从GUI何时何地调用Web服务

Question

We have an application that can be used to view the current quantity of products available for sale. The user has a GUI to create filters, for example, all black products of at least 1kg. When the user changes the filter, a service call with the filter descriptions is made and the results are shown in charts. The service call is very slow, say 5 seconds. This in itself it not a problem.

The problem lies in changing the filters. What, in the user interface code, is the right place to make the call to the webservice.
I could place a CallWebService() line in all button_pressed and lost_focus events. This does not feel right somehow.
I could also register to the collection_changed and property_changed events in the data model and call the webservice there. This results in several calls when all elements from list are removed. We currently have a controller class in between that is called from the GUI (button_press events etc.) but inside the controller several methods call each other wich has double service calls as a result.

We use a data model, a view and a controller in combination with WPF and databinding However, we have had problems like these in other applications and I am wondering what the right place/event is to place the call to the webservice. I would like to avoid asynchronous programming and in my perception my problem has nothing to do with asynchronous programming.

Answer 1

I suggest call service when user stop entering filter with some delay (say 5 seconds or even less).

Also if you want avoid ui frezing you can use Background worker class for web service calls(and while thread will work show progress image for the user).

Answer 2

You should different between triggering a web service call from actually making the call. For example, you can trigger the call on any change mechanism (for example - data model changes or UI events like button press). A trigger would (re)start the timer to make the service call. Key is to restarting the existing timer every time event is arrived. By choosing a good value for timer say 1-2 seconds, you can actual make service call when there is pause in user's action.