android - Create a NinePatch/NinePatchDrawable in runtime

Question

I have a requirement on my Android application that parts on the graphics should be customizable, by retrieving new colors and images from the server side. Some of these images are nine-patch images.

I can't find a way to create and display these nine-patch images (that have been retrieved over the network).

The nine-patch images are retrieved and kept in the application as Bitmaps. In order to create a NinePatchDrawable, you either need the corresponding NinePatch or the chunk (byte[]) of the NinePatch. The NinePatch can NOT be loaded from the Resources, since the images doesn't exist in /res/drawable/. Furthermore, in order to create the NinePatch, you need the chunk of the NinePatch. So, it all drills down to the chunk.
The question is then, how do one format/generate the chunk from an existing Bitmap (containing the NinePatch information)?

I've searched through the Android source code and the Web and I can't seem to find any examples of this. To make things worse, all decoding of a NinePatch resources seem to be done natively.

Have anyone had any experiences with this kind of issue?

I'm targeting API level 4, if that is of importance.

Answer 1

getNinePatchChunk works just fine. It returned null because you were giving Bitmap a "source" ninepatch. It needs a "compiled" ninepatch image.

There are two types of ninepatch file formats in the Android world ("source" and "compiled"). The source version is where you add the 1px transparency border everywhere-- when you compile your app into a .apk later, aapt will convert your *.9.png files to the binary format that Android expects. This is where the png file gets its "chunk" metadata. (read more)

Okay, now down to business (you're listening to DJ kanzure).

1. Client code, something like this:

   InputStream stream = .. //whatever
   Bitmap bitmap = BitmapFactory.decodeStream(stream);
   byte[] chunk = bitmap.getNinePatchChunk();
   boolean result = NinePatch.isNinePatchChunk(chunk);
   NinePatchDrawable patchy = new NinePatchDrawable(bitmap, chunk, new Rect(), null);
   

2. Server-side, you need to prepare your images. You can use the Android Binary Resource Compiler. This automates some of the pain away from creating a new Android project just to compile some *.9.png files into the Android native format. If you were to do this manually, you would essentially make a project and throw in some *.9.png files ("source" files), compile everything into the .apk format, unzip the .apk file, then find the *.9.png file, and that's the one you send to your clients.

Also: I don't know if BitmapFactory.decodeStream knows about the npTc chunk in these png files, so it may or may not be treating the image stream correctly. The existence of Bitmap.getNinePatchChunk suggests that BitmapFactory might-- you could go look it up in the upstream codebase.

In the event that it does not know about the npTc chunk and your images are being screwed up significantly, then my answer changes a little.

Instead of sending the compiled ninepatch images to the client, you write a quick Android app to load compiled images and spit out the byte[] chunk. Then, you transmit this byte array to your clients along with a regular image-- no transparent borders, not the "source" ninepatch image, not the "compiled" ninepatch image. You can directly use the chunk to create your object.

Another alternative is to use object serialization to send ninepatch images (NinePatch) to your clients, such as with JSON or the built-in serializer.

Edit If you really, really need to construct your own chunk byte array, I would start by looking at do_9patch, isNinePatchChunk, Res_png_9patch and Res_png_9patch::serialize() in ResourceTypes.cpp. There's also a home-made npTc chunk reader from Dmitry Skiba. I can't post links, so if someone can edit my answer that would be cool.

do_9patch: https://android.googlesource.com/platform/frameworks/base/+/gingerbread/tools/aapt/Images.cpp

isNinePatchChunk: http://netmite.com/android/mydroid/1.6/frameworks/base/core/jni/android/graphics/NinePatch.cpp

struct Res_png_9patch: https://scm.sipfoundry.org/rep/sipX/main/sipXmediaLib/contrib/android/android_2_0_headers/frameworks/base/include/utils/ResourceTypes.h

Dmitry Skiba stuff: http://code.google.com/p/android4me/source/browse/src/android/graphics/Bitmap.java