From 10d657b388fdc34273050506b38a3f827aa10723 Mon Sep 17 00:00:00 2001
From: Kevin Schaaf <kschaaf@google.com>
Date: Wed, 25 Jul 2018 16:19:11 -0700
Subject: [PATCH] Back to single template getter. Add more comments.

---
 lib/legacy/class.js         | 18 ----------
 lib/mixins/element-mixin.js | 68 +++++++++++++++++++++++--------------
 2 files changed, 42 insertions(+), 44 deletions(-)

diff --git a/lib/legacy/class.js b/lib/legacy/class.js
index 47793460c1..d8ff532a3e 100644
--- a/lib/legacy/class.js
+++ b/lib/legacy/class.js
@@ -148,24 +148,6 @@ function GenerateClassFromInfo(info, Base) {
       return info.observers;
     }
 
-    /**
-     * @return {HTMLTemplateElement} template for this class
-     */
-    static get template() {
-      if (!this.hasOwnProperty(JSCompiler_renameProperty('_template', this))) {
-        this._template =                  
-          // Accept template: _null to short-circuit dom-module lookup
-          info._template !== undefined ? info._template : 
-          // Look in dom-module associated with this element's is
-          this._getTemplateFromDomModule() ||
-          // Look up in the chain
-          Base.template ||
-          // Use any template set on the prototype via registered callback
-          this.prototype._template;
-      }
-      return this._template;
-    }
-
     /**
      * @return {void}
      */
diff --git a/lib/mixins/element-mixin.js b/lib/mixins/element-mixin.js
index 11f2925825..cb74216440 100644
--- a/lib/mixins/element-mixin.js
+++ b/lib/mixins/element-mixin.js
@@ -270,6 +270,29 @@ export const ElementMixin = dedupingMixin(base => {
     }
   }
 
+  /**
+   * Look up template from dom-module for element
+   * 
+   * @param {!string} is Element name to look up
+   * @return {!HTMLTemplateElement} Template found in dom module, or
+   *   undefined if not found
+   * @protected
+   */
+  function getTemplateFromDomModule(is) {
+    let template = null;
+    // Under strictTemplatePolicy in 3.x+, dom-module lookup is only allowed
+    // when opted-in via allowTemplateFromDomModule
+    if (is && (!strictTemplatePolicy || allowTemplateFromDomModule)) {
+      template = DomModule.import(is, 'template');
+      // Under strictTemplatePolicy, require any element with an `is`
+      // specified to have a dom-module
+      if (strictTemplatePolicy && !template) {
+        throw new Error(`strictTemplatePolicy: expecting dom-module or null template for ${is}`);
+      }
+    }
+    return template;
+  }
+
   /**
    * @polymer
    * @mixinClass
@@ -341,30 +364,6 @@ export const ElementMixin = dedupingMixin(base => {
       }
     }
 
-    /**
-     * Look up template from dom-module for element
-     * 
-     * @param {!string} is Element name to look up
-     * @return {!HTMLTemplateElement} Template found in dom module, or
-     *   undefined if not found
-     * @protected
-     */
-    static _getTemplateFromDomModule() {
-      let template = null;
-      const is = /** @type {PolymerElementConstructor}*/ (this).is;
-      // Under strictTemplatePolicy in 3.x+, dom-module lookup is only allowed
-      // when opted-in via allowTemplateFromDomModule
-      if (is && (!strictTemplatePolicy || allowTemplateFromDomModule)) {
-        template = DomModule.import(is, 'template');
-        // Under strictTemplatePolicy, require any element with an `is`
-        // specified to have a dom-module
-        if (strictTemplatePolicy && !template) {
-          throw new Error(`strictTemplatePolicy: expecting dom-module or null template for ${is}`);
-        }
-      }
-      return template;
-    }
-
     /**
      * Returns the template that will be stamped into this element's shadow root.
      *
@@ -402,13 +401,30 @@ export const ElementMixin = dedupingMixin(base => {
      * @return {HTMLTemplateElement|string} Template to be stamped
      */
     static get template() {
+      // Explanation of template-related properties:
+      // - constructor.template (this getter): the template for the class.
+      //     This can come from the prototype (for legacy elements), from a
+      //     dom-module, or from the super class's template (or can be overridden
+      //     altogether by the user)
+      // - constructor._template: memoized version of constructor.template
+      // - prototype._template: working template for the element, which will be
+      //     parsed and modified in place. It is a cloned version of
+      //     constructor.template, saved in _finalizeClass(). Note that before 
+      //     this getter is called, for legacy elements this could be from a
+      //     _template field on the info object passed to Polymer(), a behavior,
+      //     or set in registered(); once the static getter runs, a clone of it
+      //     will overwrite it on the prototype as the working template. 
       if (!this.hasOwnProperty(JSCompiler_renameProperty('_template', this))) {
         this._template =
+          // If user has put template on prototype (e.g. in legacy via registered 
+          // callback or info object), prefer that first
+          this.prototype.hasOwnProperty(JSCompiler_renameProperty('_template', this.prototype)) ?
+          this.prototype._template :
           // Look in dom-module associated with this element's is
-          this._getTemplateFromDomModule() ||
+          (getTemplateFromDomModule(/** @type {PolymerElementConstructor}*/ (this).is) ||
           // Next look for superclass template (call the super impl this
           // way so that `this` points to the superclass)
-          Object.getPrototypeOf(/** @type {PolymerElementConstructor}*/ (this).prototype).constructor.template;
+          Object.getPrototypeOf(/** @type {PolymerElementConstructor}*/ (this).prototype).constructor.template);
       }
       return this._template;
     }