等待文件加载(加载JavaScript) [英] Waiting for a file to load (onload JavaScript)

问题描述

我想从文件中读取文本并在函数中返回它.因此，这是我的代码的重要部分:

I want to read a text from a file and return it in a function. So here's the important part of my code:

        function getFileRequest(id, contentType, callback) {
        var val = "x";      

        if (window.File && window.FileReader && window.FileList && window.Blob) {
            var element = document.getElementById(id);

            var file = element.files[0]; 
            if(file != null) {      
                if(file.type.match("text/xml")){
                    var r;
                    r = new FileReader();

                    r.onload = function (e) {
                        val = e.target.result;                    

                    }                   
                    r.readAsText(file);
                }
                else
                    alert("Wrong file format");
            }

        } else {
            alert('The File APIs are not fully supported by your browser.');
        }

        alert(val);
        if(val == null)
                return "";
            else
                return getRequestBody(id,contentType,val);  

    }

我想将文本传递给一个名为"val"的变量.但是，至少在我看来，alert(val)始终显示默认的"x"，因为它可能不等待onload函数执行.我完全正确吗? 那么，我如何才能访问该文本?有办法等待被执行吗?

I want to pass the text to a variable called "val". But, as it seems to me at least, alert(val) is always showing default "x" because probably it's not waiting for onload function to be executed. Am I right at all? How can I get an access to that text then? Is there a way to wait for an excecution?

推荐答案

当然，警报不在onload函数中，因此会立即调用.

Of course the alert isn't in the onload function, so it's called immediately.

您可以这样做:

        var val = "x";      
        //... code to load a file variable
        var r;
        r = new FileReader();

        r.onload = function (e) {
            val = e.target.result;  
            r.readAsText(file);
            alert(val);
        };  

您不能等待并停止执行代码，因此通常的想法是使用回调来推迟执行代码.

You cannot wait and stop the execution of your code, so the general idea is to defer it using a callback.

假设您显示的代码实际上是由两部分完成的，一个部分执行文件操作，另一部分使用它，就像这样:

Supposing the code you show is really to be done in two parts, one doing file manipulation and the other one using it, it could be like this :

function fetchVal(callback) {
        var val = "x";      
        //... code to load a file variable
        var r;
        r = new FileReader();
        r.onload = function (e) {
            val = e.target.result;  
            r.readAsText(file);
            callback(val);
        };  
}

fetchVal(function(val){
   alert(val);
   // use val
});

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