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Many UIs contain data-bound controls, which are controls tied to a database result set. Data-bound controls display values contained in the result set. There are two basic kinds of data-bound controls: simple and complex. The former show only one field of the currently selected row in the result set. The latter show multiple fields from multiple rows of the result set. How should you split the workload between the Coordinator and the Worker when dealing with data-bound controls The basic guidelines are the same as before, but can rephrased like this: Coordinators are responsible for retrieving data. Workers are responsible for presenting data to the user. The guideline doesn t imply that coordinators always retrieve data directly by themselves. In many cases, coordinators need to fire events to other coordinators to get the data they need, sometimes putting in motion a whole slew of activity. Once the coordinators have the requested information, they give it to the Worker. In the case of data-bound controls, the information is in the form of a result set. User interfaces often provide a way for users to change the current row in the result set. All those UI controls that are bound to the given result set should be able to update their displayed values without calling into action a coordinator. Sometimes, changing the current row of a result set requires the retrieval of other data. For example, if the result set is the master in a master-detail relationship, changing the master row selected requires new child rows to be retrieved. In such cases, you should use a coordinator directly or indirectly to fetch the new child rows.
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In many situations, a worker may need to handle requests asynchronously, perhaps to service different clients or to do multiple things at once. Rather than using a single-worker design, it is advantageous to break the system into multiple parts, as shown back in Figure 10-2. The top-level Worker handles the incoming requests, dispatching them to subordinate workers that run on separate threads. The task of managing threading issues is assigned to a coordinator, so that the workers can concentrate just on getting their job done without worrying about timing problems. As an example, assume you have a requirement to build a service that handles concurrent requests from client applications. All the clients interact with the same service, which exposes a single ProcessRequest method. The service must be able to handle any number of requests simultaneously. You can design the system using three classes, as shown in Figure 10-8. The class named WorkerRequestDispatcher handles incoming client requests. When a request arrives, the dispatcher calls Coordinator.Run, which creates a WorkerRequestHandler and runs it on a separate thread, as shown in Figure 10-9. The design entails an asynchronous blind interaction between the top-level Worker (WorkerRequestDispatcher) and the Coordinator. The interaction is blind because the top-level Worker doesn t receive completion feedback from the subordinates. To add feedback, the simplest solution is to have the WorkerRequestHandler fire a completion event that is wired back to WorkerRequestDispatcher.
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WorkerRequestDispatcher +ProcessRequest()
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Coordinator +Run(in theHandler : WorkerRequestHandler)
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Figure 10-8. The class diagram for a concurrent Worker system
Asynchronous Call
Figure 10-9. How the top-level Worker uses the Coordinator to run subordinate workers concurrently While there is nothing inherently wrong with this approach, it is important to point out that the event handler in the top-level Worker would execute on the subordinate Worker s thread. This will cause problems if the top-level Worker is not thread-safe a situation that is more common than one might expect. In many software systems that have a UI, a significant part of the code is often executed on the UI thread. If the dispatcher is running on the UI thread and, in response to a subordinate notification, tries to update the UI when the Worker completion event fires, problems might occur. Why Because GUI operating systems typically expect the UI to be updated only by code running on the UI thread. Updates performed from other threads might cause strange behaviors or race conditions. What this all means is that you shouldn t have subordinate workers fire events to the dispatcher on their own background threads. Any notifications reaching the dispatcher must be running on the UI thread, which the dispatcher is running on. Since none of the workers should have any threadrelated code in them, you must use a separate object for the thread-switching management during notification delivery. The design shown back in Figure 10-9 uses a Coordinator to create threads to run the subordinate workers on. It is reasonable to also use the Coordinator to handle the threadswitching logic for the completion notifications. The Coordinator acts as a mediator between the subordinate workers and the top-level one. Synchronization with the UI thread is an OS-dependent operation. In the .NET Framework, it is accomplished by the method Control.Invoke. Class Control is a base class for all UI controls. The method accepts two arguments: a reference to a method and an array of objects. Control.Invoke calls the referenced method on the UI thread, passing the method the array of objects as parameters. The diagram in Figure 10-10 shows the event notification sequence.
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