The Secret Pie

Ever wondered how to properly create a custom button, which looks different when selected (or clicked) and can be resized? Fortunately it's quite easy in Android. Let's start by creating a bitmap for our button. I'm using Gimp for such things. That's what I got:



(I told you it would be fancy!)
As you can see, I created 4 variants. Accordingly: regular button, pressed button, selected (focused) button and the disabled button. I can create a 4-state button now with such XML:

Put all the files (button_menu_normal.png, button_menu_pressed.png, button_menu_selected.png, button_menu_disabled.png and the newly created button_menu.xml) in your res/drawable directory. That creates a drawable consisting of our three images, each for one state of the button. If I want to use it in a button, I have to specify my drawable as a button background (in some layout XML file):



Let's run the project! Oh, it doesn't look very nice...



That's because our buttons aren't resizing properly. Android just takes our bitmap and stretches it to fit the button. It's ugly.

Nine-patches, we need you!

Wait, nine-what? Nine-patches (or nine-patch drawables) are the Android's remedy to resizing problem. Basically a nine-patch is an almost-ordinary image, just it's treated by the system differently when resizing. Look at this:



That's a nine-patch image. Two things differ it from a regular image:

• it has a 1-pixel border with some black pixels
• it has to be saved with name like my_image.9.png

So what are those black pixels for? They indicate which parts of the image will be stretched. Look:



Android looks at those black pixels and divides the image in 9 parts using them and then resizes the image like that:

• Parts 1, 3, 7 and 9 aren't resized at all
• Parts 2 and 8 are resized horizontally
• Parts 4 and 6 are resized vertically
• Part 5 is resized horizontally and vertically

So only the pink parts are resized (in one axis) and the most pink part is resized on two axes. (that phrase sounds really weird)

That's how it looks when resized (I left your favourite pink color to show the patches):



Neat! You can create 9-patch images with any image manipulation program, but there's a little tool in the Android SDK for that purpose. Look for it in ANDROID_SDK_DIR/tools/draw9patch.



It's very simple, but it does the job. Play with it for a while or read some instructions here.

Ok, I've changed my button images to nine-patches (so button_menu_normal.png became button_menu_normal.9.png etc.). Let's change the code. The best thing is that we don't have to change anything! If Android sees the button_menu_normal.9.png file, it loads it automatically as a nine-patch image that's available under id @drawable/button_menu_normal. How cool is that?

Running the app gives us nicely resized buttons now.



Summary:

• create your button image
• create some variants for clicked, selected and disabled states
• make 9-patches of it
• create my_fancy_button.xml file in res/drawable directory
• set it as your button background in the layout xml file

Easy! I'll continue to dig this subject and add some styling in the next post.