Is there any way to force execute the OnSizeAllocated method? - xaml

Question

I have created a view using Xaml code behind. I did it using the code behind because I wanted to change the layout of the view based on the device orientation. So, the problem which I am facing is that the OnSizeAllocated method is being called after the view is loaded. So, it is unable to change the layout as per the device orientation. I just want to know if there is any way to invoke the OnSizeAllocated method before the view is loaded. Please click on the below link to view the code:
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Answer 1

1.Rearrange the Page
you could check if width is greater than height to determine if the device is now in landscape or portrait:
public partial class Page13 : ContentPage
{
private double _width ;
private double _height ;
private Grid grid;
private Label label;
private Entry entry;
private Button button;
public Page13 ()
{
_width = this.Width;
_height = this.Height;
label = new Label(){Text = "i am a laber"};
entry = new Entry(){WidthRequest = 200};
button = new Button(){Text = "Submit"};
grid = new Grid();
UpdateLayout();
StackLayout stackLayout = new StackLayout();
stackLayout.Children.Add(grid);
Content = stackLayout;
}
protected override void OnSizeAllocated(double width, double height)
{
base.OnSizeAllocated(width, height);
if (_width != width || _height != height)
{
_width = width;
_height = height;
UpdateLayout();
}
}
void UpdateLayout()
{
grid.RowDefinitions.Clear();
grid.ColumnDefinitions.Clear();
grid.Children.Clear();
if (_width > _height)
{
ScreenRotatedToLandscape();
}
else
{
ScreenRotatedToPortrait();
}
}
private void ScreenRotatedToLandscape()
{
grid.RowDefinitions.Add(new RowDefinition(){Height = new GridLength(1,GridUnitType.Auto)});
grid.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(1, GridUnitType.Auto) });
grid.ColumnDefinitions.Add(new ColumnDefinition(){Width = new GridLength(1,GridUnitType.Auto)});
grid.ColumnDefinitions.Add(new ColumnDefinition() { Width = new GridLength(1, GridUnitType.Auto) });
grid.Children.Add(label,0,0);
grid.Children.Add(entry, 1, 0);
grid.Children.Add(button, 0, 1);
Grid.SetColumnSpan(button,2);
}
private void ScreenRotatedToPortrait()
{
grid.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(1, GridUnitType.Auto) });
grid.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(1, GridUnitType.Auto) });
grid.RowDefinitions.Add(new RowDefinition() { Height = new GridLength(1, GridUnitType.Auto) });
grid.ColumnDefinitions.Add(new ColumnDefinition() { Width = new GridLength(1, GridUnitType.Auto) });
grid.Children.Add(label, 0, 0);
grid.Children.Add(entry, 0, 1);
grid.Children.Add(button, 0, 2);
}
}
This is the recommended implementation pulled right from the Xamarin.Forms documentation.
2.Using Xamarin.Essentials
It adds additional functionality to cross-platform applications built in Xamarin. One of these new features is the ability to ping the device for the current orientation by accessing the DeviceDisplay.ScreenMetrics.Orientation property. This returns the current device orientation, which can be used to determine which layout to render.
it's similar to the one above
private bool IsPortrait;
public Page13 ()
{
...
IsPortrait = DeviceDisplay.ScreenMetrics.Orientation == ScreenOrientation.Portrait;
UpdateLayout();
...
}
void UpdateLayout()
{
grid.RowDefinitions.Clear();
grid.ColumnDefinitions.Clear();
grid.Children.Clear();
if (IsPortrait)
{
ScreenRotatedToPortrait();
}
else
{
ScreenRotatedToLandscape();
}
}

Answer 2

You can't force run that since the SizeAllocation hasn't changed, but you could do this to get orientation on initial load:
If you add the Xamarin.Essentials nuget package, as you can see here, you can get the orientation using this line of code DeviceDisplay.MainDisplayInfo.Orientation and you will get Landscape, Portrait, Square, or Unknown.
If you don't want to add the package, you can just use Application.Current.MainPage.Width and Application.Current.MainPage.Height to figure out orientation.