MSDN Magazine - December 2008 - (Page 60)

custom composite activities where the emphasis is on customized control flow instead of realizing a specific domain function. WF ships with several standard control flow composites such as IfElseActivity and WhileActivity. But with WF, you can realize more exotic cases, such as execution of composite child activities based on some configurable priority level, or perhaps use of a dependency network to determine execution order (see Figure 6). In addition, almost all of the various runtime services that ship with WF are extendible or replaceable. You’re free to (for example) Figure 6 Using a Dependency Network [Designer( typeof( DependencyNetworkActivityDesigner ) )] [ActivityValidator( typeof( DependencyNetworkActivityValidator ) )] public partial class DependencyNetworkActivity : SequenceActivity { DependencyProperty.RegisterAttached( "RunAfter", typeof( string ), typeof( DependencyNetworkActivity ), new PropertyMetadata( DependencyPropertyOptions.Metadata ) ); public static object GetRunAfter( object depObj ) { return ( (DependencyObject) depObj ).GetValue( RunAfterProperty ); } public static void SetRunAfter( object depObj, object value ) { ( (DependencyObject) depObj ).SetValue( RunAfterProperty, value ); } public DependencyNetworkActivity() { InitializeComponent(); } private IList _orderedList = null; private int _currentIndex = 0; protected override ActivityExecutionStatus Execute( ActivityExecutionContext aec ) { if ( this.EnabledActivities.Count == 0 ) { return ActivityExecutionStatus.Closed; } else { _orderedList = DependencyNetworkActivityValidator.ComputeOrderedList( this ); Debug.Assert( _orderedList != null ); Debug.Assert( _orderedList.Count == this.EnabledActivities.Count ); ScheduleNextChild( aec ); } return ActivityExecutionStatus.Executing; implement your own workflow persistence layer on top of another storage medium, or perhaps implement a workflow scheduler that executes against a custom thread pool implementation instead of the built-in .NET ThreadPool. Any such custom service is configured in exactly the same way as all other WF services, so the model is consistent and straightforward. Ultimately, the goal for WF is to remain flexible in the face of evolving technologies and application architectures, instead of mandating a rigid set of requirements that might otherwise render the technology less broadly applicable. Domain Modeling and Program Design } private void ScheduleNextChild( ActivityExecutionContext aec ) { Debug.Assert( aec != null ); Debug.Assert( _orderedList != null ); Debug.Assert( _currentIndex < _orderedList.Count ); Activity child = _orderedList[ _currentIndex++ ]; Debug.Assert( child != null ); child.Closed += ChildDone; } aec.ExecuteActivity( child ); private void ChildDone( object sender, ActivityExecutionStatusChangedEventArgs e ) { ActivityExecutionContext aec = ( sender as ActivityExecutionContext ); e.Activity.Closed -= ChildDone; if ( _currentIndex < _orderedList.Count ) { ScheduleNextChild( aec ); } else { aec.CloseActivity(); } } } One of the more difficult aspects of building software with WF is mapping elements of your problem domain to elements of a workflow. The primary issue is the control flow-centric programming model of WF in which—unlike traditional imperative, or objectoriented programming—the atomic elements of your workflow (activities) don’t represent entities in your problem domain, but rather actions or processing steps. It’s a subtle but important distinction, and it makes what are already non-trivial tasks (domain modeling and program design) even more challenging, especially for WF newcomers. It’s one thing to understand the mechanics of the WF programming model—activities, services, episodic execution, and the rest— but it’s another thing entirely to take a functional specification or a bunch of use cases and turn that into one or more efficient, wellfactored, robust workflows. What domain concepts map to an activity versus service versus an entire workflow? What data should flow through the workflow, and how? Where can work be done in parallel, and where must the workflow be designed to go idle, waiting on user input? Truth be told, it’s impossible to sufficiently answer such questions in the space of a few paragraphs. But I offer you here a few general guidelines that have served me well. As with any discussion of modeling and design, take the advice that follows as a starting point and not a complete treatment of the topic, and don’t assume this is the only way to achieve WF design nirvana. It isn’t. First, start your modeling phase as you normally would with use cases, actors, and domain elements given up-front design consideration suitable for your project. Use any approach that works for you; there’s no need for WF-specific accommodations here. Next, model your actors (Customer, BankManager, and so on) and your domain elements (PurchaseOrders, InsurancePolicies, AcmeWidgets) as objects. As always, give careful consideration to the data encapsulated by each item. Expose data with properties, as necessary. Stub out actor methods, but don’t implement them just yet. At this point, you have a decision to make. Is your intended design WF-centric, such that all significant behavior will be represented as distinct activities in one or more workflows? Or do you want to use WF in a more targeted fashion, implementing key aspects in WF while keeping the rest as more traditional .NET code? Don’t consider it a binary yes-or-no question; instead, give deliberate consideration to where on the spectrum between all WF and no WF Real-World WF 60 msdn magazine

Table of Contents Feed for the Digital Edition of MSDN Magazine - December 2008

MSDN Magazine - December 2008 Contents Toolbox CLR Inside Out Advanced Basics Cutting Edge Patterns In Practice Team System Real-World WF Visual Studio OBA Tools SOA Data Access Geneva Framework Test Run Foundations Windows With C++ Going Places End Bracket

MSDN Magazine - December 2008

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