/**
 * Load page without leaving current one (hot-reload).
 */
export default class PageLoad {
    constructor(el, {loadingClass="loading", append=false}={}) {
        this.el = el
        this.append = append
        this.loadingClass = loadingClass
    }

    get target() {
        if(!this._target)
            this._target = document.querySelector(this.el)
        return this._target
    }

    reset() {
        this._target = null
    }

    /**
     * Enable hot reload: catch page change in order to fetch them and
     * load page without actually leaving current one.
     */
    enable(target=null) {
        if(this._pageChanged)
            throw "Already enabled, please disable me"

        if(!target)
            target = this.target || document.body
        this.historySave(document.location, true)

        this._pageChanged = event => this.pageChanged(event)
        this._statePopped = event => this.statePopped(event)

        target.addEventListener('click', this._pageChanged, true)
        target.addEventListener('submit', this._pageChanged, true)
        window.addEventListener('popstate', this._statePopped, true)
    }

    /**
     * Disable hot reload, remove listeners.
     */
    disable() {
        this.target.removeEventListener('click', this._pageChanged, true)
        this.target.removeEventListener('submit', this._pageChanged, true)
        window.removeEventListener('popstate', this._statePopped, true)

        this._pageChanged = null
        this._statePopped = null
    }

    /**
    * Fetch url, return promise, similar to standard Fetch API.
    * Default implementation just forward argument to it.
    */
    fetch(url, options) {
        return fetch(url, options)
    }

    /**
     * Fetch app from remote and mount application.
     */
    load(url, {mount=true,  scroll=[0,0], ...options}={}) {
        if(this.loadingClass)
            this.target.classList.add(this.loadingClass)

        if(this.onLoad)
            this.onLoad({url, el: this.el, options})
        if(scroll)
            window.scroll(...scroll)
        return this.fetch(url, options).then(response => response.text())
            .then(content => {
                if(this.loadingClass)
                    this.target.classList.remove(this.loadingClass)

                var doc = new DOMParser().parseFromString(content, 'text/html')
                var dom = doc.querySelectorAll(this.el)
                var result = {url,
                              content: dom || [document.createTextNode(content)],
                              title: doc.title,
                              append: this.append}
                mount && this.mount(result)
                return result
            })
    }

    /**
    * Mount the page on provided target element
    */
    mount({content, title=null, ...options}={}) {
        if(this.onPreMount)
            this.onPreMount({target: this.target, content, items, title})
        var items = null;
        if(content)
            items = this.mountContent(content, options)
        if(title)
            document.title = title
        if(this.onMount)
            this.onMount({target: this.target, content, items, title})
    }

    /**
    * Mount page content
    */
    mountContent(content, {append=false}={}) {
        if(typeof content == "string") {
            this.target.innerHTML = append ? this.target.innerHTML + content
                                           : content;
            // TODO
            return []
        }

        if(!append)
            this.target.innerHTML = ""

        var fragment = document.createDocumentFragment()
        var items = []
        for(var node of content)
            while(node.firstChild) {
                items.push(node.firstChild)
                fragment.appendChild(node.firstChild)
            }
        this.target.append(fragment)
        return items
    }

    /// Save application state into browser history
    historySave(url,replace=false) {
        const state = { content: this.target.innerHTML,
                        title: document.title, }
        if(replace)
            history.replaceState(state, '', url)
        else
            history.pushState(state, '', url)
    }

    dispatchPageLoaded(url) {
        var evt = new CustomEvent("pageLoaded", {detail: url})
        document.dispatchEvent(evt)
    }

    // --- events
    pageChanged(event) {
        let submit = event.type == 'submit';
        let target = submit || event.target.tagName == 'A'
                        ? event.target : event.target.closest('a');
        if(!target || target.hasAttribute('target') || (target.dataset && target.dataset.forceReload))
            return;

        let url = submit ? target.getAttribute('action') || ''
                         : target.getAttribute('href');
        let domain = window.location.protocol + '//' + window.location.hostname
        let stay = (url === '' || url.startsWith('/') || url.startsWith('?') ||
                    url.startsWith(domain)) && url.indexOf('wp-admin') == -1
        if(url===null || !stay) {
            return;
        }

        let options = {};
        if(submit) {
            let formData = new FormData(event.target);
            if(target.method == 'get')
                url += '?' + (new URLSearchParams(formData)).toString();
            else
                options = {...options, method: target.method, body: formData}
        }
        this.load(url, options).then(() => this.dispatchPageLoaded(url)).then(() => this.historySave(url))
        event.preventDefault();
        event.stopPropagation();
    }

    statePopped(event) {
        const state = event.state
        if(state && state.content)
            this.mount({ content: state.content, title: state.title });
    }
}