Home > Articles > Mobile Application Development & Programming

This chapter is from the book

This chapter is from the book

Property Change Notification

A key aspect of MVVM related to data binding is property change notification. Property change notification allows a source object (for example, a viewmodel) to signal to a target FrameworkElement that a value needs updating in the UI.

There are two ways to implement change notification in a source class: either using dependency properties or by implementing the INotifyPropertyChanged interface, which is often referred to as just INPC.

Implementing INotifyPropertyChanged: The Traditional Approach

The INotifyPropertyChanged interface has a single event called PropertyChanged. The implementation of INotifyPropertyChanged ordinarily includes the following construct:

public event PropertyChangedEventHandler PropertyChanged;

protected virtual void OnPropertyChanged(

        [CallerMemberName] string propertyName = null)
    PropertyChangedEventHandler tempEvent = PropertyChanged;
    if (tempEvent != null)
        tempEvent(this, new PropertyChangedEventArgs(propertyName));

The CallerMemberName attribute is used by a new compiler feature that automatically passes the name of the calling member to the target method. No longer is it necessary in most cases to pass the name of a property as a loosely typed string.

A property is then able to signal to a subscriber of the event that a property value needs updating, like so:

string foo;

public string Foo
        return foo;
        if (foo != value)
            foo = value;

When setting a property that is the source property of a data binding, the update must occur on the UI thread or an UnauthorizedAccessException will ensue. Source properties can be set from non-UI threads using the application’s Dispatcher as shown in the following excerpt:

            Foo = "bah";

There are a number of reasons why peppering your code with BeginInvoke calls is not a good idea. First, it imposes an unnecessary threading model on your viewmodel code. Second, it can lead to code that need not be executed on the UI thread, creeping in to the delegate. And, third, it is pretty ugly and decreases the readability of your code.

The next section looks at extracting INPC into a reusable and UI thread friendly class.

Implementing INotifyPropertyChanged: An Alternative Approach

Although there is nothing manifestly wrong with adding the OnPropertyChanged method to every class that implements INotifyPropertyChanged (apart from violating the DRY principle), it makes sense to extract the change notification code into a reusable class, because this allows you to not only reduce boilerplate code but also to add other features to the event-raising code, such as improving support for multithreaded apps and implementing INotifyPropertyChanging (as well as INotifyPropertyChanged).

The WPUnleashed project in the downloadable sample code includes such a class, named PropertyChangeNotifier. The ViewModelBase class delegates change notification to a PropertyChangeNotifier instance.

Throughout this book you frequently see viewmodel properties (with backing fields) resembling the following:

string foo;

public string Foo
        return foo;
        Assign(ref foo, value);

Here, the name of the property, the current value, and the new value are passed to the base class’s Assign method. The following excerpt shows the signature of the Assign method:

public AssignmentResult Assign<TField>(
        ref TField field,
        TField newValue,
        [CallerMemberName] string propertyName = "")

The Assign method updates the field value, while also offering the following advantages:

  • The application’s Dispatcher automatically raises the PropertyChanged event on the UI thread if called from a non-UI thread. This eliminates the need to add Dispatcher.BeginInvoke calls to a viewmodel to avoid cross-thread errors.
  • The Assign method also raises a PropertyChanging event. PropertyChangeNotifier implements INotifyPropertyChanging interface as well as INotifyPropertyChanged and allows a subscriber to cancel an update if desired.
  • PropertyChangeNotifier assists the viewmodel in remaining UI technology agnostic. That is, retrieving an application’s Dispatcher in a Windows Phone app is done differently in a WPF application.
  • PropertyChangeNotifier uses a weak reference to its owner, thereby preventing memory leaks from occurring when targets fail to unsubscribe from events.
  • The single line Assign method reduces the amount of boilerplate code in properties.

The return value of the Assign method is an AssignmentResult enum value, whose values are described in the following list:

  • Success—The assignment occurred and the field value now equals the new value.
  • Cancelled—A subscriber to the PropertyChanging event cancelled the assignment. This relies on a custom extension to the INotifyPropertyChanging event.
  • AlreadyAssigned—No assignment was made because the existing field value was already equal to the new value.
  • OwnerDisposed—The PropertyChangeNotifier uses a weak reference to the object for which it is providing property changing monitoring. This value indicates that no assignment was performed because the owner object has been disposed.

Because property change notification is such a common requirement of model and viewmodel classes, for the sake of convenience a NotifyPropertyChangeBase class is also provided in the downloadable sample code. It leverages an instance of the PropertyChangeNotifier, and can be used as a base class for any class that needs INotifyPropertyChanged to be implemented.

In particular, the ViewModelBase class inherits from this class (see Figure 2.10).

Figure 2.10

FIGURE 2.10. ViewModelBase inherits from NotifyPropertyChangeBase, which has a PropertyChangeNotifier.

The implementation details of the PropertyChangeNotifier are lengthy and are not included in this section. However, you can find an article already covering the topic at http://danielvaughan.org/post/Property-Change-Notification-using-a-Weak-Referencing-Strategy.aspx.

Before moving on to commanding, be assured that you do not need to use the property notification system presented here in your own projects. If you are happy using the traditional approach to INPC, that is perfectly fine. Be mindful, however, that a lot of the phone SDK APIs have events that do not always return on the UI thread, and you may need to rely more heavily on the Dispatcher to prevent cross-thread errors.

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