今天要学习的内容时LineJoint线段关节，通过线段关节，我们可以限制刚体在某个指定的方向上来回移动。就像算珠只能在算盘的轴上移动一样。

老规矩，我们先来看一下LineJoint的构造函数：

public function LineJoint(

body1:Body,

body2:Body,

anchor1:Vec2,

anchor2:Vec2,

direction:Vec2,

jointMin:Number,

jointMax:Number):void

每个参数的作用说明如下：

body1：和其他的关节一下，关节连接的第1个刚体

body2：和其他的关节一下，关节连接的第2个刚体

anchor1：连接刚体body1的节点，其坐标是相对于body1的本地坐标的

anchor2：连接刚体body2的节点，其坐标是相对于body2的本地坐标的

direction：线段关节中线条的方法，用一个Vec2向量表示

jointMin：线段上沿direction方法，距离anchor1的距离

jointMax：线段沿上direction方向，距离anchor2的距离

看完构造函数和参数说明，依然有些不知其解，看看下面的图示会更简单些：

如图所示，anchor1和anchor2是线段关节中的两个点，两点形成的线段即线段关节。线段关节的方向可以用anchor1.sub(anchor2)表示。线段起点与anchor1的距离是JointMin，终点与anchor1的距离是jonitMax；jointMin/jointMax小于0，在anchor1左侧，大于0在anchor1右侧。值得注意的是，body1可不能绕着anchor1旋转，而body2可以绕着anchor2旋转。

还记得distanceJoint示例吗？如果把示例中的刚体限制在水平方向移动，会是什么样子呢？下面的示例中，点击圆形刚体，可以在水平方向拖动，它通过一个distanceJoint关节与下面的矩形刚体连接。按下空格键”剪断”这个distanceJoint关节，让矩形落到下面的圆形范围内。有没有《割绳子》游戏里的样子呢？赶快动手试试吧！

Sorry, either Adobe flash is not installed or you do not have it enabled

完整的代码和注释如下：

package learnNape {

import ldEasyNape.LDEasyUserData;

import com.bit101.components.PushButton;

import nape.callbacks.InteractionCallback;

import nape.callbacks.InteractionType;

import nape.callbacks.CbEvent;

import nape.callbacks.InteractionListener;

import nape.callbacks.CbType;

import flash.ui.Keyboard;

import flash.events.KeyboardEvent;

import nape.constraint.DistanceJoint;

import nape.shape.Circle;

import flash.events.MouseEvent;

import nape.constraint.LineJoint;

import ldEasyNape.LDEasyNape;

import nape.phys.Body;

import nape.geom.Vec2;

import learnNape.AbstractNapeTest;

/**

* @author yangfei

*/

[SWF( width="400", height="500", frameRate="60")]

public class T19_LineJoint extends AbstractNapeTest {

private var anchorX1 : Number = 100;

private var anchorX2 : Number = 300;

private var anchorY1 : Number = 100;

private var anchorY2 : Number = 300;

private var anchorBody1 : Body;

private var anchorBody2 : Body;

private var joint1 : LineJoint;

private var joint2 : LineJoint;

private var distanceJoint : DistanceJoint;

private var box : Body;

private var sensorType : CbType;

private var boxType : CbType;

private var graphic : LDEasyUserData;

public function T19_LineJoint() {

super();

}

override protected function onNapeWorldReady() : void {

createStuff();

var sensorListener : InteractionListener = new InteractionListener(CbEvent.BEGIN, InteractionType.SENSOR, sensorType, boxType, onCollision);

napeWorld.listeners.add(sensorListener);

new PushButton(this, 280, 15, "reStart",function (e:MouseEvent):void{

box.position.setxy(anchorBody1.position.x+10, anchorY1-50);

distanceJoint.body1= anchorBody1;

distanceJoint.body2 = box;

distanceJoint.active = true;

});

}

private function createStuff():void{

var sensorShape : Circle = new Circle(50);

sensorShape.sensorEnabled = true;

sensorType = new CbType();

graphic = new LDEasyUserData();

graphic.setGraphicAuotmatically(0);

//创建上面的圆形刚体

anchorBody1 = LDEasyNape.createCircle(anchorX1, anchorY1, 10, false, false, graphic);

anchorBody1.shapes.add(sensorShape);

anchorBody1.cbTypes.add(sensorType);

//用线段关节限制anchorBody1只能在水平方向移动

var anchorStart1 : Vec2 = new Vec2(anchorX1, anchorY1);

var anchorEnd1 : Vec2 = new Vec2(anchorX2, anchorY1);

var direction1 : Vec2 = anchorEnd1.sub(anchorStart1);

joint1 = new LineJoint(napeWorld.world, anchorBody1, anchorStart1, Vec2.weak(), direction1, 0, direction1.length);

joint1.space = napeWorld;

//创建下面的刚体

anchorBody2 = LDEasyNape.createCircle(anchorX2, anchorY2, 10, false, false, graphic.clone());

anchorBody2.shapes.add(sensorShape.copy());

anchorBody2.cbTypes.add(sensorType);

//用线段关节限制anchorBody1只能在水平方向移动

var anchorStart2 : Vec2 = new Vec2(anchorX1, anchorY2);

var anchorEnd2 : Vec2 = new Vec2(anchorX2, anchorY2);

var direction2 : Vec2 = anchorEnd2.sub(anchorStart2);

joint2 = new LineJoint(napeWorld.world, anchorBody2, anchorStart2, Vec2.weak(), direction2, 0, direction2.length);

joint2.space = napeWorld;

graphic.setGraphicAuotmatically(0xFF0000,1);

box = LDEasyNape.createBox(anchorX1+10, anchorY1-50, 30, 30, false, false, graphic.clone());

boxType = new CbType();

box.cbTypes.add(boxType);

//创建距离关节，连接矩形和圆形

distanceJoint = new DistanceJoint(anchorBody1, box, Vec2.weak(), Vec2.weak(), 0, 100);

distanceJoint.space = napeWorld;

}

private function onCollision(cb: InteractionCallback):void{

var anchorBody : Body = cb.int1.castBody;

if(!anchorBody.constraints.has(distanceJoint)){

distanceJoint.body1 = anchorBody;

distanceJoint.body2 = box;

distanceJoint.active = true;

}

}

override protected function mouseEventHanlder(event : MouseEvent) : void {

if(event.type == MouseEvent.MOUSE_DOWN){

if(LDEasyNape.getBodyAtMouse()!=null ){

if(LDEasyNape.getBodyAtMouse().shapes.length<=1) return;

}

}

super.mouseEventHanlder(event);

}

override protected function keyBoardEventHanlder(event : KeyboardEvent) : void {

if(event.type != KeyboardEvent.KEY_DOWN) return;

if(event.keyCode == Keyboard.SPACE){

if(box.constraints.length>0){

box.constraints.at(0).active = false;

}

}

}

}

}