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Creating methods
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Name | Description
Abstract Methods | Sometimes a method is declared as abstract in a Delphi component. In the component library, abstract methods usually occur in classes whose names begin with "custom," such as TCustomGrid. Such classes are themselves abstract, in the sense that they are intended only for deriving descendant classes. While you can create an instance object of a class that contains an abstract member, it is not recommended. Calling the abstract member leads to an EAbstractError exception. The abstract directive is used to indicate parts of classes that should be surfaced and defined in descendant components; it forces component writers to redeclare... more
Avoiding Interdependencies | At all times when writing components, minimize the preconditions imposed on the developer. To the greatest extent possible, developers should be able to do anything they want to a component, whenever they want to do it. There will be times when you cannot accommodate that, but your goal should be to come as close as possible. This list gives you an idea of the kinds of dependencies to avoid: Methods that the user must call to use the component Methods that must execute in a particular order Methods that put the component into a state or mode where certain events... more
Creating Methods: Overview | Component methods are procedures and functions built into the structure of a class. Although there are essentially no restrictions on what you can do with the methods of a component, Delphi does use some standards you should follow. These guidelines include: Avoiding dependencies Naming methods Protecting methods Making methods virtual Declaring methods In general, components should not contain many methods and you should minimize the number of methods that an application needs to call. The features you might be inclined to implement as methods are often better encapsulated into properties. Properties provide an interface that suits the Delphi and are... more
Declaring Methods | Declaring a method in a component is the same as declaring any class method. To declare a new method in a component, do the following: Add the declaration to the component's object-type declaration. Implement the method in the implementation part of the component's unit.
Example of Declaring Methods | The following code shows a component that defines two new methods, one protected method and one public virtual method.
Methods That Should Be Protected | Any implementation methods for the component should be protected so that applications cannot call them at the wrong time. If you have methods that application code should not call, but that are called in derived classes, declare them as protected. For example, suppose you have a method that relies on having certain data set up for it beforehand. If you make that method public, there is a chance that applications will call it before setting up the data. On the other hand, by making it protected, you ensure that applications cannot call it directly. You can then... more
Methods That Should Be Public | Any method that application developers need to call must be declared as public. Keep in mind that most method calls occur in event handlers, so methods should avoid tying up system resources or putting the operating system in a state where it cannot respond to the user. Note: Constructors and destructors should always be public.
Naming Methods | Delphi imposes no restrictions on what you name methods or their parameters. There are a few conventions that make methods easier for application developers, however. Keep in mind that the nature of a component architecture dictates that many different kinds of people can use your components. If you are accustomed to writing code that only you or a small group of programmers use, you might not think too much about how you name things. It is a good idea to make your method names clear because people unfamiliar with your code (and even unfamiliar with coding) might have to use... more
Protecting Methods | All parts of classes, including fields, methods, and properties, have a level of protection or "visibility," as explained in Controlling access. Choosing the appropriate visibility for a method is simple. Most methods you write in your components are public or protected. You rarely need to make a method private, unless it is truly specific to that type of component, to the point that even derived components should not have access to it.
Making Methods Virtual | You make methods virtual when you want different types to be able to execute different code in response to the same method call. If you create components intended to be used directly by application developers, you can probably make all your methods nonvirtual. On the other hand, if you create abstract components from which other components will be derived, consider making the added methods virtual. This way, derived components can override the inherited virtual methods.

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