Render any value returned by #render. (#712)

* Test that returning undefined from the first call to `#render` doesn't remove shimmed adopted style sheets.

* Render any value returned by `#render`.

* Update change log.

* format

* Update lit-html dependency to v1.1.1.

* Update types for static and instance `render` methods.

* Add type for static `render` function. Fix types in comments.

* `test.only` -> `test`

CHANGELOG.md

@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 <!-- ### Added -->
+### Changed
+* The value returned by `render` is always rendered, even if it isn't a `TemplateResult`. ([#712](https://github.com/Polymer/lit-element/issues/712)
 <!-- ### Removed -->
 
 ### Added

package.json

@@ -63,7 +63,7 @@
   },
   "typings": "lit-element.d.ts",
   "dependencies": {
-    "lit-html": "^1.0.0"
+    "lit-html": "^1.1.1"
   },
   "publishConfig": {
     "access": "public"

src/lit-element.ts

@@ -11,8 +11,7 @@
  * subject to an additional IP rights grant found at
  * http://polymer.github.io/PATENTS.txt
  */
-import {TemplateResult} from 'lit-html';
-import {render} from 'lit-html/lib/shady-render.js';
+import {render, ShadyRenderOptions} from 'lit-html/lib/shady-render.js';
 
 import {PropertyValues, UpdatingElement} from './lib/updating-element.js';
 
@@ -45,15 +44,17 @@ export class LitElement extends UpdatingElement {
    * optimizations. See updating-element.ts for more information.
    */
   protected static['finalized'] = true;
+
   /**
-   * Render method used to render the lit-html TemplateResult to the element's
-   * DOM.
-   * @param {TemplateResult} Template to render.
-   * @param {Element|DocumentFragment} Node into which to render.
-   * @param {String} Element name.
+   * Render method used to render the value to the element's DOM.
+   * @param result The value to render.
+   * @param container Node into which to render.
+   * @param options Element name.
    * @nocollapse
    */
-  static render = render;
+  static render:
+      (result: unknown, container: Element|DocumentFragment,
+       options: ShadyRenderOptions) => void = render;
 
   /**
    * Array of styles to apply to the element. The styles should be defined
@@ -194,14 +195,11 @@ export class LitElement extends UpdatingElement {
    */
   protected update(changedProperties: PropertyValues) {
     super.update(changedProperties);
-    const templateResult = this.render() as unknown;
-    if (templateResult instanceof TemplateResult) {
-      (this.constructor as typeof LitElement)
-          .render(
-              templateResult,
-              this.renderRoot,
-              {scopeName: this.localName, eventContext: this});
-    }
+    (this.constructor as typeof LitElement)
+        .render(
+            this.render(),
+            this.renderRoot,
+            {scopeName: this.localName, eventContext: this});
     // When native Shadow DOM is used but adoptedStyles are not supported,
     // insert styling after rendering to ensure adoptedStyles have highest
     // priority.
@@ -216,10 +214,12 @@ export class LitElement extends UpdatingElement {
   }
 
   /**
-   * Invoked on each update to perform rendering tasks. This method must return
-   * a lit-html TemplateResult. Setting properties inside this method will *not*
-   * trigger the element to update.
+   * Invoked on each update to perform rendering tasks. This method may return
+   * any value renderable by lit-html's NodePart - typically a TemplateResult.
+   * Setting properties inside this method will *not* trigger the element to
+   * update.
    */
-  protected render(): TemplateResult|void {
+  protected render(): unknown {
+    return undefined;
   }
 }

src/test/lit-element_styling_test.ts

@@ -798,6 +798,62 @@ suite('Static get styles', () => {
         const bodyStyles = `${cssModule}`;
         assert.equal(bodyStyles, '.my-module { color: yellow; }');
       });
+
+  test(
+      'Styles are not removed if the first rendered value is undefined.',
+      async () => {
+        const localName = generateElementName();
+
+        class SomeCustomElement extends LitElement {
+          static styles = css`:host {border: 4px solid black;}`;
+
+          renderUndefined: boolean;
+
+          constructor() {
+            super();
+            this.renderUndefined = true;
+          }
+
+          static get properties() {
+            return {
+              renderUndefined: {
+                type: Boolean,
+                value: true,
+              },
+            };
+          }
+
+          render() {
+            if (this.renderUndefined) {
+              return undefined;
+            }
+
+            return htmlWithStyles`Some text.`;
+          }
+        }
+        customElements.define(localName, SomeCustomElement);
+
+        const element = document.createElement(localName) as SomeCustomElement;
+        document.body.appendChild(element);
+
+        await (element as LitElement).updateComplete;
+        assert.equal(
+            getComputedStyle(element)
+                .getPropertyValue('border-top-width')
+                .trim(),
+            '4px');
+
+        element.renderUndefined = false;
+
+        await (element as LitElement).updateComplete;
+        assert.equal(
+            getComputedStyle(element)
+                .getPropertyValue('border-top-width')
+                .trim(),
+            '4px');
+
+        document.body.removeChild(element);
+      });
 });
 
 suite('ShadyDOM', () => {