iOS上实现画箭头，如果是指定了坐标点，那是很简单的，但如果需要做到实时绘制，就需要计算一下了

需求：

在白板上，根据手势落下点和移动点，实时绘制一条箭头直线（如下图）

 

实现代码：

/// 获取箭头的点位置    ///    /// - Parameters:    ///   - fPoint: <#fPoint description#>    ///   - tPoint: <#tPoint description#>    /// - Returns: <#return value description#>    func getArrowPoint(fPoint:CGPoint,tPoint:CGPoint) -> (CGPoint,CGPoint,CGPoint) {        var p1 = CGPoint.zero           //箭头点1        var p2 = CGPoint.zero           //箭头点2        var p3 = CGPoint.zero           //箭头最前面点        //假设箭头边长20,箭头是一个等腰三角形        let line = sqrt(pow(fabs(tPoint.x-fPoint.x), 2)+pow(fabs(tPoint.y-fPoint.y), 2))        let arrowH:CGFloat = line>40 ? 20 : line/3        //线与水平方向的夹角        let angle = getAnglesWithThreePoints(p1: fPoint, p2: tPoint, p3: CGPoint(x: fPoint.x, y: tPoint.y))        let _x = CGFloat(fabs(sin(angle)))*arrowH/2        let _y = CGFloat(fabs(cos(angle)))*arrowH/2        //向右上角、水平向右        if tPoint.x >= fPoint.x && tPoint.y <= fPoint.y{            p1.x = tPoint.x-_x            p1.y = tPoint.y-_y                        p2.x = tPoint.x+_x            p2.y = tPoint.y+_y                        p3.x = tPoint.x+_y*2            p3.y = tPoint.y-_x*2                    }else if tPoint.x > fPoint.x && tPoint.y > fPoint.y{            //向右下角            p1.x = tPoint.x+_x            p1.y = tPoint.y-_y                        p2.x = tPoint.x-_x            p2.y = tPoint.y+_y                        p3.x = tPoint.x+_y*2            p3.y = tPoint.y+_x*2        }else if tPoint.x < fPoint.x && tPoint.y < fPoint.y{            //向左上角            p1.x = tPoint.x-_x            p1.y = tPoint.y+_y                        p2.x = tPoint.x+_x            p2.y = tPoint.y-_y                        p3.x = tPoint.x-_y*2            p3.y = tPoint.y-_x*2                    }else if tPoint.x < fPoint.x && tPoint.y >= fPoint.y{            //向左下角,水平向左            p1.x = tPoint.x-_x            p1.y = tPoint.y-_y                        p2.x = tPoint.x+_x            p2.y = tPoint.y+_y                        p3.x = tPoint.x-_y*2            p3.y = tPoint.y+_x*2        }else if fPoint.x==tPoint.x {            //竖直方向            p1.x=tPoint.x-arrowH/2            p1.y=tPoint.y            p2.x=tPoint.x+arrowH/2            p2.y=tPoint.y            p3.x=tPoint.x            p3.y = tPoint.y>fPoint.y ? tPoint.y+arrowH : tPoint.y-arrowH        }                return (p1,p2,p3)    }

 

其中，获取夹角方法：getAnglesWithThreePoints

/// 计算三点之间的角度    ///    /// - Parameters:    ///   - p1: 点1    ///   - p2: 点2（也是角度所在点）    ///   - p3: 点3    /// - Returns: 角度（180度制）    func getAnglesWithThreePoints(p1:CGPoint,p2:CGPoint,p3:CGPoint) -> Double {        //排除特殊情况，三个点一条线        if (p1.x == p2.x && p2.x == p3.x) || ( p1.y == p2.x && p2.x == p3.x){            return 0        }                let a = fabs(p1.x - p2.x)        let b = fabs(p1.y - p2.y)        let c = fabs(p3.x - p2.x)        let d = fabs(p3.y - p2.y)                if (a < 1.0 && b < 1.0) || (c < 1.0 && d < 1.0){            return 0        }        let e = a*c+b*d        let f = sqrt(a*a+b*b)        let g = sqrt(c*c+d*d)        let r = Double(acos(e/(f*g)))        return r        //弧度值        //        return (180*r/Double.pi)      //角度值    }

 

最后得到三个点，即箭头的三个点坐标