Hi there: I have been writing an iOS program which uses many http queries to the backend rails server, and hence there are tons of codes like below. In this case, it is updating a UITableView:
//making requests before this...
NSOperationQueue* queue = [[NSOperationQueue alloc] init];
[NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse* response, NSData* data, NSError* error)
{
NSLog(#"Request sent!");
NSHTTPURLResponse* httpResponse = (NSHTTPURLResponse*)response;
NSLog(#"Response code: %d", [httpResponse statusCode]);
if ([data length] > 0 && error == nil){
NSLog(#"%lu bytes of data was returned.", (unsigned long)[data length]); }
else if ([data length] == 0 &&
error == nil){
NSLog(#"No data was returned.");
}
else if (error != nil){
NSLog(#"Error happened = %#", error); }
id jsonObject = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:&error];
if (jsonObject != nil && error == nil){
NSLog(#"Successfully deserialized...");
if ([jsonObject isKindOfClass:[NSDictionary class]]){
NSDictionary *deserializedDictionary = (NSDictionary *)jsonObject;
NSLog(#"Dersialized JSON Dictionary = %#", deserializedDictionary);
[listOfItems addObject:deserializedDictionary];
}
else if ([jsonObject isKindOfClass:[NSArray class]]){
NSArray *deserializedArray = (NSArray *)jsonObject;
NSLog(#"Dersialized JSON Array = %#", deserializedArray);
[listOfItems addObjectsFromArray:deserializedArray];
}
else {
/* Some other object was returned. We don't know how to deal
with this situation as the deserializer only returns dictionaries
or arrays */ }
}
else if (error != nil){
NSLog(#"An error happened while deserializing the JSON data., Domain: %#, Code: %d", [error domain], [error code]);
}
[self.tableView performSelectorOnMainThread:#selector(reloadData) withObject:nil waitUntilDone:YES];
}];
//the place where never runs
NSLog(#"End of function.");
Here is the problem: the last line gets executed usually before the code block. How do I ensure that the code after block actually runs after the block?
I am aware that the block uses some other threads, which is why I use performSelectorOnMainThread function instead of a direct call of [self.tableView reloadData]. But if I want to do something else afterward, how am I supposed to do?
Also, can anyone show some better ways to do this? I am trying to figure out the best way to make massive calls to the backend. There are several ways to make asynchronous requests, including this block way and another old-fashioned way invoking delegate classes. In the progress to refactor the codes, I also tried to create my own delegate class and let other classes invoke that, but it is difficult to identify the correct behaviour of callback functions for which connection's data it returns, especially for classes that use multiple functions to call different requests. And I don't want to use synchronous calls.
Thanks very much for any answers. Also welcome to point out any bugs in the code.
You can using dispatch group
Sample code:
- (void)doSomethingAndWait {
// synchronous method
// called in main thread is not good idea.
NSAssert(! [NSThread isMainThread], #"this method can't run in main thread.");
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
//making requests before this...
NSOperationQueue* queue = [[NSOperationQueue alloc] init];
[NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse* response, NSData* data, NSError* error)
{
// your work here.
dispatch_group_leave(group);
}];
// wait for block finished
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
dispatch_release(group);
//will call until block is finished.
NSLog(#"End of function.");
}
And to call that method, you need avoid call it in main thread.
you should call it like this
dispatch_queue_t queue = dispatch_queue_create("com.COMPANYNAME.APPNAME.TASKNAME", NULL);
dispatch_async(queue, ^{
[self doSomethingAndWait];
});
Related
The following code is freezing my UI. Cant do any actions.
- (void) longPoll {
//create an autorelease pool for the thread
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSError* error = nil;
NSURLResponse* response = nil;
NSURL* requestUrl = [NSURL URLWithString:#"myurl"];
NSURLRequest* request = [NSURLRequest requestWithURL:requestUrl];
//send the request (will block until a response comes back)
NSData* responseData = [NSURLConnection sendSynchronousRequest:request
returningResponse:&response error:&error];
dispatch_async(dispatch_get_main_queue(), ^{
[self dataReceived:responseData];
});
});
//compose the request
//pass the response on to the handler (can also check for errors here, if you want)
//clear the pool
}
- (void) startPoll {
//not covered in this example: stopping the poll or ensuring that only 1 poll is active at any given time
[self performSelectorInBackground:#selector(longPoll) withObject: nil];
}
- (void) dataReceived: (NSData*) theData {
//process the response here
NSDictionary *dict=[theData JSONValue];
[self ParseJson:dict];
[self performSelectorInBackground:#selector(longPoll) withObject: nil];
}
Can anyone give me the exact reason for it or any alternative to do the similar code for continues polling.
You are creating an infinite loop:
longCall calls dataReceived calls longCall etc....
What exactly you want to do. There is infinite loop between longPool and dataReceived
there should be mechanism where you stop this call and you can use
#autorelease {} block for create autorelease pool in ARC Enabled project and
NSAutoReleasePool class obj for Without ARC.
I'm using NSURLConnection to make multiple asynchronous requests. I'd like to show a progress indicator to show how many requests have been completed out of the total number to be performed. However, when I attempt to set up and display this progress indicator either before making the request, or in another method called before performing the request, it will not show. The progress indicator displays fine when the request is commented out. But when it's not, it's as if Xcode looks ahead and sees an asynchronous request coming and blocks the main thread, thereby making UI changes impossible.
Here's the relevant code being called, including both the request and code to show the progress indicator:
- (void)getRegionalInformationFromChecked:(NSSet *)set atIndex:(NSInteger)index {
__block BOOL responseRecieved = NO;
NSString *stringForURL = [NSString stringWithFormat:#"http://www.thebluealliance.com/api/v1/event/details?event=%#",[[set allObjects] objectAtIndex:index]];
NSURL *url = [NSURL URLWithString:stringForURL];
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
NSLog(#"URL IS GO: %#", stringForURL);
[NSURLConnection sendAsynchronousRequest:[NSURLRequest requestWithURL:url] queue:queue completionHandler:^(NSURLResponse *_response, NSData *_data, NSError *_error) {
NSLog(#"CHECKED DATA RETURNED AT INDEX %i", index);
NSError *error;
NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:_data options:NSJSONReadingMutableContainers error:&error];
if (!_regionalDetails) {
_regionalDetails = [[NSMutableArray alloc] init];
}
[_regionalDetails addObject:dict];
responseRecieved = YES;
}];
regionalSchedulesToGet = [set count];
while (responseRecieved == NO) {}
[[MBProgressHUD HUDForView:[[UIApplication sharedApplication] keyWindow]] setLabelText:[NSString stringWithFormat: #"Getting regional %i of %i", index+2, [set count]]];
if (index+1 < [set count]) {
[self getRegionalInformationFromChecked:set atIndex:index+1];
} else {
[[MBProgressHUD HUDForView:[[UIApplication sharedApplication] keyWindow]] setLabelText:#"Writing to file"];
}
}
When the asynchronous request's block is commented out, the MBProgressHUD displays its value fine. But when the block is inserted, the SDK refuses to update the progress indicator, even after leaving the block (after which any threading issues should have been resolved). It does not update until there are no more requests to display, at which point it reads "Writing to file".
Why does an asynchronous request seem to block the main thread, and why can I not make changes on the main thread immediately before or after the request is called?
With
while (responseRecieved == NO) {}
you block the main thread (probably with almost 100% CPU load) until the asynchronous block has finished. Then you call your
function recursively, start another asynchronous block and block again until that has
finished. Therefore the program control does not return to the main runloop until all
operations have finished. Only then the UI updates are done.
Instead of waiting synchronously (which is always a bad idea),
you should start the next operation at the end of the completion block.
Note also that the queue argument of sendAsynchronousRequest is the queue on which
the completion handler is called, so you can just use [NSOperationQueue mainQueue].
Then your code looks roughly like this:
- (void)getRegionalInformationFromChecked:(NSSet *)set atIndex:(NSInteger)index
{
[[MBProgressHUD HUDForView:[[UIApplication sharedApplication] keyWindow]]
setLabelText:[NSString stringWithFormat:#"Getting regional %i of %i", index+1, [set count]]];
NSString *stringForURL = [NSString stringWithFormat:#"http://www.thebluealliance.com/api/v1/event/details?event=%#",[[set allObjects] objectAtIndex:index]];
NSURL *url = [NSURL URLWithString:stringForURL];
NSLog(#"URL IS GO: %#", stringForURL);
[NSURLConnection sendAsynchronousRequest:[NSURLRequest requestWithURL:url] queue:[NSOperationQueue mainQueue]
completionHandler:^(NSURLResponse *_response, NSData *_data, NSError *_error) {
NSLog(#"CHECKED DATA RETURNED AT INDEX %i", index);
NSError *error;
NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:_data options:NSJSONReadingMutableContainers error:&error];
if (!_regionalDetails) {
_regionalDetails = [[NSMutableArray alloc] init];
}
[_regionalDetails addObject:dict];
if (index+1 < [set count]) {
[self getRegionalInformationFromChecked:set atIndex:index+1];
} else {
[[MBProgressHUD HUDForView:[[UIApplication sharedApplication] keyWindow]] setLabelText:#"Writing to file"];
// ... perhaps call a completion function from here ?
}
}];
}
But note that the initial call to getRegionalInformationFromChecked will now
return almost immediately (that's how asynchronous tasks work :-).
Try to dispatch on the main thread all the methods that involve UI refresh
I've been trying to experiment with some code from a tutorial, however not having much success due to not getting my head around GCD.
I have an class named API.m and here is the code regarding GCD:
+ (API *) sharedInstance
{
static API *sharedInstance = nil;
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
sharedInstance = [[self alloc] initWithBaseURL:[NSURL URLWithString:APIHost]];
});
return sharedInstance;
}
-(void)commandWithParams:(NSMutableDictionary*)params
onCompletion:(JSONResponseBlock)completionBlock
{
NSMutableURLRequest *apiRequest = [self multipartFormRequestWithMethod:#"POST"
path:APIPath
parameters:params
constructingBodyWithBlock: ^(id <AFMultipartFormData>formData) {
//TODO: attach file if needed
}];
AFJSONRequestOperation* operation = [[AFJSONRequestOperation alloc] initWithRequest: apiRequest];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
//success!
completionBlock(responseObject);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
//failure :(
completionBlock([NSDictionary dictionaryWithObject:[error localizedDescription] forKey:#"error"]);
}];
[operation start];
}
I make a simple test by implementing a button and getting an NSArray to print it's content to the output window:
- (IBAction)test:(id)sender {
NSMutableDictionary* params =[NSMutableDictionary dictionaryWithObjectsAndKeys:
#"pending", #"command",
[[[API sharedInstance] user] objectForKey:#"UserID"] , #"userID",
nil];
[[API sharedInstance] commandWithParams:params
onCompletion:^(NSDictionary *json) {
//result returned
if ([json objectForKey:#"error"]==nil) {
// Simple example
[self.users addObject:#"1"];
} else {
//error
[UIAlertView title:#"Error" withMessage:[json objectForKey:#"error"]];
}
}];
NSLog(#"%#", self.users);
}
Now when I first click the button an empty NSArray is printed to the output window, but when I press it again it print's "1". It's clear that the program is reaching NSLog before the completion block has time to fully execute. Could someone please help me modify the code so that I have the option to have the NSLog execute after the completion block has finished?
Not sure as to what you are trying to accomplish, but if the goal is to just have NSLog execute after the completion block, you can move the NSLog statement after
[self.users addObject:#"1"];
If you have some code which you want to execute after adding it to the array, you can have
[self methodName]; in the completion block and it will get called there.
Completion block, is the code which is run after execution of the code which you wanted run. The code which you wanted run, will happen asynchronously and on another thread. After that code is run, the completion block code will get executed.
First of all the questions are failry simiple.. if you just want to see what they are skip to the bottom of this post and you will see them in bold.. for more detail then you can read the rest of this post...
I am just trying to iron out my NSURLConnection so that its working smoothly and I understand this properly. There is a profound lack of example/tutorials for Asynchronous connections on the internet or not any that I can find that explaine what is going on with any level of depth other than getting the connection up and running which after working on it seems pretty simple. Hopefully this question can full the void that I feel is out there for other users.
So, in my .h file i have imported the foundations headers and declared the methods required for the received or lack of received data (errors etc).
.h
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h> //add foundations
//.. other headers can be imported here
#interface MyViewController: UITableViewController {
//Im not setting any delegates to access the methods because Is all happening in the same
//place so I just use the key word 'self' when accessing the methods declared below
//I'm not sure if this is the best thing to do but I wasn't able to get my head around declaring the delegate or how it would help me with the way I have set up my request etc.
}
- (IBAction)setRequestString:(NSString *)string; //this method sets the request and connection methods
//these methods receive the response from my async nsurlconnection
- (void)receivedData:(NSData *)data;
- (void)emptyReply;
- (void)timedOut;
- (void)downloadError:(NSError *)error;
So thats my header file.. pretty simple not much explaining needed.
.m
//call setRequestString from some other method attached to a button click or something
[self setRequestString:#"rss.xml"];
//..
- (IBAction)setRequestString:(NSString *)string
{
//Set database address
NSMutableString *databaseURL = [[NSMutableString alloc] initWithString:#"http:www.comicbookresources/feeds/"]; // address not real jsut example
//append the string coming in to the end of the databaseURL
[databaseURL appendString:string];
//prepare NSURL with newly created string
NSURL *url = [NSURL URLWithString:databaseURL];
//AsynchronousRequest to grab the data
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
{
if ([data length] > 0 && error == nil){
[self receivedData:data];
}else if ([data length] == 0 && error == nil){
[self emptyReply];
}else if (error != nil && error.code == NSURLErrorTimedOut){ //used this NSURLErrorTimedOut from foundation error responses
[self timedOut];
}else if (error != nil){
[self downloadError:error];
}
}];
}
now set up the methods that were initialized in the .h file and called in the if statement above
- (void)receivedData:(NSData *)data
{
NSString* newStr = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSLog(#"%#", newStr); //logs recived data
//now you can see what data is coming in on your log
//do what you want with your data here (i.e. start parsing methods
}
- (void)emptyReply
{
//not sure what to do here yet?
}
- (void)timedOut
{
//also not sure what to do here yet?
}
- (void)downloadError:(NSError *)error
{
NSLog(#"%#", error);
UIAlertView *errorAlert = [[UIAlertView alloc] initWithTitle:#"Error!" message:#"A connection failure occurred." delegate:self cancelButtonTitle:#"OK" otherButtonTitles:nil, nil];
[errorAlert show];
}
Cool so that pretty much the basics of what I have done right there.. now the questions I have are as follows.
Question one:
Where I call NSURLConnection like so
[NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
{
What is happening here what is the ^ for is that executing that whole block (including the if statements) on a different thread or something? because it looks alot like grand central dispatch formatting but slightly different.
Question two:
what should I be doing inside emptyReply & timedOut methods?
Question three:
How would I incorporate caching into this? I would like to cache the responses I get back from different requests. i.e. with my setRequestString you will see there is a string input parameter, so i can request different rss feeds with the same method.. I need to figure out how to cache these responses into individual caches.. but im not sure where to start with it.
Finally
If you have made it this far, thank you very much for reading my question. Hopefully with your responses we can get a pretty nice solution going here.. that other people can use for themselves and pick and choose the bits and peices they need that works for there own solution..
Anyway thank you very much for reading and I look forward to your replies.. even if they are just refrences to tutorials or examples you think might help me.. anything is good I just want to fully understand whats going on and whats a good solution.
Read about blocks in Apple documentation. Its new. Or you can read here
You can show errors such as request timed out etc. You don't really have to handle them separately than the error one unless you have special logic.
Try this for caching
NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalAndRemoteCacheData timeoutInterval:TIMEOUT_INTERVAL];
I have a program that has multiple url request so I used the the code in http://snippets.aktagon.com/snippets/350-How-to-make-asynchronous-HTTP-requests-with-NSURLConnection and put it in it's own class (class B).To call the class I am simple initializing class B in class A, sending a url to class B's get method([classname get:url]) and then getting the server response upon return.
The problem is that I am getting defeated by race conditions due to the fact that the didReceiveData: method is not complete by the time my method is returned.
I have gone through the developer example of using NSUrlConnection and they are updating views once the response finally came in so they didn't have to fight this problem.
Thank you so much for your help.
I need to keep the calls asynchronous due to the number of them I have to make but I am open to any suggestions.
Edit (moved from answer)
I changed the code to GCD based off of a tutorial and I am still getting defeated by the race condition. Here is the code that I am using now:
I changed it to GCS based on on your suggestion but I am still getting caught by the race condition. Below is the code that I changed it to and I am calling it by:
NSString *responseStringClassA = [InitalizedInstanceOfClassA LogIn:#"username" #"password"];
//Log into the server
-(NSString *)logIn: (NSString *) username password:(NSString *) password
{
NSString* returnString;
dispatch_queue_t downloadQueue = dispatch_queue_create("Login", NULL);
dispatch_async(downloadQueue, ^{
BOOL success = YES;
NSString *urlAsString =[NSString stringWithFormat:#""URL HERE];
NSLog(#"url sent out: %#", urlAsString);
NSURL *url = [NSURL URLWithString:urlAsString];
NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];
NSError *error = nil;
NSData *connectionData = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:nil error:&error];
NSLog(#"Connection Data: %#", [[NSString alloc] initWithData:connectionData encoding:NSASCIIStringEncoding]);
[returnString isEqualToString:[NSString stringWithUTF8String:[connectionData bytes]]];
if ([connectionData length] > 0 && error == nil) {
//success
success = YES;
}
else if([connectionData length] == 0 && error == nil){
//nodata
success = YES;
}
else if(error != nil){
//error ..
success = NO;
}
dispatch_async(dispatch_get_main_queue(), ^{
[returnString isEqualToString:[[NSString alloc] initWithData:connectionData encoding:NSASCIIStringEncoding] ];
});
});
return returnString;
}
It's the very purpose of asynchronous requests that the intial method returns almost immediately without having done the work. Later, when the work has been done, you will be notified and you can access and use the result.
But obviously, you're looking for something else than asynchronous operations. And alternative would be to use synchronous URL requests but run them from separate threads. The best way to achieve this is to use GCD (grand central dispatch).
Note that you may not update the user interface from background threads. Instead, when the URL request has finished and you want to display your results, you have to call performSelectorOnMainThread (part of NSObject) for that.