Handling keyboard events in normal web applications is not so common. However, if you are writing a web game, you'll almost certainly want to catch arrow key input, and the like. This recipe shows us how we can handle those events.
Look at the keyboard project. The web page is a copy of the page used in the Preventing an onSubmit event recipe, but now submit stays enabled. Suppose we also want to ensure that pressing the Enter key provokes submit, just like clicking on the submit button.
- To that end, we add the following event listener to
main():document.onKeyPress.listen(_onKeyPress);
- The
_onKeyPressmethod is as follows:_onKeyPress(KeyboardEvent e){ if (e.keyCode == KeyCode.ENTER) submit(e); }
Now, pressing Enter will cause the form to be submitted.
The document object from dart:html has three events to work with key input: onKeyPress, onKeyDown, and onKeyUp. They all generate a stream of KeyboardEvent objects that capture user interaction with the keyboard. However, these events are also defined for window, the parent object of document, and any Element on the page, so you can use them very specifically on a certain InputElement or <div> region.
The keyCode of the event is an integer from a list of constants, which specifies a value for every key, such as KeyCode.A, KeyCode.SPACE, and KeyCode.LEFT for the left arrow key, and so on. This lends itself very easily to a switch/case construct, as follows:
_onKeyPress(KeyboardEvent e){
e.preventDefault();
print('The charCode is ${e.charCode}'),
if (e.keyCode == KeyCode.ENTER) submit(e);
if (e.ctrlKey) return;
switch(e.keyCode) {
case KeyCode.DOWN:
// move sprite down
break;
case KeyCode.UP:
// move sprite up
break;
case KeyCode.F1:
// show help
break;
}
}The event also has an integer character code getter named charCode. If the key was a special key, this can also be tested with e.ctrlKey, e.altKey, e.metaKey, e.ShiftKey, and e.altGraphKey:
if (e.ctrlKey) return;