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Centering a mediaelement according to the aspect ratio

So, I've got this mediaelement, which I want to appear in the middle of the screen, 20 pixels away from each edge, and when it's width gets to 600, it stops expanding and stays where it is. I've got the following XAML to get exactly what I want:
<Grid x:Name="video">
<MediaElement x:Name="player" AreTransportControlsEnabled="True" Margin="20,0" MaxWidth="600" HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Grid>
The only problem is that the VerticalAlignment property seems to always be set to "Stretch". How can I fix this? I've tried using a viewbox, which works flawlessly, except I then get overly large or small TransportControls. I've also tried putting the mediaelement inside another grid with centered alignments, but I still get the same problem.

What you want is
20 pixels away from each edge.
But in your code you defined Margin="20,0", if you read the FrameworkElement.Margin property, you will find the useage <frameworkElement Margin="left+right,top+bottom"/>. With your code, this usage makes your MediaElement stretch in the vertical orientation.
If you use the Live Visual Tree to get a real-time view of your running XAML code, you will find, when you use the code Margin="20,0", the MediaElement is like this:
And if you use the code Margin="20", the MediaElement is like this:
The dashed red border in each picture stands for the MediaElement control.

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<Pivot Margin="0,0,0,0" Background="LightGray">
<PivotItem Header="Formulas">
<Grid>
<GridView HorizontalAlignment="Stretch" HorizontalContentAlignment="Center" Background="#FF983636">
<GridView.ItemsPanel>
<ItemsPanelTemplate>
<VariableSizedWrapGrid Orientation="Horizontal" />
</ItemsPanelTemplate>
</GridView.ItemsPanel>
<x:String>One</x:String>
<x:String>Two</x:String>
<x:String>Three</x:String>
<x:String>Four</x:String>
</GridView>
</Grid>
</PivotItem>
</Pivot>
</Grid>
The key here is HorizontalAlignment="Stretch" on the GridView.
The HorizontalAlignment="Center" on the VariableSizedWrapGrid inside the ItemsPanelTemplate inside the Gridview; should do the trick which will always keep the content centered regardless of the width of the screen.

In the end, I found how to fix this, by keeping a constant aspect ratio of 16/9:
XAML:
<Grid SizeChanged="player_SizeChanged" Background="Black" HorizontalAlignment="Center" VerticalAlignment="Center" MinWidth="1" MinHeight="1" Margin="30" MaxWidth="1000" ScrollViewer.VerticalScrollBarVisibility="Disabled">
<MediaElement x:Name="player" Margin="0">
</Grid>
C#:
private void player_SizeChanged(object sender, SizeChangedEventArgs e)
{
(sender as Grid).Height = (e.NewSize.Width * 0.5625);
}

Fill StackPanel inside DockPanel

This should be easy: how can I stretch StackPanel inside the DockPanel, so that it would fill the whole parent's content and also maintain it's HorizontalAlignment?
Example:
<DockPanel LastChildFill="True">
<StackPanel Orientation="Horizontal" HorizontalAlignment="Center" Background="Yellow">
<Button Height="30">Button1</Button>
<Button Height="30">Button2</Button>
</StackPanel>
</DockPanel>
In this example the StackPanel's width is the same as the combined width from both buttons (and the background here is yellow), but the remaining space in DockPanel stays white. Looks like the LastChildFille property isn't working in my example.

I think by setting HorizontalAligment to Center on the StackPanel you are overriding the behavior of the DockPanel to fill the whole space with the StackPanel.
When i understand your question right, you want that the full space is yellow and the stackpanel with buttons is centered in the middle. Then you should wrap the StackPanel inside a Grid.
<DockPanel LastChildFill="True">
<Grid Background="Yellow">
<StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
<Button Height="30">Button1</Button>
<Button Height="30">Button2</Button>
</StackPanel>
</Grid>
</DockPanel>

Scrollable canvas containing imges in windows phone 8

I am working on a windows phone 8 app. I have a canvas containing images which I can drag to change their position in canvas. My problem is that canvas is not scrollable, I want a vertical scrollbar in page. I tried like this
<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
<ScrollViewer x:Name="scvImages" HorizontalAlignment="Stretch"
VerticalAlignment="Stretch" Margin="0"
HorizontalContentAlignment="Stretch" Height="Auto" Width="Auto">
<Grid Height="Auto" Width="Auto">
<Canvas x:Name="canImages" Height="Auto" Width="Auto">
</Canvas>
</Grid>
</ScrollViewer>
</Grid>

At first glance it looks like there's nothing telling the ScrollViewer to invoke since your Grid and Canvas are just going to fill the ViewPort so you might consider setting a fixed Height or something on your Grid

You must specify the height of your panel (canvas, grid, whatewer) which you want to scroll. Otherwise it will render out of the screen but 'thinks' that it is ok and scrolling is no needed (since it is infinite height).

Issue with ScrollViewer in WPF

I am working on a WPF application and using a scrollViewer to view the content that is offlimit the screen area. Working fine, no issues.
But if my window contains a Listbox or Grid or anything like that and that control has many records then instead of adding a scrollbar to itself, it just increase the height of the control and window itself because scrollviewer believes it needs to extend.
I don't want to hardcode the listbox's height, because that make it same in different resolutions, i want to make it increase its height but not always as scrollviewer make it do so.
Thanks

You can't include a variable height/width object inside a ScrollViewer without setting the height/width of the viewer.
The scroll viewer effectively has infinite height so the grid expands to fill the "available" space - hence the effect you are seeing. As #JoeWhite says in his comments, the ScrollViewer is a container that can be as tall as it needs to be to hold all of its content. Anchoring wouldn't help - in effect, your ListBox already is anchored, it's just anchored to something that says "oh, I'll be whatever size you need me to be".
You'll need to either restrict the height, move the ListBox outside the ScrollViewer or use something other than a ScrollViewer.
Again quoting #Joe "These are the hazards of putting a scrolling area inside another scrolling area - it's poor usability and it's hard to define the behaviour."

You can wrap ScrollViewer into Grid, and bind scrollviewer's Width and Height properties to grid's ActualWidth and ActualHeight. So the scrollviewer will have fixed size equal to the size of the grid which will change when window resize.
Example:
<Grid x:Name="LayoutRoot" Background="White">
<Grid Background="#FFF1F1F1" Height="49" VerticalAlignment="Top">
<Button Content="Обзор" Margin="0,13,175.25,0" VerticalAlignment="Top" FontSize="14.667" HorizontalAlignment="Right" Width="95.147">
</Button>
<Label Content="{Binding DocPath, Converter={StaticResource FileNameConverter}, FallbackValue=Выберите файл, TargetNullValue=Выберите файл}" HorizontalAlignment="Left" VerticalAlignment="Top" Width="342.603" Margin="10,10,0,0" Height="33"/>
<Button Content="Загрузить данные" HorizontalAlignment="Right" Margin="0,13,10,0" VerticalAlignment="Top" Width="151.147" FontSize="14.667">
</Button>
</Grid>
<Grid x:Name="scrollBorder" Margin="10,54,10,10">
<ScrollViewer x:Name="LogScroller" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto"
HorizontalAlignment="Left" VerticalAlignment="Top"
Height="{Binding ActualHeight, ElementName=scrollBorder}" Width="{Binding ActualWidth, ElementName=scrollBorder}" >
<ItemsControl ItemsSource="{Binding Log}" />
</ScrollViewer>
</Grid>
</Grid>

WPF Create a slide out panel

I don't know how this works technically but my requirement is as follows. I have a DataGrid and to input data into the DataGrid, I want a panel at the bottom of the DataGrid that slides out on a button click showing input options. Except, as the panel slides out, the DataGrid has to resize vertically as well. Can someone throw some light on how I can implement this?

You should be able to use a StackPanel with 2 children, your grid and your panel. Set the initial height of your panel to 0. Once the button is clicked, set the height to whatever you need it to be (e.g., MyPanel.Height = 20). You might want to wrap the grid in a ScrollViewer in case that is needed.
<StackPanel Orientation="Vertical">
<ScrollViewer Height="Auto" VerticalAlignment="Stretch">
<Grid Height="*" VerticalAlignment="Stretch" />
</ScrollViewer>
<ContentControl x:Name="MyPanel" Height="0" />
</StackPanel>
You might need to experiment with VerticalAlignment and Height="Auto" or Height="0" to get the layout you want.

You can use Expander. Please look at the following code snippet.
<DockPanel>
<Expander DockPanel.Dock="Bottom">
<StackPanel>
<TextBlock Height="25"></TextBlock>
<TextBlock Height="25"></TextBlock>
<TextBlock Height="25"></TextBlock>
</StackPanel>
</Expander>
<Border BorderBrush="LightGreen" BorderThickness="2">
<DataGrid/>
</Border>
</DockPanel >