Get values from child sprite?

rio8 · April 2014

Hi everyone, I've been using Gideros for a few days and I'm trying to add support for collision masks into a simple collision function but I'm having some trouble

I have a class, RedSquare, and I want to add a child, maskSprite, so I'm doing it like this

RedSquare = Core.class(Sprite)
 
local maskSprite
 
function RedSquare:init()
	local bmp = Bitmap.new(Texture.new("redsquare.png"))
	bmp:setAnchorPoint(0.5, 0.5)
	self:addChild(bmp)
 
	local mask = Bitmap.new(Texture.new("redsquare_mask.png"))
	mask:setAnchorPoint(0.5, 0.5)
	maskSprite = Sprite.new()
	maskSprite:addChild(mask)
 
	self:addChild(maskSprite)
end

When I run the project, it seems to work fine and the mask follows the main object wherever it moves

The problem is, when I want to get the X value of the mask in RedSquare's update function (so I can pass it to the collision function)

If I do this:

print(maskSprite:getX())

It just prints 0.... But I can see that the mask is definitely not at 0

I'm sure I'm setting up the sprite the wrong way somehow but I can't figure it out, can anyone help?

Thank you

- 'Rio

hgvyas123 · April 2014

actually maskSprite is a child of RedSquare and it is at the position of 0,0 in redsquare now if you move the redsquare masksprite will also move as redsquare is parent but still it is at 0,0 in redsquare so you will always get 0 for the print(maskSprite:getX()) .

you should try localToGlobal or local x,y,width,height = masksprite:getBounds(stage) to get the position of masksprite in actual screen or simply use the position parent sprite

rio8 · April 2014

actually maskSprite is a child of RedSquare and it is at the position of 0,0 in redsquare now if you move the redsquare masksprite will also move as redsquare is parent but still it is at 0,0 in redsquare so you will always get 0 for the print(maskSprite:getX()) .

you should try localToGlobal or local x,y,width,height = masksprite:getBounds(stage) to get the position of masksprite in actual screen or simply use the position parent sprite

I got it working perfectly with localToGlobal, thank you!

Paulo777 · February 2018

Hello!
I'm having the same issue but in my case I tried all the way possible and nothing happens. I have a sprite that starts with initial positions, but on mouse event I can change its position to whatever I want. This sprite is initiated on level class. Later, enemy is initiated, and I've set the sprite as parameter to get the values but indepently what I do, my enemy sprite receives always the sprite initial positions, never update its value... I have no idea of what this might be... If anyone has any life is appreciated.

pie · February 2018

@Paulo777 your description of the issue is not really descriptive.. could be anything from here

if you post some code it may help to help

Paulo777 · February 2018

On my ship class I have:

Ship = Core.class(Sprite)
 
function Ship:init()
 
self:setPosition(230, 200)
 
self:addEventListener(Event.TOUCHES_MOVE, self.events, self)
 
end
 
function.onMouseMove(self, event)
 
    if self:hitTestPoint(event.x, event.y)
        self.x = event.touch.x - self:getWidth()/2
        self.y = event.touch.y - self:getHeight()/2
    end
        self:setPosition(self.x, self.y)
end

On my enemy class I have:

Enemy = Core.class(Sprite)
 
function Enemy:init(sprite)
 
    self:setPosition(0, 0)
 
    self.x = sprite:getX()
    self.y = sprite:getY()
 
end

And on level code I have:

 
Level = Core.class(Sprite)
local ship
 
function Level:init()
 
    self:setPosition(0, 0)
    ship = Ship.new()
    self:addChild(ship)
    self.counter = 1
 
    self:addEventListener(Event.ENTER_FRAME, self.events, self)
 
end

In the function events I have:

 
function Level:events(event)
 
    self.counter += 1
 
    if self.counter >= 400 then
        local enemy = Enemy.new(ship)
    end

I tried everything and nothing... I even tried to create custom event, but also nothing... everything I get is the ship initial positions.... never get it to update... never updates...

Paulo777 · February 2018

I want to know why this happens and why I am doing wrong

keszegh · February 2018

minimally i'd change the order of lines:

Enemy = Core.class(Sprite)
 
function Enemy:init(sprite)
 
    self.x = sprite:getX()
    self.y = sprite:getY()
 
    self:setPosition(self.x, self.y)
end

Paulo777 · February 2018

keszegh said:

minimally i'd change the order of lines:

Enemy = Core.class(Sprite)
 
function Enemy:init(sprite)
 
    self.x = sprite:getX()
    self.y = sprite:getY()
 
    self:setPosition(self.x, self.y)
end

Hello! Thank you for your return, but by what I've got the enemy would set its position to the sprite position.. but what I want is: every time I move my "ship" sprite, is for the enemy to get this curren sprite position... that is what's not doing... firstly, it does... but when I move the sprite, the sprite on the screen updates... but on enemy doesn't... and doesn't matter what I do it never changes...

keszegh · February 2018

well, the ship does not know about these enemies, so no surprise there.
either you should add the enemies to the ship instead of to the stage as child. that's simplest.

alternatively, which could be better in some use cases, you should make an empty table inside the ship init function, like self.enemies. and then every time you create a new enemy you should add it to this table as well (you can have a function Ship.addEnemy(enemy) e.g. for this and then call this function).
finally, in the ship event function you should update along with
self:setPosition(self.x, self.y) also the position of every enemy in the table like

for i=1,#self.enemies do
self.enemies[i]:setPosition(self.x,self.y)
self.enemies[i].x=self.x
self.enemies[i].y=self.y
end

btw having properties self.x and self.y are pretty unnecessary as you could simply get the coordinates by calling self.getX() and self.getY() and then there is no issue about updating self.x and self.y.

oleg · February 2018

function Level:events(event)
 
    self.counter += 1
 
    if self.counter >= 400 then
 
        local enemy = Enemy.new()
        ship:addChild(enemy )
    end

Paulo777 · February 2018

Omg!! How I didn't think about that before!! I'll have to redo my logic about that... This actually makes sense... Really thank you for your answer... Getting home I'll do it!

Paulo777 · February 2018

Really thanks for this light! I will try all of the suggestions. If I choose one, surelly I'll use the another for other projects. Thanks @oleg and @keszegh
It really makes sense when we think about objected oriented... about relationships between them... At first I'm thinking about "level", and what happens to the level, but I must take into consideration the relation between objects...

By the way, do you know where I can find sprite hierarchy explanation or material? If anyone knows anything or some link about I'd be very glad.

oleg · February 2018

10.3 Mouse and touch events

https://docs.google.com/document/pub?id=149g_P1YlE4_6v_hHbFx3YAaYpdU0HoM70XJSLTE38ww

Paulo777 · February 2018

uhaaaaaaaau!! Exactly what I need! Thanks so much!!!

Paulo777 · February 2018

oleg said:

function Level:events(event)
 
    self.counter += 1
 
    if self.counter >= 400 then
 
        local enemy = Enemy.new()
        ship:addChild(enemy )
    end

Hello! I tried that but now I'm getting an ... unpleasant behaviour... the enemy is added "glued" with the ship... is there a way to specify its position to the stage?

keszegh · February 2018

"but what I want is: every time I move my "ship" sprite, is for the enemy to get this curren sprite position." - you wrote and then
"unpleasant behaviour... the enemy is added "glued" with the ship"
these seem to be contradicting each other, so i'm not sure what you want to achieve. try to clarify.

Paulo777 · February 2018

what I want is just the enemy to "take" ship position dinamically, whereas enemy position keeps intact, having its own movement to be coded thereafter

keszegh · February 2018

it's still not clear for me, so when the enemy is spawned, it should be at the same place as the ship, but later it moves independently, just you need the position of the ship for other purposes (like where the enemy shoots)? if yes, then in the
Enemy init add:

self.ship=sprite

and then any time the enemy can get the current position of its mothership by querying

self.ship:getPosition()

Paulo777 · February 2018

keszegh said:
it's still not clear for me, so when the enemy is spawned, it should be at the same place as the ship, but later it moves independently, just you need the position of the ship for other purposes (like where the enemy shoots)? if yes, then in the
Enemy init add:

self.ship=sprite

and then any time the enemy can get the current position of its mothership by querying

self.ship:getPosition()

Exactly! For that purpose that I want to know ship position, for wherever the ship changes its position, the enemy will target the fire, the fire class I did to shoot wherever I want and from wherever sprite that I want it to shoot... and maybe for other purposes also

Paulo777 · February 2018

OOOMG!!!! Worked like a charm!! So simple... really thank you!! I see that I have to much to learn with this...

Paulo777 · February 2018

@keszegh can you explain me how this way works and what's the behaviour behind both sprites?

keszegh · February 2018

i'm not sure what you need. guessing i'd say you may confuse
sprite1.mySprite=sprite2
and sprite hierarchy when you add a child to another like
sprite1:addChild(sprite2).

the first (sprite1.mySprite=sprite2) just creates a reference for sprite2 so sprite1.mySprite points to the same object as sprite2, and that's it, there is no other connection between them.
in the example self.ship=sprite, self refers to the sprite whose function we are inside currently (an object from the class Enemy), and self.ship is just a way to store sprite in the scope of this enemy.

the second example on the other hand ties sprite2 to sprite1 in the visual hierarchy, so whenever sprite1 moves/scales/is added or removed from stage/made invisible etc. then the same happens with sprite2.
sprite2:getParent() will still get you sprite1, so there is a way to get it (like in the first way), but the connection is now much stronger as i said, they basically move together.

to understand the first, probably you should read through a lua manual, the basics are short. for the second, the ultimate guide for gideros is a good place to learn about sprite hierarchy etc. but this concept is quite common, as actionscript works the same way.

oleg · February 2018

If I understood correctly, then you do need to create an AI for the enemy's behavior ??

Paulo777 · February 2018

thanks @keszegh for the introduction.
@oleg not quite. But from now just simple behaviors like moving and shooting, as it is a side scrolling, more of them are in my plans to add in the level

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