignalNode<T, T>;
+    [SIGNAL]: ɵInputSignalNode<T, T>;
 }
 
 // @public @deprecated

packages/core/src/core_private_export.ts

@@ -137,3 +137,5 @@ export {isPromise as ɵisPromise, isSubscribable as ɵisSubscribable} from './ut
 export {performanceMarkFeature as ɵperformanceMarkFeature} from './util/performance';
 export {stringify as ɵstringify, truncateMiddle as ɵtruncateMiddle} from './util/stringify';
 export {NOT_FOUND_CHECK_ONLY_ELEMENT_INJECTOR as ɵNOT_FOUND_CHECK_ONLY_ELEMENT_INJECTOR} from './view/provider_flags';
+
+export {type InputSignalNode as ɵInputSignalNode} from './authoring/input/input_signal_node';

packages/core/src/core_reactivity_export_internal.ts

@@ -6,6 +6,8 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
+export {SIGNAL as ɵSIGNAL} from '@angular/core/primitives/signals';
+
 export {isSignal, Signal, ValueEqualityFn} from './render3/reactivity/api';
 export {computed, CreateComputedOptions} from './render3/reactivity/computed';
 export {

packages/upgrade/src/common/src/downgrade_component.ts

@@ -105,6 +105,8 @@ export function downgradeComponent(info: {
     $injector: IInjectorService,
     $parse: IParseService,
   ): IDirective {
+    const unsafelyOverwriteSignalInputs =
+      (info as {unsafelyOverwriteSignalInputs?: boolean}).unsafelyOverwriteSignalInputs ?? false;
     // When using `downgradeModule()`, we need to handle certain things specially. For example:
     // - We always need to attach the component view to the `ApplicationRef` for it to be
     //   dirty-checked.
@@ -216,6 +218,7 @@ export function downgradeComponent(info: {
             $parse,
             componentFactory,
             wrapCallback,
+            unsafelyOverwriteSignalInputs,
           );
 
           const projectableNodes = facade.compileContents();

packages/upgrade/src/common/src/downgrade_component_adapter.ts

@@ -19,6 +19,8 @@ import {
   StaticProvider,
   Testability,
   TestabilityRegistry,
+  type ɵInputSignalNode as InputSignalNode,
+  ɵSIGNAL as SIGNAL,
 } from '@angular/core';
 
 import {
@@ -53,6 +55,7 @@ export class DowngradeComponentAdapter {
     private $parse: IParseService,
     private componentFactory: ComponentFactory<any>,
     private wrapCallback: <T>(cb: () => T) => () => T,
+    private readonly unsafelyOverwriteSignalInputs: boolean,
   ) {
     this.componentScope = scope.$new();
   }
@@ -131,7 +134,7 @@ export class DowngradeComponentAdapter {
       let expr: string | null = null;
 
       if (attrs.hasOwnProperty(inputBinding.attr)) {
-        const observeFn = ((prop) => {
+        const observeFn = ((prop, isSignal) => {
           let prevValue = INITIAL_VALUE;
           return (currValue: any) => {
             // Initially, both `$observe()` and `$watch()` will call this function.
@@ -140,11 +143,11 @@ export class DowngradeComponentAdapter {
                 prevValue = currValue;
               }
 
-              this.updateInput(componentRef, prop, prevValue, currValue);
+              this.updateInput(componentRef, prop, prevValue, currValue, isSignal);
               prevValue = currValue;
             }
           };
-        })(inputBinding.prop);
+        })(inputBinding.prop, input.isSignal);
         attrs.$observe(inputBinding.attr, observeFn);
 
         // Use `$watch()` (in addition to `$observe()`) in order to initialize the input in time
@@ -166,9 +169,9 @@ export class DowngradeComponentAdapter {
       }
       if (expr != null) {
         const watchFn = (
-          (prop) => (currValue: unknown, prevValue: unknown) =>
-            this.updateInput(componentRef, prop, prevValue, currValue)
-        )(inputBinding.prop);
+          (prop, isSignal) => (currValue: unknown, prevValue: unknown) =>
+            this.updateInput(componentRef, prop, prevValue, currValue, isSignal)
+        )(inputBinding.prop, input.isSignal);
         this.componentScope.$watch(expr, watchFn);
       }
     }
@@ -314,13 +317,19 @@ export class DowngradeComponentAdapter {
     prop: string,
     prevValue: any,
     currValue: any,
+    isSignal: boolean,
   ) {
     if (this.implementsOnChanges) {
       this.inputChanges[prop] = new SimpleChange(prevValue, currValue, prevValue === currValue);
     }
 
     this.inputChangeCount++;
-    componentRef.instance[prop] = currValue;
+    if (isSignal && !this.unsafelyOverwriteSignalInputs) {
+      const node = componentRef.instance[prop][SIGNAL] as InputSignalNode<unknown, unknown>;
+      node.applyValueToInputSignal(node, currValue);
+    } else {
+      componentRef.instance[prop] = currValue;
+    }
   }
 
   private groupProjectableNodes() {

packages/upgrade/src/common/test/downgrade_component_adapter_spec.ts

@@ -181,6 +181,7 @@ withEachNg1Version(() => {
           $parse,
           componentFactory,
           wrapCallback,
+          /* unsafelyOverwriteSignalInputs */ false,
         );
       }
 

packages/upgrade/static/test/integration/downgrade_component_spec.ts

@@ -15,6 +15,7 @@ import {
   ElementRef,
   EventEmitter,
   Injector,
+  input,
   Input,
   NgModule,
   NgModuleRef,
@@ -171,6 +172,49 @@ withEachNg1Version(() => {
       });
     }));
 
+    it('should bind properties to signal inputs', waitForAsync(() => {
+      const ng1Module = angular.module_('ng1', []).run(($rootScope: angular.IScope) => {
+        $rootScope['name'] = 'world';
+      });
+
+      @Component({
+        selector: 'ng2',
+        inputs: ['message'],
+        template: 'Message: {{message()}}',
+      })
+      class Ng2Component {
+        message = input<string>('');
+      }
+
+      @NgModule({declarations: [Ng2Component], imports: [BrowserModule, UpgradeModule]})
+      class Ng2Module {
+        ngDoBootstrap() {}
+      }
+
+      // Hack to wire up the `input()` signal correctly, since our JIT tests don't run with the
+      // transform which supports `input()`.
+      (Ng2Component as any).ɵcmp.inputs.message = ['message', /* InputFlags.SignalBased */ 1];
+
+      ng1Module.directive(
+        'ng2',
+        downgradeComponent({
+          component: Ng2Component,
+        }),
+      );
+
+      const element = html(`
+        <div>
+          <ng2 literal="Text" message="Hello {{name}}"></ng2>
+        </div>`);
+
+      bootstrap(platformBrowserDynamic(), Ng2Module, element, ng1Module).then((upgrade) => {
+        expect(multiTrim(document.body.textContent)).toEqual('Message: Hello world');
+
+        $apply(upgrade, 'name = "everyone"');
+        expect(multiTrim(document.body.textContent)).toEqual('Message: Hello everyone');
+      });
+    }));
+
     it('should bind properties to onpush components', waitForAsync(() => {
       const ng1Module = angular.module_('ng1', []).run(($rootScope: angular.IScope) => {
         $rootScope['dataB'] = 'B';