在for循环中等待承诺 [英] Waiting for promise in for loop

问题描述

进入这整个异步的东西我有点麻烦。我正在使用pdf.js来读取pdf文件的内容。一切正常，但执行顺序给我带来了麻烦。这是我的代码：

I have a bit of trouble getting into this whole async stuff. I'm using pdf.js to read the contents of a pdf file. Everything is working, however the execution order is causing me troubles. This is the code I have:

function getText(data) {
    var contents = [];
    PDFJS.getDocument(data).then(function(pdf) {
        var numPages = pdf.pdfInfo.numPages;
        for (var i = 1; i <= numPages; i++) {
            pdf.getPage(i).then(function(page) {
                page.getTextContent().then(function(content) {
                    contents.concat(content.bidiTexts);
                })
            });
        }
    })
}

这是按预期工作的，但是，当我们处理所有页面并将其bidiTexts连接到内容时，我想传递内容。如果我只是在最后一次结束} 之前将带有 contents 的函数调用作为参数，那么它会被提前调用。

This is working as intended, however, I would like to pass contents when all pages are processed and their bidiTexts are concated to contents. If I just put a function call with contents as argument before the last closing } then it gets called to early.

推荐答案

尝试使用承诺：

function getText(data) {
    var contents = [];
    var promises = [];

    PDFJS.getDocument(data).then(function(pdf) {
        var numPages = pdf.pdfInfo.numPages;

        for (var i = 1; i <= numPages; i++) {
            var deferred = $.Deferred(); //Create a deferred object
            promises.push(deferred.promise()); //push promise to the list

            pdf.getPage(i).then(function(page) {
                page.getTextContent().then(function(content) {
                    contents.concat(content.bidiTexts);
                    deferred.resolve(); //resolve the deferred object
                })
            });
        }

        $.when.apply($,promises).then(function(){ //callback executed when all deferreds are resolved
            //do your task with contents
        });
    })
}

这只是演示如何使用promises。在实际应用程序中，您必须使用 deferred.reject 来处理错误，并在第二次回调中处理它 $。when

This is just a demo how to use promises. In real applications, you have to take care of errors by using deferred.reject and handle it in the second callback to $.when

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