/*
 * Copyright (c) 2020, University of Oxford
 */
import React from 'react'
import PropTypes from 'prop-types'

/**
 * This is just an div that covers the whole of the iframe. When the surrounding
 * page is scrolled the div is moved to the new location. You should just have
 * a single one of these components in an application and all components should
 * use it through the id.
 */
class FixedIFrame extends React.Component {

  static propTypes = {
    id: PropTypes.string
  }

  static defaultProps = {
    id: 'lti-fixed-content'
  }

  state = {
    height: '0px',
    top: '0px'
  }

  /**
   * Registers to get scroll events from Canvas. There's no way to unregister
   * from scroll events and we don't want multiple registrations so we ensure
   * that we only register once.
   */
  ltiScrollEvents = () => {
    // We only want to do this once.
    if (!this.registeredScroll) {
      window.parent.postMessage(JSON.stringify({
        subject: 'lti.enableScrollEvents'
      }), '*')
      this.registeredScroll = true
    }
  }

  componentDidMount() {
    window.addEventListener('resize', this.resizeListener)
    window.addEventListener('message', this.messageListener)
    this.fetchWindowSize()
    this.ltiScrollEvents()
  }

  componentWillUnmount() {
    window.removeEventListener('message', this.resizeListener)
    window.removeEventListener('resize', this.fetchWindowSize)
  }

  resizeListener = () => this.fetchWindowSize()

  fetchWindowSize = () => {
    // When running outside of the iframe don't send the message as it goes to ourselves
    if (window.parent !== window) {
      window.parent.postMessage(JSON.stringify({
        subject: 'lti.fetchWindowSize'
      }), '*')
    }
  }

  messageListener = e => e.data && this.messageHandler(e)

  messageHandler = (event) => {
    let message
    try {
      message = typeof event.data === 'string' ? JSON.parse(event.data) : event.data
    } catch (err) {
      // unparseable message may not be meant for our handlers
      return
    }
    if (message.subject === 'lti.fetchWindowSize') {
      this.message = message
      this.scrollHandler()
    } else if (message.subject === 'lti.scroll') {
      this.message = { ...this.message, ...message }
      this.scrollHandler()
    }
  }

  scrollHandler = () => {
    const message = this.message
    // We might get a scroll event before we know the size of the window
    if (message.offset) {
      // Math.max to stop it overflowing over the beginning of the document.
      const top = Math.max(message.scrollY - message.offset.top, 0)
      // Math.min to stop it overflowing the bottom and generating a second scroll bar.
      const bottom = Math.min(message.height + message.scrollY - message.offset.top, document.documentElement.scrollHeight)
      this.setState({
        top: top + 'px',
        height: (bottom - top) + 'px'
      })
    }
  }

  render() {
    return <div id={this.props.id} style={{
      position: 'absolute',
      backgroundColor: 'transparent',
      width: '100%',
      // This isn't working quite right, a Modal needs the element to receive events otherwise it doesn't prevent
      // clicks on the mask from propagating through to the masked elements.
      // We can't then move it forward in the z-index stack as it's capturing events then and so this means modal
      // transitions aren't done quite right.
      // pointerEvents: 'none',
      // zIndex: 50,
      top: this.state.top,
      height: this.state.height
    }}>
    </div>
  }
}

export default FixedIFrame