# Tutorial:PhysicsCollisionCallbacks (简体中文)

## 教程

### main.lua 配置

```function love.load()

end
function love.update(dt)

end
function love.draw()

end```

### World 配置

```function love.load()
--用变量 "world" 存储创建好的新的物理世界
world = love.physics.newWorld(0, 200, true)  --设置 X 方向上为 0，Y 方向上为 200 的重力。
end

function love.update(dt)
--永远别忘记了这玩意——更新！
world:update(dt)
end```

```function love.load()
... -- don't include this, it's just indicating where the existing code for the function should be

ball = {}
ball.b = love.physics.newBody(world, 400,200, "dynamic")  -- set x,y position (400,200) and let it move and hit other objects ("dynamic")
ball.b:setMass(10)                                        -- make it pretty light
ball.s = love.physics.newCircleShape(50)                  -- give it a radius of 50
ball.f = love.physics.newFixture(ball.b, ball.s)          -- connect body to shape
ball.f:setRestitution(0.4)                                -- make it bouncy
ball.f:setUserData("Ball")                                -- give it a name, which we'll access later
static = {}
static.b = love.physics.newBody(world, 400,400, "static") -- "static" makes it not move
static.s = love.physics.newRectangleShape(200,50)         -- set size to 200,50 (x,y)
static.f = love.physics.newFixture(static.b, static.s)
static.f:setUserData("Block")
end```

The objects are there now, but you can't yet see them. Let's draw them.

```function love.draw()
love.graphics.polygon("line", static.b:getWorldPoints(static.s:getPoints()))
end```

Now we should see a ball fall down and hit a solid rectangle.

### World Callbacks

But what if we want more information on the two objects colliding? Now we will use World:setCallbacks to further dissect their collision(s).

First thing we do is set the world callbacks with World:setCallbacks. There are four callbacks for a collision: beginContact, endContact, preSolve, and postSolve.

beginContact gets called when two fixtures start overlapping (two objects collide).
endContact gets called when two fixtures stop overlapping (two objects disconnect).
preSolve is called just before a frame is resolved for a current collision
postSolve is called just after a frame is resolved for a current collision.
```function love.load()
...  -- substitute for the rest of love.load

world = love.physics.newWorld(0, 200, true)
--These callback function names can be almost any you want:
world:setCallbacks(beginContact, endContact, preSolve, postSolve)

text       = ""   -- we'll use this to put info text on the screen later
persisting = 0    -- we'll use this to store the state of repeated callback calls```

Now define each function you just named.

```function beginContact(a, b, coll)

end

function endContact(a, b, coll)

end

function preSolve(a, b, coll)

end

function postSolve(a, b, coll, normalimpulse, tangentimpulse)

end```

These functions are called every time one of the collision actions happen. They pass in two fixtures and a collision object. The postsolve callback also contains the normal and tangent impulse for each collision contact point. These parameters can also be named to whatever you want. In this tutorial, we choose a, b, and coll.

• a is the first fixture object in the collision.
• b is the second fixture object in the collision.
• coll is the contact object created.
• normalimpulse is the amount of impulse applied along the normal of the first point of collision. It only applies to the postsolve callback, and we can ignore it for now.
• tangentimpulse is the amount of impulse applied along the tangent of the first point of collision. It only applies to the postsolve callback, and we can ignore it for now.

Say we want to print to screen whenever a callback is called. We just need to modify the text variable we added to love.load() earlier by appending a string every time a collision action happens. We need a bit of extra code to keep the output clean too.

A list of functions you can use on contacts can be found at the Contact page.

```function love.update(dt)
... -- substitute for the rest of love.update

if string.len(text) > 768 then    -- cleanup when 'text' gets too long
text = ""
end
end

function love.draw()
... -- substitute for the rest of love.draw

love.graphics.print(text, 10, 10)
end

function beginContact(a, b, coll)
x,y = coll:getNormal()
text = text.."\n"..a:getUserData().." colliding with "..b:getUserData().." with a vector normal of: "..x..", "..y
end

function endContact(a, b, coll)
persisting = 0    -- reset since they're no longer touching
text = text.."\n"..a:getUserData().." uncolliding with "..b:getUserData()
end

function preSolve(a, b, coll)
if persisting == 0 then    -- only say when they first start touching
text = text.."\n"..a:getUserData().." touching "..b:getUserData()
elseif persisting < 20 then    -- then just start counting
text = text.." "..persisting
end
persisting = persisting + 1    -- keep track of how many updates they've been touching for
end

function postSolve(a, b, coll, normalimpulse, tangentimpulse)
-- we won't do anything with this function
end```

And now you know how to use world callbacks!

To better explore how this world behaves and see when the callbacks are invoked, add some controls to allow you to push around the ball:

```function love.update(dt)
world:update(dt)

if love.keyboard.isDown("right") then
ball.b:applyForce(1000, 0)
elseif love.keyboard.isDown("left") then
ball.b:applyForce(-1000, 0)
end
if love.keyboard.isDown("up") then
ball.b:applyForce(0, -5000)
elseif love.keyboard.isDown("down") then
ball.b:applyForce(0, 1000)
end

if string.len(text) > 768 then    -- cleanup when 'text' gets too long
text = ""
end
end```

## Finished

### 截图

Screenshot of the finished product.

### main.lua

```function love.load()
world = love.physics.newWorld(0, 200, true)
world:setCallbacks(beginContact, endContact, preSolve, postSolve)

ball = {}
ball.b = love.physics.newBody(world, 400,200, "dynamic")
ball.b:setMass(10)
ball.s = love.physics.newCircleShape(50)
ball.f = love.physics.newFixture(ball.b, ball.s)
ball.f:setRestitution(0.4)    -- make it bouncy
ball.f:setUserData("Ball")
static = {}
static.b = love.physics.newBody(world, 400,400, "static")
static.s = love.physics.newRectangleShape(200,50)
static.f = love.physics.newFixture(static.b, static.s)
static.f:setUserData("Block")

text       = ""   -- we'll use this to put info text on the screen later
persisting = 0    -- we'll use this to store the state of repeated callback calls
end

function love.update(dt)
world:update(dt)

if love.keyboard.isDown("right") then
ball.b:applyForce(1000, 0)
elseif love.keyboard.isDown("left") then
ball.b:applyForce(-1000, 0)
end
if love.keyboard.isDown("up") then
ball.b:applyForce(0, -5000)
elseif love.keyboard.isDown("down") then
ball.b:applyForce(0, 1000)
end

if string.len(text) > 768 then    -- cleanup when 'text' gets too long
text = ""
end
end

function love.draw()
love.graphics.polygon("line", static.b:getWorldPoints(static.s:getPoints()))

love.graphics.print(text, 10, 10)
end

function beginContact(a, b, coll)
x,y = coll:getNormal()
text = text.."\n"..a:getUserData().." colliding with "..b:getUserData().." with a vector normal of: "..x..", "..y
end

function endContact(a, b, coll)
persisting = 0
text = text.."\n"..a:getUserData().." uncolliding with "..b:getUserData()
end

function preSolve(a, b, coll)
if persisting == 0 then    -- only say when they first start touching
text = text.."\n"..a:getUserData().." touching "..b:getUserData()
elseif persisting < 20 then    -- then just start counting
text = text.." "..persisting
end
persisting = persisting + 1    -- keep track of how many updates they've been touching for
end

function postSolve(a, b, coll, normalimpulse, tangentimpulse)
end```