Avoiding a Javascript race condition
Solution 1
Use the semaphore (let's call it StillNeedsValidating). if the SaveForm function sees the StillNeedsValidating semaphore is up, have it activate a second semaphore of its own (which I'll call FormNeedsSaving here) and return. When the validation function finishes, if the FormNeedsSaving semaphore is up, it calls the SaveForm function on its own.
In jankcode;
function UserInputChanged(control) {
StillNeedsValidating = true;
// do validation
StillNeedsValidating = false;
if (FormNeedsSaving) saveForm();
}
function SaveForm() {
if (StillNeedsValidating) { FormNeedsSaving=true; return; }
// call web service to save value
FormNeedsSaving = false;
}
Solution 2
Disable the save button during validation. Set it to disabled as the first thing validation does, and re-enable it as it finishes.
e.g.
function UserInputChanged(control) {
// --> disable button here --<
currentControl = control;
// use setTimeout to simulate slow validation code (production code does not use setTimeout)
setTimeout("ValidateAmount()", 100);
}
and
function ValidateAmount() {
// various validationey functions here
amount = currentControl.value; // save value to collection
document.getElementById("Subtotal").innerHTML = amount; // update subtotals
// --> enable button here if validation passes --<
}
You'll have to adjust when you remove the setTimeout and make the validation one function, but unless your users have superhuman reflexes, you should be good to go.
Solution 3
I think the timeout is causing your problem... if that's going to be plain code (no asynchronous AJAX calls, timeouts etc) then I don't think that SaveForm will be executed before UserInputChanged completes.
Solution 4
A semaphore or mutex is probably the best way to go, but instead of a busy loop, just use a setTimeout()
to simulate a thread sleep. Like this:
busy = false;
function UserInputChanged(control) {
busy = true;
currentControl = control;
// use setTimeout to simulate slow validation code (production code does not use setTimeout)
setTimeout("ValidateAmount()", 100);
}
function SaveForm() {
if(busy)
{
setTimeout("SaveForm()", 10);
return;
}
// call web service to save value
document.getElementById("SavedAmount").innerHTML = amount;
}
function ValidateAmount() {
// various validationey functions here
amount = currentControl.value; // save value to collection
document.getElementById("Subtotal").innerHTML = amount; // update subtotals
busy = false;
}
Jeremy Frey
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Updated on July 09, 2022Comments
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Jeremy Frey almost 2 years
My users are presented a basically a stripped down version of a spreadsheet. There are textboxes in each row in the grid. When they change a value in a textbox, I'm performing validation on their input, updating the collection that's driving the grid, and redrawing the subtotals on the page. This is all handled by the
OnChange
event of each textbox.When they click the Save button, I'm using the button's
OnClick
event to perform some final validation on the amounts, and then send their entire input to a web service, saving it.At least, that's what happens if they tab through the form to the Submit button.
The problem is, if they enter a value, then immediately click the save button,
SaveForm()
starts executing beforeUserInputChanged()
completes -- a race condition. My code does not usesetTimeout
, but I'm using it to simulate the sluggishUserInputChanged
validation code:<script> var amount = null; var currentControl = null; function UserInputChanged(control) { currentControl = control; // use setTimeout to simulate slow validation code setTimeout(ValidateAmount, 100); } function SaveForm() { // call web service to save value document.getElementById("SavedAmount").innerHTML = amount; } function ValidateAmount() { // various validationey functions here amount = currentControl.value; // save value to collection document.getElementById("Subtotal").innerHTML = amount; } </script> Amount: <input type="text" onchange="UserInputChanged(this)"> Subtotal: <span id="Subtotal"></span> <button onclick="SaveForm()">Save</button> Saved amount: <span id="SavedAmount"></span>
I don't think I can speed up the validation code -- it's pretty lightweight, but apparently, slow enough that code tries to call the web service before the validation is complete.
On my machine, ~95ms is the magic number between whether the validation code executes before the save code begins. This may be higher or lower depending on the users' computer speed.
Does anyone have any ideas how to handle this condition? A coworker suggested using a semaphore while the validation code is running and a busy loop in the save code to wait until the semaphore unlocks - but I'd like to avoid using any sort of busy loop in my code.