無法計算異步/等待和承諾工作

Question

我正在嘗試了解async / await和Promise函數。 但是，經過大量的努力和研究，我找不到任何簡單的方法來獲得使用4 for循環的正確方法。

我給你代碼，如果Java語言不是異步的，應該如何處理。 您能告訴我在哪里（以及如何）承諾使我的代碼按預期工作？

function (courses) {
    var lines = [];
    for (var i = 0; i < courses.length; i++) {
        var course = courses[i];
        var sessions = course.sessions;
        var students = course.students;
        var skills = course.skills;
        console.log('\nCourse : ' + course);

        for (var j = 0; j < sessions.length; j++) {
            var session = sessions[j];
            console.log('\nSession : ' + session);

                for (var k = 0; k < students.length; k++) {
                    var student = students[k];
                    console.log('\nStudent : ' + student);

                    for (var l = 0; l < skills.length; l++) {
                        var skill = skills[l];
                        console.log('\nSkill : ' + skill);
                        Evaluation.findOne({
                            student_id: student._id,
                            session_id: session._id,
                            skill_id: skill._id
                        }).exec(function (err, eval) {
                            console.log('\nStudentID : ' + eval);
                            console.log('\nSessionID : ' + eval);
                            console.log('\nSkillID : ' + eval);
                            console.log('\nEval : ' + eval);
                            var line = [];
                            line.push(course.name);
                            line.push(session.date);
                            line.push(student.firstname);
                            line.push(student.name);
                            if (eval) {
                                line.push(eval.name);
                                line.push(eval.mark);
                            }
                            else {
                                line.push(skill.name);
                                line.push('Not Evaluated');
                            }
                            lines.push(line)
                        })
                    }
                }
            }
        }
    return lines;
}

Answer 1

假設您使用貓鼬，則只需更換

 Evaluation.findOne({ student_id: student._id, session_id: session._id, skill_id: skill._id }).exec(function (err, eval) { console.log('\\nStudentID : ' + eval); console.log('\\nSessionID : ' + eval); console.log('\\nSkillID : ' + eval); console.log('\\nEval : ' + eval); var line = []; line.push(course.name); line.push(session.date); line.push(student.firstname); line.push(student.name); if (eval) { line.push(eval.name); line.push(eval.mark); } else { line.push(skill.name); line.push('Not Evaluated'); } lines.push(line) }) 

通過

const eval = await Evaluation.findOne({
//           ^^^^^
    student_id: student._id,
    session_id: session._id,
    skill_id: skill._id
});
console.log('\nStudentID : ' + eval);
console.log('\nSessionID : ' + eval);
console.log('\nSkillID : ' + eval);
console.log('\nEval : ' + eval);
var line = [];
line.push(course.name);
line.push(session.date);
line.push(student.firstname);
line.push(student.name);
if (eval) {
    line.push(eval.name);
    line.push(eval.mark);
} else {
    line.push(skill.name);
    line.push('Not Evaluated');
}
lines.push(line)

因為所有方法都已經返回了類似於promise的對象，所以您可以直接await ，而不是使用exec並傳遞nodeback。

Answer 2

似乎您在這里嘗試做兩件事：您想異步地遍歷課程/課程/學生/技能，並進行相當耗時的Evaluation.findOne調用。

我建議用async.forEach替換部分或全部for循環。 您還可以將Evaluation.findOne塊放入Promise包裝的幫助器函數中，但是考慮到它在執行后被壓入lines ，因此這是一個可選步驟。

做這樣的事情：技能迭代現在使用async.forEach，並且行生成器有望實現。

function (courses) {
    var lines = [];
    for (var i = 0; i < courses.length; i++) {
        var course = courses[i];
        var sessions = course.sessions;
        var students = course.students;
        var skills = course.skills;
        console.log('\nCourse : ' + course);
        for (var j = 0; j < sessions.length; j++) {
            var session = sessions[j];
            console.log('\nSession : ' + session);
            for (var k = 0; k < students.length; k++) {
                var student = students[k];
                console.log('\nStudent : ' + student);
                skills.forEach(function(skill) {
                    console.log('\nSkill : ' + skill);
                    buildStudentLine(student, session, skill, course)
                        .then((line )=> {
                            lines.push(line);
                        })
                        .catch((error) => {
                            console.log(error);
                        })

                });
            }
        }
    }
    return lines;
}


function buildStudentLine(student, session, skill, course) {
    return new Promise((resolve, reject) => {
    Evaluation
        .findOne({
            student_id: student._id,
            session_id: session._id,
            skill_id: skill._id
        })
        .exec(function (err, evaluated) {
            var line = [];
            line.push(course.name);
            line.push(session.date);
            line.push(student.firstname);
            line.push(student.name);
            if (evaluated) {
                line.push(evaluated.name);
                line.push(evaluated.mark);
            } else {
                line.push(skill.name);
                line.push('Not Evaluated');
                // possibly put this in here if you don't want the line to enter the lines array:
                // reject('Not Evaluated');
            }
            resolve(line);
        });
    });
}