Today I had a close look at the Data binding possibilities provided by the Windows Presentation Foundation.
Before telling you more about that I’d like to show you good-old know alternatives and then compare them to WPF’s solution.
MFC with C++
DDX (Dialog Data Exchange) and DDV (Dialog Data Validation) are two great tools that allow us to easily set and access the values of certain MFC controls.
DDX allows us to link the value of a variable to a control. To use some DDX functions, you should override the DoDataExchange function of a dialog:
afx_msg void DoDataExchange(CDataExchange*);
The function itself can look like this, it just binds the “resource” to a member variable.
afx_msg void DoDataExchange(CDataExchange* pDX)
{
CDialog::DoDataExchange(pDX);
DDX_Text(pDX, TXT_NAME, m_strName);
}
The first line calls the base class, the seconds calls that function:
void DDX_Text(CDataExchange* pDX, int nIDC, CString& value);
If we want to set the value of the controls, we can call the UpdateData passing FALSE to the UpdateData function. On the other hand, if we want to retrieve the value of the control into our CString variable, we would pass TRUE to the UpdateData function.
BOOL CWnd::UpdateData(BOOL bSaveAndValidate)
{
CDataExchange dx(this, bSaveAndValidate);
try
{
DoDataExchange(&dx);
}
catch(CUserException e)
{
return FALSE;
}
return TRUE;
}
For other controls such as the list box, radio button and check box, we would use the similar DDX functions DDX_CBString, DDX_Radio and DDX_Check respectively.
When using DDX for the exchange of data between controls and variables we easily can validate them.
void CSample::DoDataExchange(CDataExchange* dx)
{
CDialog::DoDataExchange(dx);
//{{AFX_DATA_MAP(CDlg)
...
DDX_Text (dx, TXT_AGE, m_iAge);
DDV_MinMaxInt(dx, m_iAge, 0, 150); // <- validation
. . .
//}}AFX_DATA_MAP
}
The DDX_Text() call transfers the contents of the edit control into the m_age data member, converting it into an integer. After that is has to pass validation.
DDV_MinMaxInt looks as follows:
void DDV_MinMaxInt(CDataExchange* dx, int value, int min, int max)
{
if (value < min || value > max)
{
AfxMessageBox("Please enter a value between ...");
dx->Fail();
}
}
If the value doesn’t yield the condtions of the validation method a error message is shown and Fail() is invoked on the data exchange object.
Windows Forms with C#
Data binding in C# provides a more advanced way for developers to create a read/write link between the controls on a form. It’s often used for binding forms to complex data structures such as SQL databases. Windows Forms data binding allows you to access data from databases as well as data in other structures, such as arrays and collections.
Each Windows Form has at least one BindingContext object that manages the access to and from the controls. For example if you add use a textbox control to a form and bind it to a column of a table (e.g. “tblAddress.Name”) in a dataset(e.g. called “dsAddr”), the control communicates with the BindingContext object for that form.
txtname.DataBindings.Add("Tom Morello",Adresses,"tblAddress.Name");CurrencyManager cm = (CurrencyManager)this.BindingContext[dsAddr,"tblAddress"];
foreach(object s in cm)
{
Console.WriteLine(s.ToString());
}
The CurrencyManager is used to keep data-bound controls synchronized with each other.
That way of data binding is very useful when dealing with collection controls such as ListView or ListBox.
It’s common practice to bind them to any kind of Array, Collection or List.
A quick and clever way is the usage of data bindings in C# with ADO.NET Data Objects because those data structures are suitable for binding to.
DataTables such as GridView provide DataSource and DataMember properties:
gv1.DataSource = dsAddr;
gv1.DataMember = "tlbAddresses";
other appropriate objects are:
- DataColumn
- DataView
- DataSet
- DataViewManager
You also can bind controls to other controls. Binding data of a ComboBox to a ListBox follows the same model and is no problem.
DataBinding with WPF
Now we come to the point the article originally was about.
When working with collection-like data sources in WPF, you can read properties of the selected item by writing your binding expressions directly into the XAML (eXtensible Application Markup Language) code that defines the structure of every WPF-GUI.
<DockPanel DataContext="{x:Static Persons}">
<TextBlock DockPanel.Dock="Top"
Text="{Binding Lastname}" >
<ListBox ItemsSource="{Binding}"
IsSynchronizedWithCurrentItem="True" />
</DockPanel>
In that example the source is an array of persons names and the ListBox should show the lastname of every person the array contains.
When WPF encounters a binding to a non-existent property on a collection, WPF has a fallback strategy: it looks for the named property on the current item.
If source and destination are both collections you can do
to bind them to each other.
That’s nice behavoir but just the beginning of some great ideas.
There are several things you can define in the binding object:
- OneWay binding causes changes to the source property to automatically update the target property, but changes to the target property are not propagated back to the source property.
- TwoWay binding causes changes to either the source property or the target property to automatically update the other. This type of binding is appropriate for editable forms or other fully-interactive UI scenarios.
- OneWayToSource is the reverse of OneWay binding; it updates the source property when the target property changes.
- OneTime binding causes the source property to initialize the target property, but subsequent changes do not propagate. This means that if the data context undergoes a change or the object in the data context changes, then the change is reflected in the target property. This type of binding is appropriate if you are using data where either a snapshot of the current state is appropriate to use or the data is truly static.
The UpdateSourceTrigger property of the binding determines when binding should appear.
- LostFocus (default e.g. for TextBox)
- PropertyChanged
- Explecit (when calling UpdateSource)
Due data binding you could for example bind a textbox value to its background color. The property of type string in the binding source object is connected to a Color property of type Color.
IValueConverter makes it possible:
public class ColorBrushConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
Color color = (Color)value;
return new SolidColorBrush(color);
}public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return null;
}
}
To ensure that the user has entered the expected information we user input validate it with some logic.
At first we add a validation in description section of the GUI.
<TextBox Name="Birthday"
Style="{StaticResource textStyleTextBox}">
<TextBox.Text>
<Binding Path="BDay" UpdateSourceTrigger="PropertyChanged">
<Binding.ValidationRules>
<ExceptionValidationRule />
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
The ValidationRules property takes a collection of ValidationRule objects. When the user enters a value that cannot be converted to an the expected type, an exception is thrown, causing the binding to be marked as invalid.
You also can define your own rule:
<Binding.ValidationRules>
<src:MyRule />
</Binding.ValidationRules>
class MyRule : ValidationRule
{
public override ValidationResult Validate(object value, CultureInfo cultureInfo)
{
DateTime date;
try
{
date = DateTime.Parse(value.ToString());
}
catch (FormatException)
{
return new ValidationResult(false, "Invalid date!");
}
if (DateTime.Now.Date < date)
{
return new ValidationResult(false, "You cannot be born in the future.");
}
else
{
return ValidationResult.ValidResult;
}
}
}
You also might visualize a wrong input for the user.
<Style x:Key="textStyleTextBox" TargetType="TextBox">
<Setter Property="Foreground" Value="#fff000" />
<Setter Property="MaxLength" Value="10" />
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true">
<Setter Property="ToolTip"
Value="{Binding RelativeSource={RelativeSource Self},
Path=(Validation.Errors)[0].ErrorContent}"/>
</Trigger>
</Style.Triggers>
</Style>
Now a tooltip appears when the HasError property of Validation is true. You can define a so called Error Template that is used for invalid inputs, if you don’t the default is used.
There are enough and to spare features you can use when working with bindings in WPF, but telling about all of them would carry things too far.
Comparison and forecast
The way MFC programmers use is clear and easy to use. It’s not very casual to handle but it can be extented unique for the project’s necessities.
Win Forms are mostly the same - also the spreading of Win Forms applications on the software market seems to be similar to MFC apps.
From my point of view software development can be really enhanced by technolgies like those provided by the WPF. Also for non-WPF applications its always worth to think about those concepts.
ext4 is on the rise
Just after Linus Torvalds has released Linux 2.6.24 kernel the development on 2.6.25 has started. With 2.6.25 lots of changes concerning ext4 filesystem are suggested. 
