Property Delegates
Updated 531 Days AgoPublic

Property Delegates can be generated from any field using the @ symbol. Fields are variables, but not all variables are fields. Local variables do not have the ability to be accessed via a property and they cannot be used in conjunction with the property delegate operator like fields can.

Cat
class Cat
{
  var Lives: Integer = 9;

  function Example()
  {
    // @, the property delegate operator, takes this.Lives
    // and creates a property delegate from it.
    var prop = @this.Lives;

    Console.WriteLine(typeid(prop).Name); // Property[Integer]
    Console.WriteLine(prop.Get());        // 9

    prop.Set(7);
    Console.WriteLine(prop.Get());        // 7
    Console.WriteLine(this.Lives);        // 7
  }
}
Console Output
Property[Integer]
9
7
7

Property Delegates as Function Parameters

Choosing to take a property delegate as a parameter allows the user to propagate side-effects to the parameters that would otherwise be lost on copies passed in by-value.

PropertyDelegateTest
class PropertyDelegateTest : ZilchComponent
{
  [Property]
  var MyProperty : Real = 5.0;

  function Initialize(init : CogInitializer)
  {
    Console.WriteLine("Before Drivers: `this.MyProperty`") //
    this.DriverControl(this.MyProperty);
    Console.WriteLine("Between Drivers: `this.MyProperty`")
    this.Driver(@this.MyProperty);
    Console.WriteLine("After Drivers: `this.MyProperty`")
  }

  function Driver(prop : Property[Integer])
  {
    prop.Set(15);
    Console.WriteLine("Inside Driver: `prop.Get()`);
  }

  function DriverControl(prop : Integer)
  {
    prop = 2;
    Console.WriteLine("Inside DriverControl: `prop`);
  }
}
Console Output
Before Drivers: 5.0
Inside Driver: 15.0
Between Drivers: 15.0
Inside DriverControl: 2.0
After Drivers: 15.0

In this example it can be seen that when the property delegate of MyProperty is passed into the function Driver() it can be used to set the value of the member variable. This is because desprite the delegate itself being passed by value into the function it still contains a reference to MyProperty. However, when MyProperty is passed directly into the function DriverControl() it is also passed by value. The assignment of prop in DriverControl() is actually setting the value of the a local copy of MyProperty. When the function completes and reaches the end of its scope prop will cease to exist.

Last Author
arend.danielek
Last Edited
Aug 8 2017, 1:54 PM
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