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Tutorial #2: Event Driven Programming

Now that we've created a window and got it painting, let's talk briefly about how the message loop that we created in the previous tutorial works. As I mentioned, the call to DispatchEvents causes the SDL application to process any events incoming from the operating system. But how do you, as the programmer, actually respond to those events? By creating event handlers that will take care of responding to specific events.

Step 1: Respond to key presses

Go back now to the program we created. First add the following line at the top of the file below the other using statements:

using SdlSharp.Input;

This will enable us to use the classes that deal with input from the user without qualification. Then add these lines before the DispatchEvents loop:

Keyboard.KeyDown += (s, e) => Application.ShowMessageBox(MessageBoxType.Information, "Key Press", e.Keycode switch
{
    Keycode.Up => "Up!",
    Keycode.Down => "Down!",
    Keycode.Left => "Left!",
    Keycode.Right => "Right!",
    _ => "Other!"
}, window);

What this code does is hook up a handler for the KeyDown events that are fired when the user presses a key on the keyboard. The parameter s is the "sender" of the event, but in this case there is no "sender" so this value will always be null. The parameter e are the event arguments, which contain information about the event. In this case, the KeyDown event will provide a KeyboardEventArgs object that contains information about the key that was involved in the event.

So in this case, all we're doing is putting up a message box when the users presses the up, down, left or right key. You can do dotnet run and see this in action.

Step #2: Respond to mouse input

In addition to keyboard input, you can also respond to mouse events as well. Add these lines below the ones you added in the previous step:

Mouse.ButtonDown += (s, e) => Application.ShowMessageBox(MessageBoxType.Information, "Mouse Button", "Down!", window);
Mouse.ButtonUp += (s, e) => Application.ShowMessageBox(MessageBoxType.Information, "Mouse Button", "Up!", window);

Now when you click on the mouse button when you're over the window, you should get a message box that says "Down!" and then one that says "Up!".

Step #3: Explore!

Keyboard and Mouse are just two of the many objects that will raise events. Here is a list of the major objects that have events:

Type Events
Application Events related to application-level actions such as quitting, drag and drop, etc.
Clipboard Events related to the clipboard
Display and Window Events related to the screen
GameController and Joystick Events related to game controllers
Keyboard Events related to the keyboard
Mouse Events related to the mouse
Sensor Events related to system sensors (if any)
AudioDevice Events related to sound
Gesture and TouchDevice Events related to touch input