Converting a NSDictionary to a Binary PList

HowTo.txt

When I posted this on Apple’s Cocoadev listserver, I never did get pointers to code to convert a NSDictionary to a binary formatted plist, but did get pointers to look at NSPropertyListSerialization - something I had already (mis)read a few times.

The description for the dataFromPropertyList class method seems confusing if you don't understand that you can create a property list with no keys: that is, you can create one with a single compliant object, and the key becomes "root".

So, in the unlikely event that some else someday has the same mental block, here is actual tested code that takes a NSDictionary and creates a binary plist file:

   NSDictionary *dict;
    char *payload = "This is the payload";
    
    dict = [NSDictionary dictionaryWithObjectsAndKeys:
        @"Hello world", @"greeting",
        [NSData dataWithBytes:payload length:strlen(payload)], @"payload",
        [NSNumber numberWithInt:10], @"result",
        nil ];
    
    // get a text representation, for comparison
    [dict writeToFile:@"/tmp/text.plist" atomically:NO];
    
    
    NSString *err = nil;
    NSData *plist;
    NSFileManager *manager;
    BOOL ret;
    
    manager = [NSFileManager defaultManager];

    plist = [NSPropertyListSerialization dataFromPropertyList:dict
            format:NSPropertyListBinaryFormat_v1_0
            errorDescription:&err];
    if(plist == nil) {
        NSLog(@"NSPropertyListSerialization error: %@", err);
        return -1;
    }
    ret = [manager createFileAtPath:@"/tmp/binary.plist" contents:plist attributes:nil];
    if(ret == NO) {
        NSLog(@"Create failed");
        return -1;
    }

Just to prove to myself that yes, you can create a property list with other objects like NSNumbers or raw NSData, I added these two tests:

    plist = [NSPropertyListSerialization dataFromPropertyList:[NSNumber numberWithInt:1]
            format:NSPropertyListBinaryFormat_v1_0
            errorDescription:&err];
    if(plist == nil) {
        NSLog(@"NSPropertyListSerialization error: %@", err);
        return -1;
    }
    ret = [manager createFileAtPath:@"/tmp/tst1.plist" contents:plist attributes:nil];
    if(ret == NO) {
        NSLog(@"Create failed");
        return -1;
    }

    plist = [NSPropertyListSerialization dataFromPropertyList:[NSData dataWithBytes:payload length:strlen(payload)]
            format:NSPropertyListBinaryFormat_v1_0
            errorDescription:&err];
    if(plist == nil) {
        NSLog(@"NSPropertyListSerialization error: %@", err);
        return -1;
    }
    ret = [manager createFileAtPath:@"/tmp/tst2.plist" contents:plist attributes:nil];
    if(ret == NO) {
        NSLog(@"Create failed");
        return -1;
    }

Then, Ken Thomases  (on Cocoadev) provided a shorter version, by having the NSData instance write the file. Jean-Daniel Dupas suggested using CFPropertyListWriteToStream to bypass the intermediate NSData object (a speed and memory footprint improvement). Here is the above code wrapped in a short command line test program:

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSString *err = nil;
    NSData *plist;
    NSDictionary *dict;
    char *payload = "This is the payload";
    
    dict = [NSDictionary dictionaryWithObjectsAndKeys:
        @"Hello world", @"greeting",
        [NSData dataWithBytes:payload length:strlen(payload)], @"payload",
        [NSNumber numberWithInt:10], @"result",
        nil ];
    
// 1 - get a XML (text) file representation
    [dict writeToFile:@"/tmp/text.plist" atomically:NO];
    
    
    // get a binary file representation - creates an additional NSData in memory object
    plist = [NSPropertyListSerialization dataFromPropertyList:dict
            format:NSPropertyListBinaryFormat_v1_0
            errorDescription:&err];
    if(plist == nil) {
        NSLog(@"NSPropertyListSerialization error: %@", err);
        return -1;
    }
// 2 - have the NSData object save a binary representation
    [plist writeToFile:@"/tmp/binary1.plist" atomically:NO];


    // get a binary file representation - no additional memory footprint
    // NOTE: documentation does not state it, but NSOutputStream creates the file if it does not yet exist
    NSOutputStream *str = [NSOutputStream outputStreamToFileAtPath:@"/tmp/binary2.plist" append:NO];
    if(str == nil) {
        NSLog(@"cannot create output stream");
        return -1;
    }
    
// 3 - stream a binary representation
    [str open];
    CFIndex idx = CFPropertyListWriteToStream(dict, (CFWriteStreamRef)str, kCFPropertyListBinaryFormat_v1_0, (CFStringRef *)&err);
    if(idx == 0) {
        NSLog(@"CFPropertyListWriteToStream error: %x %@", err, err);
        return -1;
    }
    [str close];


    [pool drain];
    return 0;
}

Hope this is of use to someone else.

Originally posted in 2008 on my iWeb site (iWeb is going away soon).