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), )+pow(fabs(tPoint.y-fPoint.y), ))
let arrowH:CGFloat = line> ? : line/
//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/
let _y = CGFloat(fabs(cos(angle)))*arrowH/
//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*
p3.y = tPoint.y-_x* }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*
p3.y = tPoint.y+_x*
}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*
p3.y = tPoint.y-_x* }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*
p3.y = tPoint.y+_x*
}else if fPoint.x==tPoint.x {
//竖直方向
p1.x=tPoint.x-arrowH/
p1.y=tPoint.y
p2.x=tPoint.x+arrowH/
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
} 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
}
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
