async multiple items - ios

Question

So I normally write apps for android and just use an async task to call methods to run in the background while an alert dialog plays saying "Loading" or something like that. On this app Im trying to translate to iOS, Im parsing data from different websites and displaying a couple web images and I want to have my alart dialog play while all these things are being loaded. Ive been searching for hours and havent found the solution I am looking for. I was hoping someone could point me to a tutorial or somewhere in the right direction.
here is what im working with:
- (void) RSEpic{
NSURL * imageURL = [NSURL URLWithString:RSEimageURL];
NSData * imageData = [NSData dataWithContentsOfURL:imageURL];
UIImage * image = [UIImage imageWithData:imageData];
_RSEImage.image = image;
[self waterTemp];
}
- (void) waterTemp{
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
/* set headers, etc. on request if needed */
[request setURL:[NSURL URLWithString:#"http://waterdata.usgs.gov/usa/nwis/uv?02035000"]];
NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:NULL error:NULL];
NSString *html = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSScanner *scanner = [NSScanner scannerWithString:html];
NSString *token = nil;
[scanner scanUpToString:#"<table id=\"table_12_00010\"" intoString:NULL];
[scanner scanUpToString:#"&nbsp" intoString:&token];
NSArray *words = [token componentsSeparatedByCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:#":"]];
double temp = [words[1] doubleValue];
_waterTempC.Text = [NSString stringWithFormat:#"%.2f°C",temp];
_waterTempF.Text = [NSString stringWithFormat:#"%.2f°F",temp*9/5+32];
[self waterDepth];
}
- (void) waterDepth{
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
/* set headers, etc. on request if needed */
[request setURL:[NSURL URLWithString:#"http://waterdata.usgs.gov/va/nwis/uv?02037500"]];
NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:NULL error:NULL];
NSString *html = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSScanner *scanner = [NSScanner scannerWithString:html];
NSString *token = nil;
[scanner scanUpToString:#"<table id=\"table_07_00065\"" intoString:NULL];
[scanner scanUpToString:#"&nbsp" intoString:&token];
NSArray *words = [token componentsSeparatedByCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:#":"]];
double temp = [words[1] doubleValue];
_waterLevel.Text = [NSString stringWithFormat:#"%.2fFT",temp];
if (temp >= 9.0) {
_levelAlert.text = #"HIGH WATER PERMIT REQUIRED";
}
else if (temp >= 5.0){
_levelAlert.text = #"LIFE JACKET REQUIRED";
}
else {
_levelAlert.text = #"";
}
[self tempChart];
}
- (void) tempChart{
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
/* set headers, etc. on request if needed */
[request setURL:[NSURL URLWithString:#"http://waterdata.usgs.gov/nwis/uv/?dd_cd=12_00010&format=img_default&site_no=02035000&set_arithscale_y=on&period=7"]];
NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:NULL error:NULL];
NSString *html = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSScanner *scanner = [NSScanner scannerWithString:html];
NSString *token = nil;
[scanner scanUpToString:#"http" intoString:NULL];
[scanner scanUpToString:#"\"" intoString:&token];
NSLog(#"%#",token);
NSURL * imageURL = [NSURL URLWithString:token];
NSData * imageData = [NSData dataWithContentsOfURL:imageURL];
UIImage * image = [UIImage imageWithData:imageData];
_chartImage.image = image;
}
- (void) depthChart{
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
/* set headers, etc. on request if needed */
[request setURL:[NSURL URLWithString:#"http://waterdata.usgs.gov/va/nwis/uv/?dd_cd=07_00065&format=img_default&site_no=02037500&set_arithscale_y=on&period=7"]];
NSData *data = [NSURLConnection sendSynchronousRequest:request returningResponse:NULL error:NULL];
NSString *html = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSScanner *scanner = [NSScanner scannerWithString:html];
NSString *token = nil;
[scanner scanUpToString:#"http" intoString:NULL];
[scanner scanUpToString:#"\"" intoString:&token];
NSLog(#"%#",token);
NSURL * imageURL = [NSURL URLWithString:token];
NSData * imageData = [NSData dataWithContentsOfURL:imageURL];
UIImage * image = [UIImage imageWithData:imageData];
_chartImage.image = image;
}
- (void) progressAlert {
// initialize our Alert View window without any buttons
baseAlert=[[UIAlertView alloc]initWithTitle:#"Please wait,\ndownloading updates...." message:nil delegate:self cancelButtonTitle:nil otherButtonTitles:nil];
// Display our Progress Activity view
[baseAlert show];
// create and add the UIActivity Indicator
UIActivityIndicatorView
*activityIndicator=[[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
activityIndicator.center=CGPointMake(baseAlert.bounds.size.width
/ 2.0f,baseAlert.bounds.size.height-40.0f);
// initialize to tell our activity to start animating.
[activityIndicator startAnimating];
[baseAlert addSubview:activityIndicator];
// automatically close our window after 3 seconds has passed.
[self performSelector:#selector(showProgressDismiss)withObject:nil afterDelay:3.0f];
}
- (void) showProgressDismiss
{
[baseAlert dismissWithClickedButtonIndex:0 animated:NO];
}
#end
So can someone tell me how to make my baseAlert Show and Dismiss while all this stuff is loading?

Answer 1

Use a dispatch_group_t, and once all threads have been completed, they can invoke notify, like so:
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
[self doAnExpensiveOperation];
});
dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
[self doAnotherExpensiveOperation];
});
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
dispatch_async(dispatch_get_main_queue(), ^{
// called when both background threads have finished.
// Update UI elements here
});
});
To prioritize:
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
[self doAnotherExpensiveOperation];
});

Answer 2

The same concepts are applied on iOS as on Android: if you want to perform heavy computing and you want the app to be responsive do the heavy computing on a background thread and DO NOT ACCES UI FROM BACKGROUND THREADS. The only difference between the iOS and Android is the way you can perform tasks on background threads. Android has AsyncTasks or Loaders, iOS has NSOperations & NSOperationsQueue,GCD (grand central dispatch which is my favorite and the best solution in my opinion) or there are methods like performSelectorInBackground: which I don't like because is harder to return objects after thread is completed.
So my suggestion is, have a look over GCD (there are lots of other tutorials) and after that, change your code accordingly, if you have troubles changing the code or you have unexpected behaviors, come back to SO with other questions

Answer 3

Well, to make it simple and concise, you can tell your OS to execute some code in another thread with this:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//Insert your code here
});
The global queue means that you will submit a block to GCD, and the OS will execute that block in a convenient thread, except the main thread (where your UI runs). You must take care that if you want to update the UI (for example hide an alert, or stop an activity indicator), you need to do that in the main thread, (main_queue). So, a template we always use is as follows:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//Process your heavy code here.
dispatch_async(dispatch_get_main_queue(), ^{
//Update your UI here.
});
});
This is very important to avoid retain cycles if you're using ARC
As Apple says here, you have to use weak references to self inside a block, as so for any iVar you use inside a block whose retain count will be incremented.
I suggest the tutorial proposed by #danypata , it's a good one. In the same site you can find a lot of useful tutorials!!
Have a great day, and hope it helps!