Removing Dependencies

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One quality that makes components usable is the absence of restrictions on what they can do at any point in their code. By their nature, components are incorporated into applications in varying combinations, orders, and contexts. You should design components that function in any situation, without preconditions.

An example of removing dependencies is the Handle property of TWinControl. If you have written Windows applications before, you know that one of the most difficult and error-prone aspects of getting a program running is making sure that you do not try to access a windowed control until you have created it by calling the CreateWindow API function. Delphi windowed controls relieve users from this concern by ensuring that a valid window handle is always available when needed. By using a property to represent the window handle, the control can check whether the window has been created; if the handle is not valid, the control creates a window and returns the handle. Thus, whenever an application's code accesses the Handle property, it is assured of getting a valid handle.

By removing background tasks like creating the window, Delphi components allow developers to focus on what they really want to do. Before passing a window handle to an API function, you do not need to verify that the handle exists or to create the window. The application developer can assume that things will work, instead of constantly checking for things that might go wrong.

Although it can take time to create components that are free of dependencies, it is generally time well spent. It not only spares application developers from repetition and drudgery, but it reduces your documentation and support burdens.