Promise应用
【1】基础应用——单次调用
const fs = require(‘fs‘);
//Promise初始化为：等待状态 pending
let promise = new Promise((resolve, reject) => {
console.log(‘start...‘);
//异步任务
fs.readFile(‘./callback/a.txt‘, ‘utf-8‘, function(err, data) {
if (err) return reject(err);
resolve(data);
});
console.log(‘end...‘);
});
//then()实例方法 指定Promise对象状态改变时确定执行的操作
promise.then(data => {
console.log(data);
}, err => {
console.log(err);
});
【2】多次调用
【1】简单组合串行
let fs = require(‘fs‘);
//回调多次的问题
function read() {
fs.readFile(‘./callback/promise/a.txt‘, ‘utf8‘, function(err, data) {
if (err) return console.log(err);
console.log(data);
fs.readFile(‘./callback/promise/b.txt‘, ‘utf8‘, function(err, data) {
if (err) return console.log(err);
console.log(data);
})
});
}
read();
=========================================
【2】promise+then
function _read(path) {
return new Promise((resolve, reject) => {
fs.readFile(path, ‘utf8‘, function(err, data) {
if (err) return reject(err);
resolve(data);
});
});
}
_read(‘./callback/promise/a.txt‘).then((err) => {
console.log(err);
}, (data) => {
console.log(data);
});
_read(‘./callback/promise/b.txt‘).then((err) => {
console.log(err);
}, (data) => {
console.log(data);
});
=================================
【3】promise+then嵌套
_read(‘./callback/promise/a.txt‘).then((data) => {
console.log(data);
_read(‘./callback/promise/b.txt‘).then((data) => {
console.log(data);
});
});
===================================
【4】链式调用
_read(‘./callback/promise/a.txt‘).then(data => {
console.log(data);
console.log(1);
return _read(‘./callback/promise/b.txt‘);
}, err => {
console.log(err);
}).then(data => {
console.log(data);
console.log(2);
}, err => {
console.log(err);
});
Peomise.all方法应用
//同时读取a.txt,b.txt, 返回的是所有结果
Promise.all([read(‘./callback/promise/a.txt‘), read(‘./callback/promise/b.txt‘)]).then((res) => {
console.log(res);
});
// res 返回的是全部文件结果
//race ： 返回最先执行完的结果
//或者说：参数中谁的状态先改变，直接决定最后race()返回的promise实例状态
Promise.race([read(‘./callback/promise/b.txt‘), read(‘./callback/promise/a.txt‘)]).then((res) => {
console.log(res);
}, (error) => {
console.log(error);
});
catch语句
let promise = read(‘./callback/promise/ad.txt‘);
promise.then(data => {
console.log(data);
}, err => {
console.log(err);
});
promise = read(‘./callback/promise/ad.txt‘);
//捕获error
promise.then(data => {
console.log(data);
}).catch(err => {
console.log(err);
});