7. Promises
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2023-12-01
Promise 代表异步操作的最终结果。jQuery 用它自己的方式处理 promises,原生 JavaScript 遵循 Promises/A+ 标准实现了最小 API 来处理 promises。
7.1 done, fail, always
done
会在 promise 解决时调用,fail
会在 promise 拒绝时调用,always
总会调用。// jQuery
$promise.done(doneCallback).fail(failCallback).always(alwaysCallback)
// Native
promise.then(doneCallback, failCallback).then(alwaysCallback, alwaysCallback)
7.2 when
when
用于处理多个 promises。当全部 promises 被解决时返回,当任一 promise 被拒绝时拒绝。// jQuery
$.when($promise1, $promise2).done((promise1Result, promise2Result) => {
});
// Native
Promise.all([$promise1, $promise2]).then([promise1Result, promise2Result] => {});
7.3 Deferred
Deferred 是创建 promises 的一种方式。
// jQuery
function asyncFunc() {
const defer = new $.Deferred();
setTimeout(() => {
if(true) {
defer.resolve('some_value_computed_asynchronously');
} else {
defer.reject('failed');
}
}, 1000);
return defer.promise();
}
// Native
function asyncFunc() {
return new Promise((resolve, reject) => {
setTimeout(() => {
if (true) {
resolve('some_value_computed_asynchronously');
} else {
reject('failed');
}
}, 1000);
});
}
// Deferred way
function defer() {
const deferred = {};
const promise = new Promise((resolve, reject) => {
deferred.resolve = resolve;
deferred.reject = reject;
});
deferred.promise = () => {
return promise;
};
return deferred;
}
function asyncFunc() {
const defer = defer();
setTimeout(() => {
if(true) {
defer.resolve('some_value_computed_asynchronously');
} else {
defer.reject('failed');
}
}, 1000);
return defer.promise();
}