从 Q Promise 运行并获得顺序进展

Question

我有这段代码，我希望这两个 Promise 顺序运行，结果应该是这样的：

a progress: 20
a progress: 40
a progress: 60
a progress: 80
a progress: 100
a resolve: a

b progress: 20
b progress: 40
b progress: 60
b progress: 80
b progress: 100
b resolve: b

但我明白了：

a progress: 20
b progress: undefined
a progress: 40
b progress: undefined
a progress: 60
b progress: undefined
a progress: 80
b progress: undefined
a progress: 100
a resolve: a
b progress: undefined
b progress: 20
b progress: 40
b progress: 60
b progress: 80
b progress: 100
b resolve: b

这是我的全部代码：

var a =  function(){
        return q.Promise(function(resolve, reject, notify){
            var percentage = 0;
            var interval = setInterval(function() {
                percentage += 20;
                notify(percentage);
                if (percentage === 100) {
                    resolve("a");
                    clearInterval(interval);
                }
            }, 500);
        });
    };

    var b =  function(){
        return q.Promise(function(resolve, reject, notify){
            var percentage = 0;
            var interval = setInterval(function() {
                percentage += 20;
                notify(percentage);
                if (percentage === 100) {
                    resolve("b");
                    clearInterval(interval);
                }
            }, 500);
        });
    };


    a().then(function(res) {
            console.log('a resolve: '+res);
            return b();
        },
        errorHandler,
        function(progress){
            console.log('a progress: '+progress);
        })

        //call b()
        .then(function(res){
            console.log('b resolve: '+res);
        },
        errorHandler,
        function(progress){
            console.log('b progress: '+progress);
        });

    function errorHandler(err) {
        console.log('Error Handler:', err);
    }

Answer 1

您可以使用Q's chaining。

a()
.then(
    function(res) {
        b().then(function(resb) {
            console.log('b resolve: '+resb);
        },
        errorHandler,
        function(progress){
            console.log('b progress: '+progress);
        });
    },
    errorHandler,
    function(progress){
        console.log('a progress: '+progress);
    });

使用fin 也可以：

a()
.then(
    function(res) {
        console.log('a resolve: '+res);
    },
    errorHandler,
    function(progress){
        console.log('a progress: '+progress);
    })
.fin(function() {
    b().then(function(res) {
        console.log('b resolve: '+res);
    },
    errorHandler,
    function(progress){
        console.log('b progress: '+progress);
    });
});

我们看一下原始代码的流程：

a().then(function(res) {
        console.log('a resolve: '+res);
        return b(); // (0) here we only create promise for 'b'
    },
    errorHandler,
    function(progress){
        console.log('a progress: '+progress); // (1) here Q calls progress's callback
    })

    //call b()
    .then(function(res){
        console.log('b resolve: '+res); // (2) it's called in the end
    },
    errorHandler,
    function(progress){
        console.log('b progress: '+progress);  // (1) and here Q calls progress's callback
    });

'b progress: undefined' 是因为完全没有进度，但是调用了回调。在未完成承诺时调用进度回调。当您返回某些东西或调用“解决”时，承诺就会得到履行。