Listen to Cart proceed to payment event

Hi ,
In the payment API (Pay and Pay-backend) it shown that there are several steps needed to be taken. The first one is listen to the ‘button pressed’ event in the GUI but in the Cart this button is a widget property that seem to be hidden or not exposed. Is there a way to do it? and If yes, is there a way to read the cart information.
Best,
Shlomi