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I have a very simple html page which has a file upload button as well as a modal popup to show loading status and a cancel button. My problem is that the cancel button seems to be blocked while it's processing the file (making it useless). How can I have a button which will cancel the form submission.

HTML:

<body>
    <div id="myModal" class="modal">
        <div class="modal-content">
            <p>Processing File.</p>
            <div class="loader-inner pacman"><div></div><div></div><div></div><div></div><div></div></div>
            <div id="progressbar" ></div>
            <input type = "button" name="cancel" value = "cancel" onclick = "cancelRead()" />
        </div>
    </div>

    <div>
        <input type="file" id="logSelector" name="log" title="Select a log file" onchange="logSelected();"/>

JS:

function cancelRead() {
    console.log("cancelRead");  //never see this message even though the button is clicked
    fileReader.abort();
    operationAborted = true;
    document.execCommand('Stop');
}

The logSelected() method is quite lengthy and I omitted it from the above code mostly to eliminate clutter. That method uses FileReader to read the selected file than create a table from the data. However I do signal to this method to stop via the operationAborted variable. however like i said in the comments, this method is never called by clicking my button. The cancel button doesnt even animate like it was clicked.

The promise api was mentioned below, here is my attempt at it. Although it works it did not fix the issue of the cancel button not clickable due to the main/only thread being used.

var promise = $.Deferred(function (dfd) {
    FILE_READER.onload = function (event) {
        var contents = event.target.result;
        dfd.resolve(contents);
    };
    FILE_READER.readAsText(file);
}).promise();
$.when(promise).then(function (contents) {
    processFile(contents.split('\n'));
});
owen gerig
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3 Answers3

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And it's almost sure that it will never work, due to how event loop works in javascript browsers.

Salathiel Genese
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This could be because the file reader is using up all of the thread leaving to time to process the function to be called - you see, JavaScript only has one thread & thus can only handle one thing at a time, & this one thing is the file reader!

edit
However you could use a promise (as explained here) to run the file reader asynchronously & cancel the promise.
I don;t have much experience with using promises so I can't give much advice, but it is worth a look.
Jacob
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You page remain blocked until this thead (uploading the file) its done. Since its a blocking function, your user won't be able to do anything until this task end.

Your alternative is to use non-block io

Community
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Nicholas Pedroso
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