如何将对象创建限制在画布上？

Question

http://jsfiddle.net/goldrunt/SeAGU/33/ Lines 24-40 are meant to limit circle creation to the canvas. I need to check the location before allowing the new object to be created. How would I do this if the object doesn't exist yet?

function isAtWall(a) {
    return (a.x - a.radius <= rect.left || a.y + a.radius >= rect.top || a.x + a.radius >= rect.left + canvas.width || a.y - a.radius <= rect.top - canvas.height);
}

window.onmousedown = function (e) {
    // IE fixer
    e = e || window.event;
    // get event location on page offset by canvas location
    var location = {
        x: e.pageX - offset.x,
        y: e.pageY - offset.y
    };
    if (!isAtWall(location)) {
        create(location);
    }
};

Answer 1

Make it simpler to start off with and you'll get to the answer much easier:

function isAtWall(a) { 

  if ((a.x >= 0 && a.x <= rect.width) && (a.y >= 0 && a.y <= rect.height))
  {
    return false;
  }

return true;
}

http://jsfiddle.net/SeAGU/35/

That will only allow the user to place circles on the canvas.

EDIT: Just to clarify, I'm deliberately leaving out the circle radius calculation. This function will only determine if the click event is on the canvas.