Client side people picker control callback function?

Hi,

I am using client side people picker control in my custom page, I'm filling in this control with people or groups after page is loaded. However, sometimes it takes some time for the control to load people's names, I see loading icons in the control, see below screenshot,

Is there a callback function I can use when the loading is done by the control? The reason why I need this is because I have some buttons for users to click, I need to disable these button until the two people picker controls have successfully loaded people's names.

Thanks,

Michael