Love2D Lua错误：尝试调用字段“ isDown”（nil值）

Question

This is my code in Love2D:

function love.load()   
    ninja = love.graphics.newImage("Ninja.png")
    x = 0
    y = 0
    speed = 256
end

function love.update(dt)
    if love.keyboard.isDown("right") then
        ninja = love.graphics.newImage("NinjaRight.png")
        x = x + (speed * dt)
    end

    if love.keyboard.isDown("left") then
        ninja = love.graphics.newImage("NinjaLeft.png")
        x = x - (speed * dt)
    end

    if love.keyboard.isDown("down") then
        y = y + (speed * dt)
    end

    if love.keyboard.isDown("up") then
        y = y - (speed * dt)
    end

    if love.joystick.isDown(joystick, 1, 2, 3, 4) then
        a = 5
    end
end


function love.draw()
    love.graphics.draw(ninja, x, y)
end

I want to make the game to recognize a controller when connected. But when I run the game, I receive the error:

attempt to call field 'isDown'(a nil value)

Where is the problem?

Answer 1

Since LÖVE 0.9.0 Joystick related isDown() function is moved to another namespace / table /You name ir or more "object" like structure. [1]

So, in Your code You should use it something like this:

--Get table of all connected Joysticks:
local joysticks = love.joystick.getJoysticks()

--Pick first one:
local joystick = joysticks[1]

if joystick:isDown(1, 2, 3, 4) then
    a = 5
end

Where joystick is Your Joystick object. [2]

Be aware, love.keyboard.isDown() usage haven't changed yet. But, I guess, it's about to too. Sooner or later.

[1] https://love2d.org/wiki/Joystick:isDown

[2] https://love2d.org/wiki/love.joystick.getJoysticks