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216 lines
6.8 KiB
JavaScript
216 lines
6.8 KiB
JavaScript
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## Promise 的常用 API：实例方法【重要】
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Promise 自带的 API 提供了如下实例方法：
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- promise.then()：获取异步任务的正常结果。
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- promise.catch()：获取异步任务的异常结果。
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- promise.finaly()：异步任务无论成功与否，都会执行。
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代码举例如下。
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写法 1：
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```html
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8" />
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<meta name="viewport" content="width=device-width, initial-scale=1.0" />
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<title>Document</title>
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</head>
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<body>
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<script>
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function queryData() {
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return new Promise((resolve, reject) => {
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setTimeout(function () {
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var data = { retCode: 0, msg: 'qianguyihao' }; // 接口返回的数据
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if (data.retCode == 0) {
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// 接口请求成功时调用
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resolve(data);
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} else {
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// 接口请求失败时调用
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reject({ retCode: -1, msg: 'network error' });
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}
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}, 100);
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});
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}
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queryData()
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.then((data) => {
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// 从 resolve 获取正常结果
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console.log('接口请求成功时，走这里');
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console.log(data);
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})
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.catch((data) => {
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// 从 reject 获取异常结果
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console.log('接口请求失败时，走这里');
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console.log(data);
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})
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.finally(() => {
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console.log('无论接口请求成功与否，都会走这里');
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});
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</script>
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</body>
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</html>
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```
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写法 2：（和上面的写法 1 等价）
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```html
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8" />
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<meta name="viewport" content="width=device-width, initial-scale=1.0" />
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<title>Document</title>
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</head>
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<body>
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<script>
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function queryData() {
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return new Promise((resolve, reject) => {
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setTimeout(function () {
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var data = { retCode: 0, msg: 'qianguyihao' }; // 接口返回的数据
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if (data.retCode == 0) {
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// 接口请求成功时调用
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resolve(data);
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} else {
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// 接口请求失败时调用
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reject({ retCode: -1, msg: 'network error' });
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}
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}, 100);
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});
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}
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queryData()
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.then(
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(data) => {
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// 从 resolve 获取正常结果
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console.log('接口请求成功时，走这里');
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console.log(data);
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},
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(data) => {
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// 从 reject 获取异常结果
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console.log('接口请求失败时，走这里');
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console.log(data);
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}
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)
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.finally(() => {
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console.log('无论接口请求成功与否，都会走这里');
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});
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</script>
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</body>
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</html>
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```
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**注意**：写法 1 和写法 2 的作用是完全等价的。只不过，写法 2 是把 catch 里面的代码作为 then 里面的第二个参数而已。
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## Promise 的常用 API：对象方法【重要】
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Promise 自带的 API 提供了如下对象方法：
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- Promise.all()：并发处理多个异步任务，所有任务都执行成功，才能得到结果。
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- Promise.race(): 并发处理多个异步任务，只要有一个任务执行成功，就能得到结果。
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下面来详细介绍。
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### Promise.all() 代码举例
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代码举例：
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```html
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8" />
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<meta name="viewport" content="width=device-width, initial-scale=1.0" />
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<title>Document</title>
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</head>
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<body>
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<script type="text/javascript">
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/*
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封装 Promise 接口调用
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*/
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function queryData(url) {
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return new Promise((resolve, reject) => {
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var xhr = new XMLHttpRequest();
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xhr.onreadystatechange = function () {
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if (xhr.readyState != 4) return;
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if (xhr.readyState == 4 && xhr.status == 200) {
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// 处理正常结果
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resolve(xhr.responseText);
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} else {
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// 处理异常结果
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reject('服务器错误');
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}
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};
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xhr.open('get', url);
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xhr.send(null);
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});
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}
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var promise1 = queryData('http://localhost:3000/api1');
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var promise2 = queryData('http://localhost:3000/api2');
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var promise3 = queryData('http://localhost:3000/api3');
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Promise.all([promise1, promise2, promise3]).then((result) => {
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console.log(result);
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});
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</script>
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</body>
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</html>
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```
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### Promise.race() 代码举例
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代码举例：
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```html
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8" />
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<meta name="viewport" content="width=device-width, initial-scale=1.0" />
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<title>Document</title>
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</head>
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<body>
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<script type="text/javascript">
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/*
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封装 Promise 接口调用
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*/
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function queryData(url) {
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return new Promise((resolve, reject) => {
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var xhr = new XMLHttpRequest();
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xhr.onreadystatechange = function () {
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if (xhr.readyState != 4) return;
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if (xhr.readyState == 4 && xhr.status == 200) {
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// 处理正常结果
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resolve(xhr.responseText);
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} else {
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// 处理异常结果
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reject('服务器错误');
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}
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};
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xhr.open('get', url);
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xhr.send(null);
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});
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}
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var promise1 = queryData('http://localhost:3000/api1');
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var promise2 = queryData('http://localhost:3000/api2');
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var promise3 = queryData('http://localhost:3000/api3');
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Promise.race([promise1, promise2, promise3]).then((result) => {
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console.log(result);
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});
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</script>
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</body>
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</html>
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```
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