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

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;
}

Document Revision History

Date	Notes
2018-06-04	Moved to Retired Documents Library.
2005-02-23	The third parameter to "FSResolveAlias(NULL, tAliasHdl, &outFSRef, &wasChanged);" doesn't require the "&": "FSResolveAlias(NULL, tAliasHdl, outFSRef, &wasChanged);"
	New document that describes storing file references in CFPreferences