Cross platform mobile apps using .NET

Cross platform mobile apps using .NET
Jonas Follesø
NDC, June 2011

AGENDA:
Why?
8 techniques
Complete example
http://www.flickr.com/photos/20792787@N00/2248623391/

TILE FLOOD

Why cross platform..?

REUSE SKILL SET?

REUSE CODE?
privatevoidopen_Click(objectsender, RoutedEventArgse)
{
varchannel=HttpNotificationChannel.Find("NNUG");
if (channel==null)
{
channel=newHttpNotificationChannel("NNUG");
}
channel.ChannelUriUpdated+=channel_ChannelUriUpdated;
channel.ShellToastNotificationReceived+=
channel_ShellToastNotificationReceived;
channel.Open();
}

REUSE LIBRARIES
+35 libraries monotouch.info/MonoTouch/Libraries

COST OF MAINTAINING MULTIPLE CODE BASES

LIKE C#..?

... OR DISLIKE THE ALTERNATIVE

OBJECTIVE-C: DATE (NO TIME)
+ (NSDate *) stripTime:(NSDate *) date {NSCalendar *gregorian = [[NSCalendaralloc] initWithCalendarIdentifier:NSGregorianCalendar];NSDateComponents *components =
[gregorian components:
(NSYearCalendarUnit | NSMonthCalendarUnit |
NSDayCalendarUnit)
fromDate:date]; date = [gregoriandateFromComponents:components]; [gregorian release]; return date;}

C#: DATE (NO TIME)
public DateTimeStripTime(DateTime date){ return date.Date;}

TECHNIQUE 1:PORTABLE CLASS LIBRARY

HELP YOU STICK TO A COMMON SUBSET

PORTABLE CLASS LIBRARY
PROS
One class library for all platforms
Helps you stick to a common subset
CONS
No Preprocessor Directives
No Platform Specific Code in Class Lib
Not directly supported on MonoDevelop yet

TECHNIQUE 2:LINKED FILES

WORKS IN MONODEVELOP

LINKED FILES
PROS
Same code across all platforms
Ability to add platform specific code
Preprocessor Directives
CONS
Need to manage multiple projects

TECHNIQUE 3:LINK FILES USING «PROJECT LINKER»

PROJECT LINKER
PROS
Auto link files between projects
CONS
Does not work in MonoDevelop
Must have all projects in same solution

TECHNIQUE 4:VSMonoTouch Open MonoTouch projects in VS2010

VSMonoTouch
PROS
Open and build (verify) MonoTouch in VS2010
CONS
Not a «real» build
Some manual steps involved

TECHNIQUE 5:PREPROCESSOR DIRECTIVES

using System.Net;
#if MONOTOUCH || MONODROID
using System.Web;
#endif

PREPROCESSOR DIRECTIVES
PROS
Easily write platform specific code
CONS
Code smell
DRY – The #ifdefs tend to spread

TECHNIQUE 6:ABSTRACT COMMONFUNCTIONALITY

publicinterfaceIDispatchOnUIThread
{
void Invoke(Action action);
}

// Windows Phone 7
publicclassDispatchAdapter : IDispatchOnUIThread
{
publicvoid Invoke(Action action)
{
Deployment.Current.Dispatcher.BeginInvoke(action);
}
}

// Mono for Android
{
privatereadonlyActivity _owner;
public DispatchAdapter(Activity owner)
{
_owner = owner;
}
publicvoid Invoke(Action action)
{
_owner.RunOnUiThread(action);
}
}

// Monotouch
{
privatereadonly NSObject _owner;
publicDispatchAdapter(NSObject owner)
{
_owner = owner;
}
publicvoid Invoke (Action action)
{
_owner.BeginInvokeOnMainThread(newNSAction(action));
}
}

ABSTRACT FUNCTIONALITY
PROS
Share even more code
Do not need preprocessor directives
CONS
More abstractions in code

TECHNIQUE 7:LEVERAGE MVVM ON ALL PLATFORMS

Ensure that any code that manipulates presentation only manipulates presentation, pushing all domain and data source logic into clearly separated areas of the program.
Martin Fowler, Separated Presentation, July 2006

Separated Presentation Patterns

Data & Domain Logic(Model)
UI(View)
Interaction (Controller/Presenter)

MVVMPATTERN FOR BINDABLE UI

APP LOGIC IN CODE BEHIND IS HARD TO TEST AND MAINTAIN
VIEW
XAML
DataModel
Code-Behind
EventHandlers

SEPARATE USINGMVVM
VIEW
XAML
Code-Behind
Change notification
Data-binding and commands
VIEW MODEL
Data Model
State + Operations

publicclassObservableAdapter<T> : BaseAdapter<T>
{
privatereadonlyActivity _context;
privatereadonlyObservableCollection<T> _collection;
publicObservableAdapter(Activitycontext, ObservableCollection<T> collection)
{
_context = context;
_collection = collection;
_collection.CollectionChanged += (o, e) => NotifyDataSetChanged();
}
...
ListAdapter = newObservableAdapter<Airport> (this, _viewModel.Airports);

publicabstractclassObservableDataSource<T> : UITableViewDataSource
{
publicObservableDataSource (ObservableCollection<T> collection, UITableView tableView)
{
_tableView = tableView;
_collection = collection;
_collection.CollectionChanged += (o, e) => {
tableView.ReloadData();
};
}
...

APPLYING MVVM
PROS
Testable View Models
CONS
May not feel natural to platform

TECHNIQUE 8:USE PRE-BUILT ABSTRACTIONS

PHONE SPECIFIC APIS?
http://www.flickr.com/photos/howzey/2880455762/

EXAMPLES
Accelerometer
Camera
Compass
Geolocation
Notification
Storage
Contacts
And more...

MonoMobile.Extensions
https://github.com/chrisntr/MonoMobile.Extensions
One common API for phone specific functionality across all three devices
Inspired by PhoneGap(same API)
http://www.flickr.com/photos/tambako/3974809361/

privatevoid FindNearestAirport()
{
var coordinates = new MonoMobile.Extensions.Geolocation();
coordinates.GetCurrentPosition(PositionAvailable);
}
privatevoid PositionAvailable(Position position)
{
Console.WriteLine("{0} {1}",
position.Coords.Latitude,
position.Coords.Longitude);
}

privatevoid FindNearestAirport()
{
var coordinates = newMonoMobile.Extensions.Geolocation();
coordinates.GetCurrentPosition(PositionAvailable);
}
privatevoid PositionAvailable(Position position)
{
Console.WriteLine("{0} {1}",
position.Coords.Latitude,
position.Coords.Longitude);
}

3x MonoMobile.Extensions.dll
MonoDroid -> MonoMobile.Extensions.dll
Implemented using LocationManager
MonoTouch -> MonoMobile.Extensions.dll
Implemented using CLLocationManager
Windows Phone 7 -> MonoMobile.Extensions.dll
Implemented using GeoLocationWatcher

ABSTRACT FUNCTIONALITY
PROS
Same set of abstractions used in multiple apps
Well known API
CONS
Smallest common denominator

DOES IT WORK IN PRACTICE?
http://www.flickr.com/photos/kozloski/2306510520/

FLIGHTS NORWAYMONITORARRIVALS AND DEPARTURES
http://www.flickr.com/photos/svenwerk/2181849280/

SUMMARY
Portable Library
Linked Files
Project Linker
VSMonoTouch
Preprocessor Directives
Abstracting Common Functionality
Leverage MVVM
Using Pre-built Abstractions

FORK ME ON GITHUB
github.com/follesoe/FlightsNorway
Complete app for iOS, Android & WP7
56 page tutorial with step-by-step code.

Jonas Follesø
Scientist
+47 977 06660
Jonas.folleso@bekk.nojonas@follesoe.no

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Cross platform mobile apps using .NET

by Jonas Follesø

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