From eaca195467ab92d81ce5bb837b3d4a5496ab9ad9 Mon Sep 17 00:00:00 2001
From: Steven Orvell <sorvell@google.com>
Date: Wed, 29 Jan 2020 17:54:02 -0800
Subject: [PATCH] Introduce opt-out per class for `legacyNoObservedAttributes`

This is necessary when a native property that sets an attribute should be observeable via attributeChangedCallback or Polymer property reflection (e.g. element.tabIndex = 5 setting the `tabindex` attribute).
---
 lib/legacy/legacy-element-mixin.js | 11 +++++++----
 lib/legacy/polymer-fn.js           |  4 ++++
 test/unit/legacy-noattributes.html | 22 +++++++++++++++++++++-
 3 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/lib/legacy/legacy-element-mixin.js b/lib/legacy/legacy-element-mixin.js
index 8939ecbf79..e06b371ae7 100644
--- a/lib/legacy/legacy-element-mixin.js
+++ b/lib/legacy/legacy-element-mixin.js
@@ -104,6 +104,8 @@ export const LegacyElementMixin = dedupingMixin((base) => {
       this.__isUpgradeDisabled;
       /** @type {boolean|undefined} */
       this.__needsAttributesAtConnected;
+      /** @type {boolean|undefined} */
+      this._legacyForceObservedAttributes;
     }
 
     /**
@@ -143,7 +145,7 @@ export const LegacyElementMixin = dedupingMixin((base) => {
 
     /** @override */
     setAttribute(name, value) {
-      if (legacyNoObservedAttributes) {
+      if (legacyNoObservedAttributes && !this._legacyForceObservedAttributes) {
         const oldValue = this.getAttribute(name);
         super.setAttribute(name, value);
         // value coerced to String for closure's benefit
@@ -155,7 +157,7 @@ export const LegacyElementMixin = dedupingMixin((base) => {
 
     /** @override */
     removeAttribute(name) {
-      if (legacyNoObservedAttributes) {
+      if (legacyNoObservedAttributes && !this._legacyForceObservedAttributes) {
         const oldValue = this.getAttribute(name);
         super.removeAttribute(name);
         this.__attributeReaction(name, oldValue, null);
@@ -166,7 +168,8 @@ export const LegacyElementMixin = dedupingMixin((base) => {
 
     // NOTE: Inlined for perf from version of DisableUpgradeMixin.
     static get observedAttributes() {
-      if (legacyNoObservedAttributes) {
+      if (legacyNoObservedAttributes && !this._legacyForceObservedAttributes) {
+        this.prototype._legacyForceObservedAttributes = this._legacyForceObservedAttributes;
         // Ensure this element is property registered with the telemetry system.
         if (!this.hasOwnProperty(JSCompiler_renameProperty('__observedAttributes', this))) {
           this.__observedAttributes = [];
@@ -315,7 +318,7 @@ export const LegacyElementMixin = dedupingMixin((base) => {
         this.root = /** @type {HTMLElement} */(this);
         this.created();
         // Pull all attribute values 1x if `legacyNoObservedAttributes` is set.
-        if (legacyNoObservedAttributes) {
+        if (legacyNoObservedAttributes && !this._legacyForceObservedAttributes) {
           if (this.hasAttributes()) {
             this._takeAttributes();
           // Element created from scratch or parser generated
diff --git a/lib/legacy/polymer-fn.js b/lib/legacy/polymer-fn.js
index de019be6a9..4ca4511e50 100644
--- a/lib/legacy/polymer-fn.js
+++ b/lib/legacy/polymer-fn.js
@@ -39,6 +39,10 @@ const Polymer = function(info) {
   } else {
     klass = Polymer.Class(info);
   }
+  // Copy opt out for `legacyNoObservedAttributes` from info object to class.
+  if (info._legacyForceObservedAttributes) {
+    klass._legacyForceObservedAttributes = info._legacyForceObservedAttributes;
+  }
   customElements.define(klass.is, /** @type {!HTMLElement} */(klass));
   return klass;
 };
diff --git a/test/unit/legacy-noattributes.html b/test/unit/legacy-noattributes.html
index 6e5399f1cc..56bf947a12 100644
--- a/test/unit/legacy-noattributes.html
+++ b/test/unit/legacy-noattributes.html
@@ -64,7 +64,7 @@
           zot: Boolean,
           shouldIf: Boolean,
           camelCase: String,
-          disabled: {type: Boolean, value: 'true'}
+          disabled: {type: Boolean, value: 'true'},
         },
         created() {
           this.wasCreated = true;
@@ -74,6 +74,17 @@
           this.attrInfo = {name, old, value};
         }
       });
+
+      Polymer({
+        is: 'x-native-attrs-force',
+        _legacyForceObservedAttributes: true,
+        properties: {
+          tabindex: {reflectToAttribute: true, type: Number}
+        },
+        attributeChanged(name, old, value) {
+          this.attributeChangedCalled = true;
+        }
+      });
     </script>
   </dom-module>
 
@@ -106,6 +117,15 @@
         assert.ok(def);
       });
 
+      test('native property observeable via `attributeChanged` and reflected to property when `_forceObservedAttributes` is used', () => {
+        el = document.createElement('x-native-attrs-force');
+        document.body.appendChild(el);
+        el.tabIndex = 5;
+        assert.ok(el.attributeChangedCalled);
+        assert.equal(el.tabindex, 5);
+        document.body.removeChild(el);
+      });
+
       test('static attributes', () => {
         assert.equal(el.foo, 'foo');
         assert.equal(el.$.child1.getAttribute('foo'), 'x-attrs.foo');