Home > Articles > Programming > General Programming/Other Languages

This chapter is from the book

Recipe: Using Multiple Gesture Recognizers Simultaneously

Recipe 1-3 builds on the ideas presented in Recipe 1-2, but with several differences. First, it introduces multiple recognizers that work in parallel. To achieve this, the code uses three separate recognizers—rotation, pinch, and pan—and adds them all to the DragView’s gestureRecognizers property. It assigns the DragView as the delegate for each recognizer. This allows the DragView to implement the gestureRecognizer:shouldRecognize-SimultaneouslyWithGestureRecognizer: delegate method, enabling these recognizers to work simultaneously. Until this method is added to return YES as its value, only one recognizer will take charge at a time. Using parallel recognizers allows you to, for example, both zoom and rotate in response to a user’s pinch gesture.

Recipe 1-3 extends the view’s state to include scale and rotation instance variables. These items keep track of previous transformation values and permit the code to build compound affine transforms. These compound transforms, which are established in Recipe 1-3’s updateTransformWithOffset: method, combine translation, rotation, and scaling into a single result. Unlike the previous recipe, this recipe uses transforms uniformly to apply changes to its objects, which is the standard practice for recognizers.

Finally, this recipe introduces a hybrid approach to gesture recognition. Instead of adding a UITapGestureRecognizer to the view’s recognizer array, Recipe 1-3 demonstrates how you can add the kind of basic touch method used in Recipe 1-1 to catch a triple-tap. In this example, a triple-tap resets the view back to the identity transform. This undoes any manipulation previously applied to the view and reverts it to its original position, orientation, and size. As you can see, the touches began, moved, ended, and cancelled methods work seamlessly alongside the gesture recognizer callbacks, which is the point of including this extra detail in this recipe. Adding a tap recognizer would have worked just as well.

This recipe demonstrates the conciseness of using gesture recognizers to interact with touches.

Recipe 1-3 Recognizing Gestures in Parallel

@interface DragView : UIImageView <UIGestureRecognizerDelegate>
@end

@implementation DragView
{
    CGFloat tx; // x translation
    CGFloat ty; // y translation
    CGFloat scale; // zoom scale
    CGFloat theta; // rotation angle
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    // Promote the touched view
    [self.superview bringSubviewToFront:self];

    // initialize translation offsets
    tx = self.transform.tx;
    ty = self.transform.ty;
    scale = self.scaleX;
    theta = self.rotation;
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    if (touch.tapCount == 3)
    {
        // Reset geometry upon triple-tap
        self.transform = CGAffineTransformIdentity;
        tx = 0.0f; ty = 0.0f; scale = 1.0f; theta = 0.0f;
    }
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    [self touchesEnded:touches withEvent:event];
}

- (void)updateTransformWithOffset:(CGPoint)translation
{
    // Create a blended transform representing translation,
    // rotation, and scaling
    self.transform = CGAffineTransformMakeTranslation(
        translation.x + tx, translation.y + ty);
    self.transform = CGAffineTransformRotate(self.transform, theta);

    // Guard against scaling too low, by limiting the scale factor
    if (self.scale > 0.5f)
    {
        self.transform = CGAffineTransformScale(self.transform, scale, scale);
    }
    else
    {
        self.transform = CGAffineTransformScale(self.transform, 0.5f, 0.5f);
    }
}

- (void)handlePan:(UIPanGestureRecognizer *)uigr
{
    CGPoint translation = [uigr translationInView:self.superview];
    [self updateTransformWithOffset:translation];
}

- (void)handleRotation:(UIRotationGestureRecognizer *)uigr
{
    theta = uigr.rotation;
    [self updateTransformWithOffset:CGPointZero];
}

- (void)handlePinch:(UIPinchGestureRecognizer *)uigr
{
    scale = uigr.scale;
    [self updateTransformWithOffset:CGPointZero];
}

- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer
        shouldRecognizeSimultaneouslyWithGestureRecognizer:
        (UIGestureRecognizer *)otherGestureRecognizer
{
    return YES;
}

- (instancetype)initWithImage:(UIImage *)image
{
    // Initialize and set as touchable
    self = [super initWithImage:image];
    if (self)
    {
        self.userInteractionEnabled = YES;

        // Reset geometry to identities
        self.transform = CGAffineTransformIdentity;
        tx = 0.0f; ty = 0.0f; scale = 1.0f; theta = 0.0f;

        // Add gesture recognizer suite
        UIRotationGestureRecognizer *rot =
            [[UIRotationGestureRecognizer alloc]
                initWithTarget:self
                action:@selector(handleRotation:)];
        UIPinchGestureRecognizer *pinch =
            [[UIPinchGestureRecognizer alloc]
                initWithTarget:self
                action:@selector(handlePinch:)];
        UIPanGestureRecognizer *pan =
            [[UIPanGestureRecognizer alloc]
                initWithTarget:self
                action:@selector(handlePan:)];
        self.gestureRecognizers = @[rot, pinch, pan];
        for (UIGestureRecognizer *recognizer
            in self.gestureRecognizers)
                recognizer.delegate = self;
    }
    return self;
}
@end

Resolving Gesture Conflicts

Gesture conflicts may arise when you need to recognize several types of gestures at the same time. For example, what happens when you need to recognize both single- and double-taps? Should the single-tap recognizer fire at the first tap, even when the user intends to enter a double-tap? Or should you wait and respond only after it’s clear that the user isn’t about to add a second tap? The iOS SDK allows you to take these conflicts into account in your code.

Your classes can specify that one gesture must fail in order for another to succeed. Accomplish this by calling requireGestureRecognizerToFail:. This gesture recognizer method takes one argument, another gesture recognizer. This call creates a dependency between the two gesture recognizers. For the first gesture to trigger, the second gesture must fail. If the second gesture is recognized, the first gesture will not be.

iOS 7 introduces new APIs that offer more flexibility in providing runtime failure conditions via gesture recognizer delegates and subclasses. You implement gestureRecognizer:shouldRequireFailureOfGestureRecognizer: and gestureRecognizer:shouldBe-RequiredToFailByGestureRecognizer: in recognizer delegates and shouldRequireFailureOfGestureRecognizer: and shouldBeRequiredToFailByGestureRecognizer: in subclasses.

Each method returns a Boolean result. A positive response requires the failure condition specified by the method to occur for the gesture to succeed. These UIGestureRecognizer delegate methods are called by the recognizer once per recognition attempt and can be set up between recognizers across view hierarchies, while implementations provided in subclasses can define class-wide failure requirements.

In real life, failure requirements typically mean that the recognizer adds a delay until it can be sure that the dependent recognizer has failed. It waits until the second gesture is no longer possible. Only then does the first recognizer complete. If you recognize both single- and double-taps, the application waits a little longer after the first tap. If no second tap happens, the single-tap fires. Otherwise, the double-tap fires, but not both.

Your GUI responses will slow down to accommodate this change. Your single-tap responses become slightly laggy. That’s because there’s no way to tell if a second tap is coming until time elapses. You should never use both kinds of recognizers where instant responsiveness is critical to your user experience. Try, instead, to design around situations where that tap means “do something now” and avoid requiring both gestures for those modes.

Don’t forget that you can add, remove, and disable gesture recognizers on-the-fly. A single-tap may take your interface to a place where it then makes sense to further distinguish between single- and double-taps. When leaving that mode, you could disable or remove the double-tap recognizer to regain better single-tap recognition. Tweaks like this can limit interface slowdowns to where they’re absolutely needed.

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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