How to resize the canvas form center point ?

Question

Hi,

i want to resize a canvas from center point i.e., for example if i want resize a canvas 20pixels, it should resize left 20px, rigth 20px, bottom 20px, top 20px. before change heigth:100; width:100; x:0; y:0;

after change heigth:140; width:140; x:-20; y:20;

thanks...