In my last job interview I got a (rather vague) question about traversing a tree and operating on the tree nodes. I think I’ve a lot of experience in programming but I couldn’t figure out the answer on my own. The answer the guy wanted to hear was: visitor pattern.

I had never heard of it before. So, in preparation for my next job interview, I thought I take a look at it.

While trying to figure it out, I stumbled over this quote:

The Visitor pattern is possibly the most complicated design pattern you will face. (Source)

I totally agree. (And this is probably why I’ve never heard or used it before.)

But since I’m not a quitter I went on and tamed it. So, in this article I’m going to shed some light on this mysterious design pattern.

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Mono for Android let’s you write Android applications in a .NET language (for example, C#). It comes with wrappers for almost the entire Android API so building “standard” apps is easy enough. However, sometimes you may need to work with third party Java classes that aren’t part of Android itself. These classes usually come as a .jar file. Fortunately, Mono for Android provides a way to access those Java classes. Unfortunately, this way is a “little” bit tricky and documentation on this is (currently) quite limited.

This article will show some basic example of how to call a method of a Java class from Mono for Android. We will use Visual Studio 2010 in this article, as it is (in my opinion) currently superior to Mono Develop.

Update: Mono for Android v4.2 and higher provides an automatic way for doing the things described in this article. This is a lot easier than the manual way and thus the preferred way.

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Currently, MonoDroid (Mono for Android) doesn’t support native libraries (or any other Android resource type) in library projects (bug report). Fortunately, they support normal .NET assembly resources. This led me to write a workaround for this missing feature.

The Class ∞

Here’s the class that implements to loader.

MonoDroidLibraryLoader.cs

History:

• 2012-07-18: Updates documentation and added some logging

• 2012-07-11: Initial revision

How to use ∞

To use this class, you first need to create a directory structure for the native libraries, like this:

Project
+- NativeLibs
   +- armeabi-v7a
   +- armeabi
   +- other ABIs you want to support

Now place the native libraries in the apropriate directories, for example:

Project
+- NativeLibs
   +- armeabi-v7a
   |  +- libsqlite.so
   +- armeabi
      +- libsqlite.so

Then select “Embedded Resource” as Build Action in the file’s properties (Hotkey: F4).

Now use one of the Load() methods to load the library. For example, if you have a type called MyType in the project’s default namespace (specified in the project settings), you can use this call to load a library called libsqlite.so:

MonoDroidLibraryLoader.Load("sqlite", typeof(MyType));

Note: If you renamed the directory “NativeLibs” from above, you can specify it as third parameter for Load().

CodeProject

Sometimes a C/C++ function needs to store data you pass to it for later reference. If such data is a managed object (like a string or class) you need to make sure that the garbage collector doesn’t delete it while it’s still in use/still stored in the native code.

That’s what pinning is for. It prevents the garbage collector from deleting and moving the object.

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P/Invoke tries to make your life easier by automatically converting (“marshalling”) data types from managed code to native code and the other way around.

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