atom/src/pane-container-element.js

const {CompositeDisposable} = require('event-kit')

class PaneContainerElement extends HTMLElement {
  createdCallback () {
    this.subscriptions = new CompositeDisposable()
    this.classList.add('panes')
  }

  initialize (model, {views}) {
    this.model = model
    this.views = views
    if (this.views == null) {
      throw new Error('Must pass a views parameter when initializing PaneContainerElements')
    }
    this.subscriptions.add(this.model.observeRoot(this.rootChanged.bind(this)))
    return this
  }

  rootChanged (root) {
    const focusedElement = this.hasFocus() ? document.activeElement : null
    if (this.firstChild != null) {
      this.firstChild.remove()
    }
    if (root != null) {
      const view = this.views.getView(root)
      this.appendChild(view)
      if (focusedElement != null) {
        focusedElement.focus()
      }
    }
  }

  hasFocus () {
    return this === document.activeElement || this.contains(document.activeElement)
  }
}

module.exports = document.registerElement('atom-pane-container', {
  prototype: PaneContainerElement.prototype
})