Checking the Network and Creating a Connection in Objective-C

Creator ECC200 in Objective-C Checking the Network and Creating a Connection

- (void)loadPredictionXML:(id<BARTPredictionLoaderDelegate>)delegate { _delegate = delegate; // Load the predictions XML from BART's web site // Make sure that bart.gov is reachable using the current connection SCNetworkReachabilityFlags flags; SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithName(NULL, [@"www.bart.gov" UTF8String]); SCNetworkReachabilityGetFlags(reachability, &flags); // The reachability flags are a bitwise set of flags // that contain the information about // connection availability BOOL reachable = ! (flags & kSCNetworkReachabilityFlagsConnectionRequired); NSURLConnection *conn; NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"http://www.bart.gov/dev/eta/bart_eta.xml"]]; if ([NSURLConnection canHandleRequest:request] && reachable) { conn = [NSURLConnection connectionWithRequest:request delegate:self]; if (conn) { self.predictionXMLData = [NSMutableData data]; } } }

5. NSURLConnection s connectionWithRequest method also takes a delegate argument. In this case, we ll set the delegate to self, so that we can implement the connection s delegate methods right here in the BARTPredictionLoader class. NSURLConnection has several delegate methods, three of which we ll implement: didReceiveResponse, didReceiveData, and connectionDidFinishLoading. The comments in Listing 7-6 explain how each of the delegate methods works, while Figure 7-19 shows the order in which these delegate methods are called.

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse*)response { // didReceiveResponse is called at the beginning of the request when // the connection is ready to receive data. We set the length to zero // to prepare the array to receive data [self.predictionXMLData setLength:0]; } - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data { // Each time we receive a chunk of data, we'll appeend it to the // data array. [self.predictionXMLData appendData:data]; } - (void)connectionDidFinishLoading:(NSURLConnection *)connection { // When the data has all finished loading, we set a copy of the // loaded data for us to access. This will allow us to not worry about // whether a load is already in progress when accessing the data. self.lastLoadedPredictionXMLData = [self.predictionXMLData copy]; // Make sure the _delegate object actually has the xmlDidFinishLoading // method, and if it does, call it to notify the delegate that the // data has finished loading. if ([_delegate respondsToSelector:@selector(xmlDidFinishLoading)]) { [_delegate xmlDidFinishLoading]; } }

Finally, you ll need to set up a singleton instance of BARTPredictionLoader so that you can easily call it from anywhere in your application s code (see Listing 7-7). The @synchronized block around the initialization of the prediction loader ensures that the instance you create will be thread-safe. Refer to 3 for more detailed information on how threading works. With this class method in place, you ll be able to access the shared instance of the prediction loader from anywhere by simply calling [BARTPredictionLoader sharedBARTPredictionLoader]. For a full explanation of how to properly implement singletons, see Apple s Cocoa Fundamentals Guide:

http://developer.apple.com/documentation/Cocoa/Conceptual/ CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid /TP40002974-CH4-SW32

static BARTPredictionLoader *predictionLoader; + (BARTPredictionLoader*)sharedBARTPredictionLoader { @synchronized(self) { if (predictionLoader == nil) { [[self alloc] init]; } } return predictionLoader; }

Now that we have the ability to load our data on demand, we need to figure out where to use it. Let s look again at the BART feed at http://www.bart.gov/dev/eta/bart_eta.xml. You ll notice that the feed contains predictions for all the stations in one very small, quickto-load XML file. Normally, we would only load predictions for the station that a user selects, but this file contains everything, so it makes the most sense to begin loading the data when our list of stations first loads, so we can be ready with predictions when the user chooses a station. 7. Let s revisit the viewDidLoad code in RootViewController.m. First, we need to keep the user from selecting anything in the table until the predictions are done loading. Then, we ll begin loading the XML. Add #import "BARTPredictionLoader.h" to the imports in RootViewController.m, and then add the following code to the end of viewDidLoad:

self.tableView.userInteractionEnabled = NO; [[BARTPredictionLoader sharedBARTPredictionLoader] loadPredictionXML:self];