CODEWITHSUNDEEP
c++for-loopwhile-loopkeypress
James Andrew
James Andrew

Reputation: 7243

Catching keypresses C++

I am using a simple loop for catching user keypresses;

while (1)
{
    for(i = 8; i <= 190; i++)
    {
       if (GetAsyncKeyState(i) == -32767){
         //Do stuff
       }
    }
}

Which will 'do stuff' when a user presses a certain key, however obviously as it's looping indefinitely and as i'm new to C++ it's taking up 100% CPU which is not good for a simple input.

What am I doing wrong? I've tried the Sleep() function (if I put it in the 'for loop' it misses keypresses and if I put it in the 'while loop' it doesn't lower CPU at all, afaik)

How can I make it catch keypresses the same, but use not nearly as much CPU; am I missing a trick? I'm sure most programs catch keypresses and you don't see all them using 100%!

Thanks alot.

Upvotes: 4

Views: 7593

Answers (3)

Matt Phillips
Matt Phillips

Reputation: 9691

Put your infinite loop in a worker thread, and have it sleep for some reasonable interval on each iteration. C++11 makes this pretty easy:

#include <thread>
#include <chrono>

std::chrono::milliseconds THREAD_WAIT = 50;

int keypress = -1;

void GetKeyPress()
{
   while (1)
   {
       for(i = 8; i <= 190; i++)
       {
          int k = GetAsyncKeyState(i);
          if (/*whatever condition needs to be satisfied*/)
              keypress = k;
       }
       if (keypress != -1) break; //Use this only if you have td.join() below
       std::this_thread::sleep_for(THREAD_WAIT);
   }
}

int main(void)
{
   ...

   std::thread td( GetKeyPress );
   td.join(); //If you want to block until the user presses a key, otherwise remove.

   //If no join(), do the rest of your program, checking in on keypress when need be

   return 0;
}

Upvotes: 3

Jack
Jack

Reputation: 193

Please check following link that explains Proper use of GetAsyncKeyState() with example code. http://www.mpgh.net/forum/31-c-c-programming/120656-proper-use-getasynckeystate.html

Hope this link might help you to solve your problem

Edited: The GetAsyncKeyState() function is not ideal for what you are trying to do.

All it does is simply check the actual, current-at-this-nanosecond position of a key on the keyboard. Doing that is almost always incorrect.

Instead, read the console input using the proper input functions. Please find below the sample code.

#include <stdio.h>
#include <windows.h>

int main()
{
    DWORD        mode;          /* Preserved console mode */
    INPUT_RECORD event;         /* Input event */
    BOOL         done = FALSE;  /* Program termination flag */
    unsigned int counter = 0;   /* The number of times 'Esc' is pressed */

    /* Don't use binary for text files, OK?  ;-) */
    FILE* myfile = fopen( "example.txt", "w" );

    /* Get the console input handle */
    HANDLE hstdin = GetStdHandle( STD_INPUT_HANDLE );

    /* Preserve the original console mode */
    GetConsoleMode( hstdin, &mode );

    /* Set to no line-buffering, no echo, no special-key-processing */
    SetConsoleMode( hstdin, 0 );

    /* Give the user instructions */
    printf(
        "Press Escape as many times as you like.\n"
        "Press anything else to quit.\n\n"
        );

    while (!done)
    {
        if (WaitForSingleObject( hstdin, 0 ) == WAIT_OBJECT_0)  /* if kbhit */
        {
            DWORD count;  /* ignored */

            /* Get the input event */
            ReadConsoleInput( hstdin, &event, 1, &count );

            /* Only respond to key release events */
            if ((event.EventType == KEY_EVENT)
            &&  !event.Event.KeyEvent.bKeyDown)
                switch (event.Event.KeyEvent.wVirtualKeyCode)
                {
                    case VK_ESCAPE:
                        counter++;
                        fprintf( myfile, "Escape: %d\n", counter );
                        printf( "Button pressed!\n" );
                        break;
                    default:
                        done = TRUE;
                }
        }
    }

    /* All done! */
    printf( "You pressed the Escape key %d times\n", counter );
    fclose( myfile );
    SetConsoleMode( hstdin, mode );
    return 0;
}

Upvotes: 1

Sidharth Mudgal
Sidharth Mudgal

Reputation: 4264

You could use 

while (!kbhit());

This might help.

Upvotes: 0

Related Questions