Random programming things I'd want to remember

Friday, February 28, 2014

Another simple MVVM tutorial for Windows Phone explained

I will take another attempt to explain the use of MVVM on Windows Phone. I already tried it before, but this time code is available on GitHub. Clone the project and follow along with my explanations.

This time we are building a registration database. The app will be tracking people's names along with gender.

1. Model

There is a class called "Person" in the Models folder. For the sake of simplicity, the app will track the name and gender.

2. View

There is MainPage.xaml file in ViewModels folder that is the front-end of the app (to change startup page for a Windows Phone app, go to Properties/WMAppManifest.xml and change it in "Navigation Page" field to whatever you need; the sample app already has it set up properly). There is a textbox for name, and radiobuttons to select a gender. The app must only allow to register when a name is provided and a gender is selected. So it can either provide feedback to the user when not all data is supplied for registration, or we can disable the Register button until all data is present.

There is a tricky part here. TextBox's text property originally had the following binding:

Text="{Binding UserName, Mode=TwoWay}"
The problem with that is that the Text property is updated in the view model only after textbox loses focus. To mitigate the situation, we can use the solution provided by Charles Petzold in his "Programming Windows Phone 7" book:
<!-- TextBox full declaration in XAML -->
<TextBox Text="{Binding UserName, Mode=TwoWay, UpdateSourceTrigger=Explicit}" 
                     TextChanged="boundTextBox_TextChanged" />

//and then, the event in the code-behind file for the View:
private void boundTextBox_TextChanged(object sender, TextChangedEventArgs e)
  TextBox txtbox = sender as TextBox;
  BindingExpression bindingExpression = txtbox.GetBindingExpression(TextBox.TextProperty);

As a result of this modification, every time a key is pressed in that TextBox, the property in our view model will be updated. One can argue that the true MVVM has no data in the code-behind files and this event might violate that principle, but the reality is that sometimes it's not easily possible. In addition, one of the main benefits of MVVM is testability. Having such an event in the code-behind of the view only affects internal workings of the app and does not affect data or business logic, so testability is very much preserved intact.

Now let's look a little closer at bindings. RegisterViewModel exposes properties and commands, which the view uses to display data or react to user input. For example, RegisterViewModel exposes a property named ProspectiveRegistration, which it calculates as as combination of name and gender:

public string ProspectiveRegistration
        string ret = this.UserName;
        ret += this.UserName.Length > 0 ? ", " : "";

        if (this.IsALady)
            ret += "Lady";
        if (this.IsAGentleman)
            ret += "Gentleman";

        return ret;
The view binds this property to a TextBlock's Text property in the following manner:
<TextBlock Text="{Binding ProspectiveRegistration}" />
How does the view know that the property was updated? The answer to that lies in the setters of the all the other properties used in ProspectiveRegistration (UserName, IsALady, IsAGentleman). They all have the following line of code:
This line of code is the magic that updates the view with the new value of the property.

The example above was somewhat simple because the view only displayed the value of a property that belongs to the view model. What if the view needs to send data back to the ViewModel? Let's look at how the gender radiobuttons are declared:

<RadioButton Content="Lady" IsChecked="{Binding IsALady, Mode=TwoWay}" GroupName="genderGroup" />
IsChecked property is declared as a two-way property, and not only it displays the state of the property in the view model, but it also sends its value back to the view model. This is one of the ways how the view propagates data to the view model.

3. ViewModel

The view model for the project is the class called RegisterViewModel and it resides in the ViewModels folder along with the class called ViewModel. ViewModel class is the parent class for RegisterViewModel and it has the plumbing for INotifyPropertyChanged interface. It also has plumbing for navigation since it may be a bit tricky if using MVVM, but there are usage instructions.


A couple of interesting points here. First, the data store is an ObservableCollection. The difference between an ObservableCollection and List is that ObservableCollection lets know when it changed. The app conveniently subscribes to the CollectionChanged event to update properties bound to the view.

Next, let's look at the register command. Declaration:

private DelegateCommand _registerCommand;
It is an instance of a DelegateCommand. What is a DelegateCommand? Any command bound to the view must implement ICommand interface. DelegateCommand wraps up all the members of the ICommand interface so that the app does not have to re-implement them every time. DelegateCommand.cs can be found in ServiceClasses folder.

_registerCommand as it is declared in constructor of the RegisterViewModel:

_registerCommand = new DelegateCommand(this.RegisterCommandAction, CanRegister); //initialize command
Method RegisterCommandAction is where the registration actually happens. This is what the command is supposed to do:
//Registration procedure
private void RegisterCommandAction(object obj)
    people.Add(new Person
        Name = this.UserName,
        Gender = this.IsALady ? "Lady" : "Gentleman"
    this.UserName = "";
    this.IsALady = false;
    this.IsAGentleman = false;
CanRegister method defines whether the command can execute or not. Here is what it looks like:
//Data consistency conditions that must be satisfied for a successful registration
private bool CanRegister(object arg)
    return (IsALady || IsAGentleman) && UserName.Length > 0;
This method enables/disables the register button in the view without having to bind the IsEnabled property of the button on the view. By the way, IsEnabled property can be managed by binding this property on the view to a boolean property on a view model. Pretty much any property on the view can be bound to a property on the view model, sometimes one might need to use a converter class (any class that implements an IValueConverter interface).

There is just one piece missing. The app somehow needs to know when CanRegsiter changes. For that, the DelegateCommand exposes a method called RaiseCanExecuteChanged(). Every time a property changes in the RegisterViewModel, not only the app has to notify that the property value changed, but it also has to make sure we can (or cannot) register. Here is, for example, an implementation of the UserName property in the view model:

private string _userName;
public string UserName
        if (_userName == null)
            _userName = "";
        return _userName;
        if (_userName == value)
        _userName = value;
        _registerCommand.RaiseCanExecuteChanged(); //checking on RegisterCommand to see if it can execute

4. Testing

Now you can see that MVVM decouples View from ViewModel making it easy to test ViewModels. Visual Studio (even Express versions) provides tools for testing. A couple of test methods included with the Test project give a basic idea of how testing can go.

5. Conclusion

Hopefully this article explained the usage of MVVM pattern on Windows Phone. Again, the code is available on GitHub, feel free to ask questions and/or leave comments

UPDATE 3/16/2014:

I updated the code in the repository to include a technique where a Text propery (or another one for that matter) can be used as a CommandParameter. See the code for full example, but in short, here is what you need to do:

In the View, I added a TextBox, gave it an x:Name property of "typeSomethingHere" (that's the second named control on the entire page). Then I added a Button, and I assigned the following properties to it besides Content and position on the page:

Command="{Binding DisplayInputWithoutAdditionalPropertyCommand}"
CommandParameter="{Binding ElementName=typeSomethingHere, Path=Text}"
The Command property is just as usual, but the CommandParameter property shows the beauty and flexibility of XAML, it grabs the Text property from the TextBox.

The rest are details, I create a string property ShowWhatWasTyped and it displays what was typed.

What's the bottom line here? I could have bound the Text property of a typeSomethingHere TextBox and use it to get text, but instead I taught the button to grab Text directly from the TextBox and pass it to the ViewModel. This works well only if you need to pass one value into a method quickly. You can, of course, use other properties of the ViewModel in the method.

Friday, February 21, 2014

Some notes on Angular.js

I have almost finished (read: abandoned) a project using Angular.js and here are some notes of little things that I would like to keep here so that I know where to find them next time.

This code snippet changes CSS class depending on the variable state:
ng-class="{CSSClassName: scopeVariable1 == scopeVariable2}"

The following regular expression finds the use of all scope variables that does not start with the letter "s":

Let's say there is a keyboard event, and the code needs to know what key was pressed. The following statement will provide the keycode to the function:
... ng-keypress="yourEventName($event.keyCode, otherArgsIfAny)" ...

Also, gotta remember that in JavaScript, when a variable's value is 'null', it means that there is such variable, but it has no value; when value is 'undefined' it means that there is no such variable.

In JavaScript, only the getMonth() function is zero-based (so December is month number 11), all other methods to get elements of a date are 1-based. To get year, use the getFullYear() function.

A new Date can be instantiated from milliseconds. For example:

var today = new Date(); //gets the date and time at the moment
var milliSec = Date.now(); //gets the number of milliseconds since epoch
//to reconstruct the date from millisec, just do this:
var someDate = new Date(millisec);

ng-show/ng-hide directives are pretty awesome to show/hide content:
...ng-show="oneVariable == anotherVariable"...
Or a function can be plugged in or a field or a string (if it is empty, then it would return false, otherwise -- true) or whatever else returns a boolean after evaluation.

Ordering a collection using Angular.js seems to be tricky. First of all, it only sorts arrays as of now. I had an array of object literals like so:
$scope.data = [{one:'one',two:'two'},{one:'three',two:'four'},{one:'five',two:'six'}];

and this sort expression worked for me:
<div ng-repeat = "a in data|orderBy:['one']">{{a}}</div>

You can access the ordinal number of the variable from within ngRepeat directive this way:
Read more about it on Angular.js docs site.

The statement above becomes useful, if the app needs to convert an array into a comma- (or other delimiter) separated string. Here is a way to do it:
<div ng-repeat="a in itemCollection">
  <span ng-show="$index < itemCollection.length - 1">, </span>
  <span ng-show="$index >= itemCollection.length -1">. </span>
Let's say itemCollection has 3 items. $index is a 0-based variable, so for the first two items ($index is 0 and 1 and both times is less than three, so the span with a comma will be shown and the span with a period will be hidden. Then, on the last item $index equals to 2, which is >= to the (itemCollection.length-1), so the span with a comma will be hidden and the span with a dot will show up.

Wednesday, February 19, 2014

Select all text in TextBox using C#

Here is what worked for me today:
    txtContent.SelectionLength = txtContent.Text.Length;
It does not work if the TextBox does not have focus. There are other solutions, but this one is quick and easy.

Friday, February 14, 2014

JavaScript: convert milliseconds to a date

Pretty easy, as it turns out:
  var dt = new Date(milliseconds);

And then one can use the standard Date methods available here. JSFiddle.