生成器模式

问题

准备一个复杂的，多部分的对象，希望操作不止一次或有不同的配置。

解决方案

创建一个生成器封装对象的产生过程。

Todo.txt格式提供了一个先进的但还是纯文本文件的方法来维护待办事项列表。

手工输入每个项目都有损耗且容易出错，而TodoTxtBuilder类可以解决我们的麻烦：

class TodoTxtBuilder
    constructor: (defaultParameters={ }) ->
        @date = new Date(defaultParameters.date) or new Date
        @contexts = defaultParameters.contexts or [ ]
        @projects = defaultParameters.projects or [ ]
        @priority =  defaultParameters.priority or undefined
    newTodo: (description, parameters={ }) ->
        date = (parameters.date and new Date(parameters.date)) or @date
        contexts = @contexts.concat(parameters.contexts or [ ])
        projects = @projects.concat(parameters.projects or [ ])
        priorityLevel = parameters.priority or @priority
        createdAt = [date.getFullYear(), date.getMonth()+1, date.getDate()].join("-")
        contextNames = ("@#{context}" for context in contexts when context).join(" ")
        projectNames = ("+#{project}" for project in projects when project).join(" ")
        priority = if priorityLevel then "(#{priorityLevel})" else ""
        todoParts = [priority, createdAt, description, contextNames, projectNames]
        (part for part in todoParts when part.length > 0).join " "

builder = new TodoTxtBuilder(date: "10/13/2011")

builder.newTodo "Wash laundry"

# => '2011-10-13 Wash laundry'

workBuilder = new TodoTxtBuilder(date: "10/13/2011", contexts: ["work"])

workBuilder.newTodo "Show the new design pattern to Lucy", contexts: ["desk", "xpSession"]

# => '2011-10-13 Show the new design pattern to Lucy @work @desk @xpSession'

workBuilder.newTodo "Remind Sean about the failing unit tests", contexts: ["meeting"], projects: ["compilerRefactor"], priority: 'A'

# => '(A) 2011-10-13 Remind Sean about the failing unit tests @work @meeting +compilerRefactor'

讨论

TodoTxtBuilder类负责所有文本的生成，让程序员关注每个工作项的独特元素。从而，命令行工具或GUI可以插入这个代码并且之后仍保持支持，提供轻松，更高版本的格式。

前期建设

并非每次创建一个新的实例所需的对象都要从头开始，我们将负担转移到一个单独的对象，可以在对象创建过程中进行调整。

builder = new TodoTxtBuilder(date: "10/13/2011")

builder.newTodo "Order new netbook"

# => '2011-10-13 Order new netbook'

builder.projects.push "summerVacation"

builder.newTodo "Buy suntan lotion"

# => '2011-10-13 Buy suntan lotion +summerVacation'

builder.contexts.push "phone"

builder.newTodo "Order tickets"

# => '2011-10-13 Order tickets @phone +summerVacation'

delete builder.contexts[0]

builder.newTodo "Fill gas tank"

# => '2011-10-13 Fill gas tank +summerVacation'

练习

展开project-和context-tag生成代码来过滤掉重复的重复。

一些Todo.txt用户喜欢在任务描述中插入项目和某些的标签。添加代码来识别这些标签和过滤器的结束标记。