Drawing Custom Previews

By default, Navigation Services draws a preview in the Open dialog box when a file selected in the browser list contains a valid 'pnot' component. To override how previews are drawn and handled, you can create a preview-drawing function, as described in MyPreviewProc . You register your preview-drawing function by passing a Universal Procedure Pointer (UPP) in the previewProc parameter of a Navigation Services function such as NavGetFile . You obtain this UPP by calling the macro NewNavPreviewProc and passing a pointer to your preview-drawing function. When the user selects a file, Navigation Services calls your preview-drawing function. Before you attempt to create a custom preview, your application should determine whether previews are enabled by specifying the kNavCtlIsPreviewShowing constant in the NavCustomControlMessages parameter of the function NavCustomControl .

Your preview-drawing function obtains information from a structure of type NavCBRec specified in the callBackParms parameter of your event-handling function. The NavCBRec structure contains the following information:

The eventData field contains a structure of type NavEventData describing the file to be previewed. This structure provides an Apple event descriptor list (AEDescList ) that you must be able to coerce into a file specification (FSSpec) . If you cannot coerce the AEDescList into a valid FSSpec , then the object is not a file and you should not attempt to create a preview.
The previewRect field describes the preview area , which is the section of the dialog box reserved for preview drawing.
The window field identifies the window to draw in.

Once you have determined that previews are enabled, your preview-drawing function should draw the custom preview in the specified area and return a function result of true . If you don't want to draw a preview for a given file, be sure to return a function result of false .

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