framework/ember/app/components/discussions/stream-item.js

import Ember from 'ember';

// A discussion 'item' represents one item in the post stream. In other words, a
// single item may represent a single post, or it may represent a gap of many
// posts which have not been loaded.

export default Ember.Component.extend({
	classNames: ['item'],
	classNameBindings: ['item.gap:gap', 'loading', 'direction'],
	attributeBindings: [
		'start:data-start',
		'end:data-end',
		'time:data-time',
		'number:data-number'
	],

	start: Ember.computed.alias('item.indexStart'),
	end: Ember.computed.alias('item.indexEnd'),
	time: Ember.computed.alias('item.content.time'),
	number: Ember.computed.alias('item.content.number'),
	loading: Ember.computed.alias('item.loading'),
	direction: Ember.computed.alias('item.direction'),

	count: function() {
		return this.get('end') - this.get('start') + 1;
	}.property('start', 'end'),

	loadingChanged: function() {
		this.rerender();
	}.observes('loading'),

	render: function(buffer) {
		if (! this.get('item.gap')) {
			return this._super(buffer);
		}

		buffer.push('<span>');
		if (this.get('loading')) {
			buffer.push('&nbsp;');
		} else {
			buffer.push(this.get('count')+' more post'+(this.get('count') != 1 ? 's' : ''));
		}
		buffer.push('</span>');
	},

	didInsertElement: function() {
		if (! this.get('item.gap')) {
			return;
		}

		if (this.get('loading')) {
			var view = this;
			Ember.run.scheduleOnce('afterRender', function() {
				view.$().spin('small');
			});
		} else {
			var self = this;
			this.$().hover(function(e) {
				if (! self.get('loading')) {
					var up = e.clientY > $(this).offset().top - $(document).scrollTop() + $(this).outerHeight(true) / 2;
					self.set('direction', up ? 'up' : 'down');
				}
			});
		}
	},

	load: function(relativeIndex) {
		// If this item is not a gap, or if we're already loading its posts,
		// then we don't need to do anything.
		if (! this.get('item.gap') || this.get('loading')) {
			return false;
		}

		// If new posts are being loaded in an upwards direction, then when they
		// are rendered, the rest of the posts will be pushed down the page.
		// However, we want to maintain the current scroll position relative to
		// the content after the gap. To do this, we need to find item directly
		// after the gap and use it as an anchor.
        if (this.get('direction') === 'up') {
			var anchor = this.$().nextAll('.item:first');

			// Immediately after the posts have been loaded (but before they
			// have been rendered,) we want to grab the distance from the top of
			// the viewport to the top of the anchor element.
            this.get('stream').one('postsLoaded', function() {
                if (anchor.length) {
                    var scrollOffset = anchor.offset().top - $(document).scrollTop();
                }

                // After they have been rendered, we scroll back to a position
                // so that the distance from the top of the viewport to the top
                // of the anchor element is the same as before. If there is no
                // anchor (i.e. this gap is terminal,) then we'll scroll to the
                // bottom of the document.
                Ember.run.scheduleOnce('afterRender', function() {
                    $('body').scrollTop(anchor.length
                        ? anchor.offset().top - scrollOffset
                        : $('body').height());
                });
            });
        }

        // Tell the controller that we want to load the range of posts that this
		// gap represents. We also specify which direction we want to load the
		// posts from.
		this.sendAction(
			'loadRange',
			this.get('start') + (relativeIndex || 0),
			this.get('end'),
			this.get('direction') === 'up'
		);
	},

	click: function() {
		this.load();
	}
});