One of the more difficult design patterns I’ve managed to get working in Objective-C while developing an iOS app is one that requires NSStreamDelegates, Grand Central Dispatch (GCD) and NSNotificationCenter. \u00a0This design pattern comes into play in the following situation:<\/p>\n
Its a pretty common scenario and I’m sure one that has many answers. \u00a0In my case, I needed to use CFReadStreams and CFWriteStreams due to the way the client server communication works. \u00a0So if there was a simpler way to do it, I’d love to know.<\/p>\n
The design pattern that I used at a high level to accomplish this is as follows:<\/p>\n
<\/p>\n
That’s the high level. \u00a0So now lets go through and flesh out the details.<\/p>\n
This is a simple example, but its just to get you the basic idea. \u00a0This part is standard in most iOS apps. \u00a0The thing to note about this is that my FirstViewController is not an NSStreamDelegate. \u00a0It does however contain an object that is an NSStreamDelegate. \u00a0This is called my Communicator class.<\/p>\n
...\r\n#import \"Communicator.h\"\r\n@interface FirstViewController : UIViewController {\r\n UILabel *info\r\n Communicator *myComm;\r\n UIActivityIndicatorView *spinner;\r\n}\r\n...\r\n\/\/ @property stuff here...<\/pre>\nFirst View Controller viewDidLoad<\/h2>\n
Here’s where we start to get cool. \u00a0viewDidLoad looks something like this:<\/p>\n
- (void)viewDidLoad\r\n{\r\n \/\/ Let user know that something is happening.. spin the spinner..\r\n [spinner startAnimating];\r\n \/\/ register for notification of updates.\r\n [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateView) name:@\"updateView\" object:nil];\r\n \/\/ create a thread and get the server updates.\r\n dispatch_async(dispatch_get_global_queue(0, 0), ^{\r\n myConnectionInfo = [[Connection alloc] initWithStuff:@\"Stuff\"\r\n myComm = [[Communicator alloc] initWithConnection:myConnection];\r\n [myComm communicate:@\"Send this message\"]\r\n });\r\n [super viewDidLoad];\r\n}<\/pre>\nA few things to note here:<\/p>\n
1. \u00a0We add an observerer to NSNotificationCenter. \u00a0This basically says: \u00a0If someone sends the updateView message, then I’ll respond to it with my updateView method. \u00a0(We’ll show this later)<\/p>\n
2. \u00a0We dispatch an\u00a0asynchronous\u00a0task to GCD. \u00a0That’s what this whole dispatch_async business is all about. \u00a0This way, the view controller will just display while this task runs in the background.<\/p>\n
3. \u00a0We instantiate myComm and initiate it with some connection info. \u00a0This info might be something you got from the app delegate, some other view controller, or a plist. \u00a0That part is left to you to decide.<\/p>\n
NSStreamDelegate actions<\/h2>\n
Our class Communicator is a NSStreamDelegate. \u00a0Here’s some of the relevant header info:<\/p>\n
...\r\n@interface Communicator : NSObject <NSStreamDelegate> {\r\n NSInputStream *inputStream;\r\n NSOutputStream *outputStream;\r\n NSString *theResult;\r\n NSString *cmd;\r\n Connection *myConn;\r\n}\r\n\/\/... @property stuff here... and functions<\/pre>\nNothing big here, just the NSInputStream (the stream we get info from the server) and the NSOutputStream (The stream we write to the server)<\/p>\n
As part of the initialization of the Communicator, we have a function called startConnection. This can be called in the init methods. (provided you have all the server communication info: (e.g: user, password, server, port, etc).<\/p>\n
Our’s has this:<\/p>\n
self.inputStream = (NSInputStream *)readStream;\r\n self.outputStream = (NSOutputStream *)writeStream;\r\n [inputStream setDelegate:self];\r\n [outputStream setDelegate:self];\r\n\r\n \/\/ schedule on the main run loop\r\n dispatch_async(dispatch_get_main_queue(), ^ {\r\n\r\n [inputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];\r\n [outputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];\r\n\r\n [inputStream open];\r\n [outputStream open];<\/pre>\n });<\/pre>\n