Blueprint Interfaces

• This is a method to add an interface to many different types of objects.
  • Essentially you "attach" a event to any class.
• Sources
  • Unreal Reference
  • This tutorial
  • Another Unreal Reference
• Let's do a simple example.
  • Let's implementing a game where when you cast a spell, all magic items glow.
  • Many different things can be magic
    • Pickup items
    • Walls
    • Other characters
  • So deriving everything from a common class will not do.
  • This is an ideal situation for a blueprint interface.
• First create two different "things" that can be magic.
  • I made a BP_MagicItem1 which is a sphere with point light source above it.
  • And a BP_MagicItem2 which is a cube with a point light source above it.
  • Really no member functions at this point.
  • 
• Create a BPI to add to these.
  • Right click in the content browser and select blueprint, blueprint interface
  • I renamed this to be BPI_DetectMagic
• Open this up and it will have default function.
  • I renamed this to be DetectMagic
  • I added a parameter, the position of the caster
    • In this world, the detect magic spell is a ranged based spell and objects react based on their distance from the caster.
    • MagicItem1 types will glow less brightly based on the distance.
    • MagicItem2 types will change color based on the distance.
  • You can add other functions if you wish.
  • This is really it for the BPI. You are adding function prototypes to objects.
• Next go to the object you want to add the interface to.
  • Select Class Settings at the top.
  • 
  • Then add the interface in the interface section.
  • 
  • This has attached the "interface" to the object.
• After you have added the interface, you can implement functions that respond to that interface.
  • For the MagicType1, I implemented
    • A function that makes the item glow then slowly fade.
    • The intensity of the glow is based on the distance from the caster.

    • DetectMagic(vector casterLocation){
          distance = Distance( casterLocation, myLocation)/100;
          distance = clamp(distance, 1, 30);
          For(timer=0 to 2) {
              intensity = lerp(10000,0, alpha)
              intensity /= distance;
              PointLight.SetIntensity(intensity);
          }
      } 

    • 
  • And the second MagicType I implemented
    • The glow is colored by the distance from the caster.
    • It goes out after 3 seconds.

    • DetectMagic(vector casterLocation) {
          distance = Distance(casterLocation, myLocation)
          if (distance < 500) {
              PointLight.SetColor(red);
          } else if (distance < 1000) {
              PointLight.SetColor(gold);
          } else if (distance < 3000) {
              PointLight.SetColor(blue);
          }
          
          if (distance < 3000) {
              PointLight.SetIntensity(50000)
              Sleep(3);
              PointLight.SetIntensity(0);
              PointLight.SetColor(black);
          }
      } 

    • 
    • 
• Using the interface
  • I added the following function to the player character

  • OnKeypressC() {
        For each actor in GetAllActorsWith BPI_DetectMagic:
           actor.DetectMagic(myLocation);
    } 

  • 
• Let's add another function.
  • Poor planning on my part. I should have called the BPI react to spells.
  • I added another function called SummonCoin
    • This has no arguments but returns a int
    • 
    • Compile and save and it shows up in the interface.
    • 
  • Double click on this to add this function.
    • 
  • Back in the FirstPerson Character
    • I added a SummonCoin input action (Keypress x)
    • I change the Cast interaction to DetectMagic.
    • And Added the following function
    • 
    • Note this uses the function DoesImplementInterface
    • And I am looking at ALL of the actors in the level, Probably VERY BAD.
  • You can also add the interface to the Level.
    • Open the level then open the class settings.
    • Add the BPI
    • I add a function that will dim the skylight when detect magic is cast.
    • 
  • The documentation points out be you have two different instances of the BPI functions
    • 
    • The top one is intended to call the your own function.
    • The bottom one is intended to call another intsances' function
    •