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iliak's picture

[Input] : Detect a new keypress and a constant key press

Posted Wednesday, 28 October, 2009 - 08:26 by iliak
Project:The Open Toolkit library
Version:all versions
Component:Code
Category:support request
Priority:normal
Assigned:Unassigned
Status:by design
Description

It's a nice (not to say important feature) to detect the difference between a new key press (just pressed key) and a constant key press (key down for several frames) events. You can have a look here to know how I handle the case. The main idea is to poll keyboard state at each frame and store the previous / current states in an array. To detect a constant key press, both value in the array are true, and to detect a new keypress, only the current array is true, and the previous is false.

You can extented this technique for more input device (mouse and gamepad).


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the Fiddler's picture

#1

Posted Wednesday, 28 October, 2009 - 08:51 by the Fiddler
Status:open» by design

This has been requested a few times in the past. The situation is as follows:

  • The event-based interface in NativeWindow already supports this: you only get a KeyDown/KeyUp event on a "fresh" press/release of a key (unless you set KeyRepeat to true, where holding a key down will generate KeyDown+KeyUp events continuously).
  • However, the polled interface will not support this feature. The reason is that it introduces all kinds of concurrency issues:
    1. Thread 1 calls GetKeyState
    2. Thread 2 calls GetKeyState
    3. Thread 1 calls GetKeyState and receives garbage

The solution is trivial: have the user compare states and find their differences:

using OpenTK.Input;
...
KeyboardState previous_state, current_state;
...
previous_state = current_state;
current_state = Keyboard.GetState();
for (int i = 0; i < (int)Key.Last; i++)
{
    Key key = (Key)i;
    bool current = current_state.IsKeyDown(key);
    bool previous = previous_state.IsKeyDown(key);
    if (current && !previous)
        NewKeyDownDetected(key);
    else if (!current && previous)
        NewKeyUpDetected(key);
    else
        // Whatever
}

I was thinking about reusing the event-based interface (KeyboardDevice) to simplify this process, but I don't think I can implement this in time for OpenTK 1.0.