CocosCreator物理引擎Demo源码分析(3)-stick-arrow

stick-arrow示例展示了如何动态发射刚体飞往目标点。

技术点

1、触摸屏幕发射刚体,计算起点和目标点的夹角,设置刚体的线性速度。
2、在Update中不断施加一个作用力到刚体尾部,使它能一直往目标点飞去。
3、在碰撞上后,动态计算并设置WeldJoint的属性,使刚体和碰撞体按一定角度连接起来,不致于自然掉落。

源码分析

arrow.js

arrow.js代码功能是处理碰撞之后的逻辑,主要是动态计算和设置WeldJoint关节的属性。

cc.Class({
    extends: cc.Component,

    properties: {

    },

    // LIFE-CYCLE CALLBACKS:

    onLoad () {
        this.weldJoint = this.getComponent(cc.WeldJoint);
    },

    // 每次处理完碰撞体接触逻辑时被调用
    onPostSolve: function(contact, selfCollider, otherCollider) {
        // 获取冲量信息 注意:这个信息只有在 onPostSolve 回调中才能获取到
        var impulse = contact.getImpulse();
        // normalImpulses: 法线方向的冲量
        // PTM_RATIO: 物理单位与像素单位互相转换的比率,一般是 32。
        if (Math.abs(impulse.normalImpulses[0]) < cc.PhysicsManager.PTM_RATIO) return;

        let joint = this.weldJoint;

        if (joint.enabled) {
            joint.enabled = false;
            return;
        }

        if (otherCollider.node.name === 'arrow') {
            return;
        }

        let arrowBody = selfCollider.body;
        let targetBody = otherCollider.body;
        // 将 arrowBody 本地坐标系下的点转换为世界坐标系下的点
        let worldCoordsAnchorPoint = arrowBody.getWorldPoint(cc.v2(0.6, 0));

        joint.connectedBody = targetBody;
        // 将给定的世界坐标系下的点转换为 arrowBody 本地坐标系下的点
        joint.anchor = arrowBody.getLocalPoint(worldCoordsAnchorPoint);
        // 将给定的世界坐标系下的点转换为 targetBody 本地坐标系下的点
        joint.connectedAnchor = targetBody.getLocalPoint(worldCoordsAnchorPoint);
        joint.referenceAngle = targetBody.node.rotation - arrowBody.node.rotation;

        joint.enabled = true;
    },
});

shoot-arrow.js

shoot-arrow.js代码功能是发射刚体,并在刚体飞行过程中不断计算和施加作用力。

cc.Class({
    extends: cc.Component,

    properties: {
        arrow: {
            type: cc.Node,
            default: null
        }
    },

    onEnabled: function() {
        this.debugDrawFlags = cc.director.getPhysicsManager().debugDrawFlags;
        cc.director.getPhysicsManager().debugDrawFlags = 
            cc.PhysicsManager.DrawBits.e_jointBit |
            cc.PhysicsManager.DrawBits.e_shapeBit
            ;
    },

    onDisable: function() {
        cc.director.getPhysicsManager().debugDrawFlags = this.debugDrawFlags;
    },

    // LIFE-CYCLE CALLBACKS:

    onLoad () {
        this.node.on(cc.Node.EventType.TOUCH_START, this.onTouchBegan, this);
        this.arrowBodies = [];
    },

    onTouchBegan: function(event) {
        let touchLoc = event.touch.getLocation();
        let node = cc.instantiate(this.arrow);
        node.active = true;
        let vec = cc.v2(touchLoc).sub(node.position);
        // 通过反正切函数得到触摸点和arrow出生点之间的夹角角度
        // 乘以 180/3.14159 是为了把弧度转化为角度
        node.rotation = -Math.atan2(vec.y, vec.x) * 180 / Math.PI;
        cc.director.getScene().addChild(node);

        // 返回向量的长度
        let distance = vec.mag();
        // 返回归一化后的向量,再乘以800
        let velocity = vec.normalize().mulSelf(800);
        // 设置刚体的线性速度
        let arrowBody = node.getComponent(cc.RigidBody);
        arrowBody.linearVelocity = velocity;

        this.arrowBodies.push(arrowBody);
    },

    update: function (dt) {
        let dragConstant = 0.1;
        let arrowBodies = this.arrowBodies;
        for (let i = 0; i < arrowBodies.length; i++) {
            let arrowBody = arrowBodies[i];
            let velocity = arrowBody.linearVelocity;
            let speed = velocity.mag();
            if (speed === 0) {
                continue;
            }
            let direction = velocity.normalize();

            // 将世界坐标系下的(1,0)向量转换为刚体本地坐标系下的向量
            let pointingDirection = arrowBody.getWorldVector(cc.v2(1, 0));
            let flightDirection = arrowBody.linearVelocity;
            let flightSpeed = flightDirection.mag();
            // 向量归一化,让这个向量的长度为 1
            flightDirection.normalizeSelf();

            // 向量之间进行点乘
            let dot = cc.pDot(flightDirection, pointingDirection);
            let dragForceMagnitude = (1 - Math.abs(dot)) * flightSpeed * flightSpeed * dragConstant * arrowBody.getMass();
            // 得到arrowBody尾部的世界坐标
            let arrowTailPosition = arrowBody.getWorldPoint(cc.v2(-80, 0));
            // 施加一个力到arrowBody刚体的尾部(世界坐标系)
            arrowBody.applyForce(flightDirection.mul(-dragForceMagnitude), arrowTailPosition, false);
        }
    },
});

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