React Native填坑之旅--class(番外篇)

无论React还是RN都已经迈入了ES6的时代,甚至凭借Babel的支持都进入了ES7。ES6内容很多,本文主要讲解类相关的内容。 构造函数 定义侦

无论React还是RN都已经迈入了ES6的时代,甚至凭借Babel的支持都进入了ES7。ES6内容很多,本文主要讲解类相关的内容。

构造函数

定义侦探类作为例子。

ES5的“类”是如何定义的。

function ES5Detective() { console.log('##ES5Detective contructor');}

ES6定义类:

class ES6Detective { constructor() { console.log('Detective constructor'); }}

ES6使用了 class关键字,而且有专门的 constructor。ES5里的 function ES5Detective既是类的定义,也是构造函数。

属性

看看这个侦探是从哪本书出来的。

ES5:

ES5Detective.prototype.fromBookName = 'who';

ES6:

class ES6Detective { detectiveName: string; _bookName: string; constructor() { console.log('Detective constructor'); this.detectiveName = 'Detective who'; // 属性 }}

ES6 getter & setter

class ES6Detective { detectiveName: string; _bookName: string; constructor() { console.log('Detective constructor'); this.detectiveName = 'Detective who'; this._bookName = 'who'; } get fromBookName() { return this._bookName; } set fromBookName(value) { this._bookName = value; }}

如果只有getter没有setter而赋值的话就会出现下面的错误:

detective.bookAuthor = 'A C'; ^TypeError: Cannot set property bookAuthor of #<ES6Detective> which has only a getter

实例方法

侦探是如何解决案件的。

ES5:

ES5Detective.prototype.solveCase = function(caseName) { var dn = this.dectiveName; if(!caseName) { console.log('SOLVE CASE: ' + dn + ' no case to solve'); } else { console.log('SOLVE CASE: ' + dn + ' get case ' + caseName + ' is solved'); }};

或者:

function ES5Detective() { this.dectiveName = 'Detective who'; console.log('##ES5Detective contructor'); // 实例方法 this.investigate = function(scene) { console.log('investigate ' + scene); } this.assistant = "assistant who";}

ES6:

class ES6Detective { detectiveName: string; _bookName: string; constructor() { console.log('Detective constructor'); this.detectiveName = 'Detective who'; this._bookName = 'who'; } solveCase(caseName) { if(!caseName) { console.log('no case to solve'); } else { console.log('case ' + caseName + ' is solved'); } }}

ES6添加方法非常简单直接。ES5中添加实例方法有两种方法,一是在prototype里定义,一是在构造函数重定义。 在构造函数中定义的实例方法和属性在每一个实例中都会保留一份,而在原型中定义的实例方法和属性是全部实例只有一份

另外,在ES5的构造函数重定义的实例方法可以访问类的私有变量。比如:

function ES5Detective() { console.log('##ES5Detective contructor'); var available: boolean = true; // private field. default income is ZERO. this.investigate = function(scene) { if (available) { console.log('investigate ' + scene); } else { console.log(`i'm not available`); } }}

在其他的方法访问的时候就会报错。

if (!available) { ^

静态方法

ES5:

ES5Detective.countCases = function(count) { if(!count) { console.log('no case solved'); } else { console.log(`${count} cases are solved`); }};

类名后直接定义方法,这个方法就是静态方法。

ES5Detective.countCases();

ES6:

class ES6Detective { static countCases() { console.log(`Counting cases...`); }}// call itES6Detective.countCases();

继承

ES6使用 extends关键字实现继承。

ES5:

function ES5Detective() { var available: boolean = true; // private field. this.dectiveName = 'Detective who'; console.log('##ES5Detective contructor'); this.investigate = function(scene) { // 略 } this.assistant = "assistant who";}ES5Detective.prototype.solveCase = function(caseName) { // 略}// inheritancefunction ES5DetectiveConan() { // first line in constructor method is a must!!! ES5Detective.call(this); this.dectiveName = 'Conan';}// inheritanceES5DetectiveConan.prototype = Object.create(ES5Detective.prototype);ES5DetectiveConan.prototype.constructor = ES5DetectiveConan;

ES5继承的时候需要注意两个地方:

  1. 需要在子类的构造函数里调用 SuperClass.call(this[, arg1, arg2, ...])

  2. 子类的prototype赋值为: SubClass.prototype = Object.create(SuperClass.prototype),然后把构造函数重新指向自己的: SubClass.prototpye.constructor = SubClass

ES6:

class ES6Detective { constructor() { console.log('Detective constructor'); this.detectiveName = 'Detective who'; this._bookName = 'who'; } solveCase(caseName) { if(!caseName) { console.log('no case to solve'); } else { console.log('case ' + caseName + ' is solved'); } } get fromBookName() { return this._bookName; } set fromBookName(value) { this._bookName = value; } get bookAuthor() { return 'Author Who'; } static countCases() { console.log(`Counting cases...`); }}class ES6DetectiveConan extends ES6Detective { constructor() { super(); console.log('ES6DetectiveConan constructor'); }}

ES6的新语法更加易懂。

注意:一定要在子类的构造方法里调用 super()方法。否则报错。

调用super类内容

class ES6DetectiveConan extends ES6Detective { constructor() { super(); console.log('ES6DetectiveConan constructor'); } solveCase(caseName) { super.solveCase(caseName); if(!caseName) { console.log('CONAN no case to solve'); } else { console.log('CONAN case ' + caseName + ' is solved'); } }}

静态方法可以被继承

ES6的静态方法可以被继承。ES5的不可以。

class ES6Detective { static countCases(place) { let p = !place ? '[maybe]' : place; console.log(`Counting cases...solve in ${p}`); }}class ES6DetectiveConan extends ES6Detective { constructor() { super(); console.log('ES6DetectiveConan constructor'); }}// static methodES6Detective.countCases();ES6DetectiveConan.countCases('Japan');// resultCounting cases...solve in [maybe]Counting cases...solve in Japan

在子类 ES6DetectiveConan并没有定义任何方法,包括静态方法。但是,在父类和子类里都可以调用该方法。

甚至,可以在子类里调用父类的静态方法:

class ES6DetectiveConan extends ES6Detective { static countCases(place) { let p = !place ? '[maybe]' : place; super.countCases(p); console.log(`#Sub class:- Counting cases...solve in ${p}`); }}// resultCounting cases...solve in [maybe]Counting cases...solve in Japan#Sub class:- Counting cases...solve in Japan

代码

https://github.com/future-cha...

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