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Bookmarks

Bookmarks are the mechanism that enables an activity to passively wait for input without holding onto a workflow thread. When an activity signals that it is waiting for stimulus, it can create a bookmark. This indicates to the runtime that the activity’s execution should not be considered complete even when the currently executing method (which created the Bookmark) returns.

Bookmark Basics

A Bookmark represents a point at which execution can be resumed (and through which input can be delivered) within a workflow instance. Typically, a Bookmark is given a name and external (host or extension) code is responsible for resuming the bookmark with relevant data. When a Bookmark is resumed, the workflow runtime schedules the BookmarkCallback delegate that was associated with that Bookmark at the time of its creation.

Bookmark Options

The BookmarkOptions class specifies the type of Bookmark being created. The possible non mutually-exclusive values are None, MultipleResume, and NonBlocking. Use None, the default, when creating a Bookmark that is expected to be resumed exactly once. Use MultipleResume when creating a Bookmark that can be resumed multiple times. Use NonBlocking when creating a Bookmark that might never be resumed. Unlike bookmarks created using the default BookmarkOptions, NonBlocking bookmarks do not prevent an activity from completing.

Bookmark Resumption

Bookmarks can be resumed by code outside of a workflow using one of the ResumeBookmark overloads. In this example, a ReadLine activity is created. When executed, the ReadLine activity creates a Bookmark, registers a callback, and then waits for the Bookmark to be resumed. When it is resumed, the ReadLine activity assigns the data that was passed with the Bookmark to its Result argument.

public sealed class ReadLine : NativeActivity<string>
{
    [RequiredArgument]
    public  InArgument<string> BookmarkName { get; set; }

    protected override void Execute(NativeActivityContext context)
    {
        // Create a Bookmark and wait for it to be resumed.
        context.CreateBookmark(BookmarkName.Get(context), 
            new BookmarkCallback(OnResumeBookmark));
    }

    // NativeActivity derived activities that do asynchronous operations by calling 
    // one of the CreateBookmark overloads defined on System.Activities.NativeActivityContext 
    // must override the CanInduceIdle property and return true.
    protected override bool CanInduceIdle
    {
        get { return true; }
    }

    public void OnResumeBookmark(NativeActivityContext context, Bookmark bookmark, object obj)
    {
        // When the Bookmark is resumed, assign its value to
        // the Result argument.
        Result.Set(context, (string)obj);
    }
}

In this example, a workflow is created that uses the ReadLine activity to gather the user’s name and display it to the console window. The host application performs the actual work of gathering the input and passes it to the workflow by resuming the Bookmark.

Variable<string> name = new Variable<string>
{
    Name = "name"
};

Activity wf = new Sequence
{
    Variables =
    {
        name
    },
    Activities =
    {
        new WriteLine()
        {
            Text = "What is your name?"
        },
        new ReadLine()
        {
            BookmarkName = "UserName",
            Result = name
        },
        new WriteLine()
        {
            Text = new InArgument<string>((env) => "Hello, " + name.Get(env))
        }
    }
};

AutoResetEvent syncEvent = new AutoResetEvent(false);

// Create the WorkflowApplication using the desired
// workflow definition.
WorkflowApplication wfApp = new WorkflowApplication(wf);

// Handle the desired lifecycle events.
wfApp.Completed = delegate(WorkflowApplicationCompletedEventArgs e)
{
    // Signal the host that the workflow is complete.
    syncEvent.Set();
};

// Start the workflow.
wfApp.Run();

// Collect the user's name and resume the bookmark.
// Bookmark resumption only occurs when the workflow
// is idle. If a call to ResumeBookmark is made and the workflow
// is not idle, ResumeBookmark blocks until the workflow becomes
// idle before resuming the bookmark.
wfApp.ResumeBookmark("UserName", Console.ReadLine());

// Wait for Completed to arrive and signal that
// the workflow is complete.
syncEvent.WaitOne();

When the ReadLine activity is executed, it creates a Bookmark named UserName and then waits for the bookmark to be resumed. The host collects the desired data and then resumes the Bookmark. The workflow resumes, displays the name, and then completes. Note that no synchronization code is required with regard to resuming the bookmark. A Bookmark can only be resumed when the workflow is idle, and if the workflow is not idle, the call to ResumeBookmark blocks until the workflow becomes idle.

Bookmark Resumption Result

ResumeBookmark returns a BookmarkResumptionResult enumeration value to indicate the results of the bookmark resumption request. The possible return values are Success, NotReady, and NotFound. Hosts and extensions can use this value to determine how to proceed.




Build Date:

2012-08-02
Show:
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