architecture - How do I ensure a piece of code runs only once?

I have some code that I only want to run once, even though the circumstances that trigger that code could happen multiple times.

For example, when the user clicks the mouse, I want to click the thing:

void Update() {
    if(mousebuttonpressed) {
        ClickTheThing(); // I only want this to happen on the first click,
                         // then never again.
    }

}

However, with this code, every time I click the mouse, the thing gets clicked. How can I make it happen only once?

Answer

Use a boolean flag.

In the example shown, you'd modify the code to be something like the following:

//a boolean flag that lets us "remember" if this thing has happened already
bool thatThingHappened = false;

void Update() {

    if(!thatThingHappened && mousebuttonpressed) {
        //if that thing hasn't happened yet and the mouse is pressed
        thatThingHappened = true;
        ClickTheThing();
    }
}

Further, if you wanted to be able to repeat the action, but limit frequency of the action (i.e. the minimum time between each action). You'd use a similar approach, but reset the flag after a certain amount of time. See my answer here for more ideas on that.