feat(runtime): watch native HTML attributes

src/compiler/transformers/decorators-to-static/convert-decorators.ts

@@ -103,11 +103,10 @@ const visitClassDeclaration = (
   componentDecoratorToStatic(config, typeChecker, diagnostics, classNode, filteredMethodsAndFields, componentDecorator);
 
   // stores a reference to fields that should be watched for changes
-  const watchable = new Set<string>();
   // parse member decorators (Prop, State, Listen, Event, Method, Element and Watch)
   if (decoratedMembers.length > 0) {
-    propDecoratorsToStatic(diagnostics, decoratedMembers, typeChecker, program, watchable, filteredMethodsAndFields);
-    stateDecoratorsToStatic(decoratedMembers, watchable, filteredMethodsAndFields);
+    propDecoratorsToStatic(diagnostics, decoratedMembers, typeChecker, program, filteredMethodsAndFields);
+    stateDecoratorsToStatic(decoratedMembers, filteredMethodsAndFields);
     eventDecoratorsToStatic(diagnostics, decoratedMembers, typeChecker, program, filteredMethodsAndFields);
     methodDecoratorsToStatic(
       config,
@@ -119,7 +118,7 @@ const visitClassDeclaration = (
       filteredMethodsAndFields,
     );
     elementDecoratorsToStatic(diagnostics, decoratedMembers, typeChecker, filteredMethodsAndFields);
-    watchDecoratorsToStatic(config, diagnostics, decoratedMembers, watchable, filteredMethodsAndFields);
+    watchDecoratorsToStatic(decoratedMembers, filteredMethodsAndFields);
     listenDecoratorsToStatic(diagnostics, decoratedMembers, filteredMethodsAndFields);
   }
 

src/compiler/transformers/decorators-to-static/prop-decorator.ts

@@ -26,7 +26,6 @@ import { getDeclarationParameters, isDecoratorNamed } from './decorator-utils';
  * Only those decorated with `@Prop()` will be parsed.
  * @param typeChecker a reference to the TypeScript type checker
  * @param program a {@link ts.Program} object
- * @param watchable a collection of class members that can be watched for changes using Stencil's `@Watch` decorator
  * @param newMembers a collection that parsed `@Prop` annotated class members should be pushed to as a side effect of
  * calling this function
  */
@@ -35,12 +34,11 @@ export const propDecoratorsToStatic = (
   decoratedProps: ts.ClassElement[],
   typeChecker: ts.TypeChecker,
   program: ts.Program,
-  watchable: Set<string>,
   newMembers: ts.ClassElement[],
 ): void => {
   const properties = decoratedProps
     .filter(ts.isPropertyDeclaration)
-    .map((prop) => parsePropDecorator(diagnostics, typeChecker, program, prop, watchable))
+    .map((prop) => parsePropDecorator(diagnostics, typeChecker, program, prop))
     .filter((prop): prop is ts.PropertyAssignment => prop != null);
 
   if (properties.length > 0) {
@@ -55,15 +53,13 @@ export const propDecoratorsToStatic = (
  * @param typeChecker a reference to the TypeScript type checker
  * @param program a {@link ts.Program} object
  * @param prop the TypeScript `PropertyDeclaration` to parse
- * @param watchable a collection of class members that can be watched for changes using Stencil's `@Watch` decorator
  * @returns a property assignment expression to be added to the Stencil component's class
  */
 const parsePropDecorator = (
   diagnostics: d.Diagnostic[],
   typeChecker: ts.TypeChecker,
   program: ts.Program,
   prop: ts.PropertyDeclaration,
-  watchable: Set<string>,
 ): ts.PropertyAssignment | null => {
   const propDecorator = retrieveTsDecorators(prop)?.find(isDecoratorNamed('Prop'));
   if (propDecorator == null) {
@@ -120,7 +116,7 @@ const parsePropDecorator = (
     ts.factory.createStringLiteral(propName),
     convertValueToLiteral(propMeta),
   );
-  watchable.add(propName);
+
   return staticProp;
 };
 

src/compiler/transformers/decorators-to-static/state-decorator.ts

@@ -11,17 +11,12 @@ import { isDecoratorNamed } from './decorator-utils';
  * with which they can be replaced.
  *
  * @param decoratedProps TypeScript AST nodes representing class members
- * @param watchable set of names of fields which should be watched for changes
  * @param newMembers an out param containing new class members
  */
-export const stateDecoratorsToStatic = (
-  decoratedProps: ts.ClassElement[],
-  watchable: Set<string>,
-  newMembers: ts.ClassElement[],
-) => {
+export const stateDecoratorsToStatic = (decoratedProps: ts.ClassElement[], newMembers: ts.ClassElement[]) => {
   const states = decoratedProps
     .filter(ts.isPropertyDeclaration)
-    .map((prop) => stateDecoratorToStatic(prop, watchable))
+    .map(stateDecoratorToStatic)
     .filter((state): state is ts.PropertyAssignment => !!state);
 
   if (states.length > 0) {
@@ -38,18 +33,17 @@ export const stateDecoratorsToStatic = (
  * decorated with other decorators.
  *
  * @param prop A TypeScript AST node representing a class property declaration
- * @param watchable set of names of fields which should be watched for changes
  * @returns a property assignment AST Node which maps the name of the state
  * prop to an empty object
  */
-const stateDecoratorToStatic = (prop: ts.PropertyDeclaration, watchable: Set<string>): ts.PropertyAssignment | null => {
+const stateDecoratorToStatic = (prop: ts.PropertyDeclaration): ts.PropertyAssignment | null => {
   const stateDecorator = retrieveTsDecorators(prop)?.find(isDecoratorNamed('State'));
   if (stateDecorator == null) {
     return null;
   }
 
   const stateName = prop.name.getText();
-  watchable.add(stateName);
+
   return ts.factory.createPropertyAssignment(
     ts.factory.createStringLiteral(stateName),
     ts.factory.createObjectLiteralExpression([], true),

src/compiler/transformers/decorators-to-static/watch-decorator.ts

@@ -1,20 +1,12 @@
-import { augmentDiagnosticWithNode, buildError, buildWarn, flatOne } from '@utils';
+import { flatOne } from '@utils';
 import ts from 'typescript';
 
 import type * as d from '../../../declarations';
 import { convertValueToLiteral, createStaticGetter, retrieveTsDecorators } from '../transform-utils';
 import { getDeclarationParameters, isDecoratorNamed } from './decorator-utils';
 
-export const watchDecoratorsToStatic = (
-  config: d.Config,
-  diagnostics: d.Diagnostic[],
-  decoratedProps: ts.ClassElement[],
-  watchable: Set<string>,
-  newMembers: ts.ClassElement[],
-) => {
-  const watchers = decoratedProps
-    .filter(ts.isMethodDeclaration)
-    .map((method) => parseWatchDecorator(config, diagnostics, watchable, method));
+export const watchDecoratorsToStatic = (decoratedProps: ts.ClassElement[], newMembers: ts.ClassElement[]) => {
+  const watchers = decoratedProps.filter(ts.isMethodDeclaration).map(parseWatchDecorator);
 
   const flatWatchers = flatOne(watchers);
 
@@ -23,22 +15,12 @@ export const watchDecoratorsToStatic = (
   }
 };
 
-const parseWatchDecorator = (
-  config: d.Config,
-  diagnostics: d.Diagnostic[],
-  watchable: Set<string>,
-  method: ts.MethodDeclaration,
-): d.ComponentCompilerWatch[] => {
+const parseWatchDecorator = (method: ts.MethodDeclaration): d.ComponentCompilerWatch[] => {
   const methodName = method.name.getText();
   const decorators = retrieveTsDecorators(method) ?? [];
   return decorators.filter(isDecoratorNamed('Watch')).map((decorator) => {
     const [propName] = getDeclarationParameters<string>(decorator);
-    if (!watchable.has(propName)) {
-      const diagnostic = config.devMode ? buildWarn(diagnostics) : buildError(diagnostics);
-      diagnostic.messageText = `@Watch('${propName}') is trying to watch for changes in a property that does not exist.
-        Make sure only properties decorated with @State() or @Prop() are watched.`;
-      augmentDiagnosticWithNode(diagnostic, decorator);
-    }
+
     return {
       propName,
       methodName,

src/declarations/stencil-private.ts

@@ -1386,6 +1386,9 @@ export type ComponentRuntimeMetaCompact = [
 
   /** listeners */
   ComponentRuntimeHostListener[]?,
+
+  /** watchers */
+  ComponentConstructorWatchers?,
 ];
 
 /**

src/runtime/bootstrap-lazy.ts

@@ -69,7 +69,7 @@ export const bootstrapLazy = (lazyBundles: d.LazyBundlesRuntimeData, options: d.
         cmpMeta.$attrsToReflect$ = [];
       }
       if (BUILD.watchCallback) {
-        cmpMeta.$watchers$ = {};
+        cmpMeta.$watchers$ = compactMeta[4] ?? {};
       }
       if (BUILD.shadowDom && !supportsShadow && cmpMeta.$flags$ & CMP_FLAGS.shadowDomEncapsulation) {
         // TODO(STENCIL-854): Remove code related to legacy shadowDomShim field

src/runtime/proxy-component.ts

@@ -85,7 +85,7 @@ export const proxyComponent = (
     if (BUILD.observeAttribute && (!BUILD.lazyLoad || flags & PROXY_FLAGS.isElementConstructor)) {
       const attrNameToPropName = new Map();
 
-      prototype.attributeChangedCallback = function (attrName: string, _oldValue: string, newValue: string) {
+      prototype.attributeChangedCallback = function (attrName: string, oldValue: string, newValue: string) {
         plt.jmp(() => {
           const propName = attrNameToPropName.get(attrName);
 
@@ -133,25 +133,60 @@ export const proxyComponent = (
             // if the propName exists on the prototype of `Cstr`, this update may be a result of Stencil using native
             // APIs to reflect props as attributes. Calls to `setAttribute(someElement, propName)` will result in
             // `propName` to be converted to a `DOMString`, which may not be what we want for other primitive props.
+            return;
+          } else if (propName == null) {
+            // At this point we should know this is not a "member", so we can treat it like watching an attribute
+            // on a vanilla web component
+            const hostRef = getHostRef(this);
+            const flags = hostRef?.$flags$;
+
+            // We only want to trigger the callback(s) if:
+            // 1. The instance is ready
+            // 2. The watchers are ready
+            // 3. The value has changed
+            if (
+              !(flags & HOST_FLAGS.isConstructingInstance) &&
+              flags & HOST_FLAGS.isWatchReady &&
+              newValue !== oldValue
+            ) {
+              const elm = BUILD.lazyLoad ? hostRef.$hostElement$ : this;
+              const instance = BUILD.lazyLoad ? hostRef.$lazyInstance$ : (elm as any);
+              const entry = cmpMeta.$watchers$[attrName];
+              entry?.forEach((callbackName) => {
+                if (instance[callbackName] != null) {
+                  instance[callbackName].call(instance, newValue, oldValue, attrName);
+                }
+              });
+            }
+
             return;
           }
 
           this[propName] = newValue === null && typeof this[propName] === 'boolean' ? false : newValue;
         });
       };
 
-      // create an array of attributes to observe
-      // and also create a map of html attribute name to js property name
-      Cstr.observedAttributes = members
-        .filter(([_, m]) => m[0] & MEMBER_FLAGS.HasAttribute) // filter to only keep props that should match attributes
-        .map(([propName, m]) => {
-          const attrName = m[1] || propName;
-          attrNameToPropName.set(attrName, propName);
-          if (BUILD.reflect && m[0] & MEMBER_FLAGS.ReflectAttr) {
-            cmpMeta.$attrsToReflect$.push([propName, attrName]);
-          }
-          return attrName;
-        });
+      // Create an array of attributes to observe
+      // This list in comprised of all strings used within a `@Watch()` decorator
+      // on a component as well as any Stencil-specific "members" (`@Prop()`s and `@State()`s).
+      // As such, there is no way to guarantee type-safety here that a user hasn't entered
+      // an invalid attribute.
+      Cstr.observedAttributes = Array.from(
+        new Set([
+          ...Object.keys(cmpMeta.$watchers$ ?? {}),
+          ...members
+            .filter(([_, m]) => m[0] & MEMBER_FLAGS.HasAttribute)
+            .map(([propName, m]) => {
+              const attrName = m[1] || propName;
+              attrNameToPropName.set(attrName, propName);
+              if (BUILD.reflect && m[0] & MEMBER_FLAGS.ReflectAttr) {
+                cmpMeta.$attrsToReflect$.push([propName, attrName]);
+              }
+
+              return attrName;
+            }),
+        ]),
+      );
     }
   }
 

src/utils/format-component-runtime-meta.ts

@@ -38,11 +38,13 @@ export const formatComponentRuntimeMeta = (
 
   const members = formatComponentRuntimeMembers(compilerMeta, includeMethods);
   const hostListeners = formatHostListeners(compilerMeta);
+  const watchers = formatComponentRuntimeWatchers(compilerMeta);
   return trimFalsy([
     flags,
     compilerMeta.tagName,
     Object.keys(members).length > 0 ? members : undefined,
     hostListeners.length > 0 ? hostListeners : undefined,
+    Object.keys(watchers).length > 0 ? watchers : undefined,
   ]);
 };
 
@@ -56,6 +58,25 @@ export const stringifyRuntimeData = (data: any) => {
   return json;
 };
 
+/**
+ * Transforms Stencil compiler metadata into a {@link d.ComponentCompilerMeta} object.
+ * This handles processing any compiler metadata transformed from components' uses of `@Watch()`.
+ * The map of watched attributes to their callback(s) will be immediately available
+ * to the runtime at bootstrap.
+ *
+ * @param compilerMeta Component metadata gathered during compilation
+ * @returns An object mapping watched attributes to their respective callback(s)
+ */
+const formatComponentRuntimeWatchers = (compilerMeta: d.ComponentCompilerMeta) => {
+  const watchers: d.ComponentConstructorWatchers = {};
+
+  compilerMeta.watchers.forEach(({ propName, methodName }) => {
+    watchers[propName] = [...(watchers[propName] ?? []), methodName];
+  });
+
+  return watchers;
+};
+
 const formatComponentRuntimeMembers = (
   compilerMeta: d.ComponentCompilerMeta,
   includeMethods = true,

test/karma/test-app/components.d.ts

@@ -340,6 +340,8 @@ export namespace Components {
     }
     interface Tag88 {
     }
+    interface WatchNativeAttributes {
+    }
 }
 export interface EsmImportCustomEvent<T> extends CustomEvent<T> {
     detail: T;
@@ -1174,6 +1176,12 @@ declare global {
         prototype: HTMLTag88Element;
         new (): HTMLTag88Element;
     };
+    interface HTMLWatchNativeAttributesElement extends Components.WatchNativeAttributes, HTMLStencilElement {
+    }
+    var HTMLWatchNativeAttributesElement: {
+        prototype: HTMLWatchNativeAttributesElement;
+        new (): HTMLWatchNativeAttributesElement;
+    };
     interface HTMLElementTagNameMap {
         "append-child": HTMLAppendChildElement;
         "attribute-basic": HTMLAttributeBasicElement;
@@ -1309,6 +1317,7 @@ declare global {
         "svg-class": HTMLSvgClassElement;
         "tag-3d-component": HTMLTag3dComponentElement;
         "tag-88": HTMLTag88Element;
+        "watch-native-attributes": HTMLWatchNativeAttributesElement;
     }
 }
 declare namespace LocalJSX {
@@ -1649,6 +1658,8 @@ declare namespace LocalJSX {
     }
     interface Tag88 {
     }
+    interface WatchNativeAttributes {
+    }
     interface IntrinsicElements {
         "append-child": AppendChild;
         "attribute-basic": AttributeBasic;
@@ -1784,6 +1795,7 @@ declare namespace LocalJSX {
         "svg-class": SvgClass;
         "tag-3d-component": Tag3dComponent;
         "tag-88": Tag88;
+        "watch-native-attributes": WatchNativeAttributes;
     }
 }
 export { LocalJSX as JSX };
@@ -1924,6 +1936,7 @@ declare module "@stencil/core" {
             "svg-class": LocalJSX.SvgClass & JSXBase.HTMLAttributes<HTMLSvgClassElement>;
             "tag-3d-component": LocalJSX.Tag3dComponent & JSXBase.HTMLAttributes<HTMLTag3dComponentElement>;
             "tag-88": LocalJSX.Tag88 & JSXBase.HTMLAttributes<HTMLTag88Element>;
+            "watch-native-attributes": LocalJSX.WatchNativeAttributes & JSXBase.HTMLAttributes<HTMLWatchNativeAttributesElement>;
         }
     }
 }

test/karma/test-app/input-basic/karma.spec.ts

@@ -1,6 +1,6 @@
 import { setupDomTests, waitForChanges } from '../util';
 
-describe('attribute-basic', function () {
+describe('input-basic', function () {
   const { setupDom, tearDownDom } = setupDomTests(document);
   let app: HTMLElement;
 

test/karma/test-app/watch-native-attributes/cmp.tsx

@@ -0,0 +1,22 @@
+import { Component, Element, h, State, Watch } from '@stencil/core';
+
+@Component({
+  tag: 'watch-native-attributes',
+})
+export class WatchNativeAttributes {
+  @Element() el!: HTMLElement;
+
+  @State() callbackTriggered = false;
+
+  @Watch('aria-label')
+  onAriaLabelChange() {
+    this.callbackTriggered = true;
+  }
+
+  render() {
+    return [
+      <p>Label: {this.el.getAttribute('aria-label')}</p>,
+      <p>Callback triggered: {`${this.callbackTriggered}`}</p>,
+    ];
+  }
+}

test/karma/test-app/watch-native-attributes/index.html

@@ -0,0 +1,6 @@
+<!doctype html>
+<meta charset="utf8" />
+<script src="./build/testapp.esm.js" type="module"></script>
+<script src="./build/testapp.js" nomodule></script>
+
+<watch-native-attributes aria-label="myStartingLabel"></watch-native-attributes>