Making Network Calls in iOS

A very common task of any developer is to write a web services class. In this post I will present a very common pattern for organizing and incorporating network calls into iOS apps.

To give a quick overview, we will be creating a singleton web services class. This basically means we will make a new class that stores all the methods for making network requests, and that the same allocation of this class in memory will be accessible across all views in the app. If you want to learn more about this design pattern, look at my other post.

So, lets create this class. In Xcode, create a new objective c file that subclasses from NSObject.

The header file will look like this:

// WebServices.h
// DC_magazine
//
// Created by Andrew Rauh on 6/4/12.
// Copyright (c) 2012 __MyCompanyName__. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface WebServices : NSObject

@property (strong, nonatomic) NSMutableArray *blogPosts;
@property (strong, nonatomic) NSMutableArray *titles;
@property (strong, nonatomic) NSMutableDictionary *selectedPost;

- (void) grabDataAndParse;
- (void) buildTitleArray;
+(id) sharedInstance;

And here is the implementation file. (.m)

</pre>
//
// WebServices.m
// DC_magazine
//
// Created by Andrew Rauh on 6/4/12.
// Copyright (c) 2012 __MyCompanyName__. All rights reserved.
//

#import "WebServices.h"

@implementation WebServices
@synthesize blogPosts, titles,selectedPost, ;

+(id)sharedInstance
{
 static id sharedInstance = nil;
 if (sharedInstance == nil) {
 sharedInstance = [[self alloc] init];
 }
 return sharedInstance;
}

-(void)grabDataAndParse
{
 selectedPost = [[NSMutableDictionary alloc]init];
 NSURLRequest *main = [NSURLRequest requestWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://dcfaithinaction.org/?json=get_recent_posts"]]];

 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
 dispatch_async(queue, ^{
 NSURLResponse *response = nil;
 NSError *error = nil;

 NSData *data = [NSURLConnection sendSynchronousRequest:main
 returningResponse:&response
 error:&error];
 NSString *json = [[NSString alloc] initWithData:data encoding:NSASCIIStringEncoding];
 id parsedObject = nil;
 NSError *parseError = nil;
 if (json != nil && [json isKindOfClass:[NSString class]] && [json length] > 0) {
 parsedObject = [NSJSONSerialization JSONObjectWithData:[json dataUsingEncoding:NSASCIIStringEncoding]
 options:0
 error:&parseError];
 }
 if ([parsedObject isKindOfClass:[NSDictionary class]]) {
 NSDictionary *parsedDictionary = (NSDictionary *)parsedObject;

 NSArray *posts = [parsedDictionary objectForKey:@"posts"];
 blogPosts =[NSMutableArray arrayWithArray:posts];
 [self buildTitleArray];
 }
 });
}
- (void)buildTitleArray
{
 titles = [[NSMutableArray alloc] init ];

 for (NSDictionary *dict in blogPosts) {
 [titles addObject:[dict objectForKey:@"title"]];
 //NSLog(@"%@", [dict objectForKey:@"title"]);
 }
}

@end
<pre>

In this case, I used NSURLConnection with an asynchronous block to make the actual network call. After you make the call, you get NSData back. You then treat this as a generic objective-c object, and see if it is either a dictionary or an array. Once you figure out what data type the object you got back from the server is, cast it to that type of object and start iterating over its elements, and extract the needed data.

The reason this network call must be made asynchronously is pretty simple. On iOS, there is a main thread, where all of the UI code is run. If we put this networking code on that same thread, then all of the UI on the devices would completely freeze until we got the needed data back from the server.