How do I implement a multi-touch pinch motion?

The pinch motion that you find in many iPhone apps is known as a 'gesture'. Apple don't provide support for specific gestures such as pinch leaving us as developers to implement them ourselves from touch messages. This does mean we have to do a little math but we'll work through that together in this lesson.

The pinch gesture is often used to shrink and grow items like photos. In this lesson we'll use the pinch gesture to scale a plain graphic.

* The image scaling algorithms in LiveCode have not been optimized for the iPhone yet meaning that dynamic image scaling is quite slow on the device. So in this example we are scaling a graphic which is very fast.

The stack used in this lesson can be found here: https://tinyurl.com/y7jn6yvw

Setup our stack

1) Create a new stack, name it and save it

2) Stop the stack from being resized

3) Set the width and height of the stack to 320 x 460

Add the graphic we're going to resize together

1) Create a square graphic

2) Set its name to 'square'

3) Go to the colors & patterns inspector and set the 'backgroundColor' to one you like

Basic Multi-touch Message

We're going to leave the interface for a moment and look at the basic messages we receive from Rev when touches start, end and move.

on touchStart pTouchId
	# Sent each time a new touch is started
end touchStart

on touchEnd pTouchId
	# Sent each time a touch ends
end touchEnd

on touchMove pTouchId, pX, pY
	# Sent when any touch moves
end touchMove

If a user places two fingers on the device, two touch starts messages will be sent to your stack. You will also be sent a touch ID so you can tell they are different.

The Final Script

Copy this script to the card or stack.

1) Looks for two touches

2) Calculates the distance between the two touches to work out if they are getting closer or further away

3) Scales the image according to the distance of the two touches,

local sTouchArray, sFRAMEWIDTH, sFRAMEHEIGHT
on touchStart pId
	# Store the staring width and height of the graphic
	put the width of graphic "square" into sFRAMEWIDTH
	put the height of graphic "square" into sFRAMEHEIGHT
end touchStart

on touchEnd pId
	# When the touch end remove the ID from the array tracking all our touches
	delete variable sTouchArray[pId]
end touchEnd

on touchMove pId, pX, pY
	# First thing we want to do is store the start point of the touch
	# If this is the first move message for a given touch lets store
	# it in our array
	if sTouchArray[pId]["startloc"] is empty then
		put (pX & comma & pY) into sTouchArray[pId]["startloc"]
	end if
	
	# We want to store the current position of the touch
	put (pX & comma & pY) into sTouchArray[pId]["currentloc"]
	
	# If we know there are two touches on the screen we know that the 
	# user is performing a pinch
	if the number of lines of the keys of sTouchArray is 2 then
		
		# First lets get the current location of the two touches
		put line 1 of the keys of sTouchArray into tPointOne
		put line 2 of the keys of sTouchArray into tPointTwo
		
		# Get the start positions of the two touches. If the know the start
		# and current positions of the two touches who can calculate if the 
		# touches are getting closer of further from each other
		put sTouchArray[tPointOne]["startloc"] into tStartLoc1
		put sTouchArray[tPointTwo]["startloc"] into tStartLoc2
		
		if tStartLoc1 is not empty and tStartLoc2 is not empty then
			
			# Calculate the two distances. Distance at the start between two touches and the
			# distance between the two touches now
			put resizeDistance(tStartLoc1, tStartLoc2) into tStartDistance
			put resizeDistance(sTouchArray[tPointOne]["currentloc"], sTouchArray[tPointTwo]["currentloc"]) into tCurrentDistance
			
			# Now that we have the data we can resize the graphic
			resizeGraphic tStartDistance, tCurrentDistance
		end if
	end if
end touchMove

function resizeDistance pLoc1, pLoc2
	# Using basic trig we calculate the distance between two points
	put item 2 of pLoc1 - item 2 of pLoc2 into dy
	put item 1 of  pLoc1 - item 1 of pLoc2 into dx
	put sqrt((dy*dy) + (dx*dx)) into tDistance
	
	return tDistance
end resizeDistance 

on resizeGraphic pStartDistance, pNewDistance
	# Work out the percentage change between the old and new image
	put round((pNewDistance / pStartDistance) * 100) into tPercentage
	
	# Store the original location of the graphic
	put the loc of graphic "square" into tLoc
	
	# Calculate the new width and height
	set the width of graphic "square"  to round(sFRAMEWIDTH * (tPercentage / 100))
	set the height of graphic "square" to round(sFRAMEHEIGHT * (tPercentage / 100))
	
	set the loc of graphic "square"  to tLoc
	unlock screen
end resizeGraphic

 

Test in the Simulator

Test in the Simulator

1) Load the stack into the simulator

2) Hold the 'alt' key to bring up the multi-touch pointers (two grey circles in the screenshots above)

3) Click and drag to simulate touches moving apart / closer. The square will shrink and grow accordingly.

5 Comments

Jack

Does LiveCode support the use of the alt key when trying to simulate multitouch in the simulator? I cannot get it working. Thanks.

Hanson Schmidt-Cornelius

Hi Jack,

yes you can simulate multi-touch pinch using the alt key in LiveCode on the iOS simulator. If you download the sample stack that accompanies this lesson and run it without modification, then you should get the results shown in the last section of the lesson. The white circles are the locations at which the two touches are simulated.

Kind Regards,

Hanson

Chipp Walters

Looks like you're missing a lock screen in your resizeGraphic handler.

Jerry

Works ok on mobile, but how do we do this on a windows platform which native supports multi-touch? I exported as a windows exe, but it does not work with a multi-touch 4K screen. Only a single mouse touch. So how can we do this? Thanks for any help.

Elanor Buchanan

Hi Jerry

We have an enhancement request for this in our Quality Control Center

http://quality.livecode.com/show_bug.cgi?id=8446

If you would like to you can add your email address to the CC list to get email updates when the status of the bug changes.

Kind regards

Elanor

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