Technical Q&A QA1350

Storing file references in CFPreferences

Q: What is the most robust way to store a file reference into a CFPreference?

A: Convert your file reference into an Alias handle; create a CFData with the contents of that Alias handle and then use CFPreferencesSetAppValue and CFPreferencesAppSynchronize to store it into your preferences file.

The first listing shows how to save a File Reference to a Preference file, and the second how to reload the File Reference from the Preference File.

Listing 1: Saving a file reference into a CFPreference:

/*****************************************************
*
* SaveFSRefToPref(inFSRef, inKey)
*
* Purpose:  Saves a file reference in an application preference
*
* Inputs:   inFSRef     - The file reference
*           inKey       - the preference key
*
* Returns:  none
*/
void SaveFSRefToPref(const FSRef* inFSRef,
                     const CFStringRef inKey)
{
    AliasHandle tAliasHdl;
    OSErr err = FSNewAlias(NULL, inFSRef, &tAliasHdl);
    require_noerr(err, CantCreateAlias);

    CFDataRef tCFDataRef;
    tCFDataRef = CFDataCreate(kCFAllocatorDefault,
                              (UInt8*) *tAliasHdl,
                              GetHandleSize((Handle) tAliasHdl));
    require(NULL != tCFDataRef, CantCreateCFData);

    // set the preferences
    CFPreferencesSetAppValue( inKey,
                              tCFDataRef,
                              kCFPreferencesCurrentApplication);
    // sync to disk
    CFPreferencesAppSynchronize(kCFPreferencesCurrentApplication);

    CFRelease(tCFDataRef);
CantCreateCFData:
        DisposeHandle((Handle) tAliasHdl);
CantCreateAlias:
        return;
}

To restore your file reference from the preference:

Listing 2: Restoring the file reference:

/*****************************************************
*
* LoadFSRefFromPref(inFSRef, inKey)
*
* Purpose:  Loads a file reference from an application preference
*
* Inputs:   outFSRef    - Address where to store the file reference
*           inKey       - the preference key
*
* Returns:  Boolean     - true if the load was successful
*/
Boolean LoadFSRefFromPref(FSRef* outFSRef, CFStringRef inKey)
{
    Boolean result = false; // assume failure (pessimist!)

    CFDataRef tCFDataRef;
    tCFDataRef = (CFDataRef) CFPreferencesCopyAppValue(
                       inKey,
                       kCFPreferencesCurrentApplication);
    require(NULL != tCFDataRef, CantGetPreference);

    CFIndex dataSize = CFDataGetLength(tCFDataRef);
    AliasHandle tAliasHdl = (AliasHandle) NewHandle(dataSize);
    require(NULL != tAliasHdl, CantAllocateAlias);

    CFDataGetBytes(tCFDataRef,
                   CFRangeMake(0, dataSize),
                   (UInt8*) *tAliasHdl);

    FSRef tFSRef;
    Boolean wasChanged;
    OSErr err = FSResolveAlias(NULL,
                                tAliasHdl,
                                outFSRef,
                                &wasChanged);
    if (noErr == err) result = true;

    DisposeHandle((Handle) tAliasHdl);

CantAllocateAlias:
    CFRelease(tCFDataRef);
CantGetPreference:
    return result;
}

Posted: 2005-02-23

Document Revision History

Posted: 2005-02-23