javascript - Cross-browser way to get automatically repeating keydown events when key is held down

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javascript - Cross-browser way to get automatically repeating keydown events when key is held down

posted Oct 24, 2021 in Technique[技术] by 深蓝 (71.8m points)

javascript - Cross-browser way to get automatically repeating keydown events when key is held down

Using Javascript / jQuery, how can I get automatically repeating keydown events, or equivalent, when someone holds down a key?

What I actually want is to be able to check whether a key is down, but from other questions here it looks like that's not possible. The suggested workaround seems to be recording keydown and keyup events and then assuming the key is down if a keydown event has been recorded and no subsequent keyup.

That solution runs into a problem in my case. I am designing an online experiment. The user is supposed to hold down the "T" key for the entire experiment, never letting it up. The experiment consists of multiple trials and each trial has no access to information recorded by the previous trials. So, trial 1 could record keydown for T, but trial 2 wouldn't have access to that record and thus wouldn't know whether T was down or not.

Now, if holding down the T key would produce automatically repeating keydown events for T, I would have no problem because trial 2 would just catch the next keydown event for T to come along. But it looks like I don't get automatically repeating keydown events from holding the key down, at least in Firefox. From what I can see it seems there is variation in the way different browsers handle holding a key down. What is a good cross-browser way to solve my problem?

By the way, if it matters, I also need to be able to detect keyup and keydown events for other keys while all this is going on.

EDIT: after reading some of the comments I went back and verified that I do indeed get repeating keydown events under ordinary circumstances. But I really don't get them in the specific situation in which I need them. I've got some simple code which I think isolates the issue:

<!DOCTYPE html>
<html>

<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>
</head>

<body>
<div id="target"></div>
</body>

<script type="text/javascript">

var i;
function foo() {
    i++;
    $('#target').html(i);
}

function doTrial() { // do trial
    i=0;
    $(document).keydown(foo);
    $(document).keyup(endTrial);
}

function endTrial() { // end trial
    $('#target').html('');
    $(document).unbind('keydown',foo);
    $(document).unbind('keyup',endTrial);
    doTrial();
}

doTrial();

</script>

</html>

If you press a key and hold it down, then release, then press again, the behavior is as expected, i.e. there is a counter which increments while the key is held down, disappears when it's released, and then starts incrementing again when it's pressed again.

But if you press TWO keys down, then release ONE, I would have thought that the other (not released) key would continue sending keydown events so that the counter would (after resetting) continue incrementing. In fact, that doesn't happen. Any idea why and how to make it happen?

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1 Reply

深蓝 · Answer 1 · 2021-10-23T18:00:02+0000

In the browsers I tried this in, I got repeated keydown events when holding a typeable key down. I don't know if this is a problem you actually need to solve.

But, if you did think you need to solve it OR if you want to control the repeat-rate yourself, you could do it like this:

Capture the events for keydown and keyup.
On keydown, set an interval timer that fires however often you want to know that the key is still down.
On keyup for that key, stop the interval timer.
You will get repeated notification in a cross browser way as long as the key is held down.

Working demo here: http://jsfiddle.net/jfriend00/XbZYs/

var downTimer;
var lastKey;
$(document.body).keydown(function(e) {
    // if not still the same key, stop the timer
    if (e.which !== lastKey) {
        if (downTimer) {
            clearInterval(downTimer);
            downTimer = null;
        }
    }
    // remember previous key
    lastKey = e.which;
    if (!downTimer) {
        // start timer
        downTimer = setInterval(function() {
            $("#result").append("+");
        }, 125);
    }
}).keyup(function(e) {
    // stop timer
    if (downTimer) {
        clearInterval(downTimer);
        downTimer = null;
        lastKey = 0;
    }
});

If you want a key to auto-repeat forever until it is raised, even if other keys are pressed and released in the meantime and you want those other keys to do their own auto-repeating, then the OS does not ipmlement that behavior so you would have to implement it yourself. You can do something like this which calls a callback function for every key repeat event:

Working demo: http://jsfiddle.net/jfriend00/aD3Eg/

// this is called for every manufactured repeat event
// the frequency of the repeat event is determined by the time value set
// on setInterval() below
function repeatCallback(key) {
    $("#result").append(key + " ");
}

var repeatState = {};
$(document.body).keydown(function(e) {
    var key = e.which;
    // if no time yet for this key, then start one
    if (!repeatState[key]) {
        // make copy of key code because `e` gets reused
        // by other events in IE so it won't be preserved
        repeatState[key] = setInterval(function() {
            repeatCallback(key);
        }, 125);
    } else {
        // nothing really to do here
        // The key was pressed, but there is already a timer
        // firing for it
    }
}).keyup(function(e) {
    // if we have a timer for this key, then stop it 
    // and delete it from the repeatState object
    var key = e.which;
    var timer = repeatState[key];
    if (timer) {
        clearInterval(timer);
        delete repeatState[key];
    }
});
?

The repeatCallback function is called for all of these manufactured auto-repeat events and passed the key that is auto-repeating. ?

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javascript - Cross-browser way to get automatically repeating keydown events when key is held down

javascript - Cross-browser way to get automatically repeating keydown events when key is held down

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