Why do canvas operations lag?

Question

Suppose I have the following code in javascript, which interacts with a HTML canvas:

function draw(){
    canvas.width = canvas.width
    canvas.style.backgroundColor = 'red'
    alert('drawn')
}

function wait(){
    alert('waiting')
    while (true){
    }
}

function call(){
    draw()
    wait()
}

and then I call the function call(). What I'd expect to happen is that the function should call draw(), turning the canvas red, and then calling wait() and stalling. And indeed, the window alerts "drawn" and "waiting" as expected. However, the canvas doesn't turn red until the function wait() ends. (which, in this case will never happen). Why does the canvas lag like this, and, supposing that my wait() function were instead some function that takes a few seconds to execute, how can I get the canvas to fully update in the draw() function before going on to the wait() function?

I believe repaints are delayed until the end of the current tick? — Felix Kling, Jan 08 '17 at 20:33
Possible duplicate of [how to force a display update in canvas](http://stackoverflow.com/questions/4982631/how-to-force-a-display-update-in-canvas) — cartant, Jan 08 '17 at 20:41

Blindman67 · Answer 1 · 2017-01-10T04:37:54.507

The function wait will never exit.

function wait(){  
    while (true){  // infinite loop will never exit
    }
}

Maybe you want something like.

setTimeout(waitFunction,0); // call the waitFunction in 0ms

This will make the red canvas appear because you you have exited the current execution allowing for the canvas back buffer to be display. This only happens when there is no javascript running. Even if it is just 0ms it still tells the browser you have finished painting to a DOM element and you would like the results seen.

Why do canvas operations lag?

1 Answers1