Home > Articles > Programming > General Programming/Other Languages

  • Print
  • + Share This
This chapter is from the book

About Those Missing Views

It’s common for developers new to Auto Layout to “lose” views. They discover that views they have added end up offscreen or that they have a zero size due to constraints. (Incidentally, Auto Layout works with positive sizes, zero or larger. You cannot create views with negative widths or heights.) The missing views problem catches many devs. This problem happens with both underconstrained views and views with inconsistent rules.

In this section, you’ll see a little bit of constraint code, even before you’ve read about the details of the constraint class and how instances work. Please bear with me. I’ve added highlights to help explain ambiguous and underconstrained scenarios to make a point. If you work with Auto Layout, you should be aware of these situations before you start using the technology.

Underconstrained Missing Views

Underconstrained views don’t give Auto Layout enough information to build from, so it often defaults to a size of zero. Consider the following example. This code creates a new view, prepares it for Auto Layout, and then adds two sets of constraints, which I’ve highlighted in boldface:

// Create a new view and add it into the Auto Layout system
// This view goes missing despite the initWithFrame: size
UIView *view = [[UIView alloc]
    initWithFrame:CGRectMake(0.0f, 0.0f, 30.0f, 30.0f)];
[self.view addSubview:view];
view.translatesAutoresizingMaskIntoConstraints = NO;

// Add two sets of rules, pinning the view and setting height
[self.view addConstraints:[NSLayoutConstraint
    constraintsWithVisualFormat:@"V:|[view(==80)]" // 80 height
    options:0 metrics:nil
    views:NSDictionaryOfVariableBindings(view)]];
[self.view addConstraints:[NSLayoutConstraint
    constraintsWithVisualFormat:@"H:|[view]"
    options:0 metrics:nil
    views:NSDictionaryOfVariableBindings(view)]];

The first set of constraints pins the view to the top of its superview and sets the height to 80. The second set pins the view to the superview’s leading edge. (This is the left side in the United States, with English’s left-to-right writing system.) I deliberately did not specify a width. The view’s size is, therefore, underconstrained.

You might expect Auto Layout to default to the initial frame size, which was set to 30 by 30 points. It does not. When this snippet sets translatesAutoresizingMaskIntoConstraints to NO, that initialization is essentially thrown away. As the view appears onscreen, the ambiguous rules passed to Auto Layout result in a width that falls to zero, creating a view that’s not visible:

2013-01-14 10:47:40.460 HelloWorld[73891:c07]
    <UIView: 0x884dfc0; frame = (0 0; 0 80); layer = <CALayer: 0x884e020>>

Missing Views with Inconsistent Rules

Inconsistent rules may also produce views that are missing in action. For example, imagine a pair of rules that say “View A is three times the width of View B” and “View B is twice the width of View A.” The following code snippets implement these rules. I’ve boldfaced the parts of the code that tell the rule story:

NSLayoutConstraint *constraint;
constraint = [NSLayoutConstraint
    constraintWithItem:viewA
    attribute:NSLayoutAttributeWidth
    relatedBy:NSLayoutRelationEqual
    toItem:viewB
    attribute:NSLayoutAttributeWidth
    multiplier:3.0f constant:0.0f];
[self.view addConstraint:constraint];

constraint = [NSLayoutConstraint
    constraintWithItem:viewA
    attribute:NSLayoutAttributeWidth
    relatedBy:NSLayoutRelationEqual
    toItem:viewB
    attribute:NSLayoutAttributeWidth
    multiplier:2.0f constant:0.0f];
[self.view addConstraint:constraint];

Surprisingly, these two rules are neither unsatisfiable nor ambiguous, even though common sense suggests otherwise. That’s because both rules are satisfied when View A and View B have zero width. At zero, View A’s width can be three times the width of View B, and View B twice the width of View A:

0 = 0 * 3 and 0 = 0 * 2

When this code is run and the rules are applied, the views present the zero-width frames expected from this scenario:

2013-01-14 11:02:38.005 HelloWorld[74460:c07]
    <TestView: 0x8b30910; frame = (320 454; 0 50); layer = <CALayer: 0x8b309d0>>
2013-01-14 11:02:38.006 HelloWorld[74460:c07]
    <TestView: 0x8b32570; frame = (320 436; 0 68); layer = <CALayer: 0x8b32450>>

Tracking Missing Views

You can track down “missing” views with the debugger by inspecting their geometry after you expect them to appear (for example, in viewDidAppear: and awakeFromNib). You may want to add NSAssert statements about their expected size and positions. Some will be, as discussed, zero sized.

The following view, for example, had a zero-sized frame because it was underconstrained in the Auto Layout system:

2013-01-09 14:31:41.869 HelloWorld[29921:c07] View: <UIView: 0x71bb390;
frame = (30 430; 0 0); layer = <CALayer: 0x71bb3f0>>

Other views may simply be offscreen because you haven’t told Auto Layout that the views must appear onscreen. For example, this view had a positive size (20 points by 20 points), but its frame with its (–20, –20) origin lay outside its view controller’s presentation:

2013-01-09 14:33:37.546 HelloWorld[29975:c07] View: <UIView: 0x7125f70;
frame = (-20 -20; 20 20); layer = <CALayer: 0x7125fd0>>

In other cases, you might load a view from a storyboard or nib file and see only part of it onscreen, or it may occupy the entire screen at once. These are hallmarks of an underlying Auto Layout issue.

  • + Share This
  • 🔖 Save To Your Account

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