承諾然后綁定問題

Question

我在將值綁定到下一個Promise時遇到一個問題。 看下面的代碼，它將更好地說明情況。

'use strict';

function FindEvent(eventId) {
    console.log('Event:', eventId);
    return Promise.resolve({title: 'MyEvent'});
}

function FindUser(userId) {
    console.log('User:', userId);
    return Promise.resolve({username: 'MyUser'});
}

function MyServiceProblem(payload) {
    payload.userId = 'blah';
    return FindEvent(payload.eventId)
        .then((event) => {
            payload.userId = 'test';
            // setting a breakpoint here shows
            // that the value to payload.userId has been
            // assigned, i.e. 'test'

            return Promise.resolve(payload);
        })
        .then(FindUser.bind(this, payload.userId));
        // But new value doesn't reaches inside FindUser
}

MyServiceProblem({ // userId is 'blah', why not 'test'
    eventId: '456'
});

function MyServiceWorks(payload) {
    payload.userId = 'blah';
    return new Promise((resolve) => {
        payload.userId = 'test';

        return resolve(payload);
    })
        .then(FindUser.bind(this, payload.userId));
        // From here, the 'test' value reaches FindUser
}

MyServiceWorks({ // userId is 'test'
    eventId: '456'
});

問題是，為什么兩種情況下綁定的值都不同。 兩者是完全一樣的，只不過這是先解決promise的一部分，然后將值分配給payload.userId 。

Answer 1

讓我們分解一下代碼。 你有

function MyServiceProblem(payload) {
    payload.userId = 'blah';
    return FindEvent(payload.eventId)
        .then((event) => {
            payload.userId = 'test';

            return Promise.resolve(payload);
        })
        .then(FindUser.bind(this, payload.userId));
}

問題是您的.bind將在回調之前運行。 該代碼也可以寫成

function MyServiceProblem(payload) {
    payload.userId = 'blah';

    var firstThenCallback = (event) => {
        payload.userId = 'test';
        return Promise.resolve(payload);
    };
    var secondThenCallback = FindUser.bind(this, payload.userId);

    return FindEvent(payload.eventId)
        .then(firstThenCallback)
        .then(secondThenCallback);
}

該payload對象跨所有共享，但payload.userId傳遞到.bind之前firstThenCallback有機會來執行。

而是使用.bind並立即傳遞值，似乎最簡單的解決方案是使用匿名函數，以便稍后才讀取userId值。

function MyServiceProblem(payload) {
    payload.userId = 'blah';
    return FindEvent(payload.eventId)
        .then((event) => {
            payload.userId = 'test';

            return Promise.resolve(payload);
        })
        .then(() => FindUser(payload.userId));
}

完全不清楚為什么您也編寫了這種環繞式答應代碼，但是我認為這是一個精簡的示例？ 為什么要使用payload來解決問題，而不是僅在該函數中調用FindUser ，例如

function MyServiceProblem(payload) {
    payload.userId = 'blah';
    return FindEvent(payload.eventId)
        .then((event) => {
            payload.userId = 'test';

            return FindUser(payload.userId);
        });
}