Unreal教學（5）——變量，時間，事件（Variables, Timers, and Events）

設置一個倒計時計數器，可以給Editor提供接口，也可以給藍圖提供接口，這裏不講藍圖，講如何寫代碼。

兩個關鍵的文件。

Countdown.h

// Fill out your copyright notice in the Description page of Project Settings.

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "Components/TextRenderComponent.h"
#include "Countdown.generated.h"

UCLASS()
class STARTERNOCONTENT_API ACountdown : public AActor
{
	GENERATED_BODY()
	
public:	
	// Sets default values for this actor's properties
	ACountdown();

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

public:	
	// Called every frame
	virtual void Tick(float DeltaTime) override;
    
	//How long, in seconds, the countdown will run
	UPROPERTY(EditAnywhere)
    int32 CountdownTime;

    UTextRenderComponent* CountdownText;

    void UpdateTimerDisplay();

    void AdvanceTimer();

	UFUNCTION(BlueprintNativeEvent)
    void CountdownHasFinished();
	virtual void CountdownHasFinished_Implementation();

    FTimerHandle CountdownTimerHandle;
};

Countdown.cpp

// Fill out your copyright notice in the Description page of Project Settings.


#include "Countdown.h"


// Sets default values
ACountdown::ACountdown()
{
 	// Set this actor to call Tick() every frame.  You can turn this off to improve performance if you don't need it.
	PrimaryActorTick.bCanEverTick = false;

	CountdownText = CreateDefaultSubobject<UTextRenderComponent>(TEXT("CountdownNumber"));
    CountdownText->SetHorizontalAlignment(EHTA_Center);
    CountdownText->SetWorldSize(150.0f);
    RootComponent = CountdownText;

    CountdownTime = 3;
}

void ACountdown::UpdateTimerDisplay() {
	CountdownText->SetText(FString::FromInt(FMath::Max(CountdownTime, 0)));
}

// Called when the game starts or when spawned
void ACountdown::BeginPlay()
{
	Super::BeginPlay();
	UpdateTimerDisplay();
	GetWorldTimerManager().SetTimer(CountdownTimerHandle, this, &ACountdown::AdvanceTimer, 1.0f, true);
}

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

void ACountdown::AdvanceTimer() {
	--CountdownTime;
	UpdateTimerDisplay();
	if (CountdownTime < 1) {
		// We're done counting down, so stop running the timer.
		GetWorldTimerManager().ClearTimer(CountdownTimerHandle);
		CountdownHasFinished();
	}
}

void ACountdown::CountdownHasFinished_Implementation() {
	// Change to a special readout
	CountdownText->SetText(TEXT("GO!"));
}