Android Development: Simple collision detection

For my App I need to know when a user drags a specified element on top of another element. For instance when they drag one of their attack cards to the deck of the oponnent. On top of that I also need to know if the current location will result in an attack movement or just in a drag in open space. In both situations the game should do something else. So how can we detect if the current location is resulting in an attack? Simple by using the Rect.intersects method like so

this class implements a few utility methods. They bassicaly to the same thing. They convert the given parameters into two Rect instances and call the intersect method of the first Rect.  With this little tip you can easily integrate some basic collision detection. with this knowledge we can easily find out if the current user is attacking the enemy and which card is being attacked with the following code:

In this example we loop over the enemyfrontline cards and for every card we check if it intersects. If that is the case we break out of the loop and use the found card to do the stuff we want to do with it ( in my case find out which card has ‘won’ and respond accordingly.


Android Development: Loading image

In a previous post I talked about loading bitmaps. In this post I will tell something about caching the loaded resources. For this I created a little util class. To give you an example how to use it here is the code. I will explain it step by step.

When the View needs an image it calls the method ‘build’ (3).

It passes a Card object that specifies which card we want to load. We also need to tell the builder which color we need  ( two sides in this game remember? )

It will first check if bitmap belonging to the card specified already has been loaded in the past (20-26).  It therefore will get the correct cache (21-24) ( Yes I have 2 cache’s one for each side ). When the card has not been found in the cache it should be loaded (  5 and 6). Therefore we find the correct drawable (from the R object ) (29-32) and create a bitmap out of it (33-35). We scale it down using a scale matrix and we put the image back in the correct cache (37-41). After that we try to load it again (line 15-16).

By using this approach we asure the app that it will always be using the same instance of a particular bitmap.

I hope this will help you a bit developing your own game apps.

Android Development: Adding touch support

When creating an App users need to somehow interact with your program. Mostly this will include touch support. To set up touch support you need to do the following Warning lots of code !! I will try to explain everything step by step.

Lines 8, 39,73: Listen to the various events.

Line 11-28: When a mouse-down event is detected it means that we are in a new cycle of down->move->up events. In this case it means that we first have find out if the current location was not the previous location and if this is not the case we need to find out if we touched something we are interested in ( 13 ). If so we will do all kinds of stuff with that element and call invalidate. This causes the UI to redraw everything.

Line 39 – 65: If the action_up is detected it means that we are done with our current operation. In this case it means that we have to find out where currently are situated. If that location somehow intersects with another element we are interested in it will cause a change in the ui ( in this case some cards will be redrawn with another image )

73 – 83: Touch support means in most cases drag support. We do this simply by getting our current location and move the current selected element ( a card ) to that new position. Since the x and y coordinates of the event are left/top coordinates we need to change that location so that we don’t touch the top/left corner of that element but somewhat in the middle this feels a bit more natural to the user.

This concludes this little blogpost about touch…

Android development: Loading bitmaps

If you want to add graphics to your application you can use the following information

Create a custom Activity like this

Create your own View ( in this example StrategoGameView)

I personally find it a good practice to call an initialize method in your constructor. This way you can decide if you want to make it public so when needed you can reinitialize easily.

Now if you want to add a Bitmap to your application you need to do the following things

Add a res/drawable directory to your project

create some fancy images and save them in this directory.

After a rebuild you can get a pointer to the using the R.drawable object like this

to draw the bitmap to the screen you need to add a onDraw method to your View like this:


Android Development: Responding to a socket request

Warning: Worst Blog post written ever in history!

in my current project, a simple multiplayer game, I need to react on a connection request from another device. Currently both devices start their own service discovery. On one device I go to the list of found services and pick on. On the click the ServiceHelper will try to resolve the chosen device and if successful it will start communication sending a simple ‘gameplayrequest’ string.

the other device reacts to this with a OK response. This response is generated only when the user accepts the ‘invitation’. Ok and now some code 😉

These are some code snippets First we need to connect to the other device after we done that we need to send a response. This has to be done in a separate thread. The reason for this is that Android does not allow any heavy operations in the main thread. When we have notified the device that we are ok with the request the result is notified to all our listeners ( the view in this case )

And now the code to start resolving the chosen service

We get an instance of our service helper and subscribe ourselves to it

We start to resolve the service and logging which thing we clicked.

Now the update code in this activity

the update call first checks if the data that we receive is of the type notifier ( a class created by me to send data and some payload ). after we have resolved succesfully (see previous code snippet) we send a request to the server (first snippet) . The server will respond with an ok response ( or response_connection_allowed in this case ). We will log that for now but in the future this will cause a switch to another activity.

That’s it for now.

Android Development: Starting a server socket

As you already know the application i’m building is using service discovery to connect to other devices. How does this work?

Step 0:  Add activity as observer to the servicehelper class (line 4)

Step 2: Start registering the service to the network (10)

Step 3: If service has been registered successfully update ui and start a server socket that waits for any connection. (line 19)