C# Interfaces in Unity! – Intermediate Scripting Tutorial

– [Instructor] Interfaces
can be thought of as a contract on functionality. Any class that implements an interface, must have all of its
methods and properties. In exchange, the implementing class can be treated like the
interface by other classes, using polymorphism. It is important to note, that
interfaces are not classes, and can not have their own instances. While inheritance is an is a relationship, where one class inherits from another, interfaces use an implements relationship, where one class implements an interface. Interfaces are usually
declared outside of a class. When declaring interfaces,
you would commonly use one script per interface. But for the purpose of this example, we’re going to show
two in the same script. By convention, interfaces
are declared using a name that starts with a capital
I, followed by a name that starts with another capital letter. Since interfaces normally
describe some functionality that the implementing classes will have, a lot of interfaces end
in the post fix able. It is worth noting however,
that this is not mandatory, and could potentially be misleading, depending on the interface. We have declared two interfaces here, IKillable, and IDamageable. In IKillable, we have
a function called Kill, which has a return type of
void, and no parameters. Any class which implements
the IKillable interface must have a public function
which matches this signature. The IDamageable interface
has a generic type T. This means that anything in the interface can have a generic type. The function we have
in IDamageable, Damage, takes a parameter of type T. When an interface with a
generic type is implemented by a class, the type must be chosen. Then the corresponding type
must be used throughout. Implementing an interface
comes with some requirements and some benefits. In order to implement an interface, a class must publicly
declare all of the methods, properties, events, and indexers that exist in that interface. Failure to do so will result in an error. The major advantage of interfaces, is that they allow you to
define common functionality across many classes. Therefore, you are able
to safely make assumptions about what classes can do,
based on the interfaces that they implement. To implement an interface,
simply add a comma after any inheritance the class has, followed by the name of the interface. If the class doesn’t
inherit, then the comma is not required. If the interface has a generic type, then the name should be
followed by angle brackets, with the type in them. In this case, we have class Avatar, which inherits from Monobehavior,
and implements IKillable and IDamageable, with the type float. We must also declare both
of the functions required for these interfaces. Note that the body of the
functions is independent of the interfaces, and can be implemented however you would like. These source of interfaces might be useful if there was a situation in your game, where you want to damage
or kill everything. Simply by finding
everything that implemented IKillable or IDamageable,
you could be sure that they will have the
Kill or Damage functions. You may be wondering to yourself, why you would implement an interface in a class, when you could
justify in a function once in a class, and have
other classes inherit from it. The easy answer is that you can implement multiple interfaces,
but you can not inherit from multiple classes. Therefore, interfaces are a nice way to provide a wide range of functionality. The better answer though,
is that interfaces are used to define common functionality across many classes that
are unrelated to each other. Consider two classes, Wall and Car. Wall and Car have very little
in common with each other, except that they both are damageable. Since they are so
different from each other, inheriting from a parent
class makes little sense. Implementing an interface
however, works great.

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