Changing isVisible property of Xamarin Forms XAML buttons

Change your code in xmal file and write properties for start and stop button

<Button x:Name="start_btn" Clicked="startPanic" IsVisible="{Binding IsStartVisible}">
    <Button.Text>START</Button.Text>
</Button>

<Button x:Name="stop_btn" IsVisible="{Binding IsStopVisible}">
    <Button.Text>STOP</Button.Text>
</Button>

In ViewModel write following property and similar for start button and set IsStopVisible =true/false based on your logic

   private bool _isStopVisible;

    public bool IsStopVisible{
        get {
            return _isStopVisible;
        }
        set {
            _isStopVisible= value;
            RaisePropertyChanged ("IsStopVisible");
        }
    }

It should work just fine. I copied your code and cleaned it up a bit, it shows the STOP button, then I

A few remarks:

• use the short property where possible <Button Text="X"/>, it's easier to read
• when you add a XAML page the IDE adds a .xaml.cs file next to it and generates another .g.cs that you don't see. The .g.cs file contains generated code that finds all the x:Name'd elements and defines placeholders for them, no need to find them by name yourself
• all UI-initiated events are executed on the UI thread, no need to do that explicitly

Here's the XAML, same as yours just tighter and added Margin so the button is visible

<StackLayout Orientation="Vertical" Margin="20">
  <Button x:Name="start_btn" Clicked="startPanic" Text="START" />
  <Button x:Name="stop_btn" Text="STOP" IsVisible="false" />
</StackLayout>

And the code behind:

public partial class TestPage : ContentPage
{   
    public TestPage ()
    {
        InitializeComponent ();
    }

    private void startPanic(object sender, EventArgs args){
        Device.BeginInvokeOnMainThread (() => {
            start_btn.IsVisible = false;
            stop_btn.IsVisible = true;  
        });
    }
}

Maybe I'm late but I was searching this too without success. This may be useful for someone.

objectView.SetValue(IsVisibleProperty, false); // the view is GONE, not invisible
objectView.SetValue(IsVisibleProperty, true);