[FMX] loop question

I am trying to fade in a dynamic image, but when I try this bit of code, it seems to go into an infinite loop.
I’m confused, because it makes sence to me :confused:
It seems to always crash when I do the WHILE part (i’ve tried different while loops)

do {
  while (this._alpha < 100) 

fadein = function() {
if(this._alpha < 100){
                this._alpha += 5;

Any suggestions?
Thank you!