在不使用 Promise 的情况下按顺序执行回调 [英] Executing callbacks in sequential order without using Promises
问题描述
我正在尝试按顺序执行以下函数数组(避免 callbackHell)以实现函数 runCallbacksInSequence
(我需要实现自己的函数以了解回调如何工作并避免使用 Async.js).这是我到目前为止所拥有的.我不太明白回调是如何工作的,这就是我做这个练习的原因.如果您有任何想法,请告诉我我做错了什么以及如何解决.
I am trying to execute following array (avoid callbackHell) of functions in a sequential order implementing function runCallbacksInSequence
(I need to implement my own function to understand how callbacks work and avoid using Async.js). Here is what I have so far. I do not quite understand how callbacks work that is why I am doing this exercise. If you have any ideas let me know what I am doing wrong and how I can fix it.
function first(cb) {
console.log('first()');
cb();
}
function second(cb) {
console.log('second()');
cb();
}
function third(cb) {
console.log('third()');
cb();
}
function last() {
console.log('last()');
}
let fns = [first, second, third, last];
function runCallbacksInSequence(fns, cb) {
return fns.reduceRight((acc, f) => f(acc), cb);
}
runCallbacksInSequence(fns, second);
回调地狱
// first(function() {
// third(function() {
// second(function() {
// last();
// });
// });
// });
UPD
const cache = {};
function runCallbacksInSequence(fns, cb) {
fns.reduce(
function(r, f) {
return function(k) {
return r(function() {
if (cache[f]) {
return;
// f(function(e, x) {
// e ? cb(e) : k(x);
// });
} else {
cache[f] = f;
return f(function(e, x) {
return e ? cb(e) : k(x);
});
}
});
};
},
function(k) {
return k();
}
)(function(r) {
return cb(null, r);
});
}
推荐答案
让 .reduce
回调成为一个 高阶 函数,该函数在调用时调用带有回调的链中的下一个函数.最后,您将拥有一个函数链,首先调用第一个函数,然后调用第二个函数,依此类推:
Have the .reduce
callback be a higher-order function, which, when called, calls the next function in the chain with the callback. At the end, you'll have a function chain that will start by calling the first function, then the second, etc:
function first(cb) {
console.log('first()');
cb();
}
function second(cb) {
console.log('second()');
cb();
}
function third(cb) {
console.log('third()');
cb();
}
function last() {
console.log('last()');
}
let fns = [first, second, third, last];
function runCallbacksInSequence(fns, cb) {
const chainedFns = fns.reduceRight((acc, f) => () => f(acc), cb);
return chainedFns();
}
runCallbacksInSequence(fns);
如果您希望 runCallbacksInSequence
接受 另一个 回调以在所有回调结束时运行,则:
If you wanted the runCallbacksInSequence
to accept another callback to run at the end of all of them, then:
function first(cb) {
console.log('first()');
cb();
}
function second(cb) {
console.log('second()');
cb();
}
function third(cb) {
console.log('third()');
cb();
}
function last(cb) {
console.log('last()');
cb();
}
let fns = [first, second, third, last];
function runCallbacksInSequence(fns, cb) {
const chainedFns = fns.reduceRight((acc, f) => () => f(acc), cb);
return chainedFns();
}
runCallbacksInSequence(fns, () => console.log('outer call'));
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