Things to remember: opening an SSL socket using CFStream

This was a pain in the neck, but I finally figured out the answer.

So I was attempting to open a connection to a server from iOS which may have a self-signed certificate installed.

The specific steps I used to open the connection (from my test harness) was:

1. Set up the variables

	NSString *host = @"127.0.0.1";
	NSInteger port = 12345;
	CFReadStreamRef readStream;
	CFWriteStreamRef writeStream;

2. Set debugging diagnostics. Noted here so I can remember this trick.

	setenv("CFNETWORK_DIAGNOSTICS","3",1);

3. Open the sockets.

	CFStreamCreatePairWithSocketToHost(kCFAllocatorDefault, (__bridge CFStringRef)host, (uint32_t)port, &readStream, &writeStream);

	NSInputStream *inStream = (__bridge_transfer NSInputStream *)readStream;
	NSOutputStream *outStream = (__bridge_transfer NSOutputStream *)writeStream;

4. Set up SSL. The part that tripped me up: you must set the properties for kCFStreamPropertySSLSettings after setting NSStreamSocketSecurityLevelKey. It appears the NSStreamSocketSecurityLevelKey setting overwrites the kCFStreampropertySSLSettings parameter.

	// Note: inStream and outStream are linked by an underlying object, so
	// parameters only need to be set on one of the two streams.

	NSDictionary *d = @{ (NSString *)kCFStreamSSLValidatesCertificateChain: (id)kCFBooleanFalse };
	[inStream setProperty:NSStreamSocketSecurityLevelNegotiatedSSL forKey:NSStreamSocketSecurityLevelKey];
	[inStream setProperty:d forKey:(id)kCFStreamPropertySSLSettings];

5. Open and initalize as needed. Here I’m just opening (because I don’t care if this blocks; this is test code). If using blocking API, use threads. Otherwise use the runloop and delegate APIs.

	[inStream open];
	[outStream open];

On the Java (server) side, the way I set up my server socket for listening to incoming connections was:

1. Set up the variables. Note that my Config class is an internal class that reads properties, and is beyond the scope of this exercise.

	Properties p = Config.get();
	String keystore = p.getProperty("keystorefile");
	String password = p.getProperty("keystorepassword");

	int port = 12345;

2. Load the keystore and key manager. This can be a signed or (in my case) self-signed certificate.

	FileInputStream keyFile = new FileInputStream(keystore); 
	KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
	keyStore.load(keyFile, password.toCharArray());

	KeyManagerFactory keyManagerFactory = KeyManagerFactory.getInstance(KeyManagerFactory.getDefaultAlgorithm());
	keyManagerFactory.init(keyStore, password.toCharArray());

	KeyManager keyManagers[] = keyManagerFactory.getKeyManagers();

3. Create an SSLContext. Note that you cannot use “Default” for getInstance below, because that returns an already initialized context, and we want to initialize it with our parameters above. Also note that iOS 9 prefers TLS 1.2.

	SSLContext sslContext = SSLContext.getInstance("TLSv1.2");
	sslContext.init(keyManagers, null, new SecureRandom());

4. Open a ServerSocket class to listen for incoming connections. Note the constant 50 below is arbitrary.

	SSLServerSocketFactory socketFactory = sslContext.getServerSocketFactory();

	socket = socketFactory.createServerSocket(port, 50);

Note that the way I loaded the keystore is sort of the “hard way” to do this; my eventual goal is to have the Java startup code generate a self-signed certificate internally if a keystore is not provided, but I haven’t figured out how to do that yet. (There are plenty of pages out there that show how, but most of them rely on internal Java APIs, and I’m sort of allergic to using undocumented stuff.)

Creating a custom Key in Objective C

When creating a custom key in Objective C for NSDictionary or NSCache or the like, you need to create an object which does the following:

Implements <NSCopying> protocol.

If your key is invariant, you can implement the method copyWithZone: as follows:

- (id)copyWithZone:(NSZone *)zone
{
    return self;
}

Of course if your key is invariant, ideally you would create the key entirely using a custom init function, and mark all the properties (readonly).

Implement the isEqual: method.

This is part of the NSObject protocol. Note that any class (or nil) could be passed in as the argument to isEqual: method, so you may want to use the method isKindOfClass: to verify that you got what you expected as the parameter.

Implement the hash method.

This is also part of the NSObject protocol, and returns a NSUInteger value which provides a hash function of the data you passed in.

The hash function doesn’t need to be complicated. For example, if your key is three integers, your hash function could be as simple as:

- (NSUInteger)hash
{
    return (self.a << 8) ^ (self.b << 4) ^ self.c;
}

What is important is that two keys passed into your system are unlikely to have a similar value.

Also note that many of the classes that you see used routinely as keys (such as NSString or NSNumber) also follow this protocol. Meaning if your custom key has a string in it, you can use the NSString’s hash function as one of the inputs to your own hashing function:

- (NSUInteger)hash
{
    return (self.intVal << 16) ^ [self.stringVal hash];
}

Objective C subscripting operators

I keep forgetting where this link is, so I’m putting it here: CLang Reference: Objective C, Object Subscripting.

This documents the methods we need to define if we are declaring our own Objective C class and we want to implement array-style subscripting or dictionary-style subscripting.

Extracting from the text:

… Moreover, because the method names are selected by the type of the subscript, an object can be subscripted using both array and dictionary styles.

Meaning if the compiler detects the subscript is an integral type, it uses the array-style subscript method calls, and when the subscript is an Objective C pointer type, it uses a dictionary-style subscript method call.

For array subscripting, use either or both of the methods

- (id<NSObject>)objectAtIndexedSubscript:(NSInteger)index;
- (void)setObject:(id<NSObject>)value atIndexedSubscript:(NSInteger)index;

For dictionary-style subscripting, use either or both of the methods

- (id<NSObject>)objectForKeyedSubscript:(id<NSObject>)key;
- (void)setObject:(id<NSObject>)value forKeyedSubscript:(id<NSObject>)key;

This goes hand-in-hand with my earlier post Objective C declaration shortcuts, and has been a public service announcement.

First pass at a more formal language for JSON.

So the single most common thing I run into, which is a source of all sorts of headaches when writing custom software for clients, is hooking into their back end system.

A very common pattern for me is to create a single interface which can perform a HTTP ‘get’ or ‘post’ call in order to obtain the contents, run everything through a JSON parser, and then handing the resulting NSDictionary or NSArray to an object which converts the results into a set of Objective C classes.

Up until now I’ve been using JSON Accelerator, which is a really nice little tool for converting JSON into a set of classes. But this runs into a couple of problems.

(1) A number of sites I integrate with have multiple JSON endpoints, each which return subtly different JSON results. Using JSON Accelerator and I wind up generating a lot of duplicate classes which represent more or less the same thing.

(2) Often those sites will change; after all, the back end is under development as well as the front end. I often have a hard time seeing the structure from the JSON; sometimes buried in a few hundred lines is a field that contains a null pointer or which was changed from a string to a JSON field–and tracking those bugs down can be a pain in the ass.

It seemed to me the best way to handle this is to have an intermediate representational language which allows me to see what it is that I’m working with, and to allow allow me to ‘tweak’ the results, so I can point out that the ‘Person’ record in call A is the exact same thing as the ‘Person’ record in call B, except for one of the fields being omitted.

So I built a simple analysis app and a simple compiler app to resolve this problem.

You can download the compiled tools and read the documentation (such as it is) from here.

The representational language is fairly simple: a set of objects, which can be compiled into Objective C and (when I have time) into Java. Each field in an object can be a primitive, an object or an array of objects. So, for example:

/*  Feed 
 *
 *      Top level of the feed
 */

Feed {
    id: integer,
    name: string,
    date: string,
    active: boolean,
    addressList: arrayof Address,
    phoneList: arrayof Phone,
}

/*  Address
 *
 *      The user's address
 */

Address {
    id: integer,
    name: string,
    address: string,
    address2: (optional) string, // optional in the data stream
    city: string,
    state: string,
    zip: string
}

/*  Phone
 *
 *      The user's phone
 */

Phone {
    id: integer,
    name: string,
    phone: string
}

Note that fields can also be marked as ‘nullable’:

Feed {
    id: integer,
    name: string,
    value: (nullable) real
}

This will translate into an NSNumber * field rather than into a double.

There are also a couple of tools: one that generates the Objective C code, and one which reads in a bunch of JSON (in fact, it will read multiple JSON objects all in a row), and makes a best guess at the underlying structure, collapsing common objects as needed, and even noting when the same field appears to contain ambiguous content.


At some point I will need to clean this up, add Java support, and push this out to GIT. But for now, there you go.

Let me know if this seems useful.

Moving views around when the keyboard shows in iOS

When the keyboard shows or hides in iOS, we receive an event to notify us that the keyboard is being shown and being hidden.

Ideally we want to get the animation parameters and the size of that keyboard so we can rearrange the views inside of our application to fit the keyboard. I’m only covering the case of the keyboard on the iPhone; on the iPad you also have the problem of the split keyboard, but the same ideas should hold there as well.

Step 1:

When the view controller that may show a keyboard appears, register for notifications for the keyboard being shown and hidden:

- (void)viewDidLoad
{
    [super viewDidLoad];

	// Do any additional setup after loading the view.
	[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardShowHide:) name:UIKeyboardWillShowNotification object:nil];
	[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardShowHide:) name:UIKeyboardWillHideNotification object:nil];

	// other stuff here
}

Step 2:

Remember to unregister notifications when this goes away, to prevent problems.

- (void)dealloc
{
	[[NSNotificationCenter defaultCenter] removeObserver:self];
}

Step 3:

Receive the event in our new method, and extract the keyboard parameters and animation parameters.

- (void)keyboardShowHide:(NSNotification *)n
{
	CGRect krect;

	/* Extract the size of the keyboard when the animation stops */
	krect = [n.userInfo[UIKeyboardFrameEndUserInfoKey] CGRectValue];

	/* Convert that to the rectangle in our primary view. Note the raw
	 * keyboard size from above is in the window's frame, which could be
	 * turned on its side.
	 */
	krect = [self.view convertRect:krect fromView:nil];

	/* Get the animation duration, and animation curve */
	NSTimeInterval duration = [[n.userInfo objectForKey:UIKeyboardAnimationDurationUserInfoKey] doubleValue];
	UIViewAnimationCurve curve = [[n.userInfo objectForKey:UIKeyboardAnimationCurveUserInfoKey] intValue];

	/* Kick off the animation. What you do with the keyboard size is up to you */
	[UIView animateWithDuration:0 delay:duration options:UIViewAnimationOptionBeginFromCurrentState | curve animations:^{
			/* Set up the destination rectangle sizes given the keyboard size */
			Do something interesting here
		} completion:^(BOOL finished) {
			/* Finish up here */
			Do something interesting here
		}];
}

Wrapping text with ellipsis on iOS

This is an extremely common pattern I encounter quite a few times, and worthy of it’s own “things to remember” post.

So you want to have some text which wraps, but when it hits the bottom of the window, you get an ellipsis, right? But [NSString drawInRect…] doesn’t do the trick, right?

Well, it turns out the answer is using an NSAttributedString.

The first step is to turn your label string into an attributed string. At this point you need to know the font and color of the text that will be rendered, so you need to bake these attributes into the string. So, for example:

- (NSAttributedString *)attributedStringWithString:(NSString *)str
{
	UIFont *font = [UIFont boldSystemFontWithSize:19]; /* Some font */
	UIColor *color = [UIColor blackColor]; /* Some color */
	NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:
		font, NSFontAttributeName,
		color, NSForegroundColorAttributeName,
		nil];
	return [[NSAttributedString alloc] initWithString:str attributes:d];
}

The next step is to draw the string in the rectangle area where it belongs:

	NSAttributedString *astr = [self attributedStringWithString:self.myLabel];
	[astr drawWithRect:r
		options:NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingTruncatesLastVisibleLine
		context:nil];

And this will wrap the text, truncating the last line with an ellipsis.


The advantage of using NSAttributedString is that you can also easily insert segments of text which are of a different color or style. I find it easiest to build up the attributed string through concatenation of attributed segments into an NSMutableAttributedString object. So, for example, if you’re displaying a tweet from Twitter and you wish to bold all the hashtags, you simply parse the string (scanning for sequences staring with whitespace and a ‘#’, and including all the alphanumeric characters past the ‘#’), and create an attributed string of that segment as bold, concatenating all the strings into a single mutable string.

And for optimization purposes you can then hang onto the attributed string rather than the original string.