Programming, Xamarin

[SOLVED] Unable to cast object of type ‘Xamarin. Forms. Xaml. ElementNode’ to type ‘Xamarin. Forms. Xaml. ValueNode’.

When writing XAML for Xamarin Forms, you may across the error:

Unable to cast object of type 'Xamarin.Forms.Xaml.ElementNode' to type 'Xamarin.Forms.Xaml.ValueNode'

This is nearly always caused by assigning a value to an event in XAML, instead of specifying a method name.

A common example is:

<Switch Toggled="{Binding IsToggled}" />

Toggled is the name of an event. The property that was probably intended is called IsToggled.

<Switch IsToggled="{Binding IsToggled}" />

Computers, Entertainment, Games, Technology, Video Games

Overcooked – Fun Coop Multiplayer Action

I recently a recorded a bunch of videos of the game Overcooked on Xbox One.

Your goal is to assemble meals out of various ingredients, cook them, and serve them. Over time the meals get a bit more complicated and the levels get a lot more complicated. It is strongly designed to be played cooperatively with up to four people, and even supports two players on a single controller.

My only complaint would be the difficulty is based too much on complicated level design (and jumps up a bit too quickly). Some times the controls are not exactly tight and you can end up selecting the wrong thing – having levels with moving targets or slippery floors for instance just accentuates an otherwise minor problem. I would have preferred more meal variations (that are also more complicated) on simpler levels.

But despite all that it’s a fun party game that almost anyone can play. And of course it is made in Unity.

One final note. The first video in the playlist above was generated by Google Photos. It turned out well, except for its automatic cropping.

Entertainment, Games, Programming, Unity, Video Games

Visualizing Gravity

Unity has a pretty cool feature called “gizmos”, that are things rendered only in the scene view of the Unity editor. Many built in game object types render a gizmo of some sort, but you can freely add your own. This can be very useful for debugging.

This is a visualization of the gravity (more precisely it’s the low resolution grid of gravity mentioned in the previous post). The direction of the line is the direction of the gravity, and the length is the strength.

Entertainment, Games, Programming, Unity, Video Games

The Gravity of the Situation

The most important feature in Gravitas is clearly the gravity, so getting it right is crucial. One important principal to be aware of when making games though is that “right” doesn’t necessarily mean “technically perfect” or “scientifically accurate”. “Feeling right” and being fun to play with is often more important.

Despite that, the gravity in Gravitas is essentially correct. By correct, I mean a numerical approximation of Newton’s law of universal gravitation. The formula for calculating the gravitational force between two masses is:



  • F is the force between the masses;
  • G is the gravitational constant (6.674×10−11 N · (m/kg)2);
  • m1 is the first mass;
  • m2 is the second mass;
  • r is the distance between the centers of the masses.

For simplicity and performance reasons, there are a few things to be aware of though. Firstly, I decided not to worry about real world units since the planets, ships and torpedoes are not realistically scaled to each other (either by size, mass or distance from each other). This means I can decide that my torpedoes have a mass of 1, thereby eliminating a multiplication for the most common case. This also means I can choose the value of G to be whatever looks or feels right.

This is still quite a significant calculation that has to be performed every frame for every torpedo/planet pair.

As it turns out though, the maximum number of torpedo/planet pairs is quite low. Based on the limits of the previous version of Gravitas, there were at most 9 ships, firing at most 3 torpedoes, with at most 11 planets. This means 9 × 3 × 11 = 297 times to calculate the strength of the gravity each frame. In practice I don’t think I ever so that many torpedoes (every ship using a triple-shot special power at once).

But then I wanted to add dust. By dust, I mean a trail from the torpedoes, that is also affected by gravity. At a minimum this should generate one particle per frame and be at least a couple of seconds long at normal torpedo speed. This means each torpedo could easily have a hundred dust particles all needing the same gravity calculation. All of a sudden there could be 300,000 gravitation calculations per frame.

On my laptop it ran fine. On my phone, not so much. There are a number of possible ways of solving this issue, but the one I settled on is based on the principle that accuracy of simulation of the dust particles (as opposed to the torpedoes) is not so important. That is, if the dust particles don’t behave perfectly, it doesn’t really matter.

One thing to notice about the gravity is that the only dynamic data it depends on is the position of the dust. The mass of the dust is fixed, and the position and mass of the planets are fixed (at least per round). My solution was to pre-calculate the strength of gravity on a dust particle for all the positions in a low-resolution grid. This means I can “calculate” the gravity by doing a relatively cheap lookup.

Android, Programming, Xamarin

[SOLVED] The BindableProperty “Triggers” is readonly

TLDR: When setting the Triggers property in XAML, use the actual type of the parent tag, not a supertype.

After recently updating Xamarin Forms from to, I started getting an InvalidOperationException: The BindableProperty "Triggers is readonly" inside InitializeComponent.

Unlike many problems, this was quite easy to track down. InitializeComponent errors are generally XAML, and in the page in question there was a single Trigger. In this case the solution was simple. The Trigger was on a custom Button type, but I was setting it specifically using Button.Triggers. Changing it to be the actual type fixed it.

So, I changed it from

    <DataTrigger ... />


    <DataTrigger ... />

I believe the original should be valid (and previously was) but the change is simple enough to not be a big problem.

Entertainment, Games, Programming, Unity, Video Games

Gravitas approaching playability

Gravitas is rapidly approaching something playable, now that ships can be destroyed and new levels are generated.

“Something playable” is still quite far away from being an actual game. In terms of features required for even an alpha release, it still needs: scoring, match state, player lobby, AI, and better level generation.

Some other features I would rather have but aren’t strictly required include: special powers (especially warping, to get players out of impossible situations), better UI feedback for touch controls (which was not necessary for the original) and some match options.

But overall, progress is good.