ASP.NET PDF Viewer using C#, VB/NET

That will foil the developer who wrote Example 3-15 and Example 3-18. But doesn t it also break our constructor again In fact, it doesn t: read-only fields behave differently from read-only properties. A read-only property can never be modified. A read-only field can be modified, but only by a constructor. Since read-only fields only become truly read-only after construction completes, it makes them perfect for properties that need to be able to be different from one instance to another, but which need to be fixed for the lifetime of an instance. Before we move on from const and readonly fields, there s another property our Plane needs for which const seems like it could be relevant, albeit in a slightly different way. In addition to monitoring the speed of an aircraft, we also need to know whether it is approaching or heading away from the airport. We could represent that with a bool property called something like IsApproaching (where true would mean that it was approaching, and false would, by implication, indicate that it was heading away). That s a bit clumsy, though. You can often end up having to negate Boolean properties you might need to write this sort of thing:

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Custom widgets are what make your application different from the rest. The special task that your application will perform is often handled through a special widget. Having said this, I recommend that you pick standard widgets whenever possible because it can be difficult for the users of your application to learn how to use your special widget. Designing widgets that fit into the Qt way of writing applications is not hard. First, you need to find a widget to inherit from the starting point. If there is no given starting point, you have to start from the QWidget class. After you have picked a suitable starting point, you must decide which events you want to pay attention to. This helps you decide which event handling functions to override. The event handlers can be considered your interface with users. When you have decided on your interface, you need to tend to the rest of the application, including setters, getters, signals, and slots (as well as setting up size policies and creating size hints). Make sure to think through usage scenarios other than the current one to make your widget reusable. An investment in time when writing a widget can help you in future projects because you can save having to reinvent the wheel time after time. After having discussed all these software development issues, I must emphasize the most important aspect of your widgets: usability. Try thinking as a user and make sure to test your design on real users before putting it in your production software.

if (!plane.IsApproaching) { ... }

That reads as if not plane is approaching which sounds a bit awkward. We could go with:

You ve used the ScriptManager control already to create references on the client side to the Atlas script libraries. Using the control is simple. When you drag and drop the control onto a page, you get a design-time user interface that allows you to set up some of the common elements of the ScriptManager control (see Figure 6-7).

if (somePlane.IsApproaching == false) { ... }

That s if is approaching is false which isn t much better. We could offer a second, calculated property called IsNotApproaching, but our code is likely to be simpler and easier to read (and therefore likely to contain fewer bugs) if, instead of using bool, we have a Direction property whose value could somehow be either Approaching or Leaving. We ve just seen a technique we could use for that. We could create two constant fields of any type we like (int, for example), and a property of type int called Direction (see Example 3-20).

ll painting in Qt is performed through the QPainter class in one way or another. Widgets, pictures, delegates everything uses the same mechanism. There is actually one exception to the rule (to use OpenGL directly), but you ll start with the QPainter class.

class Plane { public const int Approaching = 0; public const int Leaving = 1; // ... } public int Direction { get; set; }

This lets us write code that reads a bit more naturally than it would if we had used just true and false:

someBoeing777.Direction = Plane.Approaching; if (someAirbusA380.Direction == Plane.Leaving) { /* Do something */ }