App Inventor extensions are developed in Java, so a bit of knowledge is required. Developing extensions is only possible because the App Inventor source code is open source. Most extensions work on all App Inventor platforms. The exact instructions strongly depend on your OS, but I will try to keep them as general as possible. I am a beginner myself, so any help and suggestions are welcome. This tutorial will give you a general view from newbie to newbie.
Download the above mentioned software and unpack them respectively install them. Then set your Environment Variables. Under Windows 8 and higher do right click on Windows logo -> System -> Advanced Configuration -> Enviroment Variables. Add your ant and Java home directories to your path variable (use the /bin folder for that). Then add two new variables ANT_HOME and JAVA_HOME with the corresponding folder paths. Last but not least you need to add ANT_OPTS with the parameter -Xms512m. Detailed information can be found here
Make sure that the JRE and JDK are the same version. Make sure that your environment variables are set correctly. It is easier to put JDK, ant and appengine in a folder that does not contain spaces. If you do that, you have to add apostrophes before and after every path you write in the command line.
You can use Git for easier version control. You can use any Git software you want e.g.
Clone the App Inventor Sources if you are a beginner and do not plan to change App Inventor's core components. Fork them if you plan to contribute to the App Inventor Sources or if you think you'll mess up something. Branches have the advantage that you can easily switch back to a working branch if you mess up something.
For command line tools use the instructions at the bottom of the official instructions (Appendix A). Instructions for GitHub for Desktop: Sign up or in to your GitHub account. Go to the official App Inventor Sources repository on GitHub. Click "Clone or download" and then "Open in Desktop". Follow the instructions to clone the repository. If you want to to create a new branch, in GitHub desktop, click "Branch" and then "New branch..." and follow the instructions. Alternatively, use the shortcut "Ctrl+Shift+N" to directly create a new branch. You can also download a .zip file if you don't want to create a GitHub account (not recommended).
The App Inventor team did sum this up very good:
Here are the steps we recommend in building an extension component. [...]
antto make sure that everything is working correctly. It takes a few minutes, but should display "BUILD SUCCESSFUL Total time: xxx minutes"
For your first extension, add your own lines to the description under
@DesignerComponent of an existing component. You find all existing
components in the folder /appinventor-sources/appinventor/components/src/com/google/appinventor/components. Save the file and launch a local
App Inventor instance (see command below) to see your changes. Make sure to run
ant MakeAuthKey before your first launch.
Then start with a new file called ExistingExtensionAlt.java (e.g. CameraAlt.java) in the appinventor/components/src/com/ directory. The folder structure behind src/ will be your package name. I recommend com/yourname/ as folder structure and com.yourname.extensionname as package name. These should be written in lowercase. Note: Your package name should be unique to prevent incompatibility issues. Copy the code of ExistingExtension.java to your file. Now this is "your" component and you have to make some changes to make it working. First and most important, change all appearances of ExistingComponent to ExistingComponentAlt. Your file name is your class name and extension name and must be in CamelCase. Now you have to modify some lines of code to make this an external component.
category = ComponentCategory.EXTENSION, nonVisible = trueare necessary
@SimpleObject(external = true), all before your class.
ant extensionsto pack your extensions and share them.
Now the only thing left is learning Java (if you have not already done that) and rock the community! You can now write extensions with the help of Android Java. Awesome! If you don't know where to start, take a look at the open source extensions below. And if that doesn't help, look in the forum what extensions people need. See this documentation to see how to add components. You can download my example extension here.
@UsesPermissionspersmission names empty, it prevents users of your extension to upload it to google play
@SimpleProperty(userVisible = false)to hide it from the user
All commands must be executed in your appinventor folder (the folder that contains e.g. the build.xml and the Readme). Use the command line tool of your OS for them.
(path_to_appengine)/bin/dev_appserver._(file)_ --port=8888 --address=0.0.0.0 appengine/build/war/
ant MakeAuthKeyonce to use your Local App Inventor Instance