command to read all current pressed keyboard keys

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Roland Stange 2017-7-6

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https://ww2.mathworks.cn/matlabcentral/answers/347593-command-to-read-all-current-pressed-keyboard-keys

回答： Bruno Luong 2021-9-10

Hi, i did not find an appropriate answer... is there a simple way without using callbacks to get the current pressed keys on the keyboard.

best, Roland

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KSSV 2017-7-6

Try this: https://in.mathworks.com/matlabcentral/fileexchange/7465-getkey

Roland Stange 2017-7-6

编辑：Roland Stange 2017-7-6

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very nice function! Thanks for that. However, not exactly what i was searching for:-). The function shell not wait for an input, but read currently pressed keys. e.g.: a = whichKeysArePressed()

a =

1×2 cell array
    '3'    'k'

for the case that the '3' and the 'k' keys are currently pressed by the user.

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Answer 1

Guillaume 2017-7-6

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https://ww2.mathworks.cn/matlabcentral/answers/347593-command-to-read-all-current-pressed-keyboard-keys#answer_273117

编辑：Guillaume 2017-7-6

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From base matlab itself, it's not possible. There may be something in the psych toolbox.

Otherwise, on Windows, you would have to call the Win32 API GetKeyboardState either through loadlibrary or a mex file. Note that GetKeyboardState will only notice key changes that matlab itself has already seen.

Another option is to use .Net GetKeyStates. .Net is easier to call from matlab, but with this function you'll have to iterate over all the keys to get their status. Something like (completely untested code, I don't have matlab on this computer):

Net.addAssembly('System.Windows.Input');
akey = System.Windows.Input.Key.A;  %use any key to get the enum type
keys = System.Enum.GetValues(akey.GetType);  %get all members of enumeration
keynames = System.Enum.GetNames(akey.GetType);
keystates = arrayfun(@(key) bitand(System.Windows.Input.Keyboard.GetKeyStates(key), ...
                                   System.Windows.Input.KeyStates.Down), ...
                     keys);

edit: after testing on computer with matlab, there was a number of bugs with the above, the two major ones being you can't arrayfun a .Net array, and some of the keys may not be valid keys to pass to GetKeyStates. In any case, since we're iterating over the keys, you can use IsKeyDown which has simpler syntax. So, without bugs:

NET.addAssembly('PresentationCore');
akey = System.Windows.Input.Key.A;  %use any key to get the enum type
keys = System.Enum.GetValues(akey.GetType);  %get all members of enumeration
keynames = cell(System.Enum.GetNames(akey.GetType))';
iskeyvalid = true(keys.Length, 1);
iskeydown = false(keys.Length, 1);
for keyidx = 1:keys.Length
   try
       iskeydown(keyidx) = System.Windows.Input.Keyboard.IsKeyDown(keys(keyidx));
   catch
       iskeyvalid(keyidx) = false;
   end
end

Once you've run the above once (to get the list of valid keys) you can get the state of the keys with the simpler:

iskeydown(iskeyvalid) = arrayfun(@(keyidx) System.Windows.Input.Keyboard.IsKeyDown(keys(keyidx)), find(iskeyvalid));

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Walter Roberson 2017-7-6

I do not know the current state of the art, but earlier on it used to be the case that keyboards could typically only reliably distinguish two pressed keys beyond the modifier keys. A contact grid used to be used that had the property that if any three points of a rectangle of keys were pressed then the keyboard could not distinguish which three of the four were pressed.

Keyboards themselves typically encode events rather than key presses: "during this cycle, such-and-such a key was just pressed and this other key was released". The set of keys down at any one time was considered to be the set of keys for which a key-press event had been seen but a key release event had not been seen yet.

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