How to create an event driven SDL2 application [closed] - c++

Question

Closed. This question needs to be more focused. It is not currently accepting answers.
Want to improve this question? Update the question so it focuses on one problem only by editing this post.
Closed 3 years ago.
Improve this question
How do I create an SDL2 application, which is event based? (C++)
My current model has an update loop, which just polls input devices, right when I need them, or just leave them alone if the logic does not need it.
However I want to move to an event driven model, however there are a lot of systems which don't update on some event, just update on every tick of the update loop.
How do I deal with these?
Do I add a new thread and interrupt the update loop somehow, when I receive a new event?

Answer 1

There are many ways to implement what you're trying to do, one way I can think of is an events dispatch system. Suppose you have an event manager. Your application can register various handlers for various events with the event manager. Your event manager would listen/poll for events, it then dispatch (call handlers to those that matches the event). Your event manager would have a lookup mapping to keep track of all the handlers to corresponding event type.

Answer 2

To do event-based programming in SDL2, you need to implement part of it yourself. Right now, you probably have a basic outline that looks something like this (+ error handling, etc):
SDL_Init(SDL_INIT_VIDEO);
auto window = SDL_CreateWindow(/*arguments*/);
//create something in the window, possibly in a loop
SDL_DestroyWindow(window);
SDL_Quit();
To do event handling, you need to create a main loop yourself. In this loop, you repetitively ask SDL for the next event, and process that event as soon as it is given to you. You also re-draw the contents of the window in this loop. In game programming (and general UI, to a lesser extent) the standard system is to, over and over, clear an offscreen buffer (an offscreen buffer is just an image that isn't visible to the user), draw to it, and then make that buffer visible (while usually taking the buffer (buffers are just images) that was visible and making that be the offscreen buffer for the next iteration of the loop). The number of times that this loop runs is your FPS value in a video game, and you can measure FPS by counting how many times your loop runs in a second.
bool quit = false;
SDL_Event event;
while(!quit){
while(SDL_PollEvent(&e)){
switch(e.type){
case SDL_QUIT:
quit = true;
break;
//add more event types here as you see fit
}
}
//it is customary to re-draw the window as fast as possible here, to allow for the window contents to change
}
You can optionally write a more complex event handling system to abstract this loop, but a simple loop + switch as shown above is usually sufficient. noobius's answer gives some more detailed pointers on how to make a more complex system if a simple loop+switch is insufficient (Make sure that it actually is! Complexity is the enemy of software development, so simpler is always better all else being equal.)
Reference, to learn more: https://lazyfoo.net/tutorials/SDL/03_event_driven_programming/index.php