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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 . You register your preview-drawing function by passing a Universal Procedure Pointer (UPP) in the
previewProc
parameter of a Navigation Services function such as . 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 .
Your preview-drawing function obtains information from a structure of type specified in the
callBackParms
parameter of your event-handling function. The structure contains the following information:
eventData
field contains a structure of type 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
, the object is not a file and you should not attempt to create a preview.
previewRect
field describes the preview area, which is the section of the dialog box reserved for preview drawing.
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
.