Lessons learned from developing a Windows 8 Metro application in C# Frode Nilsen Nilsen Labs Ticki

Agenda  Async-await done right  Applying the MVVM pattern in Windows 8  Unscientific comparison of MS XAML technologies

The async-await pattern  What it is  What it not is  Pitfalls  Applying it properly

Background  Feb 2012: Introduced in.Net 4.5  Spring 2011: Async CTP  June 2008: Task Parallel Library (TPL,.NET 4.0)

The basics  Two new c# keywords: async and await  Makes asyncronous programming easy  Makes asyncronous code clean

The old way

The new way

1 2

Defintion: Asynchrony  «Not at the same time»  When method returns to the caller  When the caller gets the return value Threads

Going deeper  Async part 1, part 2 NDC 2012 presentation Lucian Wischik (Microsoft Senior Program Manager)part 1part 2  The Task Asynchronous Pattern (TAP) paper by Stephen Toub Feb 2012 (Microsoft Async Guru)Task Asynchronous Pattern  Progress reporting  Cancellation  Retrying  Interleaving  Throttling

The power of asynchronous programming - example

Option 1 – Serially Rq 1Rq 2Rq 3 Time Rq 4

Option 2 – In Parallel Rq 1 Rq 2 Rq 3 Time Rq 4

Request slot 2 Request slot 1 Option 3 – Throttled parallelism Rq 1 Rq 2Rq 3 Rq 4 Time  Google «AsyncSemaphore»AsyncSemaphore

DEMO – the powers of Tasks

Async pitfalls 1 : Blocking threads Task.Wait() Task.Result

Async pitfalls 2 : Deadlocks

Async pitfalls 3 : Exceptions

Async pitfalls 4 : async void

Rule of thumbs for async await  Never return async void from a method, unless it’s a UI event handler  Always await async methods. If you want to run several tasks in parallel, use await Task.WhenAll() or await Task.WhenAny()  Never use Task.Wait() or Task.Result to «synchronify» async methods, except in unit tests.  Never do async calls from constructors

MVVM in Windows 8  Recap of the gist  Applying it to Windows 8  Pitfalls  Tools and tips

MVVM Model View ViewModel Send notifications Data binding and commands Send notifications Updates From MSDN

MVVM Model View ViewModel Send notifications Data binding and commands Send notifications Updates DATABINDING

Things that are impossible to do directly from the ViewModel  Give a visual element focus  Select text in a text box  Scroll an item into view  Show / hide Charms, App-or Navigation-bar

MVVM “MVVM is targeted at modern UI development platforms which support Event-driven programming” Quotes from the Wikipedia article

MVVM “MVVM is targeted at modern UI development platforms which support Event-driven programming” Quotes from the Wikipedia article

DEMO - events

Controls and DataContext A B C A ViewModels Controls B C D E A ListView

Controls and DataContext A B C A ViewModels Controls B C D E A ListView

Controls and DataContext A B C A ViewModels Controls B C D E A ListView D E F

Controls and DataContext A B C A ViewModels Controls B C D E A ListView D E F

Solution: Pub/sub-pattern Messenger Publisher 1 Publisher 2 Subscriber 1 Subscriber 2

Solution: Pub/sub-pattern Messenger Publisher 1 Publisher 2 Subscriber 1 Subscriber 2 User controls ViewModels

Solution: Pub/sub-pattern Messenger Publisher 1 Publisher 2 Subscriber 1 Subscriber 2 MVVM Light Toolkit link

DEMO – pub/sub

MVVM “MVVM was designed to make use of data binding functions in WPF to better facilitate the separation of view layer development from the rest of the pattern by removing virtually all GUI code (“code-behind”) from the view layer” Quotes from the Wikipedia article

Things that are impossible to do directly from the ViewModel  Give a visual element focus  Select text in a text box  Scroll an item into view  Show / hide Charms, App-or Navigation-bar

Things I thought were impossible, but were not  Trigger animations  Achieved through the use of databound visual states and attached propertiesdatabound visual states attached properties  Alter the layout of a grid  You can databind Grid.ColumnDefinition.Width  You can databind visibility of sections within the grid

Windows 8 XAML vs «other XAML»

Example 1: Bundled controls

WinFormSLWin8 ButtonXXX DatePickerXX DataGridXX TreeViewXX ComboBoxXX TabControlXX GridXX StackPanelXX MediaElementXX

Making your own controls  Templates Templates  Visual tree vs Logical tree Visual tree vs Logical tree  Custom dependency properties Custom dependency properties  Custom events

Example 2: Databinding  Binding update on property changed

WinForms

WPF

Silverlight No UpdateSourceTrigger

WinRT No UpdateSourceTrigger No BindingOperations.GetBinding()

WinRT 2 (a slightly better alternative) link

Microsoft XAML technologies  WPF  Silverlight  Windows Phone 7  Windows 8 (WinRT) Time Quality

Thank you  Mail:   Blog:

Q & A