Building the Architecture to Suit the Application in Visual Studio .NET

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6.7 Building the Architecture to Suit the Application
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The User then submits data through the Customer screen, the data is then analyzed to see if it passes the integrity tests on the data is it of the correct format for the eld involved and if there are problems, the response is to send an error to the screen. If the data passes the integrity check, the system then queries the database to see if the record already exists, and if it does it will generate another error (different screen). If it is a genuine new customer, the system will update the database with this information. The GUI will comprise different screens for display to the user: some will be the screens listed on the XXM diagram, and others will be error screens associated with business logic clauses that fail the type of data submitted is invalid, the data con icts with that on the database, and so forth. To explain these different actions, we can envision them in the following sequence diagrams. The simplest case is a direct interaction between the User Interface and the Business Logic layer that does not require the involvement of the database. The functions to control the slider bar, to resize the screen, and to migrate between screens using mouse clicks and mouse movements are of this nature. Thus transitions in an XXM are triggered by events such as button clicks or the results of queries, and the results of these transitions are actions, which include changing the screen, committing data to a database, sending data to some other actor or component in the system, and so on. We can see how this general model works in a few examples. A simple class will handle the sort of function shown in Fig. 6.19. It will listen for the speci ed event and change the interface accordingly. A more complex situation will arise if the interface view has to re ect some other factor, for example to present speci c information and functions that depend on the current state of the database and system generally. Thus the system may cause the GUI to disable or gray out a button or data eld because it is not valid to have access to the function beneath it under the speci c circumstances pertaining at the time.
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A simple interface action.
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6 Bringing the System Together
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Figure 6.20 A conditional user interface action involving a database query.
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An example of a general process for doing this is to select the speci c customer and check whether there are any current orders and to present links to these orders on a new screen (Fig. 6.20). Similarly for a function that submits data, carries out a check with the database, and then allows a commit action with re ection of success onto the screen. Another example might be the validation of the format of the data supplied by the user. The business logic needs to check that the input data is in a valid format, for example a string of characters of length less than 30, a correct date format, and so forth. If necessary, the model will query the database to see if there is already data there in this eld; depending on the outcome of this query, the logic will either commit the data or report an error to the user interface. After successful database update, a con rmatory message may be sent to the user. The analysis of these events and actions will provide us with some guidance on how the program should be structured and what classes will need to be developed. If there is a number of places where there has to be a check that the entry data submitted satis es some prede ned format, for example a text string of length less than 30, then it makes sense to write a class with a method that does just this. This class is then available to the other parts of the program that require this check to be made. This is better than embedding this check separately in all the methods that need it. The overall machine perspective is useful for the development of a simple system metaphor. In the next chapter, we will look at the way in which we might derive the class structure from it. Because projects vary enormously, it is hard to provide examples of useful metaphors that will be applicable to them all. In the next section, we look at an area that provides an increasing number of projects, that of the e-commerce application.
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