Alongside the basics of game development is something called Events. All videogames, from basic free poker applications to the highly complex PC games and console titles, use events to interact with the player. These events can come from keyboards, mice, joysticks, gamepads, and so on, or events from our operating system. It’s important to understand how events work if we are to appropriately interact a user with a game. We’ve already been using events, but only for closing our window, now we’ll look at how to receive events from the user.
If you haven’t caught on, each tutorial builds off the previous, so we’ll be using the code from the previous lesson. In order to track all these events and throw them into nice functions, lets create a new class. Create two files called CEvent.h and CEvent.cpp. These two files will handle our events, and call the appropriate function. CApp will inherit this class, so when we need to catch an event we simply override a function.
Open up CEvent.h and add the following code:
#define _CEVENT_H_
#include <SDL.h>
class CEvent {
public:
CEvent();
virtual ~CEvent();
virtual void OnEvent(SDL_Event* Event);
virtual void OnInputFocus();
virtual void OnInputBlur();
virtual void OnKeyDown(SDLKey sym, SDLMod mod, Uint16 unicode);
virtual void OnKeyUp(SDLKey sym, SDLMod mod, Uint16 unicode);
virtual void OnMouseFocus();
virtual void OnMouseBlur();
virtual void OnMouseMove(int mX, int mY, int relX, int relY, bool Left,bool Right,bool Middle);
virtual void OnMouseWheel(bool Up, bool Down); //Not implemented
virtual void OnLButtonDown(int mX, int mY);
virtual void OnLButtonUp(int mX, int mY);
virtual void OnRButtonDown(int mX, int mY);
virtual void OnRButtonUp(int mX, int mY);
virtual void OnMButtonDown(int mX, int mY);
virtual void OnMButtonUp(int mX, int mY);
virtual void OnJoyAxis(Uint8 which, Uint8 axis, Sint16 value);
virtual void OnJoyButtonDown(Uint8 which, Uint8 button);
virtual void OnJoyButtonUp(Uint8 which, Uint8 button);
virtual void OnJoyHat(Uint8 which, Uint8 hat, Uint8 value);
virtual void OnJoyBall(Uint8 which, Uint8 ball, Sint16 xrel, Sint16 yrel);
virtual void OnMinimize();
virtual void OnRestore();
virtual void OnResize(int w,int h);
virtual void OnExpose();
virtual void OnExit();
virtual void OnUser(Uint8 type, int code, void* data1, void* data2);
};
#endif
Pretty hefty class right? Alright, open up CEvent.cpp, and add the following code:
CEvent::CEvent() {
}
CEvent::~CEvent() {
//Do nothing
}
void CEvent::OnEvent(SDL_Event* Event) {
switch(Event->type) {
case SDL_ACTIVEEVENT: {
switch(Event->active.state) {
case SDL_APPMOUSEFOCUS: {
if ( Event->active.gain ) OnMouseFocus();
else OnMouseBlur();
break;
}
case SDL_APPINPUTFOCUS: {
if ( Event->active.gain ) OnInputFocus();
else OnInputBlur();
break;
}
case SDL_APPACTIVE: {
if ( Event->active.gain ) OnRestore();
else OnMinimize();
break;
}
}
break;
}
case SDL_KEYDOWN: {
OnKeyDown(Event->key.keysym.sym,Event->key.keysym.mod,Event->key.keysym.unicode);
break;
}
case SDL_KEYUP: {
OnKeyUp(Event->key.keysym.sym,Event->key.keysym.mod,Event->key.keysym.unicode);
break;
}
case SDL_MOUSEMOTION: {
OnMouseMove(Event->motion.x,Event->motion.y,Event->motion.xrel,Event->motion.yrel,(Event->motion.state&SDL_BUTTON(SDL_BUTTON_LEFT))!=0,(Event->motion.state&SDL_BUTTON(SDL_BUTTON_RIGHT))!=0,(Event->motion.state&SDL_BUTTON(SDL_BUTTON_MIDDLE))!=0);
break;
}
case SDL_MOUSEBUTTONDOWN: {
switch(Event->button.button) {
case SDL_BUTTON_LEFT: {
OnLButtonDown(Event->button.x,Event->button.y);
break;
}
case SDL_BUTTON_RIGHT: {
OnRButtonDown(Event->button.x,Event->button.y);
break;
}
case SDL_BUTTON_MIDDLE: {
OnMButtonDown(Event->button.x,Event->button.y);
break;
}
}
break;
}
case SDL_MOUSEBUTTONUP: {
switch(Event->button.button) {
case SDL_BUTTON_LEFT: {
OnLButtonUp(Event->button.x,Event->button.y);
break;
}
case SDL_BUTTON_RIGHT: {
OnRButtonUp(Event->button.x,Event->button.y);
break;
}
case SDL_BUTTON_MIDDLE: {
OnMButtonUp(Event->button.x,Event->button.y);
break;
}
}
break;
}
case SDL_JOYAXISMOTION: {
OnJoyAxis(Event->jaxis.which,Event->jaxis.axis,Event->jaxis.value);
break;
}
case SDL_JOYBALLMOTION: {
OnJoyBall(Event->jball.which,Event->jball.ball,Event->jball.xrel,Event->jball.yrel);
break;
}
case SDL_JOYHATMOTION: {
OnJoyHat(Event->jhat.which,Event->jhat.hat,Event->jhat.value);
break;
}
case SDL_JOYBUTTONDOWN: {
OnJoyButtonDown(Event->jbutton.which,Event->jbutton.button);
break;
}
case SDL_JOYBUTTONUP: {
OnJoyButtonUp(Event->jbutton.which,Event->jbutton.button);
break;
}
case SDL_QUIT: {
OnExit();
break;
}
case SDL_SYSWMEVENT: {
//Ignore
break;
}
case SDL_VIDEORESIZE: {
OnResize(Event->resize.w,Event->resize.h);
break;
}
case SDL_VIDEOEXPOSE: {
OnExpose();
break;
}
default: {
OnUser(Event->user.type,Event->user.code,Event->user.data1,Event->user.data2);
break;
}
}
}
void CEvent::OnInputFocus() {
//Pure virtual, do nothing
}
void CEvent::OnInputBlur() {
//Pure virtual, do nothing
}
void CEvent::OnKeyDown(SDLKey sym, SDLMod mod, Uint16 unicode) {
//Pure virtual, do nothing
}
void CEvent::OnKeyUp(SDLKey sym, SDLMod mod, Uint16 unicode) {
//Pure virtual, do nothing
}
void CEvent::OnMouseFocus() {
//Pure virtual, do nothing
}
void CEvent::OnMouseBlur() {
//Pure virtual, do nothing
}
void CEvent::OnMouseMove(int mX, int mY, int relX, int relY, bool Left,bool Right,bool Middle) {
//Pure virtual, do nothing
}
void CEvent::OnMouseWheel(bool Up, bool Down) {
//Pure virtual, do nothing
}
void CEvent::OnLButtonDown(int mX, int mY) {
//Pure virtual, do nothing
}
void CEvent::OnLButtonUp(int mX, int mY) {
//Pure virtual, do nothing
}
void CEvent::OnRButtonDown(int mX, int mY) {
//Pure virtual, do nothing
}
void CEvent::OnRButtonUp(int mX, int mY) {
//Pure virtual, do nothing
}
void CEvent::OnMButtonDown(int mX, int mY) {
//Pure virtual, do nothing
}
void CEvent::OnMButtonUp(int mX, int mY) {
//Pure virtual, do nothing
}
void CEvent::OnJoyAxis(Uint8 which,Uint8 axis,Sint16 value) {
//Pure virtual, do nothing
}
void CEvent::OnJoyButtonDown(Uint8 which,Uint8 button) {
//Pure virtual, do nothing
}
void CEvent::OnJoyButtonUp(Uint8 which,Uint8 button) {
//Pure virtual, do nothing
}
void CEvent::OnJoyHat(Uint8 which,Uint8 hat,Uint8 value) {
//Pure virtual, do nothing
}
void CEvent::OnJoyBall(Uint8 which,Uint8 ball,Sint16 xrel,Sint16 yrel) {
//Pure virtual, do nothing
}
void CEvent::OnMinimize() {
//Pure virtual, do nothing
}
void CEvent::OnRestore() {
//Pure virtual, do nothing
}
void CEvent::OnResize(int w,int h) {
//Pure virtual, do nothing
}
void CEvent::OnExpose() {
//Pure virtual, do nothing
}
void CEvent::OnExit() {
//Pure virtual, do nothing
}
void CEvent::OnUser(Uint8 type, int code, void* data1, void* data2) {
//Pure virtual, do nothing
}
Lots of code, but all the SDL events should be covered. What we basically are doing is taking an SDL_Event pointer, and switching through the types, and then calling the appropriate function. It just looks like a lot since they are quite a bit of events.
Now that this has been all setup, let’s jump over to CApp.h and link the two together:
#define _CAPP_H_
#include <SDL.h>
#include "CEvent.h"
#include "CSurface.h"
class CApp : public CEvent {
private:
bool Running;
SDL_Surface* Surf_Display;
SDL_Surface* Surf_Test;
public:
CApp();
int OnExecute();
public:
bool OnInit();
void OnEvent(SDL_Event* Event);
void OnLoop();
void OnRender();
void OnCleanup();
};
#endif
Everything should compile just fine. We have the event class setup, now lets actually link events to our new class structure. Open up CApp_OnEvent.cpp and edit the following function:
void CApp::OnEvent(SDL_Event* Event) {
CEvent::OnEvent(Event);
}
We’re passing the Event structure to our class now, it will take care of the correct function calls. Now when we want to check for an event, we override the function. Since we got rid of checking for the SDL_QUIT event, lets use the function instead. Open up CApp.h again, and add the following function:
#define _CAPP_H_
#include <SDL.h>
#include "CEvent.h"
#include "CSurface.h"
class CApp : public CEvent {
private:
bool Running;
SDL_Surface* Surf_Display;
SDL_Surface* Surf_Test;
public:
CApp();
int OnExecute();
public:
bool OnInit();
void OnEvent(SDL_Event* Event);
void OnExit();
void OnLoop();
void OnRender();
void OnCleanup();
};
#endif
The OnExit function handles the SDL_QUIT events. Now that we have the prototype, lets define what it does. Open up CApp_OnEvent.cpp, and add the following:
void CApp::OnEvent(SDL_Event* Event) {
CEvent::OnEvent(Event);
}
void CApp::OnExit() {
Running = false;
}
Recompile, and try it out. You should be able to close the application just like before.
I encourage you to look through some of the various events that we can check. Later on we’ll use some of these events in our games, but for now try to familiarize yourself with them.
Jump on over to the next tutorial to take a look at creating our first game, Tic-Tac-Toe.
SDL Events – Tutorial Files:
Win32: Zip, Rar
Linux: Tar (Thanks Gaten)
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Excellent tutorials. Thank you so much for making this wonderful resource available Mr. Jones.
Here’s a question for anyone who has been using MingW32 for compiling and running these SDL tutorials.
Up to this tutorial, I have been working on these files through MingW32 (The “Minimalist GNU for Windows”) and the GCC compiler, without any problems. The code we add in this tutorial does not compile for me though, and returns me this error: “undefined reference to __gxx_personality_sj0″ for CApp.cpp. This error is not uncommon for GCC, and often signifies that something has not been linked properly. I’m pretty sure I link all the files and required libraries in this case, though. I did nothing different when I went through Tutorials 1 and 2, and they compiled fine.
So what I eventually did was I simply took my code as-is into CodeBlocks, and it instantly compiled there without any problems. Does anyone happen to know what might be that troubles the GCC compiler under MingW32? The command I used is: gcc sourcefile1.cpp sourcefile2.cpp `sdl-config –cflags –libs`
I figured out what the problem was. I have been trying to compile a .cpp file with the “gcc” command, which is meant for compiling .c files. In this case, “g++” should be used instead of “gcc”. With the following command, the code compiles without errors:
g++ sourcefile1.cpp sourcefile2.cpp `sdl-config –cflags –libs`
Wonderfull layout! Really easy to use and a very effective lesson.
I even donated a whole $5…
I’m a student give me a break.
Thanks!
These tutorials are awesome! very nicely written and clear enough to understand everything. good job.
Fantastic! Clear informative and good style. Mr. Jones you rock!
Thanks everyone!
Enigma,
Take a look at SPriG (https://www.ohloh.net/p/SPriG) and SDL_gfx (http://www.ferzkopp.net/joomla/content/view/19/14/).
Thanks Tim Jones for those primitives links!
Thanks for the awesome tutorials! I’m finally starting to make the switch from Java2D to SDL. The only thing I’m missing so far is the ability to draw primitives. Is there a way to draw primitives like Java2D (Graphics2D.drawRect(), etc)?
I have found these tutorials to be excellent. Great job.
Great tutorials! Now I’m getting actually getting somewhere with SDL!
cheers
Bojan,
I override the OnEvent simply to demonstrate how to do so, if you don’t want to, don’t override it. And yes, technically those functions are not pure virtuals, but how much of a pain it would be to actually have to define all those methods in CApp. In that sense, it’s a semi-pure virtual, because it’s basically empty methods, but you don’t want to have to code all of those into CApp.
And another thing: You put in the comments “//pure virtual”, but those functions aren’t declared as pure virtual. Function is pure virtual only if you declare it with “=0″, right?