How to structure nested Promises
I have a situation where I think the only choice for me is to nest some Promises within each other. I have a Promise that needs to be performed and a method that does something until that Promise is complete. Something like this:
let promise = new Promise((resolve, reject) => {
// Do some stuff
});
doSomethingUntilPromiseisDone(promise);
However, within my Promise, I need to execute another method that returns another Promise:
let promise = new Promise((resolve, reject) => {
fetchValue(url)
.then((value) => {
// Do something here
}).catch((err) => {
console.error(err);
});
});
doSomethingUntilPromiseisDone(promise);
But now, in the fetchValue method's then statement, I have another method I need to execute that, guess what, returns another Promise:
let promise = new Promise((resolve, reject) => {
fetchValue(url)
.then((value) => {
saveToCache(value)
.then((success) => {
console.log('success!!');
resolve('success');
});
}).catch((err) => {
console.error(err);
});
});
doSomethingUntilPromiseisDone(promise);
So in the end, I have a Promise, within a Promise, within a Promise. Is there someway I can structure this better so that it is more straightforward? It seems like nesting them within each other is counter to Promise's intended chaining approach.
使用.then()
let doStuff = (resolve, reject) => {/* resolve() or reject() */};
let promise = new Promise(doStuff);
doSomethingUntilPromiseisDone(
promise
.then(value => fetchValue(url))
.then(value => value.blob())
.then(saveToCache)
)
.then(success => console.log("success!!"))
.catch(err => console.error(err))
您可以使用generator将您的嵌套承诺（Bluebird.couroutine或Generators）
//Bluebird.couroutine
const generator = Promise.coroutine(function*() {
try {
const value = yield fetchValue(url);
const success = yield saveToCache(value);
console.log('success:', success);
} catch(e) {
console.error(err);
}
}));
generator();
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