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Using Device Messenger

FOperatingSystemDeviceMessengerSystem (accessed via Find Messenger from Operating System / Find Messenger for Device) scopes events to a single running device. Use it when Mail, Video Player, portable battery handlers, or your own programs need to talk inside the OS without reaching level actors.

Full reference: Device Messenger Subsystem.

flowchart TB
  subgraph row1[" "]
    direction LR
    Mail["BP_Prog_Mail"] -->|Broadcast MailReceived| DM["FOperatingSystemDeviceMessengerSystem"]
  end
  subgraph row2[" "]
    direction LR
    Taskbar["UOperatingSystemTaskbarButton"] --- Battery["Portable device battery HUD"]
  end
  DM --> Taskbar
  DM --> Battery

When to use which messenger

	Device Messenger	Global Messenger
Scope	One device / OS instance	Entire world
Identifiers	`FName` event names	Gameplay tags
Typical use	Program ↔ program, battery UI	Terminal → door, OS → gameplay

Blueprint — notify when mail arrives

Mail program broadcasts

(On new mail received)
  → Find Messenger from Operating System
  → Broadcast (Event Name: "MailReceived", Payload: mail data object)

Event Construct
  → Find Messenger for Device
  → Begin Listening (Event Name: "MailReceived")
      Callback → Show unread badge on taskbar button

Device messenger broadcast node

C++ broadcast

#include "Devices/OperatingSystemDeviceMessenger.h"
#include "Statics/OperatingSystemStatics.h"

void UMyMailProgram::NotifyNewMail()
{
    if (auto* Messenger = UOperatingSystemStatics::FindMessengerFromOperatingSystem(GetOperatingSystem()))
    {
        Messenger->Broadcast(FName("MailReceived"), this);
    }
}

C++ listen

UFUNCTION()
void UMyTaskbarWidget::OnMailReceived(UObject* Payload);

void UMyTaskbarWidget::NativeConstruct()
{
    Super::NativeConstruct();
    if (auto* Messenger = UOperatingSystemStatics::FindMessengerFromOperatingSystem(OS))
    {
        FOperatingSystemDeviceMessageDelegate Delegate;
        Delegate.BindDynamic(this, &UMyTaskbarWidget::OnMailReceived);
        Messenger->BeginListening(FName("MailReceived"), Delegate);
    }
}

Bundled examples

Study plugin implementations:

Using Global Messenger