drawBackground();这个函数为什么是在gameloop里调用，gameloop()具体是怎么循环的

function gameloop(){

window.requestAnimFrame(gameloop); //setInterval,setTimeout,frame per second

//console.log("gameloop");

var now = Date.now();

deltaTime = now - lastTime;

lastTime = now;

//console.log(deltaTime);

drawBackground();

}