状态机？ 可以看这个。。。 https://book.douban.com/subject/6886605/ 在最后有关于状态机 UI 编程。。
代码大概这样

let rec drawingLoop(clr, from) = async {
let! move = Async.AwaitEvent(form.MouseMove) #1
if (move.Button &&& MouseButtons.Left) = MouseButtons.Left then
drawRectangle(clr, from, (move.X, move.Y)) #2
return! drawingLoop(clr, from) #2
else
return (move.X, move.Y)
} #3

let waitingLoop() = async {
while true do #4
let! down = Async.AwaitEvent(form.MouseDown)
let downPos = (down.X, down.Y)
if (down.Button &&& MouseButtons.Left) = MouseButtons.Left then
let! upPos = drawingLoop(Color.IndianRed, downPos) #5
do printfn "Drawn rectanlge (%A, %A)" downPos upPos
}

#A Wait for the next mouse action
#2 Refresh rectangle and continue in the 'Drawing' state
#3 Return end location to the 'Waiting' state
#4 Repeat after drawing finishes
#5 Transition to the 'Drawing' state