Reddit Watcher

Consolidated mutation observer handler for use by other reddit userscripts. By itself doesn't do much, but other scripts and use hooks to avoid redundant observers on the same thing.
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// ==UserScript==
// @name         Reddit Watcher
// @namespace    https://lawrenzo.com/p/reddit-watcher
// @description  Consolidated mutation observer handler for use by other reddit userscripts. By itself doesn't do much, but other scripts and use hooks to avoid redundant observers on the same thing.
// @version      1.3.2
// @author       Lawrence Sim
// @license      WTFPL (http://www.wtfpl.net)
// @grant        unsafeWindow
// @match        *://*.reddit.com/*
// ==/UserScript==
(function() {
    if(unsafeWindow && !unsafeWindow.redditWatcher) {
        var debug = false;
        function watcherAbstract(funcs, opts, name) {
            if(typeof funcs === "function") funcs = {get: funcs};
            return {
                name: name || "unnamed",
                defaultOpts: opts || {childList:true},
                hooks: {
                    update: [],
                    change: []
                },
                onUpdate: function(task) {
                    this.hooks.update.push(task);
                },
                update: function(mutated) {
                    if(debug) console.log(this.name + " updated");
                    this.hooks.update.forEach(task => task(this.watching, mutated));
                },
                onChange: function(task) {
                    this.hooks.change.push(task);
                },
                changed: function() {
                    if(debug) console.log(this.name + " changed");
                    this.hooks.change.forEach(task => task(this.watching));
                },
                un: function(func) {
                    let index = this.hooks.update.indexOf(func);
                    while(~index) {
                        this.hooks.update = this.hooks.splice(index, 1);
                        index = this.hooks.update.indexOf(func);
                    }
                    index = this.hooks.change.indexOf(func);
                    while(~index) {
                        this.hooks.change = this.change.splice(index, 1);
                        index = this.hooks.change.indexOf(func);
                    }
                },
                watching: null,
                watcher: null,
                get: funcs.get,
                reset: function() {
                    if(debug) console.log(this.name + " reset");
                    if(!this.watcher) {
                        this.watcher = new MutationObserver(this.update.bind(this));
                    } else {
                        this.watcher.disconnect();
                    }
                    this.update();
                    this.observe(this.watching, this.defaultOpts);
                },
                observe: function(elem, opts) {
                    if(this.watcher) this.watcher.observe(elem, opts);
                },
                wasChanged: funcs.changed,
                check: function() {
                    if(debug) console.log("checking " + this.name);
                    if(!this.watching || !document.body.contains(this.watching)) {
                        let requery = this.get();
                        if(requery && requery !== this.watching) {
                            this.watching = requery;
                            this.changed();
                            this.reset();
                            return true;
                        }
                    }
                    if(this.wasChanged && this.wasChanged(this.watching)) {
                        this.changed();
                        return true;
                    }
                }
            };
        }
        var lastFeed = null,
            lastFirstPost = null;
        var feedWatcher = watcherAbstract(
            {
                get: function() {
                    let listing = document.querySelector(".ListingLayout-outerContainer"),
                        posts = listing.querySelectorAll("div[data-testid='post-container']"),
                        feed = posts && posts.length > 1 && posts[0].parentNode;
                    while(feed && !feed.nextSibling) {
                        if(feed == listing) return null;
                        feed = feed.parentNode || null;
                    }
                    return feed && feed.parentNode;
                },
                changed: function(feed) {
                    if(lastFeed && lastFeed != feed) {
                        lastFeed = feed;
                        return false;
                    }
                    if(!lastFeed) lastFeed = feed;
                    let firstPost = lastFeed && lastFeed.querySelector("div[data-testid='post-container']");
                    if(firstPost !== lastFirstPost) {
                        lastFirstPost = firstPost;
                        return true;
                    }
                }
            },
            {childList:true},
            "feed"
        );
        var listingWatcher = watcherAbstract(
            {get: () => document.querySelector(".ListingLayout-outerContainer")},
            {childList:true, subtree:true},
            "listing"
        );
        var bodyWatcher = watcherAbstract(
            {get: () => document.body},
            {childList:true},
            "body"
        );
        listingWatcher.onChange(() => feedWatcher.check());
        listingWatcher.onUpdate(() => feedWatcher.check());
        listingWatcher.check();
        var wasInFeed = !!listingWatcher.watching;
        bodyWatcher.onUpdate(body => {
            if(body.querySelectorAll(".ListingLayout-outerContainer div[data-testid='post-container']").length > 1) {
                wasInFeed ? listingWatcher.check() : listingWatcher.reset();
                wasInFeed = true;
            } else {
                if(wasInFeed) listingWatcher.watcher && listingWatcher.watcher.disconnect();
                wasInFeed = false;
            }
        });
        bodyWatcher.check();
        unsafeWindow.redditWatcher = {
            body:    bodyWatcher,
            listing: listingWatcher,
            feed:    feedWatcher,
            update:  function() { this.body.update(); }
        };
    }
})();
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