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Writing and Installing Coercion Handlers

Coercion is the process of converting a descriptor and the data it contains from one type to another. When coercing between types, by definition the descriptor type is changed to the new type. However, if the underlying data representation is the same, data conversion is not required.

Functions that perform coercions are referred to as coercion handlers. The Mac OS provides default coercion handlers to convert between many different descriptor types. Default handlers can, for example, convert aliases to file system specifications, integers to Boolean data types, and characters to numeric data types. These handlers may be implemented by the Apple Event Manager, the Open Scripting framework, or other frameworks. Table C-2 lists descriptor types and the available default coercions.

You can also provide your own coercion handlers to perform coercions that the default handlers don’t support. This chapter describes how to write coercion handlers and how to install them so that they are available to your application.

For many of the Apple Event Manager functions that your application uses to extract data from an Apple event, you can specify a desired descriptor type for the returned data. If the original data is of a different type, the Apple Event Manager attempts to coerce the data to the requested descriptor type. For more information on functions that let you specify a desired descriptor type, see “Coercing Data From an Apple Event.”

Contents:

How Coercion Handlers are Installed and Dispatched
Writing a Coercion Handler
Installing a Coercion Handler
Testing a Coercion Handler

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