How to handle communication between objects? (Send messages, trigger actions)

Mells · May 2012

Hi community,

I would like to get a good understanding of the basics -> communication between objects, trigger actions.
If I understand well this would have to do with event listeners, but I can't see how to structure the logic with efficiency.

Here is the situation :
Let's say I have a moving object that contains a property (ex : "enemy").
When enemy gets closer to a friend (object with "friend" property), I want to send a message to other "friends" objects to notify them.
Then a message is sent to the HUD to display a "Under fire" mode (let's say, a red light sign showing up in the corner).

I am not asking for precise code, but how would you handle that scenario? How would you structure the listeners/triggers?
Are there some things to know?

I am not asking for the community to do the work for me, but I don't want to reinvent the wheel if there are best practices that "every developer" should know about

Thank you!

Michal · May 2012

I guess there is many ways of doing it, and it depends on your code.

For example:

function FriendsContainer:init()
  self.FriendsTable={} -- all friend objects go here
end
 
function FriendsContainer:onEnemyWarning(enemy)
  for n, friend in self.FriendsTable() do
    friend:onEnemyWarning(enemy)
  end
end
 
if isClose(enemy, friend) then
  FriendsContainer:onEnemyWarning(enemy)
  HUD:onUnderFire()
end

deniz · May 2012

Hello Mells,

The best way to do this is by events and listeners. Whenever you want to notify some class about a change or condition, (in this example enemy gets closer), you create a specific Event about it and dispatch that event. You also add this events listener to the objects that you want to notify about this event. So whenever the events is dispatch, the listeners know it happened and do the action.

Say you have a MovingObject class:

function MovingObject:enemyGetsCloser()
    -- you need to create a event here. The event is fired and the all the classes that 
    -- listens to the event, catches it.
    closerEvent = Event.new("enemyIsClose")   --with the specific keyword for the event.
    self:dispatchEvent(closerEvent)
end

For the objects that want to hear about this event...

friend= FriendClass.new()
movingObject:addEventListener("enemyIsClose", friend.enemyAlertFunction, friend)
 
function FriendClass:enemyAlertFunction(event)
    print("Enemy is close!")
end

You need to add this event listener to every friend object and your HUD class.

I hope this helps.
Cheers,

deniz · May 2012

I think there is a better way:

If you have a Game class, which handles enemies and friends, you can let it catch the event. It knows who are friends and notifies them.

alexzheng · May 2012

also consider if you have two or more moving objects, it will become complicated for adding event listeners, I think broadcast events will make our life easier.

Mells · May 2012

@Michal, @deniz
I have built some tools that help me achieve what I had in mind in a very convenient way, thanks to your help

@alexzheng
I am not sure I understand what you mean by broadcast events?
I have created a class (ex: Game like suggested by Deniz) that handles the attribution of triggers, listeners and actions to groups of objects. When a trigger sends an event, the listeners get automatically activated and an action is performed.
The Game class stores and pairs groups of objects.

Ex : when a moving lamp with a "light" property sends an event ("close"), a wall that has the same "light" property gets the message and its color changes.
Those triggers and listeners are grouped by "light" property and this allows me to loop and check distance between all objects contained.

Is it much different than what you are suggesting, Alex?

Mells · May 2012

Also I have another question, I have tried some variations to test, but now I can't get the basics to work.
Can you tell me why isn't the following displaying "Enemy is close"?

-- main.lua
game = Game.new()
game:testListener()
game:testEvent()

-- game.lua
Game = Core.class(Sprite)
 
function Game:testListener()
	local function displayAlert(event)
		print("Enemy is close!")
	end	
	local image = Sprite.new()		
	image:addEventListener("enemyIsClose", displayAlert, image)
end
 
function Game:testEvent()
	local image = Sprite.new()	
	closerEvent = Event.new("enemyIsClose")
        image:dispatchEvent(closerEvent)
end

ndoss · May 2012

The problem is that you're adding the event listener to a different image than the one you're dispatching the event on. Here's an example that works for me -- it's your example modded to use one image:

-- game.lua
Game = Core.class(Sprite)
function Game:init()
	self.image = Sprite.new()
end
 
function Game:testListener()
	local function displayAlert(event)
		print("Enemy is close!")
	end	
	self.image:addEventListener("enemyIsClose", displayAlert, image)
end
 
function Game:testEvent()
	closerEvent = Event.new("enemyIsClose")
    self.image:dispatchEvent(closerEvent)
end
 
game = Game.new()
game:testListener()
game:testEvent()

Mells · May 2012

@ndoss thank you,
If I understand well in your example self.image is telling to itself "enemy is close" right?

But how would "enemy" send a message to "friend" -> "I'm close, fear me!"
And "friend" would be listening, get the message, and react -> "We are under attack!"

I would like the sender and the listener to be different objects.

In fact I'm working on some tests with moving lights.
When the light is close to the wall object, "light" tells "wall" to light up. Wall changes color.
When the light is going away, the light on the wall reverts to normal.

My goal is to have a lot of different lights and walls.

Any idea how I should tackle this? I have tried without success..

ndoss · May 2012

Any idea how I should tackle this? I have tried without success..

I don't have a good answer

OZApps · May 2012

@Mr. Mells,
1. How do you determine proximity? if you want some pointers on that, read up on Levenshtein Distance.
2. Loop through a list of all the objects and determine the Levenshtein Distance of each of the objects in reference to the wall.
3. Check if the object that does have the allowable distance (proximity) is a friend or an enemy, and based on that act accordingly.

Hope that helps you to proceed. In case you have different walls, run this for each wall to determine the distance and the lights that fit the criteria.

Mells · May 2012

hey @OZApps thanks

1. How do you determine proximity?

Like this, it seems to work :

local function checkDistance (source, target)
   local srcX = source:getX()
   local srcY = source:getY()
   local targetX = source:getX()
   local targetY = target:getY()
 
   local xdiff = srcX - targetX
   local ydiff = srcY - targetY
   local dist = (xdiff*xdiff+ydiff*ydiff)^0.5		
return dist
end

2. and 3.
This is what I have and I am able to loop, check the allowable distance, check if friend/enemy ("light" and "walls" in the following example), but the last part (act accordingly) is not working.

game = Game.new()
 
-- Game:addTriggerProp(triggerProperty, objArray, eventMessage, defaultProperties to add to the object)
-- I add the trigger property light to light1 and light2. The event they dispatch is lightEvent. Also, I add extra default properties
game:addTriggerProp("light", {light1, light2}, "lightEvent", {"distance", "power"})
 
-- Game:addListenerProp(listenerProperty, objArray, actionFunction, defaultProperties)
-- I add the trigger property light to wall1, and wall2. The function to run when the listener gets the message is lightUp. Also I add extra default properties.
game:addListenerProp("light", {wall1, wall2}, lightUp, {"distance", "power"})

For each light I'm able to print "I'm lightx and I'm close to wallx : distance is xxx", only if within allowable distance and "too far" when leaving that perimeter.
So I believe the distance check is ok.

But I can't send the message to wallx so wallx runs lightUp.
The part that is not working is "send message -> get message, run function".

So that's why I was asking for a very simple example of working send/listen message between two different objects so I can adapt it to my own code (that would be two long to post here).

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