Unreal 抛体组件(ProjectileMovementComponent)解析

一.引言

　　因为工作需要，领导指定我使用抛体组件来实现某功能。故而翻阅抛体组件，刚开始看第一眼，感觉特别复杂。众所周知，UE对于玩家角色移动做的同步非常精妙，没想到随便一个抛物线组件也如此复杂。

因为是运动，所以首先看的是他如何运动，直接看Tick中逻辑。如下(拉的源码，随便大致浏览一下即可)

void UProjectileMovementComponent::TickComponent(float DeltaTime, enum ELevelTick TickType, FActorComponentTickFunction *ThisTickFunction)
{
    QUICK_SCOPE_CYCLE_COUNTER( STAT_ProjectileMovementComponent_TickComponent );

    // Still need to finish interpolating after we've stopped simulating, so do that first.
    if (bInterpMovement && !bInterpolationComplete)
    {
        QUICK_SCOPE_CYCLE_COUNTER(STAT_ProjectileMovementComponent_TickInterpolation);
        TickInterpolation(DeltaTime);
    }

    // Consume PendingForce and reset to zero.
    // At this point, any calls to AddForce() will apply to the next frame.
    PendingForceThisUpdate = PendingForce;
    ClearPendingForce();

    // skip if don't want component updated when not rendered or updated component can't move
    if (HasStoppedSimulation() || ShouldSkipUpdate(DeltaTime))
    {
        return;
    }

    Super::TickComponent(DeltaTime, TickType, ThisTickFunction);

    if (!IsValid(UpdatedComponent) || !bSimulationEnabled)
    {
        return;
    }

    AActor* ActorOwner = UpdatedComponent->GetOwner();
    if ( !ActorOwner || !CheckStillInWorld() )
    {
        return;
    }

    if (UpdatedComponent->IsSimulatingPhysics())
    {
        return;
    }

    float RemainingTime    = DeltaTime;
    int32 NumImpacts = 0;
    int32 NumBounces = 0;
    int32 LoopCount = 0;
    int32 Iterations = 0;
    FHitResult Hit(1.f);
    
    while (bSimulationEnabled && RemainingTime >= MIN_TICK_TIME && (Iterations < MaxSimulationIterations) && !ActorOwner->IsPendingKill() && !HasStoppedSimulation())
    {
        LoopCount++;
        Iterations++;

        // subdivide long ticks to more closely follow parabolic trajectory
        const float InitialTimeRemaining = RemainingTime;
        const float TimeTick = ShouldUseSubStepping() ? GetSimulationTimeStep(RemainingTime, Iterations) : RemainingTime;
        RemainingTime -= TimeTick;
        
        // Logging
        UE_LOG(LogProjectileMovement, Verbose, TEXT("Projectile %s: (Role: %d, Iteration %d, step %.3f, [%.3f / %.3f] cur/total) sim (Pos %s, Vel %s)"),
            *GetNameSafe(ActorOwner), (int32)ActorOwner->GetLocalRole(), LoopCount, TimeTick, FMath::Max(0.f, DeltaTime - InitialTimeRemaining), DeltaTime,
            *UpdatedComponent->GetComponentLocation().ToString(), *Velocity.ToString());

        // Initial move state
        Hit.Time = 1.f;
        const FVector OldVelocity = Velocity;
        const FVector MoveDelta = ComputeMoveDelta(OldVelocity, TimeTick);
        FQuat NewRotation = (bRotationFollowsVelocity && !OldVelocity.IsNearlyZero(0.01f)) ? OldVelocity.ToOrientationQuat() : UpdatedComponent->GetComponentQuat();

        if (bRotationFollowsVelocity && bRotationRemainsVertical)
        {
            FRotator DesiredRotation = NewRotation.Rotator();
            DesiredRotation.Pitch = 0.0f;
            DesiredRotation.Yaw = FRotator::NormalizeAxis(DesiredRotation.Yaw);
            DesiredRotation.Roll = 0.0f;
            NewRotation = DesiredRotation.Quaternion();
        }

        // Move the component
        if (bShouldBounce)
        {
            // If we can bounce, we are allowed to move out of penetrations, so use SafeMoveUpdatedComponent which does that automatically.
            SafeMoveUpdatedComponent( MoveDelta, NewRotation, bSweepCollision, Hit );
        }
        else
        {
            // If we can't bounce, then we shouldn't adjust if initially penetrating, because that should be a blocking hit that causes a hit event and stop simulation.
            TGuardValue<EMoveComponentFlags> ScopedFlagRestore(MoveComponentFlags, MoveComponentFlags | MOVECOMP_NeverIgnoreBlockingOverlaps);
            MoveUpdatedComponent(MoveDelta, NewRotation, bSweepCollision, &Hit );
        }
        
        // If we hit a trigger that destroyed us, abort.
        if( ActorOwner->IsPendingKill() || HasStoppedSimulation() )
        {
            return;
        }

        // Handle hit result after movement
        if( !Hit.bBlockingHit )
        {
            PreviousHitTime = 1.f;
            bIsSliding = false;

            // Only calculate new velocity if events didn't change it during the movement update.
            if (Velocity == OldVelocity)
            {
                Velocity = ComputeVelocity(Velocity, TimeTick);                
            }

            // Logging
            UE_LOG(LogProjectileMovement, VeryVerbose, TEXT("Projectile %s: (Role: %d, Iteration %d, step %.3f) no hit (Pos %s, Vel %s)"),
                *GetNameSafe(ActorOwner), (int32)ActorOwner->GetLocalRole(), LoopCount, TimeTick, *UpdatedComponent->GetComponentLocation().ToString(), *Velocity.ToString());
        }
        else
        {
            // Only calculate new velocity if events didn't change it during the movement update.
            if (Velocity == OldVelocity)
            {
                // re-calculate end velocity for partial time
                Velocity = (Hit.Time > KINDA_SMALL_NUMBER) ? ComputeVelocity(OldVelocity, TimeTick * Hit.Time) : OldVelocity;
            }

            // Logging
            UE_CLOG(UpdatedComponent != nullptr, LogProjectileMovement, VeryVerbose, TEXT("Projectile %s: (Role: %d, Iteration %d, step %.3f) new hit at t=%.3f: (Pos %s, Vel %s)"),
                *GetNameSafe(ActorOwner), (int32)ActorOwner->GetLocalRole(), LoopCount, TimeTick, Hit.Time, *UpdatedComponent->GetComponentLocation().ToString(), *Velocity.ToString());

            // Handle blocking hit
            NumImpacts++;
            float SubTickTimeRemaining = TimeTick * (1.f - Hit.Time);
            const EHandleBlockingHitResult HandleBlockingResult = HandleBlockingHit(Hit, TimeTick, MoveDelta, SubTickTimeRemaining);
            if (HandleBlockingResult == EHandleBlockingHitResult::Abort || HasStoppedSimulation())
            {
                break;
            }
            else if (HandleBlockingResult == EHandleBlockingHitResult::Deflect)
            {
                NumBounces++;
                HandleDeflection(Hit, OldVelocity, NumBounces, SubTickTimeRemaining);
                PreviousHitTime = Hit.Time;
                PreviousHitNormal = ConstrainNormalToPlane(Hit.Normal);
            }
            else if (HandleBlockingResult == EHandleBlockingHitResult::AdvanceNextSubstep)
            {
                // Reset deflection logic to ignore this hit
                PreviousHitTime = 1.f;
            }
            else
            {
                // Unhandled EHandleBlockingHitResult
                checkNoEntry();
            }
            
            // Logging
            UE_CLOG(UpdatedComponent != nullptr, LogProjectileMovement, VeryVerbose, TEXT("Projectile %s: (Role: %d, Iteration %d, step %.3f) deflect at t=%.3f: (Pos %s, Vel %s)"),
                *GetNameSafe(ActorOwner), (int32)ActorOwner->GetLocalRole(), Iterations, TimeTick, Hit.Time, *UpdatedComponent->GetComponentLocation().ToString(), *Velocity.ToString());
            
            // Add unprocessed time after impact
            if (SubTickTimeRemaining >= MIN_TICK_TIME)
            {
                RemainingTime += SubTickTimeRemaining;

                // A few initial impacts should possibly allow more iterations to complete more of the simulation.
                if (NumImpacts <= BounceAdditionalIterations)
                {
                    Iterations--;

                    // Logging
                    UE_LOG(LogProjectileMovement, Verbose, TEXT("Projectile %s: (Role: %d, Iteration %d, step %.3f) allowing extra iteration after bounce %u (t=%.3f, adding %.3f secs)"),
                        *GetNameSafe(ActorOwner), (int32)ActorOwner->GetLocalRole(), LoopCount, TimeTick, NumBounces, Hit.Time, SubTickTimeRemaining);
                }
            }
        }
    }

    UpdateComponentVelocity();
}

二.分析

　    抛物线运动的逻辑就如上，可真是多啊。

　　直接说重点吧。　　

　　　　①既然抛体运动，就是受重力加速度影响，其实就是匀变速运动,那么肯定是需要知道 公式：V = Vo+ a*t

　　　　②因为是移动组件，所以需要计算出每帧需要做多少位移。

　　　　③需要做多少位移。因为是抛体组件，运动公式是知道的，所以根据推算，需要了解下述公式

　　匀变速运动的位移公式：S = V*t +0.5*a * t^2,即可以得出做多少位移就是 

　　　　I. 匀变速直线运动的速度与时间关系的公式：V=V0+a*t → a = (V - v0)/t

　　　　II. 匀变速直线运动的位移与时间关系的公式：x=v0*t+1/2*a*t^2 → x=v0*t+1/2*(V - v0)/t * t^2 →  x = v0*t+1/2*(V - v0) * t

　　　　

FVector UProjectileMovementComponent::ComputeMoveDelta(const FVector& InVelocity, float DeltaTime) const
{
    const FVector NewVelocity = ComputeVelocity(InVelocity, DeltaTime);
    const FVector Delta = (InVelocity * DeltaTime) + (NewVelocity - InVelocity) * (0.5f * DeltaTime);
    return Delta;
}

　　　　④最后根据算出的MoveDelta，直接赋给SceneComponent，即可。

三.注意

　　　　①关于如何使用这个组件纳，那就看看初始化函数 UProjectileMovementComponent::InitializeComponent() 

if (InitialSpeed > 0.f)
        {
            Velocity = Velocity.GetSafeNormal() * InitialSpeed;
        }

        if (bInitialVelocityInLocalSpace)
        {
            SetVelocityInLocalSpace(Velocity);
        }

受初始化 InitialSpeed 影响，显而易见，当然也受 MaxSpeed 影响，所以你把Velocity设置的再大，也没用，这个只是方向而已。

　　　　②这是一个纯工具类组件，搜索UProjectileMovementComponent.h 并没有发现replicated的变量。所以，如果你的需求是在DS 也跑，Client也跑，那么肯定会有异常。最为简单的做法是你DS设置好，Client也设置好，可以使用，但是会存在一定误差，DS和Client的位移始终相差一段误差，这段误差是DS 同步到Client的误差时间 t, 平均速度 v，误差就大概等于 s = v *t,因此如果你的速度非常大，那么这个误差也就越大，如果需求是纯表现那还好，但是如果有DS上交互，出现的问题，就显而易见。那怎么办纳，还有ReplicateMovement可以帮忙同步模拟，这里扯得有点远了，就不说了，可以参考:https://www.cnblogs.com/haisong1991/p/11305783.html

 　完全不同的运动轨迹,当然如果你非要使用抛体，完成抛物线，那么就需要动态修改Velocity，需要加Tick之类逻辑。那么还不如自己撸，直接指定好抛物线轨迹，只需要同步float的 time 即可。

　　④抛体组件究竟干嘛的？

            既然是抛体，自然是落地后出现反弹等一系列效果，使用抛体最佳。(具体使用，待补充)。