考虑下面以串行/顺序方式读取文件数组的代码。readFiles返回一个承诺，只有在顺序读取所有文件后才会解析这个承诺。
var readFile = function(file) {
... // Returns a promise.
};
var readFiles = function(files) {
return new Promise((resolve, reject) => {
var readSequential = function(index) {
if (index >= files.length) {
resolve();
} else {
readFile(files[index]).then(function() {
readSequential(index + 1);
}).catch(reject);
}
};
readSequential(0); // Start with the first file!
});
};
上面的代码可以工作，但是我不喜欢为了使事情按顺序发生而进行递归。是否有一种更简单的方法可以重写这段代码，这样我就不必使用奇怪的readSequential函数了?
最初我尝试使用Promise。但是这会导致所有的readFile调用并发发生，这不是我想要的:
var readFiles = function(files) {
return Promise.all(files.map(function(file) {
return readFile(file);
}));
};
我使用以下代码扩展Promise对象。它处理承诺的拒绝并返回一个结果数组
Code
/*
Runs tasks in sequence and resolves a promise upon finish
tasks: an array of functions that return a promise upon call.
parameters: an array of arrays corresponding to the parameters to be passed on each function call.
context: Object to use as context to call each function. (The 'this' keyword that may be used inside the function definition)
*/
Promise.sequence = function(tasks, parameters = [], context = null) {
return new Promise((resolve, reject)=>{
var nextTask = tasks.splice(0,1)[0].apply(context, parameters[0]); //Dequeue and call the first task
var output = new Array(tasks.length + 1);
var errorFlag = false;
tasks.forEach((task, index) => {
nextTask = nextTask.then(r => {
output[index] = r;
return task.apply(context, parameters[index+1]);
}, e=>{
output[index] = e;
errorFlag = true;
return task.apply(context, parameters[index+1]);
});
});
// Last task
nextTask.then(r=>{
output[output.length - 1] = r;
if (errorFlag) reject(output); else resolve(output);
})
.catch(e=>{
output[output.length - 1] = e;
reject(output);
});
});
};
例子
function functionThatReturnsAPromise(n) {
return new Promise((resolve, reject)=>{
//Emulating real life delays, like a web request
setTimeout(()=>{
resolve(n);
}, 1000);
});
}
var arrayOfArguments = [['a'],['b'],['c'],['d']];
var arrayOfFunctions = (new Array(4)).fill(functionThatReturnsAPromise);
Promise.sequence(arrayOfFunctions, arrayOfArguments)
.then(console.log)
.catch(console.error);
这是我在各种项目中使用的顺序实现:
const file = [file1, file2, file3];
const fileContents = sequentially(readFile, files);
// somewhere else in the code:
export const sequentially = async <T, P>(
toPromise: (element: T) => Promise<P>,
elements: T[]
): Promise<P[]> => {
const results: P[] = [];
await elements.reduce(async (sequence, element) => {
await sequence;
results.push(await toPromise(element));
}, Promise.resolve());
return results;
};
你可以使用这个函数来获取promiseFactories List:
function executeSequentially(promiseFactories) {
var result = Promise.resolve();
promiseFactories.forEach(function (promiseFactory) {
result = result.then(promiseFactory);
});
return result;
}
Promise Factory是一个简单的函数，返回一个Promise:
function myPromiseFactory() {
return somethingThatCreatesAPromise();
}
它之所以有效，是因为承诺工厂直到被要求才创建承诺。它的工作方式与then函数相同——事实上，它是一样的!
你根本不想在一组承诺上操作。根据Promise规范，一旦创建了Promise，它就开始执行。所以你真正想要的是一系列的承诺工厂…
如果你想了解更多的承诺，你应该检查这个链接:
https://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html
2017年更新:如果环境支持，我会使用异步函数:
async function readFiles(files) {
for(const file of files) {
await readFile(file);
}
};
如果你愿意，你可以延迟读取文件，直到你需要使用异步生成器(如果你的环境支持它):
async function* readFiles(files) {
for(const file of files) {
yield await readFile(file);
}
};
更新:在第二个想法-我可能会用一个for循环代替:
var readFiles = function(files) {
var p = Promise.resolve(); // Q() in q
files.forEach(file =>
p = p.then(() => readFile(file));
);
return p;
};
或者更确切地说，用reduce:
var readFiles = function(files) {
return files.reduce((p, file) => {
return p.then(() => readFile(file));
}, Promise.resolve()); // initial
};
在其他承诺库(如when和Bluebird)中，您有用于此的实用程序方法。
例如，蓝鸟将是:
var Promise = require("bluebird");
var fs = Promise.promisifyAll(require("fs"));
var readAll = Promise.resolve(files).map(fs.readFileAsync,{concurrency: 1 });
// if the order matters, you can use Promise.each instead and omit concurrency param
readAll.then(function(allFileContents){
// do stuff to read files.
});
尽管现在没有理由不使用async await。
我使用以下代码扩展Promise对象。它处理承诺的拒绝并返回一个结果数组
Code
/*
Runs tasks in sequence and resolves a promise upon finish
tasks: an array of functions that return a promise upon call.
parameters: an array of arrays corresponding to the parameters to be passed on each function call.
context: Object to use as context to call each function. (The 'this' keyword that may be used inside the function definition)
*/
Promise.sequence = function(tasks, parameters = [], context = null) {
return new Promise((resolve, reject)=>{
var nextTask = tasks.splice(0,1)[0].apply(context, parameters[0]); //Dequeue and call the first task
var output = new Array(tasks.length + 1);
var errorFlag = false;
tasks.forEach((task, index) => {
nextTask = nextTask.then(r => {
output[index] = r;
return task.apply(context, parameters[index+1]);
}, e=>{
output[index] = e;
errorFlag = true;
return task.apply(context, parameters[index+1]);
});
});
// Last task
nextTask.then(r=>{
output[output.length - 1] = r;
if (errorFlag) reject(output); else resolve(output);
})
.catch(e=>{
output[output.length - 1] = e;
reject(output);
});
});
};
例子
function functionThatReturnsAPromise(n) {
return new Promise((resolve, reject)=>{
//Emulating real life delays, like a web request
setTimeout(()=>{
resolve(n);
}, 1000);
});
}
var arrayOfArguments = [['a'],['b'],['c'],['d']];
var arrayOfFunctions = (new Array(4)).fill(functionThatReturnsAPromise);
Promise.sequence(arrayOfFunctions, arrayOfArguments)
.then(console.log)
.catch(console.error);
