Forcing layout update - wpf

Question

How to force the layout measurements update?
I have simplified layout I am problem with; when you click the button first time you get one measurement and on the second click different one.
private void Window_Loaded(object sender, RoutedEventArgs e)
{
var w = mywindow.ActualWidth;
gridx.Width = w;
}
private void Button_Click(object sender, RoutedEventArgs e)
{
btn3.Width = 100;
var w = mywindow.ActualWidth;
gridx.Width = w - btn3.Width;
InvalidateArrange();
InvalidateMeasure();
MessageBox.Show(btn1.ActualWidth.ToString());
}
Window
<Window x:Class="resizet.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525" Loaded="Window_Loaded" Name="mywindow">
<DockPanel HorizontalAlignment="Stretch" LastChildFill="False">
<Grid HorizontalAlignment="Stretch" DockPanel.Dock="Left" Name="gridx">
<Button HorizontalAlignment="Stretch" Content="btn in grid" Click="Button_Click" />
</Grid>
<Button Name="btn2" Content="btn2" Width="0" DockPanel.Dock="Right" HorizontalAlignment="Left"></Button>
</DockPanel>
</Window>

Answer 1

This fixes the problem:
btn3.Width = 100;
btn3.Dispatcher.Invoke(DispatcherPriority.Render, EmptyDelegate);
var w = mywindow.ActualWidth;
gridx.Width = w - btn3.Width;
with additional
private static Action EmptyDelegate = delegate() { };

Answer 2

Changing the Width property must invalidate the layout on its own, you don't need to call InvalidateXXX() yourself.
The catch is that the layout is not updated immediately, but on the next iteration of the message loop. So the ActualWidth will not be changed immediately.
If you want the Grid to resize automatically when the button width is increasing, why not use the layout management and put the both into different columns of an outer Grid?
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid x:Name="gridx"
Grid.Column="0">
<Button HorizontalAlignment="Stretch"
Click="Button_Click"/>
</Grid>
<Button x:Name="btn2"
Content="btn2"
Width="0"
Grid.Column="1"/>
</Grid>
And in code-behind
private void Button_Click(object sender, RoutedEventArgs e)
{
btn2.Width = 100;
}

Answer 3

In a strict sense, #Daniel has provided some code that fixes the problem posed by the question. But the result is rather bad, putting layouting code into an event handler. The grid and the button might look good after the button got pressed, but once the user makes the window size bigger, the grid will not grow and will not use the available size. The user would have to press the button again to make the grid grow. That's most likely not how things should be and that's why #Vlad's answer is better.
WPF uses just one thread to process events and layouting, but they get executed in different phases. If width gets changed, the MeasureDirty flag of the control gets set, then the processing of the event continues immediately. Once this event and all other events needing processing are completed, only then starts WPF with the layouting (i.e. measure, arrange, render). Here is an overview how that works:
For a detailed description what happens in every step, see my article on CodeProject Deep Dive into WPF Layouting and Rendering
btn3.Dispatcher.Invoke(DispatcherPriority.Render, EmptyDelegate);
This statement halts the execution of the button event handler and forces a whole layouting / render phase to run, only then continues with the event handler code to change the width of the grid, which will force another layouting / render cycle to run.
Recommendations:
Do not set Height and Width in event handlers, unless you want them to be fixed and not to change, even the available space changes.
Use WPF controls like Grid, etc. to make best use of the available space
If you cannot find a WPF control like Grid which matches your layouting needs, write your own Control and put all the layouting code into MeasureOverride() and ArrangeOverride().