resize a canvas according to its children's position

hi, I have a canvas and its width and height are set to auto. When it's rendered, its size are fixuped. Now I need put visual children on it on the fly and I want it resize according to its children's position. For example, its height is 400 at first. Now I put a button of top set to 500 on it, so I want the canvas resize so I can see the button. I have tried surround it with a srollview but it doesn't work. Because the size not enlarged. Could you please help me? thank you.