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Showing content from https://docs.microsoft.com/en-us/uwp/cpp-ref-for-winrt/event below:

winrt::event struct template (C++/WinRT) - Windows UWP applications

A type that you can use to declare and implement an event of a specified delegate type. Subscribers pass their event-handling delegates to an event; the event registers those delegates in a collection; then, when it is raised, the event invokes its registered delegates in turn so that subscribers can handle the event. For more info about authoring events, and code examples, see Author events in C++/WinRT.

If you need events internal to your project, and not limited to Windows Runtime types, then use event<winrt::delegate>. For more info, see Parameterized delegates, simple signals, and callbacks within a project.

For more info about handling events, and code examples, see Handle events by using delegates in C++/WinRT.

Syntax

template <typename Delegate>
struct event

Template parameters

typename Delegate The type of delegate that can register to handle the event.

Requirements

Minimum supported SDK: Windows SDK version 10.0.17134.0 (Windows 10, version 1803)

Namespace: winrt

Header: %WindowsSdkDir%Include<WindowsTargetPlatformVersion>\cppwinrt\winrt\base.h (included by default)

Member type aliases Alias name Type event::delegate_type A synonym for the typename Delegate template parameter. Constructors Member functions Member operators event::event constructor

Initializes a new instance of the event struct.

Syntax

event();

event::add function

Registers a delegate with the event object.

Syntax

winrt::event_token add(Delegate const& delegate);

Parameters

delegate A delegate to register with the event object.

Return value

A winrt::event_token that can subsequently be used to revoke the delegate's registration.

event::operator() (function call operator)

Invokes all of the event object's registered delegates with the provided arguments.

Syntax

template<typename... Arg>
void operator()(Arg const&... args)

Template parameters

typename... Arg A variadic template parameter pack containing the types of the parameters that the delegate is passed when it's called.

Parameters

args A variable argument list containing the arguments that the delegate is passed when it's called.

event::operator bool

Checks whether the event object has any delegates registered with it.

Syntax

explicit operator bool() const noexcept;

Return value

true if the event object has any registered delegates, otherwise false.

event::remove function

Revokes a delegate's registration from the event object.

Syntax

void remove(winrt::event_token const token);

Parameters

token A winrt::event_token that identifies the delegate whose registration to revoke.