Angular 中的链式 promise [英] Chained promises in Angular

问题描述

我有一个要按顺序处理的记录列表.需要将根"承诺传回以作为另一个父链的一部分添加.

I have a list of records I want to process in sequence. A "root" promise needs to be passed back to be added as part of another parent chain.

processCategories = function(categories) {
    var deferred = $q.defer();
    var promise = deferred.promise;

    // Process each category in sequence
    angular.forEach(categories, function (data, classIndex) {
        promise = promise.then(doSomethingAndReturnAPromise(categories, data, classIndex));

        if (classIndex === categories.length - 1) {
            // Last category has been reached, resolve this unit of work
            promise.then(function() {
                deferred.resolve(categories);
            });
        }
    });

    // Pass the root promise back to the chain
    return promise;
}

doSomethingAndReturnAPromise = function(categories, data, index) {
    console.log('item : ' + index + ' = started');
    var deferred = $q.defer();

    executeAnAsynchronousCall(function () {
            console.log('item : ' + index + ' = done');
            deferred.resolve(categories);

    }, function(status, text) {
            console.log('item : ' + index + ' = error');
            // Handle error here

    }, data);

    console.log('item : ' + index + ' = returned');
    return deferred.promise;
}

我从异步调用中收到一些错误，因为执行是同时发生的(而不是顺序发生的).我添加了上面的日志记录并观察到以下内容:

I'm getting some errors from my asynchronous call because the executions are occurring at the same time (and not sequentially). I added the logging above and observed the following:

2014-08-08 11:38:02.702 item : 1 = started
2014-08-08 11:38:02.702 item : 1 = returned
2014-08-08 11:38:02.702 item : 2 = started
2014-08-08 11:38:02.703 item : 2 = returned
2014-08-08 11:38:02.703 item : 3 = started
2014-08-08 11:38:02.704 item : 3 = returned
2014-08-08 11:38:02.704 item : 4 = started
2014-08-08 11:38:02.705 item : 4 = returned
2014-08-08 11:38:02.705 item : 5 = started
2014-08-08 11:38:02.706 item : 5 = returned
2014-08-08 11:38:02.707 item : 6 = started
2014-08-08 11:38:02.707 item : 6 = returned
2014-08-08 11:38:02.708 item : 7 = started
2014-08-08 11:38:02.709 item : 7 = returned
2014-08-08 11:38:02.709 item : 8 = started
2014-08-08 11:38:02.710 item : 8 = returned

2014-08-08 11:38:02.716 item : 2 = error
2014-08-08 11:38:02.718 item : 3 = error
2014-08-08 11:38:02.719 item : 4 = error
2014-08-08 11:38:02.720 item : 5 = error
2014-08-08 11:38:02.722 item : 1 = done // (This has finished, the server is now expecting #2)
2014-08-08 11:38:02.723 item : 6 = error
2014-08-08 11:38:02.735 item : 8 = error
2014-08-08 11:38:02.754 item : 7 = error 

我期待 .then 确保承诺按顺序运行，但每次我运行代码时，它们似乎都被同时触发.

I was expecting the .then to ensure the promises are run in sequence, but each time I run the code it appears they are all being fired at once.

谁能看出我哪里出错了?

Can anyone see where I'm going wrong here?

推荐答案

你的第二行是调用函数，而不是安排它稍后调用

Your second line is invoking the function rather than scheduling it to be invoked later

变化:

promise = promise.then(doSomethingAndReturnAPromise(categories, data, classIndex));

致:

promise = promise.then(doSomethingAndReturnAPromise.bind(this,categories, data, index));

或者一个完整的匿名函数:

Or a full blown anonymous function:

promise = promise.then(function(){
    return doSomethingAndReturnAPromise(categories, data, classIndex));
});

附带说明，如果您需要使用类别来解决承诺，那么直接更改返回值比将该检查放入 forEach 会更简单.

As a side note, if you need the promise to resolve with the categories, it would be simpler to change the return value directly rather than put that check in the forEach.

processCategories = function(categories) {
    return categories.reduce(function (prev, data, index) {
            return prev.then(function(){
                return doSomethingAndReturnAPromise(categories, data, index));
            });
    }, $q.when()).then(function(){
        return categories;
    });
}

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