Understanding Keyboard State

Handling Key Releases with Polling

How can I handle key releases using the polling method?

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Handling key releases using the polling method involves checking the state of the keys continuously and comparing the current state with the previous state to detect changes.

Steps to Handle Key Releases

  1. Store Previous State: Keep track of the previous state of the keyboard in an array.
  2. Compare States: Compare the current state with the previous state to detect key releases.
  3. Update State: Update the previous state after each comparison.

Example

Here’s an example that demonstrates how to handle key releases:

#include <SDL.h>
#include <iostream>
#include <cstring>

class Window {
public:
  Window() {
    SDL_Init(SDL_INIT_VIDEO);
    SDLWindow = SDL_CreateWindow(
      "Key Release Polling",
      SDL_WINDOWPOS_CENTERED,
      SDL_WINDOWPOS_CENTERED,
      640, 480, SDL_WINDOW_SHOWN
    );
    std::memset(prevState, 0, sizeof(prevState)); 
  }

  ~Window() {
    SDL_DestroyWindow(SDLWindow);
    SDL_Quit();
  }

  void HandleKeyboard() {
    const Uint8* state = SDL_GetKeyboardState(NULL);
    if (prevState[SDL_SCANCODE_A]
      && !state[SDL_SCANCODE_A]) {
      std::cout << "A key released\n";
    }
    std::memcpy(prevState, state, sizeof(prevState)); 
  }

private:
  SDL_Window* SDLWindow{nullptr};
  Uint8 prevState[SDL_NUM_SCANCODES]; 
};

int main(int argc, char* argv[]) {
  Window GameWindow;
  bool running = true;
  SDL_Event event;

  while (running) {
    while (SDL_PollEvent(&event)) {
      if (event.type == SDL_QUIT) {
        running = false;
      }
    }
    GameWindow.HandleKeyboard();
    // Rendering code here
  }

  return 0;
}
A key released

Explanation

  • Store Previous State: An array prevState is used to store the previous keyboard state. This array is initialized to zero using std::memset().
  • Compare States: In the HandleKeyboard() method, the current state of the keyboard is compared with the previous state. If a key was pressed in the previous state but is not pressed in the current state, it indicates a key release.
  • Update State: After processing, the previous state array is updated to the current state using std::memcpy().

Considerations

  • Initialization: Ensure that the previous state array is properly initialized to zero to avoid false positives on the first check.
  • Efficiency: This method can be efficient for a small number of keys, but for applications with complex input handling, consider using event-driven methods or optimizing the state comparison logic.

Conclusion

By storing and comparing the previous and current states of the keyboard, you can effectively handle key releases using the polling method.

This technique allows you to detect when a key is released and perform the necessary actions in response.

This Question is from the Lesson:

Understanding Keyboard State

Learn how to detect and handle keyboard input in SDL2 using both event-driven and polling methods. This lesson covers obtaining and interpreting the keyboard state array.

Answers to questions are automatically generated and may not have been reviewed.

This Question is from the Lesson:

Understanding Keyboard State

Learn how to detect and handle keyboard input in SDL2 using both event-driven and polling methods. This lesson covers obtaining and interpreting the keyboard state array.

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