Home > Articles > Programming > C/C++

This chapter is from the book

Completing a Long-Running Background Task

In our final tutorial of the hour, we need to create a project from scratch. Our book isn't about building applications that require a great deal of background processing. Sure, we could demonstrate how to add code to an existing project and allow a method to run in the background, but we don't have any long running methods that could make use of it.

To demonstrate how we can tell iOS to allow something to run in the background, we'll create a new application, SlowCount, that does nothing but count to 1,000—slowly. We'll use the task-completion method of background to make sure that, even when the application is in the background, it continues to count until it reaches 1,000—as shown in Figure 21.8.

Figure 21.8

Figure 21.8 To simulate a long-running task, our application will count. Slowly.

Preparing the Project

Create a new view-based iPhone application named SlowCount. We'll move through development fairly quickly because, as you can imagine, this application is pretty simple.

The application will have a single outlet, a UILabel named theCount, which we'll use to present the counter onscreen. In addition, it will need a an integer to use as a counter (count), an NSTimer object that will trigger the counting at a steady interval (theTimer), and a UIBackgroundTaskIdentifier variable (not an object!) that we'll use to reference the task we have running in the background (counterTask).

Open the SlowCountViewController.h file and implement it as shown in Listing 21.6.

Listing 21.6.

#import <UIKit/UIKit.h>

@interface SlowCountViewController : UIViewController {
    int count;
    NSTimer *theTimer;
    UIBackgroundTaskIdentifier counterTask;
    IBOutlet UILabel *theCount;

@property (nonatomic,retain) UILabel *theCount;


Next, clean up after the UILabel object in the SlowCountViewController.m dealloc method. The other instance variables either aren't objects (count, counterTask) or will be allocated and released elsewhere in the application (NSTimer):

- (void)dealloc {
    [theCount release];
    [super dealloc];

Creating the User Interface

It's a bit of a stretch to claim that this application has a "user interface," but we still need to prepare the SlowCountViewController.xib to show the theCount label on the screen.

Open the XIB file in Interface Builder, and drag a UILabel into the center of the view. Set the label's text to read 0. With the label selected, use the Attributes Inspector (Command+1) to set the label alignment to center and the font size to 36. Finally, align the right and left sides of the label with the right and left sizing guides. You've just created a UI masterpiece, as shown in Figure 21.9.

Figure 21.9

Figure 21.9 Add a UILabel to the view to hold the current count.

Finish the view by Control-dragging from the File's Owner icon in the Document window to the UILabel in the view. Choose theCount when prompted to make the connection.

Implementing the Counter Logic

To finish our applications core functionality (counting!), we need to do two things. First, we need to set the counter (count) to 0 and allocate and initialize NSTimer that will fire at a regular interval. Second, when the timer fires, we will ask it to invoke a second method, countUp. In the countUp method, we'll check to see whether count is 1000. If it is, we'll turn off the timer and we're done, if not, we'll update count and display it in our UILabel theCount.

Initializing the Timer and Counter

Let's start with initializing the counter and timer. What better place to do this than in SlowCount.m's viewDidLoad method. Implement viewDidLoad as shown in Listing 21.7.

Listing 21.7.

 1: - (void)viewDidLoad {
 2:     [super viewDidLoad];
 3:     count=0;
 4:     theTimer=[NSTimer scheduledTimerWithTimeInterval:0.1
 5:                     target:self
 6:                     selector:@selector(countUp)
 7:                     userInfo:nil
 8:                     repeats:YES];
 9: }

Line 3 initializes our integer counter, count, to 0.

Lines 4–8 initialize and allocate the theTimer NSTimer object with an interval of 0.1 seconds. The selector is set to use the method countUp, which we'll be writing next. The timer is set to keep repeating with repeats:YES.

All that remains is to implement countUp so that it increments the counter and displays the result.

Updating the Counter and Display

Add the countUp method, as shown in Listing 21.8, before the viewDidLoad method in SlowCountViewController.m. This should be quite straightforward—if the count equals 1000, we're done and it's time to clean up—otherwise, we count!

Listing 21.8.

 1: - (void)countUp {
 2:     if (count==1000) {
 3:         [theTimer invalidate];
 4:         [theTimer release];
 5:     } else {
 6:         count++;
 7:         NSString *currentCount;
 8:         currentCount=[[NSString alloc] initWithFormat:@"%d",count];
 9:         theCount.text=currentCount;
10:         [currentCount release];
11:     }
12: }

Lines 2–4 handle the case that we've reached the limit of our counting (count==1000). If it has, we use the timer's invalidate method to stop it, and then release it.

Lines 5–11 handle the actual counting and display. Line 5 updates the count variable. Line 7 declares the currentCount string, which is then allocated and populated in line 8. Line 9 updates our theCount label with the currentCount string. Line 10 releases the string object.

Build and Run the application—it should do exactly what you expect—count slowly until it reaches 1,000. Unfortunately, if you background the application, it will suspend. The counting will cease until the application returns to the foreground.

Enabling the Background Task Processing

To enable the counter to run in the background, we need to mark it as a background task. We'll use this code snippet to mark the beginning of the code we want to execute in the background:

counterTask = [[UIApplication sharedApplication]
                 // If you're worried about exceeding 10 minutes, handle it here

And we'll use this code snippet to mark the end:

[[UIApplication sharedApplication] endBackgroundTask:counterTask];

Let's update our viewDidLoad and countUp methods to include these code additions. In viewDidLoad, we'll start the background task right before we initialize the counter. In countUp, we end the background task after count==1000 and the timer is invalidated and released.

Update viewDidLoad as shown in Listing 21.9.

Listing 21.9.

- (void)viewDidLoad {
    [super viewDidLoad];

    counterTask = [[UIApplication sharedApplication]
                 // If you're worried about exceeding 10 minutes, handle it here
    theTimer=[NSTimer scheduledTimerWithTimeInterval:0.1

Then make the corresponding additions to countUp, demonstrated in Listing 21.10.

Listing 21.10.

- (void)countUp {
    if (count==1000) {
        [theTimer invalidate];
        [theTimer release];
        [[UIApplication sharedApplication] endBackgroundTask:counterTask];
    } else {
        NSString *currentCount;
        currentCount=[[NSString alloc] initWithFormat:@"%d",count];
        [currentCount release];

Save your project files, then Build and Run the application on your iPhone or in the simulator. After the counter starts counting, pressing the Home button to move the application to the background. Wait a minute or so, and then re-open the application through the task manager or the application icon. The counter will have continued to run in the background!

Obviously, this isn't a very compelling project itself, but the implications for what can be achieved in real-world apps is definitely exciting!

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020