【1. Creating an Actor that uses a timer | Unreal Engine】

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If you are new to Unreal Engine 4, you might want to read our 编程快速入门 tutorial first. For this tutorial, we will assume you are familiar with creating a project and adding C++ code to it.


We will begin by creating a new, Basic Code project, with starter content, named "HowTo_VTE", and then adding an Actor class to it. We'll name it "Countdown" in this tutorial.

We'll start by creating a simple countdown timer that we can see in-game. In Countdown.h, add the following lines to the end of our class definition:

int32 CountdownTime;UTextRenderComponent* CountdownText;void UpdateTimerDisplay();
In Countdown.cpp, we can create our renderable text Component and initialize our countdown time to 3 seconds. We can also turn Ticking off for this type of Actor, since we will not need it. ACountdown::ACountdown should look like this:

PrimaryActorTick.bCanEverTick = false;CountdownText = CreateDefaultSubobjectUTextRenderComponent(TEXT("CountdownNumber"));CountdownText-SetHorizontalAlignment(EHTA_Center);CountdownText-SetWorldSize(150.0f);RootComponent = CountdownText;CountdownTime = 3;
ACountdown::UpdateTimerDisplay should update our text display to show the time remaining, or zero if the time is up. This code should run when we first spawn our ACountdown into the game, and once per second until our CountdownTime variable hits zero.

void ACountdown::UpdateTimerDisplay(){    CountdownText-SetText(FString::FromInt(FMath::Max(CountdownTime, 0)));}
Whenever we assign a Timer to run a function, we are given a Timer Handle. We need to hold onto that handle so that we can shut the Timer down when the countdown finishes. Let's add the function to count time down, and the Timer Handle we'll need to control it, to the class definition in Countdown.h. While we're there, let's also add a function to do something special when the countdown ends:

void AdvanceTimer();void CountdownHasFinished();FTimerHandle CountdownTimerHandle;
We can also write the body of ACountdown::AdvanceTimer and ACountdown::CountdownHasFinished in Countdown.cpp now:

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(){    //Change to a special readout    CountdownText-SetText(TEXT("GO!"));}
Let's initialize the text display in ACountdown::BeginPlay by adding a call to our new update function, and setting a timer to advance and update the countdown once per second:

UpdateTimerDisplay();GetWorldTimerManager().SetTimer(CountdownTimerHandle, this, ACountdown::AdvanceTimer, 1.0f, true);

We are updating the display in ACountdown::BeginPlay rather than ACountdown::ACountdown because values set to variables in the Unreal Editor will be assigned after the constructor, but before BeginPlay. We will want to respect those values later, when we expose CountdownTime to the editor.


Let's check our progress so far by going to the Unreal Editor and pressing Compile.

We can then drop our updated ACountdown class from the Content Browser into the Level Editor.

Because we set our countdown text during ACountdown::BeginPlay and not ACountdown::ACountdown, the default "Text" is shown in the Level Editor.
When we press Play, the countdown will progress as expected, saying "3", "2", "1", and finally "GO!"


At this point, we've already created a simple class that uses a timer. Non-programming users would get much more out of it if they could set the countdown time, or change the behavior when the countdown finishes. Next, we'll expose these features to the editor.


Work-In-Progress Code


Countdown.h

// Copyright 1998-2016 Epic Games, Inc. All Rights Reserved.#pragma once#include "GameFramework/Actor.h"#include "Countdown.generated.h"UCLASS()class HOWTO_VTE_API ACountdown : public AActor{    GENERATED_BODY()public:     // Sets default values for this actor's properties    ACountdown();    // Called when the game starts or when spawned    virtual void BeginPlay() override;    // Called every frame    virtual void Tick( float DeltaSeconds ) override;    //How long, in seconds, the countdown will run    int32 CountdownTime;    UTextRenderComponent* CountdownText;    void UpdateTimerDisplay();    void AdvanceTimer();    void CountdownHasFinished();    FTimerHandle CountdownTimerHandle;};
Countdown.cpp

// Copyright 1998-2016 Epic Games, Inc. All Rights Reserved.#include "HowTo_VTE.h"#include "Countdown.h"// Sets default valuesACountdown::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 = CreateDefaultSubobjectUTextRenderComponent(TEXT("CountdownNumber"));    CountdownText-SetHorizontalAlignment(EHTA_Center);    CountdownText-SetWorldSize(150.0f);    RootComponent = CountdownText;    CountdownTime = 3;}// Called when the game starts or when spawnedvoid ACountdown::BeginPlay(){    Super::BeginPlay();    UpdateTimerDisplay();    GetWorldTimerManager().SetTimer(CountdownTimerHandle, this, ACountdown::AdvanceTimer, 1.0f, true);}// Called every framevoid ACountdown::Tick( float DeltaTime ){    Super::Tick( DeltaTime );}void ACountdown::UpdateTimerDisplay(){    CountdownText-SetText(FString::FromInt(FMath::Max(CountdownTime, 0)));}void ACountdown::AdvanceTimer(){    --CountdownTime;    UpdateTimerDisplay();    if (CountdownTime  1)    {
// We're done counting down, so stop running the timer.
GetWorldTimerManager().ClearTimer(CountdownTimerHandle);
//Perform any special actions we want to do when the timer ends.
CountdownHasFinished();    }}void ACountdown::CountdownHasFinished(){    //Change to a special readout    CountdownText-SetText(TEXT("GO!"));}

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