What function is called when the Check in button is clicked

Question

Which function is called when you click on the "Check In" button of a page? I am referring to the button that opens the following modal dialog window.

It does not reference a page (already checked it with fiddler).

Update

The function that Howard specified will have the following result:

If you want to try it out, here is the code:

var ctx = new SP.ClientContext.get_current()
var str=ctx.get_url()+"/_layouts/checkin.aspx?"+"List=Pages&FileName="+document.location;
OpenPopUpPage(str, CheckInNotifyAndRefreshPage);  

Answer 1

When you click Check In button, method handleCommand called from javascript object SP.Ribbon.PageState.PageStateHandler (sp.ribbon.js) with commandId = "PageStateGroupCheckin"
If you debug it, you can find that it executes this line at least:

return SP.Ribbon.PageState.Handlers.showStateChangeDialog(properties['CommandValueId'], SP.Ribbon.PageState.ImportedNativeData.CommandHandlers[properties['CommandValueId']]);  

EDIT
Ok, somebody down voted, I'll explain how to find this function.
First of all inspect check in button using firebug or google chrome dev tools. Check in button is a link with id Ribbon.WikiPageTab.EditAndCheckout.Checkout.Menu.Checkout.Checkin-Menu16. Next, go to cmdui.xml file in 14 hive. Here using search find button with id Ribbon.WikiPageTab.EditAndCheckout.Checkout.Menu.Checkout. You will find this menu:

<Menu Id="Ribbon.WikiPageTab.EditAndCheckout.Checkout.Menu">  

and button with definition:

<Button
  Id="Ribbon.WikiPageTab.EditAndCheckout.Checkout.Menu.Checkout.Checkin"
  MenuItemId="Ribbon.WikiPageTab.EditAndCheckout.Checkout.Menu.Checkout.Checkin"
  Sequence="20"
  Alt="$Resources:core,ButCheckin;"
  Command="PageStateGroupCheckin"
  CommandValueId="PageStateGroupCheckin"
  Image16by16="/_layouts/$Resources:core,Language;/images/formatmap16x16.png" Image16by16Top="-104" Image16by16Left="-32"
  Image32by32="/_layouts/$Resources:core,Language;/images/formatmap32x32.png" Image32by32Top="-416" Image32by32Left="-192"
  LabelText="$Resources:core,ButCheckin;"
  ToolTipTitle="$Resources:core,ButCheckin;"
  ToolTipDescription="$Resources:core,cui_STT_ButCheckin_page;"
 />  

We interested in CommandValueId attribute. Next go to sp.ribbon.debug.js and using search find object, which handle this particular commandid - this is SP.Ribbon.PageState.PageStateHandler. All commands in page component handle method handleCommand, setup break and click check in in browser - consider screen shot below:

if you step further you got to line, that I mentioned above:

It means, that when you press Check In in the ribbon method SP.Ribbon.PageState.Handlers.showStateChangeDialog fires.
Hope this helps.

Answer 2

I think it's the CheckInSingleItem function which can be found in \Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\INPLVIEW.js