How do I drag and drop specific objects in Love2d? - lua

Question

I'm new to Lua and coding in general so I decided to write a Chess Program to learn. I have setup a class and created objects from it to represent the pieces. Now I want to begin moving the pieces with my mouse. I looked at a tutorial, but it only handled one rectangle. My first though was to use a "for" loop in the love.mousePressed() function to go though each of the objects until it found an object with a matching x, y coordinate. This obviously did not work the way I did it. Instead, it only goes to the next object every time the mouse is pressed or at least it would if the program didn't immediately crash once the button was released. So my question is, what is the right way to be going about this?
local blackPawn = love.graphics.newImage("Textures/Blackpawn.png")
local blackRook = love.graphics.newImage("Textures/Blackrook.png")
local blackKnight = love.graphics.newImage("Textures/Blackknight.png")
local blackBishop = love.graphics.newImage("Textures/Blackbishop.png")
local blackQueen = love.graphics.newImage("Textures/Blackqueen.png")
local blackKing = love.graphics.newImage("Textures/BlackKing.png")
local whitePawn = love.graphics.newImage("Textures/Whitepawn.png")
local whiteRook = love.graphics.newImage("Textures/Whiterook.png")
local whiteKnight = love.graphics.newImage("Textures/Whiteknight.png")
local whiteBishop = love.graphics.newImage("Textures/Whitebishop.png")
local whiteQueen = love.graphics.newImage("Textures/Whitequeen.png")
local whiteKing = love.graphics.newImage("Textures/WhiteKing.png")
local chessboard = love.graphics.newImage("Textures/ChessBoard.png")
local register = {}
local id = 0
piece = {
xSquare = 0, ySquare = 0,
x = 0, y = 0,
height = 64, width = 64,
pawn = false,
Rook = false,
Knight = false,
Bishop = false,
Queen = false,
King = false,
color = "",
texture = whitePawn,
dragging = {active = false, diffx = 0, diffy = 0}
}
function piece.new()
newPiece = {}
for k, v in pairs(piece) do
newPiece[k] = v
end
return newPiece
end
function piece:draw()
end
function getMouse()
local x, y = love.mouse.getPosition()
local isDown = love.mouse.isDown(1,2)
return x, y, isDown
end
function createBoard(id)
for x = 1, 8 do
for y = 1, 8 do
if y ~= 3 and y ~= 4 and y ~=5 and y ~= 6 then
id = id + 1
register[id] = piece.new()
register[id].x = x * 64 - 48
register[id].y = (y - 1) * 64
if y == 2 then
register[id].pawn = true
register[id].color = "white"
register[id].texture = whitePawn
print("item " .. id .. " is here x = " .. register[id].x .. " y = " .. register[id].y .. " Is pawn = " .. tostring(register[id].pawn) ..
" Color is " .. register[id].color)
elseif y == 7 then
register[id].pawn = true
register[id].color = "black"
register[id].texture = blackPawn
print("item " .. id .. " is here x = " .. register[id].x .. " y = " .. register[id].y .. " Is pawn = " .. tostring(register[id].pawn) ..
" Color is " .. register[id].color)
elseif y == 1 then
register[id].color = "white"
if x == 1 or x == 8 then
register[id].Rook = true
register[id].texture = whiteRook
elseif x == 2 or x == 7 then
register[id].Knight = true
register[id].texture = whiteKnight
print("knight is here")
elseif x == 3 or x == 6 then
register[id].Bishop = true
register[id].texture = whiteBishop
elseif x == 5 then
register[id].King = true
register[id].texture = whiteKing
elseif x == 4 then
register[id].Queen = true
register[id].texture = whiteQueen
end
elseif y == 8 then
register[id].color = "black"
if x == 1 or x == 8 then
register[id].Rook = true
register[id].texture = blackRook
elseif x == 2 or x == 7 then
register[id].Knight = true
register[id].texture = blackKnight
elseif x == 3 or x == 6 then
register[id].Bishop = true
register[id].texture = blackBishop
elseif x == 5 then
register[id].King = true
register[id].texture = blackKing
elseif x == 4 then
register[id].Queen = true
register[id].texture = blackQueen
end
end
end
end
end
end
function drawBoard(id, register)
love.graphics.draw(chessboard, 0, 0)
for id = 1, 32 do
love.graphics.draw(register[id].texture, register[id].x, register[id].y)
end
end
function love.load()
createBoard(id)
end
function love.update(dt)
for id = 1, 32 do
if register[id].dragging.active == true then
register[id].x = love.mouse.getX() - register[id].dragging.diffx
register[id].y = love.mouse.getY() - register[id].dragging.diffy
end
end
end
function love.draw()
drawBoard(id, register)
end
function love.mousepressed(x, y, button)
for id = 1, 32 do
if (button == 1 or button == 2)
and x > register[id].x and x < register[id].x + register[id].width
and y > register[id].y and y < register[id].y + register[id].height
then
register[id].dragging.active = true
register[id].dragging.diffx = x - register[id].x
register[id].dragging.diffy = y - register[id].y
end
end
end
function love.mousereleased(x, y, button)
for id = 1, 32 do
if button == 1 or button == 2 then register[id].dragging.active = false end
end
end
function love.keypressed(key, unicode)
end
function love.keyreleased(key)
end
function love.focus(bool)
end
function love.quit()
end
Update:
I fixed the crashing, but I still have the weird bug where it changes the dragged piece into a different piece
Update 2: After a little more debugging I have figured out that the major issue is that it for some reason does not correctly check if the dragging is active. As the code stands right now I need an else dragging.active = false after to correctly set it, but now that it is correctly set it won't drag anything at all despite the correct object have dragging set to active (unless I try and drag the object with value 32 where it drags everything at once). I am very confused as to what's wrong. Why isn't Lua able to check value like this?

Answer 1

First, I'd create a global boolean for if a piece has been selected and then a variable to hold the piece selected
local selected = false
local selectedPiece = {}
Then create a playing board and split it into a grid, with each square being of equal size. Something like this
board = {
size = { 8, 8 }, -- 8x8 grid
squareSize = 40, -- 40 pixels long sides
pieces = {
{ -- First row contains which pieces?
Piece:Rook(),
Piece:Bishop(),
...
},
{ -- Second row
Piece:Pawn(),
...
},
{ -- etc.
Piece:Empty(),
...
}
}
}
I advise not using nil in your table for empty squares due to the odd behavior of tables with nil indexes.
In your love.mousepressed() method, you check where the click was based on its position (this is assuming the board takes up the whole window)
function love.mousepressed(x, y, btn)
-- If a piece hasn't been clicked on.
if (not selected) then
-- This line is assuming that since all board squares are equal size, then the mouse click has to be in at least one square.
-- Therefore, if we take the floor of the position/board.squareSize, we will always get a value from 0 - 7 (8 values) on the board.
local piece = board.pieces[math.floor(x/board.squareSize)][math.floor(y/board.squareSize)]
-- If there is a piece here.
if (piece:isNotAnEmpty()) then
selectedPiece = piece -- Select the piece.
selected = not selected -- Notify program that a piece is selected to handle such things accordingly in other methods.
end
else
-- Assuming you wrote a method that determines if a piece can be moved to a certain spot on the board.
if (board:CanMovePieceHere(selectedPiece, x/board.size, y/board.size)) then
-- Do your stuff here.
...
-- Eventually, reset your variables.
selected = not selected
selectedPiece = {}
end
end
end
This is how I'd approach it, but your question is very open to interpretation.