Android APT (compile-time code generation) best practices

APT(Annotation Processing Tool) is a Tool for Processing annotations. It detects the annotations in source code files and uses annotations to perform additional Processing. When processing annotations, the Annotation processor can generate additional source files and other files according to the annotations in source files (the specific content of files is determined by the Annotation processor writer). APT will also compile the generated source files and original source files. Put them together to generate a class file.

Apply plugin: 'Java' sourceCompatibility = 1.7 targetCompatibility = 1.7 dependencies {compile fileTree(dir: 'libs', include: ['*.jar']) }Copy the code

Apply plugin: 'Java' sourceCompatibility = 1.7 targetCompatibility = 1.7 dependencies {compile fileTree(dir: 'libs', include: [' *. Jar ']) compile 'com. Google. Auto. Services: auto - service: 1.0 rc2' compile 'com. Squareup: javapoet: 1.7.0 compile project(':annotation') }Copy the code

1. It is important to define the compiled JDK version as 1.7, otherwise an error will be reported.
2. The main function of AutoService is to annotate the Processor class and generate meta-INF configuration information for it.
3. The main purpose of the JavaPoet library is to help generate code in the form of class calls.
4. Rely on the Annotation Module created above. #### the logic that defines the code generated by the Processor class is here.

@AutoService(Processor.class) public class TestProcessor extends AbstractProcessor { @Override public Set getSupportedAnnotationTypes() { return Collections.singleton(Test.class.getCanonicalName()); } @Override public boolean process(Set annotations, RoundEnvironment roundEnv) { return false; }}Copy the code

apply plugin: 'com.android.application'
apply plugin: 'com.neenbedankt.android-apt'
//...
dependencies {
    //..
    compile project(':annotation')
    apt project(':compiler')
}
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So far we haven’t used annotations and @test above is not actually used, so let’s do some more practical code generation. Implement annotation-based View instead of findByView in the project. This is just to learn how to use APT. If you really want to use DI framework, ButterKnife is recommended. 1. First, create @diActivity and @DiView annotations in the Annotation Module.

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.CLASS)
public @interface DIActivity {

}
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@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface DIView {
    int value() default 0;
}
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@AutoService(Processor.class) public class DIProcessor extends AbstractProcessor { private Elements elementUtils; @ Override public Set getSupportedAnnotationTypes () {/ / annotations need to deal with the return Collections.singleton(DIActivity.class.getCanonicalName()); } @Override public boolean process(Set annotations, RoundEnvironment roundEnv) { System.out.println("DIProcessor"); Set elements = roundEnv.getElementsAnnotatedWith(DIActivity.class); For (Element: elements) {Class TypeElement TypeElement = (TypeElement) Element; List members = elementUtils.getAllMembers(typeElement); MethodSpec.Builder bindViewMethodSpecBuilder = MethodSpec.methodBuilder("bindView") .addModifiers(Modifier.PUBLIC, Modifier.STATIC) .returns(TypeName.VOID) .addParameter(ClassName.get(typeElement.asType()), "activity"); for (Element item : members) { DIView diView = item.getAnnotation(DIView.class); if (diView == null){ continue; } bindViewMethodSpecBuilder.addStatement(String.format("activity.%s = (%s) activity.findViewById(R.id.text)",item.getSimpleName(),ClassName.get(item.asType()).toString())); } TypeSpec typeSpec = TypeSpec.classBuilder("DI" + element.getSimpleName()) .superclass(TypeName.get(typeElement.asType())) .addModifiers(Modifier.PUBLIC, Modifier.FINAL) .addMethod(bindViewMethodSpecBuilder.build()) .build(); JavaFile javaFile = JavaFile.builder(getPackageName(typeElement), typeSpec).build(); try { javaFile.writeTo(processingEnv.getFiler()); } catch (IOException e) { e.printStackTrace(); } } return true; } private String getPackageName(TypeElement type) { return elementUtils.getPackageOf(type).getQualifiedName().toString(); } @Override public synchronized void init(ProcessingEnvironment processingEnv) { super.init(processingEnv); elementUtils = processingEnv.getElementUtils(); } @Override public SourceVersion getSupportedSourceVersion() { return SourceVersion.RELEASE_7; }}Copy the code

@DIActivity
public class MainActivity extends Activity {
    @DIView(R.id.text)
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        DIMainActivity.bindView(this);
        textView.setText("Hello World!");
    }
}
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In effect, apt generates the following code

public final class DIMainActivity extends MainActivity { public static void bindView(MainActivity activity) { activity.textView = (android.widget.TextView) activity.findViewById(R.id.text); }}Copy the code

1. TypeElement: class
2. ExecutableElement: Member method
3. VariableElement: Member variable

TypeName type = TypeName.get(element.asType());

List members = processingEnv.getElementUtils().getAllMembers(typeElement);

Github.com/taoweiji/De…

AptPreferences is a fast persistence framework designed to simplify the use of SharePreferences and reduce the writing of code. You can save basic types and objects very quickly. AptPreferences is based on APT technology, which implements code generation during compilation and distinguishes persistent information according to different users.

Github.com/joyrun/AptP…