SDL - Why moving the mouse changes button state? - c

Question

I'm having a problem with the Simple Directmedia Layer library. The following code draws a block on the screen when a mouse button is pressed:
SDL_Event event;
while(running){
while(SDL_PollEvent(&event)){
while(event.button.state == SDL_PRESSED){
SDL_PollEvent(&event);
//where to draw
boxRect.x = event.motion.x;
boxRect.y = event.motion.y;
//Draw to screen
SDL_FillRect(display,&boxRect,boxColor);
SDL_Flip(display);
}
// ...
}
// ...
}
It works fine until I move the mouse, why moving the mouse makes event.button.state untrue?
How can I work with both simultaneously (i.e. keep drawing while the button is pressed)?

Answer 1

The problem with your code is that you're calling SDL_PollEvent (documented here) twice. As said in the documentation:
If event is not NULL, the next event is removed from the queue and
stored in the SDL_Event structure pointed to by event.
Rearranging your code a bit, like getting rid of the second SDL_PollEvent, creating the proper flow for clicking, moving, releasing and extracting rendering from the input pumping should give you something like this:
SDL_Event Event;
while(running)
{
while(SDL_PollEvent(&Event))
{
switch(Event.type)
{
// Handle your drawing with a simple state machine:
case SDL_MOUSEBUTTONDOWN:
{
if(Event.button.button == SDL_BUTTON_LEFT)
{
if(stateMachine == Released)
{
// ... begin drawing
stateMachine = Dragging
}
}
break;
}
case SDL_MOUSEMOTION:
{
if(stateMachine == Dragging)
{
// ... update the extends of your rect
}
}
case SDL_MOUSEBUTTONUP:
{
if(Event.button.button == SDL_BUTTON_LEFT)
{
if(stateMachine != Released)
{
// ... finalize drawing... add the rect to a list? flush it?
stateMachine = Released;
}
}
}
case SDL_QUIT:
{
running = false;
break;
}
}
}
// Outside of your event pumping, update the graphics
SDL_FillRect(display,&boxRect,boxColor);
SDL_Flip(display);
}

Answer 2

event.button.state is a bit mask. It is declared as a char which can hold eight bits representing different states of the buttons.
typedef struct{
Uint8 type;
Uint8 state;
Uint16 x, y;
Sint16 xrel, yrel;
} SDL_MouseMotionEvent;
More information here,
http://www.libsdl.org/docs/html/sdlgetmousestate.html
http://www.libsdl.org/docs/html/sdlmousemotionevent.html

Answer 3

Thanks to emartel's answer, it's working as expected, here's the code:
SDL_Event event;
int drawing = 0;
while(running){
while(SDL_PollEvent(&event)){
switch(event.type){
//starts drawing when any mouse button is pressed
case SDL_MOUSEBUTTONDOWN:
drawing = 1;
boxRect.x = event.button.x;
boxRect.y = event.button.y;
break;
//keeps drawing while no button is released
case SDL_MOUSEMOTION:
if(drawing == 1){
boxRect.x = event.motion.x;
boxRect.y = event.motion.y;
}
break;
//stops drawing when any button is released
case SDL_MOUSEBUTTONUP:
drawing = 0;
break;
case SDL_QUIT:
running = 0;
break;
}
}
//draws to screen
if(drawing == 1){
SDL_FillRect(display,&boxRect,boxColor);
SDL_Flip(display);
}
}