Stop Button (Again)

I asked this ~10 years ago, but have forgotten what I did back then.

I have a panel interface where the user set some directories and starts the loop with a button.  Since the process can be long (1 hour +), there is a request for a stop button.  I would like to exit the loop after an image is processed and then continue to write a log file with the status of each image processed. I have the function and loop below.

Question: What do I need to do to read the button push once this function starts?

Function Run_Image_workflow()

	wave/T Images_to_Process
	string basepath,QP
	Pathinfo QuantPath
	QP = s_path
	NVAR ImagesLeft
	Nvar Halt =root:halt
	variable i,imax
	imax = dimsize(Images_to_process,0)
	for(i=0;i<imax;i+=1)
		//should image be run
		
		if (Stringmatch(Images_to_Process[i][%status], "not run")) //done and mask missing are skipped
			
			//summary
			Newpath/C/Q/O Pathsummary, (QP+Images_to_Process[i]+ ":summary")
			//image
			NewPath/C/Q/O PathImageOut, (QP+Images_to_Process[i]+ ":OutputImages")
			//sizes
			NewPath/C/Q/O Pathsizes, (QP+Images_to_Process[i]+ ":size")
			//textures
			NewPath/C/Q/O Pathtexture, (QP+Images_to_Process[i]+ ":texture")	
			
			process_image(i)
			ImagesLeft -=1	
			Images_To_process[i][%Status] ="done"
			if(halt)
				break
			endif
		endif
			
	endfor
	Save_log_file()
end

Function StopButton(ba) : ButtonControl
	STRUCT WMButtonAction &ba

	switch( ba.eventCode )
		case 2: // mouse up
			// click code here
			nvar halt =root:halt
			halt =1
			break
		case -1: // control being killed
			break
	endswitch

	return 0
End

Andy