A Service Oriented ViewModel

Jacob Foshee

MonkeySpace 2013

Summary

• Service not reinvented by 3rd party
• VM not reinvented by 2nd device
• VM Responsible for wrapping DTOs and User's Intent
• Patterns yield one-liners for mapping VM property(s) to DTO(s)
• Email me if interested in the code:

  jacobf@gmail.com

Goals of the Talk

• Give back
• Synergy of ideas
• Result of a natural refactoring
• API Review
• On-Demand (Lazy) Open-Source

Goals of SOA+MVVM

• Take advantage of strong types and reflection
• Non-UI-blocking
• Use standard MVVM patterns (INotifyPropertyChanged)
• Observe little difference between a property being updated from a local calculation vs. fetching from a service.

Common Issues

• Synchronize local VM properties with remote persistence
• Disable part of UI while waiting for response (e.g. avoid double submit)
• Errors (log or display, don't allow an exception to go uncaught especially on background thread)
  • Developer needs details
  • User usually just needs to check connection

Background

• MVVM

• ServiceStack

MVVM

public class PropertyChangedEventArgs : EventArgs
{
    public virtual string PropertyName { get; }
}

public interface INotifyPropertyChanged
{
    event PropertyChangedEventHandler PropertyChanged;
}

ServiceStack

• Message Based
• IReturn <T>
• IHasLongId
• ServiceClientBase
• ILog

Patterns

Property is DTO

• Request & Set Property

• Post & Set Property

Property is part of DTO

• Post Update for Property

• (and disable while waiting)

Property is IList of DTO

• Post & Add

• Delete & Remove

Plus

• Raise PropertyChanged

• Log Errors

  For all of the above

A DTO

public class Monkey : IHasLongId, 
                      IReturn<Monkey>
{
    public long   Id        { get; set; }
    public bool   IsPrimate { get; set; }
    public string Text      { get; set; }
}

Initialize Property

public class MonkeyRequest : 
  IReturn<Monkey> { … }

public class MonkeyViewModel : 
  ServiceClientViewModelBase
{
  public Monkey Monkey { get; set; }

  public void OnInitialize(long id) {
    var req = new MonkeyRequest(id);
    RequestAndSetProperty(req, () => Monkey);
  }
}

New Record with …

Monkey Monkey        { get; set; }
string Text          { get; set; }
bool   IsTextEnabled { get; set; }

void SaveText() {
    if (Monkey.Id == 0) {
        Monkey.Text = Text;
        PostAndSetProperty(Monkey, () => Monkey,
          "IsTextEnabled");
    }
    else // post update …
}

Update Part of a Record

public class MonkeyTextUpdate : IReturn<Monkey> {
  public long   Id   { get; set; } 
  public string Text { get; set; } 
}

// …
else /* Monkey.Id != 0 */ {
  var req = new MonkeyTextUpdate(Monkey.Id, Text);
  PostAndSetProperty(req, () => Monkey, 
    "IsTextEnabled");
}

List of DTOs

public class MonkeyListRequest : 
  IReturn<IList<Monkey>> { ... }

public class MonkeyListViewModel : ServiceClientViewModelBase
{
    public IList<Monkey> Monkeys { get; set; }

    void RequestMonkeyList() {
      var req = new MonkeyListRequest();
        RequestAndSetProperty(req, () => Monkeys);
    }
}

Add / Remove

IList<Monkey> Monkeys { get; set; }

public void AddMonkey() {
    var newMonkey = new Monkey { … };
    PostAndAdd(newMonkey, () => Monkeys);
}

public void RemoveMonkey(Monkey monkey) {
    DeleteAndRemove(monkey, () => Monkeys);
}

Add Advanced

IList<Monkey> Monkeys { get; set; }
string NewMonkeyText  { get { … }; set { … Notify(() => NewMonkeyText); }
bool CanAddMonkey     { get { … }; set { … Notify(() => CanAddMonkey); }

public void AddMonkey() {
    CanAddMonkey = false;
    var newMonkey = new Monkey { Text = NewMonkeyText };
    PostAndAdd(newMonkey, () => Monkeys, 
    () => { // On Success
        NewMonkeyText = null;
        CanAddMonkey = true;
    }, // On Error
    () => CanAddMonkey = true);
}

Update Entire Record w/ wrapped DTO property

Monkey Monkey  { get; set; }
bool IsPrimate {
    get { return Monkey.IsPrimate; }
    set { Monkey.IsPrimate = value; }
}
bool IsPrimateIsEnabled { get; set; }

public void ToggleIsPrimate() {
    bool newPrimate = !IsPrimate;
    // If the update fails, we haven't changed 
    // the local copy misleadingly
    var req = new Monkey(Monkey) { 
      IsPrimate = newPrimate };
    PostUpdateForProperty(req, () => IsPrimate, 
      newPrimate);
}

Considerations

When & Where…

Service

• Integrity of the Persistent System
• Up to date
• Not reinvented by 3rd Party
• Example: Sign-off operations should be bundled together as a single service request

ViewModel

• Integrity of the Presentation
• User's Intent
• Out of date (snapshot)
  • Only update part of record
  • Don't send update if none intended (nothing changed)
• Not reinvented for another device

Examples

• One user on multiple devices
• Toggle checkbox
  • The toggling should be done in the VM so two "toggles" don't cancel out accidentally
  • Can represent failed intent by not checking the box
• Text box
  • Shouldn't erase the text when save failed, so need to represent failed intent another way

More VM Thoughts

• Implicit Save
• Fear the prospect of debugging network-related race conditions
• Do you wrap DTO properties?

View [Controller]

• Respect the ViewModel
• Respect the Device/Paradigm
• Widget-specific mechanics
• Update on Main/UI Thread

Future Work

• Email me for the code:

  jacobf@gmail.com

• ViewModel Factory stuff (hint: Segues)
• Convention for SaveFailedFor__ Properties
• Pattern for wrapping collection of DTOs
• Pub/Sub patterns

Acknowledgments

• MonkeySpace Organizers & Sponsors
• New York Presbyterian Hospital (now hiring…)

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