Add support for wrapping elm rendered dom in shadow dom

Author: Matthias Devlamynck

README.md

@@ -81,6 +81,17 @@ And now in your HTML you can use the component:
 
 Any attributes are passed into your Elm app as Flags.
 
+## Shadow Dom
+
+By default Elm will render inside your custom element directly, if you want to isolate the Elm renderer dom using shadow dom you can register the custom element like this:
+
+```js
+import elmWebComponents from '@teamthread/elm-web-components'
+import ElmApp from './Main.elm'
+
+elmWebComponents.register('demo-elm-component', ElmApp.Main, {useShadowDom: true})
+```
+
 ## Ports
 
 You can also hook up a component that uses ports. The third argument to `elmWebComponents.register` is an object that can take a function that will be called with the ports object that Elm provides, so you can then hook into it and `subscribe` and `send` to them as you would normally:

src/index.js

@@ -42,6 +42,7 @@ const elmWebComponents = {
       onDetached = () => {},
       mapFlags = flags => flags,
       onSetupError,
+      useShadowDom = false,
     } = {}
   ) {
     if (!this.__elmVersion) {
@@ -70,23 +71,30 @@ const elmWebComponents = {
           const flags = mapFlags(props)
           context.flags = flags
 
+          var elmDiv = this;
+          var parentDiv = this;
+
+          if (useShadowDom) {
+            parentDiv = this.attachShadow({mode: 'open'});
+          }
+
           if (elmVersion === '0.19') {
             /* a change in Elm 0.19 means that ElmComponent.init now replaces the node you give it
              * whereas in 0.18 it rendered into it. To avoid Elm therefore destroying our custom element
              * we create a div that we let Elm render into, and manually clear any pre-rendered contents.
              */
-            const elmDiv = document.createElement('div')
+            elmDiv = document.createElement('div')
 
-            this.innerHTML = ''
-            this.appendChild(elmDiv)
+            parentDiv.innerHTML = ''
+            parentDiv.appendChild(elmDiv)
 
             const elmElement = ElmComponent.init({
               flags,
               node: elmDiv,
             })
             setupPorts(elmElement.ports)
           } else if (elmVersion === '0.18') {
-            const elmElement = ElmComponent.embed(this, flags)
+            const elmElement = ElmComponent.embed(elmDiv, flags)
             setupPorts(elmElement.ports)
           }
         } catch (error) {