MonoDroid: Error when calling constructor of custom view - TwoDScrollView

android, android-custom-view, android-layout, scrollview, xamarin.android

Solution

Congratulations! You've hit a leaky abstraction. :-/

The problem is this: for better or worse, virtual method calls from constructors invoke the most derived method implementation. C# is the same as Java in this respect; consider the following program:

using System;

class Base {
    public Base ()
    {
        Console.WriteLine ("Base..ctor");
        M ();
    }

    public virtual void M ()
    {
        Console.WriteLine ("Base.M");
    }
}

class Derived : Base {

    public Derived ()
    {
        Console.WriteLine ("Derived..ctor");
    }

    public override void M ()
    {
        Console.WriteLine ("Derived.M");
    }
}

static class Demo {
    public static void Main ()
    {
        new Derived ();
    }
}

When run, the output is:

Base..ctor
Derived.M
Derived..ctor

That is, the `Derived.M()` method is invoked before the `Derived` constructor has executed.

In Mono for Android, things get more...complicated. The Android Callable Wrapper (ACW)'s constructor is invoked by Java and is responsible for creating the peer C# instance and mapping the Java instance to the C# instance. However, if a virtual method is invoked from the Java constructor, then the method will be dispatched before there is a C# instance to invoke the method upon!

Let that sink in a bit.

I don't know which method is triggering the scenario for your specific code (the code fragment you provided works fine), but we do have a sample which hits this scenario: LogTextBox overrides the TextView.DefaultMovementMethod property, and the `TextView` constructor invokes the `getDefaultMovementMethod()` method. The result is that Android tries to invoke `LogTextBox.DefaultMovementMethod` before a `LogTextBox` instance even exists.

So what does Mono for Android do? Mono for Android created the ACW, and thus knows which C# type the `getDefaultMovementMethod()` method should be delegated to. What it doesn't have is an instance, because one hasn't been created. So Mono for Android creates an instance of the appropriate type...via the `(IntPtr, JniHandleOwnership)` constructor, and generates an error if this constructor cannot be found.

Once the (in this case) `TextView` constructor finishes executing, the `LogTextBox`'s ACW constructor will execute, at which point Mono for Android will go "aha! we've already created a C# instance for this Java instance", and will then invoke the appropriate constructor on the already created instance. Meaning that for a single instance, two constructors will be executed: the `(IntPtr, JniHandleOwnership)` constructor, and (later) the `(Context, IAttributeSet, int)` constructor.

Problem

Original source