Mouse events

To know when the player is using the mouse, we use events. For example, when the player moves the mouse, an event of type pygame.MOUSEMOTION joins the event queue. It will include useful information, such as its position or what buttons were pressed.

This is an example of use:

for event in pygame.event.get():
    if event.type == pygame.MOUSEMOTION:
        print("The mouse moved")
        print("The X position is now" + event.pos[0])
        print("The Y position is now" + event.pos[1])

This is what the program does:

1. It loops through every event in the event queue.
2. For each event, it checks if its about mouse movement.
3. If it's about mouse movement, we print a message telling us, as well as two other messages reporting its position.

Challenges:

• Try the example above in your program. Where do you think it goes?
• Ensure that the program responds both to mouse movement and to the window being closed.

The position of the mouse (and a bit about lists)

The example above does something strange to read the position. It uses event.pos[0] and event.pos[1], which look a bit complicated. Let's look into that.

Each event comes with a few things inside. For example, it has event.type, which tells what type of event it is. Remember that the dot . means "inside", so the type is something inside event, therefore we use it as event.type. The same with MOUSEMOTION: it lives inside pygame, so we refer to it as pygame.MOUSEMOTION.

Apart from the type, each event will have other things inside, which depend on the type of event. A MOUSEMOTION event has a pos, which is the position of the mouse. This position is represented as a list of two elements: X and Y.

Remember how lists work? This is an example of a list with two elements:

position = [123, 456]

We have seen how to loop through a list and do something with each element, but we haven't seen how to work with a specific element of the list. For example, how to do something only with the first element, or only with the hundredth element. Let's see that.

Computers have a funny way to count. When people count (for example with their fingers), we start counting at 1, and then go 2, 3, 4... Computers don't do that: they normally start at 0 and then go 1, 2, 3... This is important now. For a computer, the first element of a list is the element number 0, and the second element is the element number 1. The 100th element is element 99 and so on.

We can read inside a list using square brackets [] and a number, like this:

position = [123, 456]
print("The first element is " + position[0])
print("The second element is " + position[1])

Challenges

Let's see how well you understood all that:

• Write a program where you move a small image using the mouse.
  • Nothing complicated: simply that the image will be in the same position as the mouse.
  • You will have to read the mouse position from the event, and use it to set the position to the image.
  • If you follow the example from the "Objects" chapter, you should be able to do something like actor.x = event.pos[0].