How to use dispatch_sync correctly? - ios

Question

I am getting twitter time line data from a custom NSObject class and this class has all the coe for calling the API and parse the data.
I am calling this class from my view controller which has a table view and need to populate the tableview with the data coming from twitter. But due to some issues with the dispatch_sync my view controller calls the twitter class and control comes back to the view controller before the Array (which i am using to populate the tableview) is populated with the data.
Here's some code :
#import <UIKit/UIKit.h>
#import "SecondViewController.h"
#import "SpinnerController.h"
#import "proAlertView.h"
#import "SBJson.h"
#import <Accounts/Accounts.h>
#import <Twitter/Twitter.h>
#interface TwitterDataLoad : NSObject<UIAlertViewDelegate>
{
NSMutableData * receivedData;
NSArray * results;
NSArray * cellContent;
NSMutableArray * totalContent;
SpinnerController * spinner;
proAlertView * av;
NSString *json_string;
NSDictionary * jsonResults;
SBJsonParser * parser;
NSMutableArray * dataForTable;
}
#property(nonatomic, strong) ACAccount * account;
#property(nonatomic, strong) NSArray * accounts;
#property(nonatomic, strong) ACAccountStore * accountStore;
#property (nonatomic ,retain) SecondViewController * tbView;
- (void)loadData;
#end
#import "TwitterDataLoad.h"
#interface TwitterDataLoad ()
#end
#implementation TwitterDataLoad
#synthesize tbView;
-(id) init {
self = [super init];
if (self) {
[self loadData];
}
return self;
}
- (void)loadData
{
dataForTable = [[NSMutableArray alloc]init];
//Twitter new code
if (_accounts == nil) {
if (_accountStore == nil) {
_accountStore = [[ACAccountStore alloc] init];
}
ACAccountType *accountTypeTwitter =
[_accountStore
accountTypeWithAccountTypeIdentifier:ACAccountTypeIdentifierTwitter];
[_accountStore requestAccessToAccountsWithType:accountTypeTwitter
withCompletionHandler:^(BOOL granted, NSError *error) {
if(granted) {
dispatch_sync(dispatch_get_main_queue(), ^{
_accounts = [_accountStore
accountsWithAccountType:accountTypeTwitter];
[self sendRequest];
});
}
}];
}
}
-(void) sendRequest {
totalContent = [[NSMutableArray alloc]init];
_account = [_accounts objectAtIndex:0];
TWRequest *postRequest = [[TWRequest alloc]
initWithURL:
[NSURL URLWithString:#"https://api.twitter.com/1/statuses/user_timeline.json?screen_name=test&count=20"]
parameters:nil
requestMethod:TWRequestMethodGET];
av = [[proAlertView alloc]initWithTitle:nil message:#"Getting latest news..." delegate:self cancelButtonTitle:nil otherButtonTitles:nil, nil];
UIActivityIndicatorView * indicator = [[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
indicator.frame = CGRectMake(120, 55, 35, 35);
[av addSubview:indicator];
[indicator startAnimating];
[av setBackgroundColor:[UIColor clearColor] withStrokeColor:[UIColor blackColor]];
[av show];
[postRequest setAccount:_account];
[postRequest performRequestWithHandler:^(NSData *responseData,
NSHTTPURLResponse *urlResponse,
NSError *error) {
if ([urlResponse statusCode] == 200) {
NSError *jsonError = nil;
results = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:&jsonError];
dispatch_sync(dispatch_get_main_queue(), ^{
[av dismissAlert];
[self parseJson];
});
}
else {
[self showMessage];
}
}];
}
-(void)showMessage {
av = [[proAlertView alloc]initWithTitle:#"Connection Problem" message:#"Please confirm the device is connected to the Internet." delegate:self cancelButtonTitle:#"OK" otherButtonTitles:nil, nil];
[av setBackgroundColor:[UIColor clearColor] withStrokeColor:[UIColor blackColor]];
[av show];
}
-(void)parseJson {
NSMutableArray * complete = [[NSMutableArray alloc]init];
for (NSDictionary * tweets in results)
{
NSString * status = [tweets objectForKey:#"text"];
NSString * date = [tweets objectForKey:#"created_at"];
NSDictionary * user = [tweets objectForKey:#"user"];
NSString * artistName = [user objectForKey:#"name"];
NSString * screenName = [user objectForKey:#"screen_name"];
NSString * artistImage = [user objectForKey:#"profile_image_url"];
cellContent = [NSArray arrayWithObjects:status,date,artistName,screenName,artistImage, nil];
[complete addObject:cellContent];
}
SecondViewController * tmpView = [[SecondViewController alloc]initWithNibName:#"SecondViewController_iPhone" bundle:nil];
tmpView.dataToDisplay = complete;
}
This class has been called like this:
- (void)viewDidLoad
{
[super viewDidLoad];
TwitterDataLoad * data = [[TwitterDataLoad alloc] init];
[data loadData];
NSArray * temp = dataToDisplay;
}
I know I am returning the value in a wrong way but i tried returning it through the loadData message itself to the view controller but it didn't work either so i was just trying this. Please don't care about that.
Thanks

Answer 1

Alright i figured out the problem with this.
dispatch_sync is doing what it is supposed to do the problem is with the other two statements under which dispatch_sync is been called (the other blocks )
[_accountStore requestAccessToAccountsWithType:accountTypeTwitter
withCompletionHandler:^(BOOL granted, NSError *error)
so this is an asynchronous call and dispatch sync gets called when the program control is back on the main queue as we have it defined like this : dispatch_sync(dispatch_get_main_queue())
and the method returns before the control is back on the main queue and hence the array doesn't gets any value.
So to solve this i wrote a block in my calling class which gets the return call once this data call is over. email me if you want the whole code #ghostsmitm#gmail.com

Answer 2

You should call reloadData on your UITableView once the data has been loaded. You can do this after calling loadData.