ClojureScript编程练习： études进阶

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"Etudes.for.ClojureScript.B0184" 这本书《Etudes for ClojureScript》是针对想要提升ClojureScript编程技能的开发者所设计的一本实践指南。作者J. David Eisenberg通过30多个精心设计的编程练习（études），帮助读者在特定的编程领域中磨练技术，涵盖了从基础的函数和变量到复杂的异步处理等多个方面。每个étude都包含一个项目描述，读者需要编写ClojureScript代码来完成。书中的章节结构清晰，涉及的主题包括： 1. **函数与变量**：学习`def`和`let`的用法，理解它们在定义和管理函数及变量中的作用。 2. **与JavaScript和Web页面交互**：利用不同库与JavaScript环境进行互动，实现网页操作。 3. **列表、向量与高阶函数**：探讨列表、向量以及`map`、`filter`和`reduce`等高阶函数的应用。 4. **数据映射**：了解ClojureScript中如何进行数据映射操作。 5. **React编程**：使用ClojureScript库与React框架结合进行前端开发。 6. **记录与协议**：运用`defprotocol`和`defrecord`处理有理数和复数的加减乘除。 7. **异步处理**：深入学习ClojureScript的核心库`core.async`，掌握异步编程技巧。 8. **开放性项目**：第6章提供了一个开放性的项目，让你整合所学，应用到实际场景中。书中还包括附录，提供了études的解决方案、设置ClojureScript环境的指南、使用Leiningen创建ClojureScript项目的步骤，以及关于ClojureScript在服务器端的应用介绍。通过这本书，读者不仅可以深入理解ClojureScript的语法和特性，还能通过实际动手编程，提高解决实际问题的能力。每个étude的设计旨在挑战读者，超越传统教程和在线参考资料中的基础知识，让开发者在实践中不断提升。

Listening for Events

In JavaScript, if you want an HTML element to respond to an event, you add an event

listener to that element, tell it what type of event you want to listen for, and give it the

name of a function that handles the event. That event-handling function must have

one parameter to hold the event object. In ClojureScript, you need to define functions

before you use them, so you have to write the event handler first and then invoke

addEventListener. Here is an example of what I did in the REPL (my project name

was daylight-js):

cljs.user=> (in-ns 'daylight-js.core)

nil

daylight-js.core=> (defn testing [evt] (.alert js/window "You clicked me!!!"))

#'daylight-js/testing

daylight-js.core=> (let [btn (.getElementById js/document "calculate")]

(.addEventListener btn "click" testing))

nil

The first line switches to the correct namespace for the project. The second line

defines the event handler, which calls JavaScript’s alert() function to display a mes‐

sage. The third line tells the “Calculate” button to listen for click events and call the

testing function when they occur.

Given this information, complete the code for the project such that, when you click

the “Calculate” button, the program will read the values from the latitude and Julian

day field, calculate the number of daylight hours, and place the result in the <span

id="result">. (Hint: use the innerHTML property.) You may also want to write a

function that takes a form field name as its argument and returns the floating-point

value from that field.

See a suggested solution: “Solution 2-1” on page 67.

Étude 2-2: Using Google Closure

Using JavaScript directly is all well and good; one advantage is that if you’re a Java‐

Script programmer, you already know this stuff. The bad news is that you have all the

problems of getting JavaScript to work on multiple browsers and platforms. Enter

Google Closure, a library of JavaScript utilities that has all of those nasty compatibil‐

ity parts all figured out for you. In this étude, you’ll use Closure for the interaction.

Putting Google Closure into Your Project

To use Google Closure, you need to change the first lines of your core.cljs file to

require the code that maniuplates the DOM and handles events. In this example, the

project has been named daylight-gc:

10 | Chapter 2: Interacting with JavaScript and Web Pages

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