CH01_重构、第一个示例

概述

在这一章节，作者给出了一个戏剧演出团售票的示例：剧目有悲剧（tragedy）和喜剧（comedy）；为了卖出更多的票，剧团则更具观众的数量来为下次演出打折扣（大致意思是这次的观众越多，下次的票价越低）

设计

采用JSON存储数据（因为代码是用Javascript写的），plays.json 存储剧目，invoices.json存储账单

plays.json

{"hamlet": {"name": "Hamlet", "type": "tragedy"},"as-like": {"name": "As You Like It", "type": "comedy"},"othello": {"name": "Othello", "type": "tragedy"}
}

invoices.json

[{"customer": "BigCo","performances": [{"playID": "hamlet","audience": 55 },{"playID": "as-like","audience": 35},{"playID": "othello","audience": 40}]
}]

下面这个函数用于打印账单详情

function statement(invoice, plays) {let totalAmount = 0;let volumeCredits = 0;let result = `Statement for ${invoice.customer}\n`;const format = new Intl.NumberFormat("en-US", {style: "currency",currency: "USD",minimumFractionDigits: 2}).format;for (let perf of invoice.performances) {const play = plays[perf.playID];let thisAmount = 0;switch (play.type) {case "tragedy":thisAmount = 40000;if (perf.audience > 30) {thisAmount += 1000 * (perf.audience - 30);}break;case "comedy":thisAmount = 30000;if (perf.audience > 20) {thisAmount += 10000 + 500 * (perf.audience - 20);}thisAmount += 300 * perf.audience;break;default:throw new Error(`unknown type: ${play.type}`);}// add volume creditsvolumeCredits += Math.max(perf.audience - 30, 0);// add extra credit for every ten comedy attendeesif ("comedy" === play.type) volumeCredits += Math.floor(perf.audience / 5);// print line for this orderresult += ` ${play.name}: ${format(thisAmount/100)} (${perf.audience} seats)\n`;totalAmount += thisAmount;}result += `Amount owed is ${format(totalAmount/100)}\n`;result += `You earned ${volumeCredits} credits\n`;return result;
}

函数statement实现的打印账单的功能，能正常工作；但是其结构“不甚清晰”。对于这个几十行的代码，我们要读懂、理解并不困难，如果业务比较复杂、函数很长（上百行）时（笔者在维护既有项目时见过2000多行的函数，阅读代码就像是在考古）再去阅读，或者让其他人阅读，要弄清逻辑确实需要花费一番功夫。

如果没有新的需求导入，statement保持现状也可忍受！

新的需求导入

要求输出部分以HTML的格式
扩充新的剧目类型（如：历史剧、田园剧等）
……

需求会源源不断的导入，如果每次导入一个的需求，都在statement函数中修改，随着时间的推移，statement函数将变得臃肿不堪、难以阅读和理解；这也是我们工作中经常做的（目的是懒省事儿，需求来了，在既有的函数中改改能实现功能就完事）。

分解函数statement

提取amountFor函数（for循环中的代码，注意提取参数啊Performance、play）
提取playFor函数（play参数不是for循环的变量，而是计算出来的）
修改amountFor函数，减去play参数
volumeCredits累加变量提取volumeCreditsFor函数
提取format函数（格式化数据）
……

具体过程参考《重构改善既有代码设计第二版》第一章

function statement(invoice, plays) {let result = `Statement for ${invoice.customer}\n`;for (let perf of invoice.performances) {result += ` ${playFor(perf).name}: ${usd(amountFor(perf))} (${perf.audience} s
eats)\n`;}result += `Amount owed is ${usd(totalAmount())}\n`;result += `You earned ${totalVolumeCredits()} credits\n`;return result;function totalAmount() {let result = 0;for (let perf of invoice.performances) {result += amountFor(perf);}return result;}function totalVolumeCredits() {let result = 0;for (let perf of invoice.performances) {result += volumeCreditsFor(perf);}return result;}function usd(aNumber) {return new Intl.NumberFormat("en-US", {style: "currency",currency: "USD",minimumFractionDigits: 2}).format(aNumber / 100);}function volumeCreditsFor(aPerformance) {let result = 0;result += Math.max(aPerformance.audience - 30, 0);if ("comedy" === playFor(aPerformance).type) result += Math.floor(aPerformance.audience / 5);return result;}function playFor(aPerformance) {return plays[aPerformance.playID];}function amountFor(aPerformance) {let result = 0;switch (playFor(aPerformance).type) {case "tragedy":result = 40000;if (aPerformance.audience > 30) {result += 1000 * (aPerformance.audience - 30);}break;case "comedy":result = 30000;if (aPerformance.audience > 20) {result += 10000 + 500 * (aPerformance.audience - 20);}result += 300 * aPerformance.audience;break;default:throw new Error(`unknown type: ${playFor(aPerformance).type}`);}return result;}
}