Mouse State

Click and Drag Detection

How do I detect if the user is clicking and dragging?

Abstract art representing computer programming

Detecting click-and-drag interactions involves tracking both the mouse button state and movement. Here's how to implement it:

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5class DragDetector {
6 public:
7  void Tick() {
8    int x, y;
9    Uint32 Buttons = SDL_GetMouseState(&x, &y);
10
11    if (Buttons & SDL_BUTTON_LMASK) {
12      if (!IsDragging) {
13        // Start of drag
14        DragStartX = x;
15        DragStartY = y;
16        IsDragging = true;
17        std::cout << "Started dragging at ("
18          << x << ", " << y << ")\n";
19      } else {
20        // Continue dragging
21        int DragDistanceX = x - DragStartX;
22        int DragDistanceY = y - DragStartY;
23        std::cout << "Dragging - Distance: ("
24          << DragDistanceX << ", "
25          << DragDistanceY << ")\n";
26      }
27    } else if (IsDragging) {
28      // End of drag
29      IsDragging = false;
30      std::cout << "Stopped dragging\n";
31    }
32  }
33
34 private:
35  bool IsDragging{false};
36  int DragStartX{0};
37  int DragStartY{0};
38};
39
40int main(int argc, char** argv) {
41  SDL_Init(SDL_INIT_VIDEO);
42  Window GameWindow;
43  DragDetector Detector;
44  SDL_Event E;
45
46  while (true) {
47    while (SDL_PollEvent(&E)) {
48      // Handle other events...
49    }
50
51    Detector.Tick();
52    GameWindow.Update();
53  }
54
55  SDL_Quit();
56  return 0;
57}

1Started dragging at (100, 150)
2Dragging - Distance: (5, 2)
3Dragging - Distance: (10, 5)
4Dragging - Distance: (15, 8)
5Stopped dragging

How It Works

The drag detection system works by:

Checking if the left mouse button is pressed using SDL_GetMouseState()
When the button is first pressed, storing the starting position
While dragging, calculating the distance from the start position
Detecting when the button is released to end the drag

You can enhance this basic system by:

Adding a minimum drag distance before considering it a drag (to distinguish from clicks)
Tracking drag velocity
Storing the drag path for more complex gestures
Adding support for right-button dragging

This approach using SDL_GetMouseState() is often more reliable than using events for drag detection, as it ensures you don't miss any mouse movement between frames.

This Question is from the Lesson: