Delegates

Declaring Delegates

To declare a delegate, use one of the macros below; select the macro based on the signature of the function (or functions) that you intend to bind to the delegate. Each macro features parameters for the new delegate type name, as well as the function's return type (if not void) and its parameters. Currently, delegate signatures support any combination of the following:

• Functions returning a value
• Functions declared as const
• Up to four "payload" variables
• Up to eight function parameters

Use this table to find the declaration macro to use to declare your delegate.

	UDELEGATE(BlueprintAuthorityOnly)
	DECLARE_DYNAMIC_MULTICAST_DELEGATE_FourParams(FInstigatedAnyDamageSignature, float, Damage, const UDamageType*, DamageType, AActor*, DamagedActor, AActor*, DamageCauser);

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Variations of the macros above for multi-cast, dynamic, and wrapped delegates are as follows:

• DECLARE_MULTICAST_DELEGATE...
• DECLARE_DYNAMIC_DELEGATE...
• DECLARE_DYNAMIC_MULTICAST_DELEGATE...
• DECLARE_DYNAMIC_DELEGATE...
• DECLARE_DYNAMIC_MULTICAST_DELEGATE...

Delegate signature declarations can exist at global scope, within a namespace, or even within a class declaration. These declarations may not exist within the body of a function.

See Dynamic Delegates and Multi-cast Delegates for more information on declaring these types of delegates.

Delegate functions support The same Specifiers as UFunctions, but use the UDELEGATE macro instead of UFUNCTION.

Binding Delegates

The delegate system understands certain types of objects, and additional features are enabled when using these objects. If you bind a delegate to a member of a UObject or shared pointer class, the delegate system can keep a weak reference to the object, so that if the object gets destroyed out from underneath the delegate, you will be able to handle these cases by calling IsBound or ExecuteIfBound functions. Note the special binding syntax for the various types of supported objects.

See DelegateSignatureImpl.inl (located in ..\Engine\Source\Runtime\Core\Public\Templates\) for the variations, arguments, and implementations of these functions.

Payload Data

When binding to a delegate, you can pass payload data along. These are arbitrary variables that will be passed directly to any bound function when it is invoked. This is really useful as it allows you to store parameters within the delegate itself at bind-time. All delegate types (except for "dynamic") support payload variables automatically. This example passes two custom variables, a bool and an int32 to a delegate. Then when the delegate is invoked, these parameters will be passed to your bound function. The extra variable arguments must always be accepted after the delegate type parameter arguments.

	MyDelegate.BindRaw( &MyFunction, true, 20 );

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Executing Delegates

Example Usage

Suppose you have a class with a method that you would like to be able to call from anywhere:

	class FLogWriter
	{
		void WriteToLog(FString);
	};

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To call the WriteToLog function, we will need to create a delegate type for that function`s signature. To do this, you will first declare the delegate using one of the macros below. For example, here is a simple delegate type:

	DECLARE_DELEGATE_OneParam(FStringDelegate, FString);

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This creates a delegate type called FStringDelegate that takes a single parameter of type FString.

Here is an example of how you would use this FStringDelegate in a class:

	class FMyClass
	{
		FStringDelegate WriteToLogDelegate;
	};

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This allows your class to hold a pointer to a method in an arbitrary class. The only thing the class really knows about this delegate is its function signature.

Now, to assign the delegate, simply create an instance of your delegate class, passing along the class that owns the method as a template parameter. You will also pass the instance of your object and the actual function address of the method. So, here we will create an instance of our FLogWriter class, then create a delegate for the WriteToLog method of that object instance:

	TSharedRef<FLogWriter> LogWriter(new FLogWriter());

	WriteToLogDelegate.BindSP(LogWriter, &FLogWriter::WriteToLog);

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You have just dynamically bound a delegate to a method of a class! Pretty simple, right?

Note that the SP part of BindSP stands for shared pointer, because we are binding to an object that is owned by a shared pointer. There are versions for different object types, such as BindRaw() and BindUObject().

Now, your WriteToLog method can be called by FMyClass without it even knowing anything about the FLogWriter class! To call your delegate, just use the Execute() method:

	WriteToLogDelegate.Execute(TEXT("Delegates are great!"));

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If you call Execute() before binding a function to the delegate, an assertion will be triggered. In many cases, you will instead want to do this:

	WriteToLogDelegate.ExecuteIfBound(TEXT("Only executes if a function was bound!"));
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