How to writean annotation processor

in AndroidIvan Kušt

• Generics http://docs.oracle.com/javase/tutorial/java/generics/

• Reflectionhttp://docs.oracle.com/javase/tutorial/reflect/

• Annotationshttp://docs.oracle.com/javase/1.5.0/docs/guide/apt/mirror/overview-summary.html

How to write code faster?

Let’s look at our options

Generics Reflection Annotations

Speed Fast Slow Fast (if using compile time)

Ease of use Easy Medium Advanced

“Freedom" - generic types - modify everything at run-time

- generate code during compile

• use generics for type agnostic structures when possible (e.g. lists) - it’s easy and fast

• when that isn’t enough, use annotation processing - generate code that is boring to write

• generated code isn’t “visible” - use reflection to access it at run time from the code that you write

Approach

Let’s get our hands dirty• actually only the keyboard gets dirty over time

• prerequisites:- Android Studio (tested on version 0.4.0)- enable annotation processing(Open preference and navigate to Compiler → Annotation Processors. Check "Enable annotation processing”)- apt plugin https://bitbucket.org/hvisser/android-apt

• what is an annotation processor made of?- definition of annotations using @interface- a class that extends AbstractProcessor- entry containing processor name in META-INF/services/javax.annotation.processing.Processor

Project structure

• API project - annotation definitions - other non-generated code

• Compiler project- annotation processor implementation- annotation definitions (from API project) on classpath

• Application project - android application project- references to API and Compiler projects

API project

• android library project

• contains annotation definitions, for example:

Compiler project

• standard java library project- this is required in order to see APT classes

• API project sources must be in classpath (build.gradle):

Compiler project• AbstractProcessor implementation

- override process(…) method- return true if processing was successfull- add following annotations to processor class: @SupportedAnnotationTypes(“your.package.AnnotationName”)@SupportedSourceVersion(SourceVersion.RELEASE_7)

• create file: src/main/resources/META-INF/services/javax.annotation.processing.Processor

• add fully qualified class name of your processor class to the file (e.g. co.infinum.annotations.processor.TestProcessor)

Application project

• referencing API and Compiler projects in build.gradle:

Butterknife library

• UI dependency injection library

• View injection on Activities or arbitrary objects- no more writing findViewById(…)

• simplifying view holder pattern

• on click listener injection

View injection

class ExampleActivity extends Activity { @InjectView(R.id.title) TextView title; @InjectView(R.id.subtitle) TextView subtitle; @InjectView(R.id.footer) TextView footer;! @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.simple_activity); ButterKnife.inject(this); // TODO Use "injected" views... }}

• use @InjectView on fields representing view elements- fields can’t be private or protected

• call ButterKnife.inject(this) in onCreate()

Resources• annotations example:

https://github.com/ikust/hello-annotations

• http://turbomanage.wordpress.com/2012/09/28/annotation-processor-android-eclipse/

• https://bitbucket.org/hvisser/android-apt

• http://www.javacodegeeks.com/2012/11/java-annotations-tutorial-with-custom-annotation.html

• http://blog.retep.org/2009/02/13/getting-class-values-from-annotations-in-an-annotationprocessor/

• http://jakewharton.github.io/butterknife/