/** // constructor this.subtree = new SubtreeRetainer( () => this.props.post.freshness, () => this.showing ); this.subtree.add(() => this.props.user.freshness); // view this.subtree.retain() || 'expensive expression' */ export default class SubtreeRetainer { constructor() { this.old = []; this.callbacks = [].slice.call(arguments); } retain() { var needsRebuild = false; this.callbacks.forEach((callback, i) => { var result = callback(); if (result !== this.old[i]) { this.old[i] = result; needsRebuild = true; } }); return needsRebuild ? false : {subtree: 'retain'}; } add() { this.callbacks = this.callbacks.concat([].slice.call(arguments)); } }