[UE4]事件处理(Handling Events)和委托（Delegate）代码示例（一）

1. 通过重写虚函数来处理事件

MyTriggerVolume.h

自定义一个Actor类，添加一个 Box 组件作为触发区域，然后通过重写虚函数——NotifyActorBeginOverlap, NotifyActorEndOverlap来响应事件

#pragma once  

#include "GameFramework/Actor.h"
#include "MyTriggerVolume.generated.h"  

UCLASS()
class TEST_API AMyTriggerVolume : public AActor
{
    GENERATED_BODY()  

public:
    // Sets default values for this actor's properties
    AMyTriggerVolume();  

    // Called when the game starts or when spawned
    virtual void BeginPlay() override;  

    // Called every frame
    virtual void Tick( float DeltaSeconds ) override;  

    UPROPERTY()
    UBoxComponent* TriggerZone;  

    UFUNCTION()
    virtual void NotifyActorBeginOverlap(AActor* OtherActor) override;
    UFUNCTION()
    virtual void NotifyActorEndOverlap(AActor* OtherActor) override;
};  

MyTriggerVolume.cpp

#include "Test.h"
#include "UE4TestGameMode.h"
#include "MyTriggerVolume.h"  

// Sets default values
AMyTriggerVolume::AMyTriggerVolume()
{
    // Set this actor to call Tick() every frame.  You can turn this off to improve performance if you don't need it.
    PrimaryActorTick.bCanEverTick = true;
    TriggerZone = CreateDefaultSubobject<UBoxComponent>("TriggerZone");
    TriggerZone->SetBoxExtent(FVector(, , ));// 设置触发区域的范围
}  

// Called when the game starts or when spawned
void AMyTriggerVolume::BeginPlay()
{
    Super::BeginPlay();  

}  

// Called every frame
void AMyTriggerVolume::Tick( float DeltaTime )
{
    Super::Tick( DeltaTime );  

}  

// 重写虚函数来响应事件
void AMyTriggerVolume::NotifyActorBeginOverlap(AActor* OtherActor)
{
    GEngine->AddOnScreenDebugMessage(-, , FColor::Red, FString::Printf(TEXT("%s entered me"), *(OtherActor->GetName())));// 注意FString::Format需要解引用
}
// 重写虚函数来响应事件
void AMyTriggerVolume::NotifyActorEndOverlap(AActor* OtherActor)
{
    GEngine->AddOnScreenDebugMessage(-, , FColor::Red, FString::Printf(TEXT("%s left me"), *(OtherActor->GetName())));
}  

2. 绑定在 UFUNCTION 函数上的委托（不带参数）

委托的好处在于，我们不用知道当前指派的函数的细节就可以调用它，它是一种安全版本的函数指针。

以下代码将展示如何关联 UFUNCTION 到一个委托上，即委托执行时，UFUNCTION 将被调用。
（效果为 当玩家进入触发区域，点光源亮）

首先在 UE4TestGameMode.h 中添加一个委托声明（在 UCLASS 之前），如下：

DECLARE_DELEGATE(FStandardDelegateSignature)  

然后，为 UE4TestGameMode 类添加一个新成员

FStandardDelegateSignature MyStandardDelegate;  

接着，我们新建一个 Actor 类——DelegateListener，主要实现具体方法，以及负责委托的绑定和解绑

DelegateListener.h

UCLASS()
class TEST_API ADelegateListener : public AActor
{
    GENERATED_BODY()  

public:
    // Sets default values for this actor's properties
    ADelegateListener();  

    // Called when the game starts or when spawned
    virtual void BeginPlay() override;  

    // Called every frame
    virtual void Tick( float DeltaSeconds ) override;  

    UFUNCTION()
    void EnableLight();  

    UFUNCTION()
    virtual void EndPlay(const EEndPlayReason::Type EndPlayReason) override;  

    UPROPERTY()
    UPointLightComponent* PointLight;  

};  

DelegateListener.cpp

#include "Test.h"
#include "UE4TestGameMode.h" // 注意 include 的位置
#include "DelegateListener.h"  

// Sets default values
ADelegateListener::ADelegateListener()
{
    // Set this actor to call Tick() every frame.  You can turn this off to improve performance if you don't need it.
    PrimaryActorTick.bCanEverTick = true;
    PointLight = CreateDefaultSubobject<UPointLightComponent>("PointLight");
    RootComponent = PointLight;  

    PointLight->SetVisibility(false);
}  

// Called when the game starts or when spawned
void ADelegateListener::BeginPlay()
{
    Super::BeginPlay();
    UWorld* TheWorld = GetWorld();
    if (TheWorld != nullptr)
    {
        AGameMode* GameMode = Cast<AGameMode>(UGameplayStatics::GetGameMode(TheWorld));
        AUE4TestGameMode * MyGameMode = Cast<AUE4TestkGameMode>(GameMode);
        if (MyGameMode != nullptr)
        {
            // ❤ 绑定一个基于 UObject 的成员函数的委托。UObject 委托保留了一个弱引用在你的对象上，可以通过.ExecuteIfBound() 来调用委托的函数
            MyGameMode->MyStandardDelegate.BindUObject(this, &ADelegateListener::EnableLight);// 其实就是将 EnableLight 函数绑定在了委托上。
                 }
    }  

}  

// Called every frame
void ADelegateListener::Tick( float DeltaTime )
{
    Super::Tick( DeltaTime );  

}  

void ADelegateListener::EnableLight()
{
    PointLight->SetVisibility(true);
}  

void ADelegateListener::EndPlay(const EEndPlayReason::Type EndPlayReason)
{
    Super::EndPlay(EndPlayReason);
    UWorld* TheWorld = GetWorld();
    if (TheWorld != nullptr)
    {
        AGameMode* GameMode = Cast<AGameMode>(UGameplayStatics::GetGameMode(TheWorld));
        AUE4TestGameMode * MyGameMode = Cast<AUE4TestGameMode>(GameMode);
        if (MyGameMode != nullptr)
        {
            // 解绑委托
            MyGameMode->MyStandardDelegate.Unbind();
        }
    }
}  

值得注意的是，如果我们绑定的是普通的C++函数，那么就应该将 BindUObject 改为 BindRaw，如果是静态方法，那就改为 BindStatic。

最后，回到我们之前的 MyTriggerVolume.cpp， 利用 GameMode（我们之前声明委托和定义委托成员的地方） 执行委托，

在 NotifyActorBeginOverlap 方法中添加以下代码：

UWorld* TheWorld = GetWorld();
if (TheWorld != nullptr)
{
    AGameMode* GameMode = Cast<AGameMode>(UGameplayStatics::GetGameMode(TheWorld));
    AUE4TestGameMode * MyGameMode = Cast<AUE4TestGameMode>(GameMode);
    // ❤ 执行委托的函数
    MyGameMode->MyStandardDelegate.ExecuteIfBound();
}