How to detect total available/free disk space on the iPhone/iPad device?

I have written a class to get available/used memory using Swift. Demo at: https://github.com/thanhcuong1990/swift-disk-status
Swift 4 updated.

import UIKit

class DiskStatus {

    //MARK: Formatter MB only
    class func MBFormatter(_ bytes: Int64) -> String {
        let formatter = ByteCountFormatter()
        formatter.allowedUnits = ByteCountFormatter.Units.useMB
        formatter.countStyle = ByteCountFormatter.CountStyle.decimal
        formatter.includesUnit = false
        return formatter.string(fromByteCount: bytes) as String
    }


    //MARK: Get String Value
    class var totalDiskSpace:String {
        get {
            return ByteCountFormatter.string(fromByteCount: totalDiskSpaceInBytes, countStyle: ByteCountFormatter.CountStyle.file)
        }
    }

    class var freeDiskSpace:String {
        get {
            return ByteCountFormatter.string(fromByteCount: freeDiskSpaceInBytes, countStyle: ByteCountFormatter.CountStyle.file)
        }
    }

    class var usedDiskSpace:String {
        get {
            return ByteCountFormatter.string(fromByteCount: usedDiskSpaceInBytes, countStyle: ByteCountFormatter.CountStyle.file)
        }
    }


    //MARK: Get raw value
    class var totalDiskSpaceInBytes:Int64 {
        get {
            do {
                let systemAttributes = try FileManager.default.attributesOfFileSystem(forPath: NSHomeDirectory() as String)
                let space = (systemAttributes[FileAttributeKey.systemSize] as? NSNumber)?.int64Value
                return space!
            } catch {
                return 0
            }
        }
    }

    class var freeDiskSpaceInBytes:Int64 {
        get {
            do {
                let systemAttributes = try FileManager.default.attributesOfFileSystem(forPath: NSHomeDirectory() as String)
                let freeSpace = (systemAttributes[FileAttributeKey.systemFreeSize] as? NSNumber)?.int64Value
                return freeSpace!
            } catch {
                return 0
            }
        }
    }

    class var usedDiskSpaceInBytes:Int64 {
        get {
            let usedSpace = totalDiskSpaceInBytes - freeDiskSpaceInBytes
            return usedSpace
        }
    }

}

Demo

Revised source using unsigned long long:

- (uint64_t)freeDiskspace
{
    uint64_t totalSpace = 0;
    uint64_t totalFreeSpace = 0;

    __autoreleasing NSError *error = nil;  
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);  
    NSDictionary *dictionary = [[NSFileManager defaultManager] attributesOfFileSystemForPath:[paths lastObject] error: &error];  

    if (dictionary) {  
        NSNumber *fileSystemSizeInBytes = [dictionary objectForKey: NSFileSystemSize];  
        NSNumber *freeFileSystemSizeInBytes = [dictionary objectForKey:NSFileSystemFreeSize];
        totalSpace = [fileSystemSizeInBytes unsignedLongLongValue];
        totalFreeSpace = [freeFileSystemSizeInBytes unsignedLongLongValue];
        NSLog(@"Memory Capacity of %llu MiB with %llu MiB Free memory available.", ((totalSpace/1024ll)/1024ll), ((totalFreeSpace/1024ll)/1024ll));
    } else {  
        NSLog(@"Error Obtaining System Memory Info: Domain = %@, Code = %d", [error domain], [error code]);  
    }  

    return totalFreeSpace;
}

EDIT: it seems someone edited this code to use 'uint64_t' instead of 'unsigned long long'. While in the foreseeable future this should be just fine, they are not the same. 'uint64_t' is 64 bits and will always be that. In 10 years 'unsigned long long' might be 128. its a small point but why I used unsignedLongLong.

UPDATE: Since a lot of time has passed after this answer and new methods/APIs have been added, please check the updated answers below for Swift etc; Since I've not used them myself, I can't vouch for them.

Original answer: I found the following solution working for me:

-(uint64_t)getFreeDiskspace {
    uint64_t totalSpace = 0;
    uint64_t totalFreeSpace = 0;
    NSError *error = nil;  
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);  
    NSDictionary *dictionary = [[NSFileManager defaultManager] attributesOfFileSystemForPath:[paths lastObject] error: &error];  

    if (dictionary) {  
        NSNumber *fileSystemSizeInBytes = [dictionary objectForKey: NSFileSystemSize];  
        NSNumber *freeFileSystemSizeInBytes = [dictionary objectForKey:NSFileSystemFreeSize];
        totalSpace = [fileSystemSizeInBytes unsignedLongLongValue];
        totalFreeSpace = [freeFileSystemSizeInBytes unsignedLongLongValue];
        NSLog(@"Memory Capacity of %llu MiB with %llu MiB Free memory available.", ((totalSpace/1024ll)/1024ll), ((totalFreeSpace/1024ll)/1024ll));
    } else {  
        NSLog(@"Error Obtaining System Memory Info: Domain = %@, Code = %ld", [error domain], (long)[error code]);
    }  

    return totalFreeSpace;
}

It returns me exactly the size that iTunes displays when device is connected to machine.