ASP.NET Web PDF Document Viewer/Editor Control Library

Until now you ve relied on classes available in non-F# libraries. In this and the following sections, you ll learn how to define classes of your own. First, you ll learn how to define fields and constructors so you can create instances. Then, in the following sections, you ll learn about inheritance and methods associated with classes. In F#, classes are essentially just special kinds of types, so you define them using the type keyword followed by an equals sign and then the keyword class. To terminate your class definition, you use the keyword end. The next example shows the simplest class definition possible, of a class with nothing in it, or an empty class. It then attempts to instantiate it. #light type Empty = class end let emptyItem = new Empty() This code will not compile because of the last line. You didn t provide a constructor for our class, so there s no way to create an instance of it. To enable you to create an instance of

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To move a component, simply click and hold anywhere in the colored title bar area, drag the component to the desired location, and then release the mouse button DOM-Drag allows users to place the components wherever they choose This enhancement, while seemingly small, gives Ajax Dashboard a feel more akin to a multiple document interface (MDI) client application than a Web page You may have also noticed the small plus and minus signs on the right side of the title bar for the weather forecast, stock ticker, and news stories components These plus and minus signs, respectively, maximize and minimize the content of the component Minimizing the component s content collapses the data so that only the title bar is visible, saving valuable screen real estate (see Figure 8-9).

The RCX Touch Sensor (see Figure 1-21) uses the same conductive rubber technology found in the buttons of calculators and remote controls (see Figure 1-22). The operator lacks a way to connect directly to it, and it also doesn t move back and forth very far, which makes constructing reliable bumpers difficult.

the class, you must explicitly add a constructor To do this, you need to add a member, which is always named new and is followed the constructor within parentheses After this comes an equals sign followed by a block (delimited by braces), which contains expressions to initialize every field in the class The following example defines a simple constructor for an empty class: #light type JustConstruct = class new() = {} end let constructed = new JustConstruct() This may come as a surprise to experienced object-oriented programmers since most object-oriented languages provide a default constructor for any class that doesn t define one F# s philosophy is to provide a more powerful construct called implicit class construction that often subsumes the need for explicit constructors altogether You ll return to implicit class construction in the next section.

Furthermore, default constructors can easily leave some fields uninitialized and therefore null and can leave some at risk of causing a NullReferenceException This is why a constructor in F# must initialize all fields defined by a class Fields are defined using the keyword val, followed the name of the field, and then the name of the type separated from the property name by a colon The next example shows a simple class, file1, that has two fields, path and innerFile, that are initialized in the constructor, which has one parameter, path: #light open SystemIO type File1 = class val path: string val innerFile: FileInfo new(path) = { path = path ; innerFile = new FileInfo(path) } end let myFile1 = new File1("whatevertxt") It s possible to overload constructors; one simply adds a second constructor with a different number of parameters.

This may not seem like an important feature here, since Ajax Dashboard has only four components, but imagine if the application was enhanced to include several more components A screen full of components may seem busy and hard to read, and minimizing the less frequently used components can simplify the application s appearance while still keeping the minimized components data a single mouse click away..

The RCX Light Sensor also contains an LED light source, but it s always turned on (see Figure 1-23).

If you want to overload with parameters of different types, then you must provide type annotations The following example shows a class, File2, with two constructors, one with no parameters and one with one parameter: #light open SystemIO type File2 = class val path: string val innerFile: FileInfo.