I have a Page where I want to display a string property in a Label.
This is my code, but nothing will appear in the label.
This is my .xaml
<Page x:Class="MyProject.PageOne"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="300"
Title="PageOne"
Name="pageOne>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
</Grid>
<Label Grid.Column="0" Content="{Binding ElementName=pageOne, Path=aStr}" FontWeight="Normal" FontSize="43" HorizontalAlignment="Left" Margin="0,00,0,0" VerticalAlignment="Center" Foreground="White"/>
</Grid>
</Page>
And this is my .cs code
public partial class PageOne: Page, IPageInterface
{
public String aStr{get;set;}
public PageOne()
{
InitializeComponent();
}
public void Start()
{
aStr = "Test";
}
}
The only thing really wrong with the code you posted, in terms of the problem you describe, is that you have not implemented some way for property change notifications to occur. Because the aStr property is not set to the new value until after the Label content has been initially set, without a way to receive notification, the framework has no way to know it needs to update the Label content.
In WPF the two main ways this is typically done (indeed, AFAIK the only two fully supported ways) are to create DependencyProperty instances, or to implement INotifyPropertyChanged. Either will work fine.
Here is an example of how your code should look with INotifyPropertyChanged implemented:
public partial class PageOne : Page, IPageInterface, INotifyPropertyChanged
{
private string _astr;
public String aStr
{
get { return _astr; }
set { _astr = value; OnPropertyChanged(); }
}
public Page1()
{
InitializeComponent();
}
public void Start()
{
aStr = "Test";
}
private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
Implementing the interface involves a couple of simple steps:
Declare the event named PropertyChanged
Any time a property is changed, raise that event, passing the name of the property that's changing.
Note that to do this, you can't use auto-implemented properties. You need to implement each property yourself, with a backing field, and a call to a method that will raise the property.
.NET offers the convenient [CallerMemberName] attribute, which I show here. So in the setter method for your aStr property, after setting the backing field's value, you simply call the method without any parameters, and the runtime automatically fills in the correct property name for you.
Now, the code you posted has some other problems as well, in the XAML. First it won't compile because you left out a " character, and because you've got an extra </Grid> closing tag.
One other possible problem, though it's not possible to know for sure since we are missing the full context of how you display this Page object, is that the text's color is white. If you're putting the Label instance on a white background, then of course you won't be able to see the text, even if it were set correctly.
I note that commenter Franck has suggested that you should set the DataContext. The truth is, given the code you posted this is actually not necessary, and doing so wouldn't actually fix the problem you are having.
But if you do fix the underlying notification issue, then his suggestions are an alternative way that you can achieve the binding. By setting the DataContext to the object containing the property (here, your PageOne class), then when you are binding you can just specify the property name alone, without having to include the ElementName at all, and without having to use the Path= with the property name. You may find this technique more convenient, at least some of the time.
In the future, please take the time to provide a good, minimal, complete code example that reliably reproduces the problem. You are more likely to get an answer that way, and you will ensure that any answer you do get is as good as it can be.
Related
I'm working on a UWP application and I realized that the default UpdateSourceTrigger mode for the TextBox control, which is LostFocus, can't be changed when using a compiled binding.
This means that whenever I want the binding to update for a TextBox, I have to use all this repeated boilerplate:
<TextBox
Text="{x:Bind ViewModel.Title, Mode=TwoWay}"
TextChanged="TextBox_OnTextChanged"/>
private void TextBox_OnTextChanged(object sender, TextChangedEventArgs e)
{
ViewModel.Title = ((TextBox)sender).Text;
}
Now, this is not too bad, but having to remember to create the TextChanged handler every single time a TextBox is used is annoying and error prone.
This would work fine with a classic binding:
<TextBox Text="{Binding Title, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
But of course, here there would be the additional overhead of usinc classic bindings (involving runtime reflections, etc.).
Is there a way to get the same behaviour of UpdateSourceTrigger=PropertyChanged as well? I'd be completely fine with, say, writing a custom attached property that sets things up, as long as I can do everything I need directly from XAML, with no code behind involved.
Thanks!
UPDATE: (in response to Nico Zhu - MSFT's answer)
For my testing, it works well.
It doesn't for me, at all, As I said multiple times already, using UpdateSourceTrigger with x:Bind is just not possible. It doesn't compile, the property is shown in red in the XAML editor, it just isn't there. I really don't know where are you trying that, if you say it's working for you. I'm currently targeting 17763 as minimum and I can 100% guarantee that that does not work.
Compiled Binding is used with the {x:Bind} syntax as opposed to the {Binding} syntax of Classic Binding.
I'm well aware of the difference, I've already mentioned this multiple times, both in my original question here (with code snippets too) as well as in my comments.
It still uses the notifying interfaces (like INotifyPropertyChanged) to watch for changes
As I said, I'm aware of this too. But again, as from this question, this isn't the problem here at all. The issue is not with updates from the viewmodel to the bound property, but from the bound property (TextBox.Text in this case) to the viewmodel.
{x:Bind} is by default OneTime compared to {Binding} which is OneWay. so you need to declare bind Mode OneWay or TwoWay for {x:Bind}.
I'm sorry, but I have to say at this point I'm starting to wonder if you've actually read my initial question at all. I'm aware of this, and in fact you can see in both my original code snippets that I had already used the explicit Mode=TwoWay property in both my bindings.
And once again, this was not what the question was about, at all.
To reiterate: the issue here is that the TextBox.Text property defaults to the LostFocus trigger, and that the UpdateSourceTrigger property is not available for compiled bindings. So I'd like to know if there's a way to achieve the same, with a compiled binding, in XAML-only, without having to manually create a TextChanged handler every single time (and if not, if you plan to eventually add the UpdateSourceTrigger property to compiled bindings too).
Side note: I didn't mean to sound disrespectful here, and I hope we've now solved the existing misunderstandings with my question.
UPDATE #2: turns out the issue was causing by the ReSharper plugin, which was marking the UpdateSourceTrigger property as error in compiled bindings.
I've opened an issue for that here: https://youtrack.jetbrains.com/issue/RSRP-474438
Please check UpdateSourceTrigger documentation.
The default UpdateSourceTrigger value is Default. And
using default behavior from the dependency property that uses the binding. In Windows Runtime, this evaluates the same as a value with PropertyChanged. If you used Text="{x:Bind ViewModel.Title, Mode=TwoWay}", the Title will be changed when text changes. we have not need modify the viewmode in TextChanged even handler.
The premise is that we need implement INotifyPropertyChanged like the follow.
public class HostViewModel : INotifyPropertyChanged
{
private string nextButtonText;
public event PropertyChangedEventHandler PropertyChanged = delegate { };
public HostViewModel()
{
this.NextButtonText = "Next";
}
public string NextButtonText
{
get { return this.nextButtonText; }
set
{
this.nextButtonText = value;
this.OnPropertyChanged();
}
}
public void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
// Raise the PropertyChanged event, passing the name of the property whose value has changed.
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
For more detail please refer Data binding in depth document.
Update
<TextBox Text="{x:Bind Title, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" /> doesn't compile at all, as I said the UpdateSourceTrigger property isn't available at all when using a compiled binding.
For my testing, it works well. Compiled Binding is used with the {x:Bind} syntax as opposed to the {Binding} syntax of Classic Binding. It still uses the notifying interfaces (like INotifyPropertyChanged) to watch for changes but {x:Bind} is by default OneTime compared to {Binding} which is OneWay. so you need to declare bind Mode OneWay or TwoWay for {x:Bind}.
Xaml
<StackPanel Orientation="Vertical">
<TextBox Text="{x:Bind Title, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
<TextBlock Text="{x:Bind Title, Mode=OneWay}" /> <!--declare bind mode-->
</StackPanel>
Code behind
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
private string _title;
public string Title
{
get
{
return _title;
}
set
{
_title = value;
OnPropertyChanged();
}
}
I am very sorry that this question is very basic. I just learned WPF and I failed to make simple two way binding to textbox.text to string property.
XAML Code:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApplication1"
mc:Ignorable="d"
Title="MainWindow" Height="350" Width="525">
<Grid x:Name="StuInfo">
<TextBox x:Name="textBox" HorizontalAlignment="Left" Height="23" Margin="10,26,0,0" TextWrapping="Wrap" Text="{Binding Path=str,Mode=TwoWay}" VerticalAlignment="Top" Width="120"/>
<Button x:Name="button" Content="Check" HorizontalAlignment="Left" Margin="10,67,0,0" VerticalAlignment="Top" Width="75" Click="button_Click"/>
</Grid>
C# Code
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
str = "OK";
}
public string str { get; set; }
private void button_Click(object sender, RoutedEventArgs e)
{
Console.WriteLine(str);
}
}
First, the textbox does not show "OK", but it is blank. Then, I typed a different text into the textbox, for ex:"blablabla" without the quotes. Then I click the button to check if my str property has been updated. Apparently, str still contains "OK".
What did I do wrong here? What did I miss to make the binding work?
As a newcomer to WPF, all this Binding and DataContext jazz can be quite confusing. Let's start with your binding expression first...
<TextBox Text="{Binding Path=str, Mode=TwoWay}"/>
What this is saying is that you want to bind your Text property to whatever the DataContext of the TextBox is. DataContext is essentially the "thing" your TextBox is getting it's data from. Now here's the rub. DataContext is inherited from the element "above" it in the visual tree if not explicitly set. In your code, TextBox inherits it's DataContext from the Grid element, which in turn inherits it's DataContext from the Window element. Seeing that DataContext is not set in your Window the default value of the DataContext property will be applied, which is null. The DataContext is also not set in any of the child elements of your window, which, via inheritance, will set the DataContext of all children of that window to null.
It is important to note that you've left out the Source property in your binding expression.
<TextBox Text="{Binding Source=left_out, Path=str, Mode=TwoWay}"/>
When this property is left out, the binding's source is implied to be the elements DataContext, which in this case is null, for the reasons mentioned above. Basically, what your expression is saying here is that you want to bind your text property to DataContext.str which resolved by WPF is null.str.
OK, cool. Now, how do we set the DataContext of your TextBox.Text binding to the Code Behind for the window so we can get at that str property? There are several ways to do this, but for our purposes we'll focus on setting it explicitly in the binding of the TextBox.Text property. Now, there are three different "source" type properties of bindings. "Source" being where we want our control/element's binding to get it's data from. We have Source, RelativeSource, and ElementName. We're only going to focus on ElementName here, but the others are essential to research and understand.
So, let's name our Window element so we can access it through the ElementName property.
<Window x:Class="WpfApplication1.MainWindow"
x:Name="_window"
...
Now we can set the ElementName property on the TextBox.Text binding to refer to the window.
<TextBox Text="{Binding ElementName=_window, Path=str, Mode=TwoWay}"/>
This means the binding will look for the _window.str property when trying to resolve it's binding. At this point, you still probably won't see your str value reflected in the TextBox. This is because it's value is set after the InitializeComponent method in the window's constructor. This function is where bindings are resolved for the first time. If you were to set the value of str before calling InitializeComponent, you would see the value reflected in the TextBox.
This brings us to Dependency Properties. For now, just know that Dependency Properties have built in change notification, which your binding needs so it "knows" when the binding has changed and when to resolve the binding value again. Yes, you could use INotifyPropertyChanged in your code behind, but there are good arguments for using DependencyProperties in this case, which will only confuse the issue at this point. But, it is another one of those things that is essential to understand.
Here is the code for a DependencyProperty for your str property.
public static readonly DependencyProperty StrProperty
= DependencyProperty.Register("Str", typeof(string), typeof(MainWindow),
new FrameworkPropertyMetadata(FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
public string Str
{
get{return (string)GetValue(StrProperty);}
set{SetValue(StrProperty,value);}
}
Now you'll be able to set the value like such and have it reflect through the binding to your TextBox.
public MainWindow()
{
InitializeComponent();
Str = "OK";
}
At this point, all should be well. I hope this helps out. It took me a while get the hang of WPF. My suggestion would be to read as much as you can on DataContext, Binding, and DependencyProperty as these are the core of WPF. Good luck!
The problem is that, you dont bind to codebehind of Window, but to DataContext.
Try this:
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
DataContext = new DC();
}
public class DC
{
public string str { get; set; }
public DC()
{
str = "OK";
}
}
}
Normally, you would have two different files, but for test, you can do it in one file.
After that, your DC (DataContext) should implement INotifyPropertyChanged interface.
Try to find some article about MVVM like this http://www.codeproject.com/Articles/165368/WPF-MVVM-Quick-Start-Tutorial
I have a code in wich i need to be able to access to a different amount of prebuilt grids in XAMl and make them visible or collapsed
All grid are named like grid1,grid2,grid3 etc. I have the ability in code to obtain the string name via a random number and get the name od the grid i'd like to show.
I searched online and people suggest to use the reflect method, but i'm having a hard time trying to figure out the syntax that i have to use.
Best regards
The most straight forward way of doing this is to just declare a Name value for each Grid...:
<Grid Name="Grid1">
...
</Grid>
... and then you can access them by that name from the code behind:
Grid1.Visibility = Visibility.Hidden;
However, this is WPF and that is generally not recommended. A preferred method would be to add some bool properties to your code behind or view model...:
public bool IsGrid1Visible { get; set; } // Implement INotifyPropertyChanged interface
... and then to bind these directly to the Grid1.Visibility property using a BooleanToVisibilityConverter:
<Grid Grid1.Visibility="{Binding IsGrid1Visible, Converter={StaticResource
BooleanToVisibilityConverter}}">
...
</Grid>
Then you can change the Grid.Visibility value by simply setting the IsGrid1Visible property to true or false.
Greetings folks!
I'm running into a problem with WPF databinding that I hope you can help out with. I'm new to WPF but an expereienced developer (VB 3.0-6.0, C#).
Here's the scenario:
I have a C# project called MasterPartsData which contains a number of classes which reprsent different types of parts (capacitor, diode, etc). They inherit from a base class called clsPart.
I have another C# WPF project which contains WPF UserControls (as well as a MainWindow) to visually represent the values stored in an individual MasterPartsData (MPD) object. I've created a private field in the usercontrol to hold the object with a getter and setter.
If I create a binding explicitly in the setter for the populated object:
_capacitor = value;
Binding binding = new Binding();
binding.Source = _capacitor;
binding.Path = new PropertyPath("C0uf");
this.txtC0uf.SetBinding(TextBox.TextProperty, binding);
(with _capacitor being the private object variable and C0uf being the property name)
the value correctly displays.
However I don't wish to have to explicitly create each binding in the code behind. My preference is to create the bindings inline in XAML, perhaps with a DataContext pointing to the object.
Unfortunately every different permutation I've tried fails to work; the text box doesn't show data.
I have a couple of suspicions:
1) The binding is correct, but the text box needs to be refreshed.
2) The binding is confused between the private variable and the properties.
3) Maybe the fact that the class is defined in a different project is causing issues.
4) I'm going mad and should check myself into an asylum before someone gets hurt. :)
Any help you can provide would be most appreciated. I'm more than happy to add more information, but didn't want to clutter the question with pages and pages of source.
With respect to your suspicions:
1) I think the default binding behavior of a TextBox is TwoWay, with a LostFocus update trigger, meaning that your UI focus will have to change to another control before the binding will update, if changes are made in the UI.
If changes are made in the code you need to raise the NotifyPropertyChanged event in order for the binding system to see it.
2) This is probably not the case, but it leaves the impression that you're trying to set bindings on your UserControl properties, which is not the way data binding was designed to be used in this particular kind of use case. What you want is to bind data from non-UI classes to dependency properties on your UserControls.
3) This will never matter, as long as your UI project has a reference to your classes.
4) This is a common reaction people have when beginning to use XAML and WPF. It's like instead of being handed a box of Legos, you just got handed an injection molding machine with insufficient instructions, isn't it?
Overall, this is a situation where you might need to examine your design; elements of the "Model-View-ViewModel" pattern will come in handy. If you're unfamiliar with this, it's a development pattern in which you introduce a "ViewModel" class, perhaps you can call it MasterPartsVM which contains an implementation of INotifyPropertyChanged.
The DataContext of your UserControl would be set to this MasterPartsVM class.
A brief code example, using some generic names. Given a ViewModel class with a small backing class that looks like this:
class PartViewModel : INotifyPropertyChanged
{
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
public PartClass Data { get; set; }
public String SomeVMProperty
{
get { return Data.SomeProperty; }
set
{
if (Data.SomeProperty != value)
Data.SomeProperty = value;
this.PropertyChanged(this, new PropertyChangedEventArgs("SomeVMProperty"));
}
}
}
class PartClass
{
public string SomeProperty { get; set; }
}
The XAML of a basic UserControl would look like this:
<UserControl x:Class="WpfApplication1.PartUserControl"
... >
<Grid>
<TextBox Text="{Binding SomeVMProperty}" Margin="68,77,104,176" />
</Grid>
</UserControl>
To connect your data class to this UserControl, you set the UserControl's DataContext property. If you do this in code, it's a matter of having a reference to your user control and the ViewModel, and then setting the property:
MyUserControlInstance.DataContext = new PartViewModel(); // or some existing PartViewModel
That combination of code should work to produce a textbox whose Text property changes every time the SomeVMProperty property is changed.
In a basic binding scenario, if your class looks like this
public class MasterPartsData
{
private string _c0uf;
public string C0uf
{
get { return _c0uf;}
set { _c0uf = value;}
}
public MasterPartsData()
{
C0uf = "Hello World!";
}
}
your XAML would look like this
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1"
Title="MainWindow" >
<Window.DataContext>
<local:MasterPartsData />
</Window.DataContext>
<Grid>
<TextBlock Text="{Binding Path=C0uf}" />
</Grid>
</Window>
Note, there are many different approaches to setting the DataContext, you don't necessarily just have to do it in the XAML
Also, typically your MasterDataParts class would implement INotifyPropertyChanged
<UserControl
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:myapp="clr-namespace:MyPlayer.Model"
mc:Ignorable="d"
x:Class="MyPlayer.VolumeButtons"
x:Name="UserControl"
d:DesignWidth="640" d:DesignHeight="480">
<UserControl.Resources>
<myapp:MusicPlayerModel x:Key="Model"/>
</UserControl.Resources>
<Grid x:Name="LayoutRoot" DataContext="{StaticResource Model}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="35px"/>
<ColumnDefinition Width="1.0*"/>
</Grid.ColumnDefinitions>
<Slider Value="{Binding Volume}" Margin="0,0,0,0" Grid.Column="1" VerticalAlignment="Center" x:Name="volumeSlider"/>
<Button Margin="4,4,4,4" Content="Button" x:Name="muteButton" Click="MuteButton_Click"/>
</Grid>
</UserControl>
Now the thing is, databinding is working correct when I am moving my slider(the model is updated when I am moving the slider).
However when a button is clicked I change the value in the model and expects it to update the view.
Code below:
private void MuteButton_Click(object sender, RoutedEventArgs e)
{
musicPlayerModel.Volume = 0;
}
Code in model:
public double Volume
{
get { return this.volume; }
set
{
this.volume = value;
this.OnPropertyChanged("SomeTestText");
this.OnPropertyChanged("Volume");
}
}
But in OnPropertyChanged, the event is null and therefor nothing happens. Why is the event null and not when my slider is moving and how to solve it?
You should not call the event directly. What you are doing is correct, but only if the OnPropertyChanged method is implemented correctly. In the pattern recommended by Microsoft and used throughout the BCL, the OnPropertyChanged (and any OnXXXXEventName) should look as follows:
protected override void OnPropertyChanged(PropertyChangedEventArgs e)
{
// get handler (usually a local event variable or just the event)
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, e);
}
}
If this is correct, you shouldn't need to worry about the event being null etc. But your code shows this.OnPropertyChanged("SomeTestText"); which is not legal. The string is not a valid parameter. According to the event pattern, the OnPropertyChanged event should look as above, which means you should call it as follows:
this.OnPropertyChanged(new PropertyChangedEventArgs("Volume"));
Note: if the handler (event) is null, then the calling application has not been registered to the event. Through code, this can be done with somevar.PropertyChanged += handlerMethod.
Edit: about Slider / WPF and events
In a comment you suggest that it "should" go automatically. But in the code above, you call OnPropertyChanged with a string. As mentioned before, that's not legal code, because OnXXX methods should have one parameter which inherits from EventArgs. Though I considered a normal PropertyChanged event in your case, XAML and WPF gave room for another interpretation (sorry for only getting to this now).
You (and I) were mistaken about one thing. This quote from MSDN explains that:
"Note that there is an identically
named OnPropertyChanged method with a
different signature (the parameter
type is PropertyChangedEventArgs) that
can appear on a number of classes.
That OnPropertyChanged is used for
data object notifications, and is part
of the contract for
INotifyPropertyChanged."
What you need to do in the code where you override the Volume property, you should call PropertyChangedCallback instead, or use your OnXXX code with as parameter a DependencyPropertyChangedEventArgs structure. If you ask me, your current approach is way easier, even though you don't use the original WPF methods ;-)