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If the flow and navigation of your application are simple enough so that anyone can visit any screen in pretty much any order, there's little value in a Application Controller The strength of an Application Controller comes from definite rules about the order in which pages should be visited and different views depending on the state of objects
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A good signal to use an Application Controller is if you find yourself having to make similar changes in lots of different places when your application's flow changes
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Further Reading
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Most of the ideas that underlie the writing of this pattern came from [Knight and Dai] Although their ideas aren't exactly new, I found their explanations remarkably clear and compelling
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Example: State Model Application Controller (Java)
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State models are a common way of thinking about user interfaces They're particularly appropriate when you need to react differently to events depending on the state of some object In this example I have a simple state model for a couple of commands on an asset (Figure 1411) ThoughtWork's leasing experts would faint at the virulent oversimplification of this model, but it will do as an example of a state-based Application Controller
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Figure 1411 A simple state diagram for an asset
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As far as the code is concerned our rules are these:
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When we receive a return command and we're in the On lease state, we display a page to capture information about the return of the asset A return event in the in Inventory state is an error, so we show an illegal action page When we receive a damage command we show different pages depending on whether the asset is in the Inventory or the On lease state
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The input controller is a Front Controller (344) It services the request like this:
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class FrontServlet public void service(HttpServletRequest request, HttpServletResponse response)
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throws IOException, ServletException { ApplicationController appController = getApplicationController(request); String commandString = (String) requestgetParameter("command"); DomainCommand comm = appControllergetDomainCommand(commandString, getParameterMap(request)); commrun(getParameterMap(request)); String viewPage = "/" + appControllergetView(commandString, getParameterMap(request)) + "jsp"; forward(viewPage, request, response); }
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The flow of the service method is pretty straightforward: We find the right application controller for a given request, ask the application controller for the domain command, execute the domain command, ask the application controller for a view, and, finally, forward to the view
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In this scheme I'm assuming a number of Application Controllers, all of which implement the same interface
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interface ApplicationController DomainCommand getDomainCommand (String commandString, Map params); String getView (String commandString, Map params);
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For our commands the appropriate Application Controller is an asset application controller It uses a response class to hold the domain commands and view references For the domain command I use a reference to a class; for the view I use a string, which the front controller will turn into a URL for a JSP
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class Response private Class domainCommand; private String viewUrl; public Response(Class domainCommand, String viewUrl) { thisdomainCommand = domainCommand; thisviewUrl = viewUrl; } public DomainCommand getDomainCommand() { try { return (DomainCommand) domainCommandnewInstance(); } catch (Exception e) {throw new ApplicationException (e); } } public String getViewUrl() { return viewUrl; }
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The application controller holds on to the responses using a map of maps indexed by the command string and an asset status (Figure 1412)
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class AssetApplicationController private Response getResponse(String commandString, AssetStatus state) { return (Response) getResponseMap(commandString)get(state); } private Map getResponseMap (String key) { return (Map) eventsget(key); } private Map events = new HashMap();
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Figure 1412 How the asset application controller stores its references to domain commands and views
When asked for a domain command, the controller looks at the request to figure out the asset ID, goes to the domain to determine the status of that asset, looks up the appropriate domain command class, instantiates that class, and returns the new object
class AssetApplicationController public DomainCommand getDomainCommand (String commandString, Map params) { Response reponse = getResponse(commandString, getAssetStatus(params)); return reponsegetDomainCommand(); } private AssetStatus getAssetStatus(Map params) { String id = getParam("assetID", params); Asset asset = Assetfind(id); return assetgetStatus(); } private String getParam(String key, Map params) { return ((String[]) paramsget(key))[0]; }