How to detect if a NSURL can be accessed on a Sandboxed app?

Question

I'm writing an OS X application that has an open panel with a preview accessory view.

The problem is that when I select a file that is not on a folder with sandbox permissions I cannot preview it.

Is there any way to tell if a file can would be restricted by the sandbox permissions before trying to open it?

I already tried - (BOOL)checkResourceIsReachableAndReturnError:(NSError **)error of the NSURL class but it returns true even when the file is protected.

Answer 1

The recommended method is to obtain the path from the URL, convert to a C string, and then call access (documented in section 2 of man) to check for the permission you require. E.g., in outline to check for read access:

#include <unistd.h>

if (access([[url path] UTF8String], R_OK) == 0)
{
   // have access rights to read
}

Answer 2

For those who like to have as less as possible Plain-Old-C Lines in the Code:

NSURLIsReadableKey  in NSURL - (BOOL)getResourceValue:(out id *)value forKey:(NSString *)key 

alternatively

NSFileManager - (BOOL)isReadableFileAtPath:(NSString *)path

The sandbox will cause these to return NO if it restricts your access and are merely Cocoa-Wrappers around access(). The sandbox will cause these to return NO if it restricts your access.