How a function can be automatically called upon a variable change?

Question

I have a class consisting of variable members and a function member. The variable member occasionally changes. I want the function to be called automatically upon the variable changes. In other words, how can I tie the variables inside a class?

class line
{
   double x, y; // The poition of the lind end. The line starts at the origin (0,0)
   double l; // The length of the line
   void length()
   {
      l = Math.sqrt(x*x+y*y);
   }
}

In example above, I need the length to be updated when x and y change.

How would you envision this mechanism working? This isn't magic...something has to be checking the values of x and y and calling your method once they change... — rory.ap, Mar 29 '17 at 17:34
Possible duplicate of [How to trigger event when a variable's value is changed?](http://stackoverflow.com/questions/5842339/how-to-trigger-event-when-a-variables-value-is-changed) — BetterLateThanNever, Mar 29 '17 at 17:35

score 3 · Accepted Answer · answered Mar 29 '17 at 17:35

3

Make your variables into properties, then put your functions in the set accesors.

class line
{
    double _x, _y;

    double x
    {
        get { return _x; }
        set
        {
            _x = value;
            length();
        }
    }

    double y
    {
        get { return _y; }
        set
        {
            _y = value;
            length();
        }
    }

    double l; // The length of the line

    void length()
    {
        l = Math.Sqrt(_x * _x + _y * _y);
    }
}

answered Mar 29 '17 at 17:35

Ben J

1,040
1
5
13

Thanks, Ben. Now I have another problem. I wrapped x and y over a class. Therefore x and y are updated without setting. Since I never set the variables, length() never be called. – Hossein Dehghani Mar 29 '17 at 18:13
You may want to look at events and event handlers – Ben J Mar 30 '17 at 12:12

spender · Answer 2 · 2017-03-29T17:45:08.640

2

If you define properties, on your class, you can make X and Y autoprops, then make a read-only property L that is calculated from these values:

public class Line //class names should be Capitalized
{
   public double X{ get; set; } //prop names should be Capitalized
   public double Y{ get; set; }
   public double L{
    get{
      return Math.Sqrt(X * X + Y * Y);
    }
   }
}

edited Mar 29 '17 at 17:45

answered Mar 29 '17 at 17:39

spender

106,080
28
202
324

score 1 · Answer 3 · answered Mar 29 '17 at 17:38

1

you can you properties

int x
int X {
   get { return x; }
   set { x = value; YouMethod();}
}

answered Mar 29 '17 at 17:38

Alex S

111
6

score 1 · Answer 4 · answered Mar 29 '17 at 17:42

You can achieve pretty similar behavior using calculated property like
```
double Length
{
    get { return Math.sqrt(x*x+y*y); }
}
```
The only caveat is that calculation is performed upon each call to Length even if x and y haven't changed.

You can encapsulate you x and y fields into properties and call length function from setter like

double X
{
    get { return x; }
    set 
    {
        x = value;
        length();
    }
}

double Y
{
    get { return y; }
    set 
    {
        y = value;
        length();
    }
}

and then change x and y ONLY via X and Y properties.

score 0 · Answer 5 · answered Mar 29 '17 at 17:41

As BenJ noted, you can use properties.

Instead of declaring x and y as simple fields inside the class. You can declare them as properties the following way :

private double x;
public double X
get
    {
        return this.x;
    }

    set
    {
        this.x = value;

        this.length()
        //Above line will call your desired method
    }

How a function can be automatically called upon a variable change?

5 Answers5