feat(core): add support for model inputs (angular#54252)

Adds support for model inputs in the framework. `model()` returns a writable signal that implicitly defines a input/output pair that can be used either in two-way bindings to keep two values in sync or by binding individually to the input and output. When the value of the `model` changes, it will emit an event with the current value.

Furthermore, these changes expand two-way bindings to accept `WritableSignal`. This will make it easier to transition existing code to signals in a backwards-compatible way.

Example:

```ts
@directive({
  selector: 'counter',
  standalone: true,
  host: {
    '(click)': 'increment()',
  }
})
export class Counter {
  value = model(0);

  increment(): void {
    this.value.update(current => current + 1);
  }
}

@component({
  template: `<counter [(value)]="count"/> The current count is: {{count()}}`,
})
class App {
  count = signal(0);
}
```

PR Close angular#54252

Author: crisbeto

goldens/public-api/core/errors.md

@@ -125,6 +125,8 @@ export const enum RuntimeErrorCode {
     // (undocumented)
     REQUIRED_QUERY_NO_VALUE = -951,
     // (undocumented)
+    REQUIRED_MODEL_NO_VALUE = -952,
+    // (undocumented)
     RUNTIME_DEPS_INVALID_IMPORTED_TYPE = 1000,
     // (undocumented)
     RUNTIME_DEPS_ORPHAN_COMPONENT = 1001,

goldens/public-api/core/index.md

@@ -1075,6 +1075,32 @@ export enum MissingTranslationStrategy {
     Warning = 1
 }
 
+// @public
+export const model: ModelFunction;
+
+// @public
+export interface ModelFunction {
+    <T>(): ModelSignal<T | undefined>;
+    // (undocumented)
+    <T>(initialValue: T, opts?: ModelOptions): ModelSignal<T>;
+    required<T>(opts?: ModelOptions): ModelSignal<T>;
+}
+
+// @public
+export interface ModelOptions {
+    alias?: string;
+}
+
+// @public
+export interface ModelSignal<T> extends WritableSignal<T> {
+    // (undocumented)
+    [ɵINPUT_SIGNAL_BRAND_READ_TYPE]: T;
+    // (undocumented)
+    [ɵINPUT_SIGNAL_BRAND_WRITE_TYPE]: T;
+    // (undocumented)
+    [SIGNAL]: ModelSignalNode<T>;
+}
+
 // @public @deprecated
 export class ModuleWithComponentFactories<T> {
     constructor(ngModuleFactory: NgModuleFactory<T>, componentFactories: ComponentFactory<any>[]);

packages/core/src/authoring.ts

@@ -6,12 +6,13 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-// Note: `input` is exported in `core.ts` due to:
+// Note: `input` and `model` are exported in `core.ts` due to:
 // https://docs.google.com/document/d/1RXb1wYwsbJotO1KBgSDsAtKpduGmIHod9ADxuXcAvV4/edit?tab=t.0.
 
 export {InputFunction} from './authoring/input/input';
 export {InputOptions, InputOptionsWithoutTransform, InputOptionsWithTransform, InputSignal, InputSignalWithTransform, ɵINPUT_SIGNAL_BRAND_WRITE_TYPE} from './authoring/input/input_signal';
 export {ɵUnwrapDirectiveSignalInputs} from './authoring/input/input_type_checking';
 export {ModelFunction} from './authoring/model/model';
+export {ModelOptions, ModelSignal} from './authoring/model/model_signal';
 export {output as ɵoutput, OutputEmitter as ɵOutputEmitter, OutputOptions as ɵOutputOptions} from './authoring/output';
 export {ContentChildFunction, ViewChildFunction} from './authoring/queries';

packages/core/src/core.ts

@@ -13,14 +13,12 @@
  */
 
 export * from './authoring';
-// Input and query authoring functions are exported separately as this file is exempted from
+// Authoring functions are exported separately as this file is exempted from
 // JSCompiler's conformance requirement for inferred const exports. See:
 // https://docs.google.com/document/d/1RXb1wYwsbJotO1KBgSDsAtKpduGmIHod9ADxuXcAvV4/edit?tab=t.0
 export {input} from './authoring/input/input';
 export {contentChild, contentChildren, viewChild, viewChildren} from './authoring/queries';
-
-// TODO(crisbeto): export as a public API
-// export {model} from './authoring/model/model';
+export {model} from './authoring/model/model';
 
 export * from './metadata';
 export * from './version';

packages/core/test/bundling/defer/bundle.golden_symbols.json

@@ -1556,6 +1556,15 @@
   {
     "name": "init_misc_utils"
   },
+  {
+    "name": "init_model"
+  },
+  {
+    "name": "init_model_signal"
+  },
+  {
+    "name": "init_model_signal_node"
+  },
   {
     "name": "init_module"
   },

packages/core/test/bundling/forms_template_driven/bundle.golden_symbols.json

@@ -881,6 +881,9 @@
   {
     "name": "diPublicInInjector"
   },
+  {
+    "name": "elementPropertyInternal"
+  },
   {
     "name": "enterDI"
   },
@@ -1289,6 +1292,9 @@
   {
     "name": "isReadableStreamLike"
   },
+  {
+    "name": "isSignal"
+  },
   {
     "name": "isStylingMatch"
   },
@@ -1331,6 +1337,9 @@
   {
     "name": "leaveViewLight"
   },
+  {
+    "name": "listenerInternal"
+  },
   {
     "name": "lookupTokenUsingModuleInjector"
   },