Detecting if a key was pressed, not if it is always down - c++

Question

I'm new to SFML and I have trouble finding a solution to checking if a key is pressed during one frame. The problem I've been facing is the fact that with the Keyboard and Mouse classes, it seems impossible to use a system where one first checks for the current input state before any Update() call of objects and then after all Update() you get a previous input state for the next frame so that one can do the following:
bool Entity::KeyboardCheckPressed(sf::Keyboard::Key aKey)
{
//this part doesn't work
if (KeyboardState->isKeyPressed(aKey) and !PreviousKeyboardState->isKeyPressed(aKey))
{
return true;
}
return false;
}
But this doesn't seem to work with SFML, and other sources tell me that I'm suppose to use the Event class with its type and key.codelike the following example:
bool Entity::KeyboardCheckPressed(sf::Keyboard::Key aKey)
{
if (Event->type == sf::Event::KeyPressed)
{
if (Event->key.code == aKey)
{
return true;
}
}
return false;
}
But this results in the sf::Event::KeyPressed doing the same as KeyboardState->isKeyPressed(aKey), so then I tried the method where you set key repeat to false: window.setKeyRepeatEnabled(false);with no results what so ever. I also found out that the sf::Event::KeyPressed works only as intended inside of this part in the main.cpp:
while (window.pollEvent(event))
{
}
The problem with this is that I want to handle Input inside of my Entity objects' Update()function, and I can't put the whole Update loop inside of the while (window.pollEvent(event)). So here I am, struggling to find a solution. Any help is appreciated.

Answer 1

In general, if you have a thing which you can check the current state of, and you want to check if that state changed between frames, you simply use a variable, declared outside the application loop, to store the previous state, and compare it to the current state.
bool previousState = checkState();
while (true) {
// your main application loop
bool newState = checkState();
if (newState == true && previousState == false) {
doThingy("the thing went from false to true");
} else if (newState == false && previousState == true) {
doThingy("the thing went from true to false");
} else {
doThingy("no change in the thing");
}
// this is done unconditionally every frame
previousState = newState;
}