One growing trend that I have seen in mobile & tablet applications is the creation of tools that enable your workforce to perform their job better. This can be in the case of mobile data retrieval, streamlined sales process with apps for door-to-door sales, mobile business process efficiency, etc…

One of the topics that comes up is how do you capture a signature and store it within your application? This might be for validation that the signer is who they say they are, or for legal/contractual reasons. Imagine a few scenarios:

• Your cable TV can’t be installed until you sign the digital form on the installation tech’s tablet device
• You agree to purchase a service from a sales person (door to door, or in-store kiosk) – your signature is required to make this legally binding.
• Your signature is required to accept an agreement before confidential data is presented to you.

These are just a few random scenarios, I’m sure there are many more.   In this post, I will focus on 2 (yes, I said two) cross-platform solutions to handle this task – one built with Adobe Flex & AIR, and one built with HTML5 Canvas & PhoneGap.  

Source for both solutions is available at: https://github.com/triceam/Mobile-Signature-Capture

Watch the video below to see this in action, then we’ll dig into the code that makes it work.

The basic flow of the application is that you enter an email address, sign the interface, then click the green “check” button to submit to the signature to a ColdFusion server.  The server then sends a multi-part email to the email address that you provided, containing text elements as well as the signature that was just captured.

If you’d like to jump straight to specific code portions, use the links below:

• The Server Solution
• The Flex/AIR Solution
• The HTML5/PhoneGap Solution

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The Server Solution

Let’s first examine the server component of the sample application.   The server side is powered by ColdFusion. There’s just a single CFC that is utilized by both the Flex/AIR and HTML/PhoneGap front-end applications.   The CFC exposes a single service that accepts two parameters: the email address, and a base-64 encoded string of the captured image data.

[cf]<cffunction name="submitSignature" access="remote" returntype="boolean">
<cfargument name="email" type="string" required="yes">
<cfargument name="signature" type="string" required="yes">

<cfmail SUBJECT ="Signature"
FROM="#noReplyAddress#"
TO="#email#"
username="#emailLoginUsername#"
password="#emailLoginPassword#"
server="#mailServer#"
type="HTML" >

<p>This completes the form transaction for <strong>#email#</strong>.</p>

<p>You may view your signature below:</p>
<p><img src="cid:signature" /></p>

<p>Thank you for your participation.</p>

<cfmailparam
file="signature"
content="#toBinary( signature )#"
contentid="signature"
disposition="inline" />

</cfmail>

<cfreturn true />
</cffunction>[/cf]

Note: I used base-64 encoded image data so that it can be a single server component for both user interfaces. In Flex/AIR you can also serialize the data as a binary byte array, however binary serialization isn’t quite as easy with HTML/JS… read on to learn more.

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The Flex/AIR Solution

The main user interface for the Flex/AIR solution is a simple UI with some form elements. In that UI there is an instance of my SignatureCapture user interface component. This is a basic component that is built on top of UIComponent (the base class for all Flex visual components), which encapsulates all logic for capturing the user signature. The component captures input based on mouse events (single touch events are handled as mouse events in air). The mouse input is then used to manipulate the graphics content of the component using the drawing API. I like to think of the drawing API as a language around the childhood game “connect the dots”. In this case, you are just drawing lines from one point to another.

When the form is submitted, the graphical content is converted to a base-64 encoded string using the Flex ImageSnapshot class/API, before passing it to the server.

You can check out a browser-based Flex version of this in action at http://tricedesigns.com/portfolio/sigCaptureFlex/ – Just enter a valid email address and use your mouse to sign within the signature area. When this is submitted, it will send an email to you containing the signature.

You can check out the SignatureCapture component code below, or check out the full project at https://github.com/triceam/Mobile-Signature-Capture/tree/master/flex%20client. This class will also work in desktop AIR or browser/based Flex applications. The main application workflow and UI is contained with Main.mxml.

[as3]package
{
import flash.display.DisplayObject;
import flash.display.Graphics;
import flash.display.Sprite;
import flash.events.MouseEvent;
import flash.geom.Point;

import mx.core.UIComponent;
import mx.graphics.ImageSnapshot;
import mx.managers.IFocusManagerComponent;

import spark.primitives.Graphic;

public class SignatureCapture extends UIComponent
{
private var captureMask : Sprite;
private var drawSurface : UIComponent;
private var lastMousePosition : Point;

private var backgroundColor : int = 0xEEEEEE;
private var borderColor : int = 0x888888;
private var borderSize : int = 2;
private var cornerRadius :int = 25;
private var strokeColor : int = 0;
private var strokeSize : int = 2;

public function SignatureCapture()
{
lastMousePosition = new Point();
super();
}

override protected function createChildren():void
{
super.createChildren();

captureMask = new Sprite();
drawSurface = new UIComponent();
this.mask = captureMask;
addChild( drawSurface );
addChild( captureMask );

this.addEventListener( MouseEvent.MOUSE_DOWN, onMouseDown );
}

protected function onMouseDown( event : MouseEvent ) : void
{
lastMousePosition = globalToLocal( new Point( stage.mouseX, stage.mouseY ) );
stage.addEventListener( MouseEvent.MOUSE_MOVE, onMouseMove );
stage.addEventListener( MouseEvent.MOUSE_UP, onMouseUp );
}

protected function onMouseMove( event : MouseEvent ) : void
{
updateSegment();
}

protected function onMouseUp( event : MouseEvent ) : void
{
updateSegment();
stage.removeEventListener( MouseEvent.MOUSE_MOVE, onMouseMove );
stage.removeEventListener( MouseEvent.MOUSE_UP, onMouseUp );
}

protected function updateSegment() : void
{
var nextMousePosition : Point = globalToLocal( new Point( stage.mouseX, stage.mouseY ) );
renderSegment( lastMousePosition, nextMousePosition );
lastMousePosition = nextMousePosition;
}

public function clear() : void
{
drawSurface.graphics.clear();
}

override public function toString() : String
{
var snapshot : ImageSnapshot = ImageSnapshot.captureImage( drawSurface );
return ImageSnapshot.encodeImageAsBase64( snapshot );
}

override protected function updateDisplayList(w:Number, h:Number):void
{
super.updateDisplayList(w,h);

drawSurface.width = w;
drawSurface.height = h;

var g : Graphics = this.graphics;

//draw rectangle for mouse hit area
g.clear();
g.lineStyle( borderSize, borderColor, 1, true );
g.beginFill( backgroundColor, 1 );
g.drawRoundRect( 0,0,w,h, cornerRadius, cornerRadius );

//fill mask
g.clear();
g = captureMask.graphics;
g.beginFill( 0, 1 );
g.drawRoundRect( 0,0,w,h, cornerRadius, cornerRadius );
}

protected function renderSegment( from : Point, to : Point ) : void
{
var g : Graphics = drawSurface.graphics;
g.lineStyle( strokeSize, strokeColor, 1 );
g.moveTo( from.x, from.y );
g.lineTo( to.x, to.y );
}
}
}[/as3]

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The HTML5/PhoneGap Solution

The main user interface for the HTML5/PhoneGap solution is also a simple UI with some form elements. In that UI there is a Canvas element that is used to render the signature. I created a SignatureCapture JavaScript class that encapsulates all logic for capturing the user signature. In browsers that support touch events (mobile browsers), this is based on the touchstart, touchmove and touchend events. In browsers that don’t support touch (aka desktop browsers), the signature input is based on mousedown, mousemove and mouseup events. The component captures input based on touch or mouse events, and that input is used to manipulate the graphics content of the Canvas tag instance. The canvas tag also supports a drawing API that is similar to the ActionScript drawing API. To read up on Canvas programmatic drawing basics, check out the tutorials at http://www.adobe.com/devnet/html5/html5-canvas.html

When the form is submitted, the graphical content is converted to a base-64 encoded string using the Canvas’s toDataURL() method. The toDataURL() method returns a base-64 encoded string value of the image content, prefixed with “data:image/png,”. Since I’ll be passing this back to the server, I don’t need this prefix, so it is stripped, then sent to the server for content within the email.

You can check out a browser-based version of this using the HTML5 Canvas in action at http://tricedesigns.com/portfolio/sigCapture/ – Again, just enter a valid email address and use your mouse to sign within the signature area. When this is submitted, it will send an email to you containing the signature. However, this example requires that your browser supports the HTML5 Canvas tag.

You can check out the SignatureCapture code below, or check out the full project at https://github.com/triceam/Mobile-Signature-Capture/tree/master/html%20client. This class will also work in desktop browser applications that support the HTML5 canvas. I used Modernizr to determine whether touch events are supported within the client container (PhoneGap or desktop browser). The main application workflow is within application.js.

Also a note for Android users, the Canvas toDataURL() method does not work in Android versions earlier than 3.0. However, you can implement your own toDataURL() method for use in older OS versions using the technique in this link: http://jimdoescode.blogspot.com/2011/11/trials-and-tribulations-with-html5.html (I did not update this example to support older Android OS versions.)

[js]function SignatureCapture( canvasID ) {
this.touchSupported = Modernizr.touch;
this.canvasID = canvasID;
this.canvas = $("#"+canvasID);
this.context = this.canvas.get(0).getContext("2d");
this.context.strokeStyle = "#000000";
this.context.lineWidth = 1;
this.lastMousePoint = {x:0, y:0};

this.canvas[0].width = this.canvas.parent().innerWidth();

if (this.touchSupported) {
this.mouseDownEvent = "touchstart";
this.mouseMoveEvent = "touchmove";
this.mouseUpEvent = "touchend";
}
else {
this.mouseDownEvent = "mousedown";
this.mouseMoveEvent = "mousemove";
this.mouseUpEvent = "mouseup";
}

this.canvas.bind( this.mouseDownEvent, this.onCanvasMouseDown() );
}

SignatureCapture.prototype.onCanvasMouseDown = function () {
var self = this;
return function(event) {
self.mouseMoveHandler = self.onCanvasMouseMove()
self.mouseUpHandler = self.onCanvasMouseUp()

$(document).bind( self.mouseMoveEvent, self.mouseMoveHandler );
$(document).bind( self.mouseUpEvent, self.mouseUpHandler );

self.updateMousePosition( event );
self.updateCanvas( event );
}
}

SignatureCapture.prototype.onCanvasMouseMove = function () {
var self = this;
return function(event) {

self.updateCanvas( event );
event.preventDefault();
return false;
}
}

SignatureCapture.prototype.onCanvasMouseUp = function (event) {
var self = this;
return function(event) {

$(document).unbind( self.mouseMoveEvent, self.mouseMoveHandler );
$(document).unbind( self.mouseUpEvent, self.mouseUpHandler );

self.mouseMoveHandler = null;
self.mouseUpHandler = null;
}
}

SignatureCapture.prototype.updateMousePosition = function (event) {
var target;
if (this.touchSupported) {
target = event.originalEvent.touches[0]
}
else {
target = event;
}

var offset = this.canvas.offset();
this.lastMousePoint.x = target.pageX – offset.left;
this.lastMousePoint.y = target.pageY – offset.top;

}

SignatureCapture.prototype.updateCanvas = function (event) {

this.context.beginPath();
this.context.moveTo( this.lastMousePoint.x, this.lastMousePoint.y );
this.updateMousePosition( event );
this.context.lineTo( this.lastMousePoint.x, this.lastMousePoint.y );
this.context.stroke();
}

SignatureCapture.prototype.toString = function () {

var dataString = this.canvas.get(0).toDataURL("image/png");
var index = dataString.indexOf( "," )+1;
dataString = dataString.substring( index );

return dataString;
}

SignatureCapture.prototype.clear = function () {

var c = this.canvas[0];
this.context.clearRect( 0, 0, c.width, c.height );
}[/js]

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Source for the ColdFusion server, as well as Flex/AIR and HTML5/PhoneGap clients is available at: https://github.com/triceam/Mobile-Signature-Capture