ListenMessage family

ListenMessage family

Every form of registering a listener.

Declared in Plugins/GMP/Source/GMP/GMP/GMPUtils.h (FGMPHelper)

template<typename KeyT, typename T, typename F>
static FGMPKey ListenMessage(const TMSGKEYTyped<KeyT>& K, T* Listener, F&& f,
                             GMP::FGMPListenOptions Options = {});

template<typename KeyT, typename T, typename F>
static FGMPKey ListenObjectMessage(FSigSource InSigSrc, const TMSGKEYTyped<KeyT>& K, T* Listener, F&& f,
                                   GMP::FGMPListenOptions Options = {});

template<typename KeyT, typename T, typename F>
static FGMPKey ListenWorldMessage(const UWorld* InWorld,       const TMSGKEYTyped<KeyT>& K, T* Listener, F&& f,
                                  GMP::FGMPListenOptions Options = {});
template<typename KeyT, typename T, typename F>
static FGMPKey ListenWorldMessage(const UObject* WorldContext, const TMSGKEYTyped<KeyT>& K, T* Listener, F&& f,
                                  GMP::FGMPListenOptions Options = {});

Unregistering:

static void UnListenMessage(const FMSGKEYFind& K, FGMPKey id);            // one listener
static void UnListenMessage(const FMSGKEYFind& K, const UObject* Owner);  // all of an owner's

The last two arguments are always (owner, callable)

Listener is the lifetime owner, f is what runs. The owner may be:

Owner	Lifetime handled by
UObject*	weak-pointer staleness checked on dispatch
GMP::FSigHandle*	its destructor — see FSigHandle
shared-pointer types	CreateSPLambda and friends

A raw this from a non-UObject, non-handle class has nothing to check. That is the one combination to avoid.

Options

GMP::FGMPListenOptions — Plugins/GMP/Source/GMP/GMP/GMPKey.h:29:

struct FGMPListenOptions : public FGMPListenOrder
{
    int32 Times = -1;                 // -1 = forever
    // int32 Order = 0;               // from FGMPListenOrder, under GMP_WITH_SIGNAL_ORDER
    static FGMPListenOptions Default;
};

ListenMessage(KEY, this, cb, { .Times = 3, .Order = -10 });

• Times — decremented per call; reaching zero unregisters the listener automatically.
• Order — lower runs first, default 0. Equal orders keep registration order, because the value is packed into the high bits of the FGMPKey and the pre-fire sort is stable. FGMPListenOrder::MinOrder and MaxOrder are provided for the extremes.

Order is compiled out entirely when GMP_WITH_SIGNAL_ORDER is off.

Callback shape

The callable's parameters determine the signature this listener claims. It may take fewer parameters than the send provides — see Parameter compatibility. Declaring FGMPResponder& last makes it the responding listener — see FGMPResponder.

Return value

An FGMPKey identifying this registration. Keep it if you intend to unregister that one listener; ignore it if the owner's lifetime is doing the work.

See also

FSigHandle · FGMPKey · Parameter compatibility · Dispatch layers