{"id":3073,"date":"2021-01-02T13:51:55","date_gmt":"2021-01-02T05:51:55","guid":{"rendered":"http:\/\/blog.coolcoding.cn\/?p=3073"},"modified":"2021-01-02T19:44:10","modified_gmt":"2021-01-02T11:44:10","slug":"implementing-a-native-code-construction-script","status":"publish","type":"post","link":"https:\/\/blog.coolcoding.cn\/?p=3073","title":{"rendered":"Implementing a native code Construction Script"},"content":{"rendered":"\n

Within Blueprint, a\u00a0Construction Script\u00a0is an\u00a0Event Graph\u00a0that runs any time a property is changed on the object it is attached to\u00a0\u2013\u00a0whether it’s being dragged in the editor viewport\u00a0or changed via a direct entry in a\u00a0Details\u00a0panel. Construction Scripts allow the object in question to\u00a0rebuild<\/em>\u00a0itself based on its new location, for instance, or to change the components it contains based on user-selected options. When coding in C++ with Unreal Engine, the equivalent concept is the\u00a0OnConstruction\u00a0function. <\/p>\n\n\n\n

How to do it…<\/h1>\n\n\n\n
  1. Create a new Actor called OnConstructionActor based on StaticMeshActor:<\/li><\/ol>\n\n\n\n
    \"\"\/<\/figure>\n\n\n\n
    1. Update the header file to the following: <\/li><\/ol>\n\n\n\n
      #pragma once

      #include \"CoreMinimal.h\"
      #include \"Engine\/StaticMeshActor.h\"
      #include \"OnConstructionActor.generated.h\"

      UCLASS()
      class CHAPTER_09_API AOnConstructionActor : public AStaticMeshActor
      {
        GENERATED_BODY()

      public:<\/strong>
          AOnConstructionActor();<\/strong>

          virtual void OnConstruction(const FTransform& Transform) override;<\/strong>

          UPROPERTY(EditAnywhere)<\/strong>
              bool ShowStaticMesh;<\/strong>
        
      };<\/pre>\n\n\n\n
      1. Go to the implementation file (OnConstructionActor.cpp) and implement the class constructor:<\/li><\/ol>\n\n\n\n
        #include \"OnConstructionActor.h\"
        #include \"ConstructorHelpers.h\"<\/strong>

        AOnConstructionActor::AOnConstructionActor()<\/strong>
        { <\/strong>
            \/\/ Set this actor to call Tick() every frame. You can turn
            \/\/ this off <\/strong>to improve performance if you don't need it.<\/strong>
            PrimaryActorTick.bCanEverTick = true;<\/strong>

            auto MeshAsset = ConstructorHelpers::FObjectFinder<UStaticMesh>( <\/strong>
                          TEXT(\"StaticMesh'\/Engine\/BasicShapes\/Cone.Cone'\"));<\/strong>

            UStaticMeshComponent * SM = GetStaticMeshComponent();<\/strong>

            if (SM != nullptr)<\/strong>
            {<\/strong>
                if (MeshAsset.Object != nullptr)<\/strong>
                {<\/strong>
                    SM->SetStaticMesh(MeshAsset.Object);<\/strong>
                    SM->SetGenerateOverlapEvents(true);<\/strong>
                }<\/strong>
                SM->SetMobility(EComponentMobility::Movable);<\/strong>
            }<\/strong>

            \/\/ Default value of property<\/strong>
            ShowStaticMesh = true;<\/strong>
        }<\/strong><\/pre>\n\n\n\n
        1. Implement OnConstruction:<\/li><\/ol>\n\n\n\n
          void AOnConstructionActor::OnConstruction(const FTransform& Transform) \n{ \n  GetStaticMeshComponent()->SetVisibility(ShowStaticMesh); \n} <\/pre>\n\n\n\n
          1. Compile your code and launch the editor.<\/li>
          2. Drag an instance of your class into the game world, and verify that toggling the Boolean value for ShowStaticMesh toggles the visibility of the mesh in the editor viewport:<\/li><\/ol>\n\n\n\n
            \"\"\/<\/figure>\n\n\n\n
            1. OnConstruction does not currently run for C++ actors that are placed in a level if they are moved.<\/li>
            2. To test this, place a breakpoint in your OnConstruction function, and then move your actor around the level.<\/li><\/ol>\n\n\n\n
              To place a breakpoint, place your cursor on the desired line and hit F9<\/em> in Visual Studio.<\/p>\n\n\n\n
              1. You’ll notice that the function doesn’t get called, but if you toggle the ShowStaticMesh Boolean, it does, causing your breakpoint to trigger.<\/li><\/ol>\n\n\n\n
                To see why, take a look at the beginning of the AActor::PostEditMove function: <\/p>\n\n\n\n
                void AActor::PostEditMove(bool bFinished)
                {
                    if ( ReregisterComponentsWhenModified() && !FLevelUtils::IsMovingLevel())
                    {
                        UBlueprint* Blueprint = Cast<UBlueprint>(GetClass()->ClassGeneratedBy);
                        if (bFinished || bRunConstructionScriptOnDrag || (Blueprint && Blueprint->bRunConstructionScriptOnDrag))
                        {
                            FNavigationLockContext NavLock(GetWorld(), ENavigationLockReason::AllowUnregister);
                            RerunConstructionScripts();
                        }
                    }

                    \/\/ .... <\/pre>\n\n\n\n
                The top line here casts\u00a0UClass\u00a0for the current object to\u00a0UBlueprint, and will only run the construction scripts and\u00a0OnConstruction\u00a0again if the class is a Blueprint. <\/p>\n\n\n\n
                How it works…<\/h1>\n\n\n\n
                We create a new Actor based on StaticMeshActor for easy access to a visual representation via the Static Mesh.<\/p>\n\n\n\n
                UPROPERTY is added to give us a property to change, which causes PostEditChangeProperty events to be triggered.<\/p>\n\n\n\n
                OnConstruction is a virtual function that’s defined in Actor.<\/p>\n\n\n\n
                As a result, we override the function in our class.<\/p>\n\n\n\n
                Within our class constructor, we initialize our mesh as usual, and set the default state of our bool property to match the visibility of the component that it controls.<\/p>\n\n\n\n
                Inside OnConstruction, the actor rebuilds itself using any properties that are required to do so.<\/p>\n\n\n\n
                For this simple example, we set the visibility of the mesh to match the value of our ShowStaticMesh property.<\/p>\n\n\n\n
                This could also be extended to changing other values based on the value of the ShowStaticMesh variable.<\/p>\n\n\n\n
                You’ll note that we don’t explicitly filter on a particular property being changed, like the previous recipe does with PostEditChangeProperty.<\/p>\n\n\n\n
                The OnConstruction script runs in its entirety for every property that gets changed on the object.<\/p>\n\n\n\n
                It has no way of testing which property was just edited, so you need to be judicious about placing computationally intensive code within it.<\/p>\n\n\n\n
                <\/p>\n","protected":false},"excerpt":{"rendered":"
                Within Blueprint, a\u00a0Construction Script\u00a0is an\u00a0Event Gra […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[27,1],"tags":[26],"_links":{"self":[{"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=\/wp\/v2\/posts\/3073"}],"collection":[{"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3073"}],"version-history":[{"count":2,"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=\/wp\/v2\/posts\/3073\/revisions"}],"predecessor-version":[{"id":3348,"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=\/wp\/v2\/posts\/3073\/revisions\/3348"}],"wp:attachment":[{"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3073"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3073"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.coolcoding.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3073"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}