APT的使用

1.声明注解

创建一个名字为apt-annotation的javalib，并在lib里创建一个BindView注解

@Retention(RetentionPolicy.CLASS)
@Target(ElementType.FIELD)
public @interface BindView {
    int value();
}

2.继承AbstractProcessor实现自己的注解处理类

创建另外一个名为apt-processor的javalib，在lib中创建MyProcessor 来根据注解动态生成代码

package com.example.apt_processor;

import com.example.apt_annotation.BindView;
import com.google.auto.service.AutoService;
import com.squareup.javapoet.ClassName;
import com.squareup.javapoet.JavaFile;
import com.squareup.javapoet.MethodSpec;
import com.squareup.javapoet.TypeSpec;

import java.io.IOException;
import java.lang.annotation.ElementType;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.Filer;
import javax.annotation.processing.Messager;
import javax.annotation.processing.ProcessingEnvironment;
import javax.annotation.processing.Processor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.Modifier;
import javax.lang.model.element.Name;
import javax.lang.model.element.PackageElement;
import javax.lang.model.element.TypeElement;
import javax.lang.model.element.VariableElement;
import javax.lang.model.util.Elements;
import javax.swing.JList;
import javax.tools.Diagnostic;
//必须写的，用来声明这个类是注解处理类
@AutoService(Processor.class)
@SupportedSourceVersion(SourceVersion.RELEASE_8)
//必须在javalib中创建否则无法继承AbstractProcessor类
public class MyProcessor extends AbstractProcessor {

    private Elements elementUtils;
    private Map<TypeElement, List<VariableElement>> map = new HashMap<>();
    private Messager messager;
    private Filer filer;


    //返回要处理的注解 可以添加多个
    @Override
    public Set<String> getSupportedAnnotationTypes() {
        Set<String> set = new HashSet<>();
        set.add(BindView.class.getCanonicalName());
        return set;
    }

    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);
        processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE,"init()");
        //日志打印工具
        messager = processingEnv.getMessager();
        elementUtils = processingEnv.getElementUtils();
        //用于生成类文件
        filer = processingEnv.getFiler();

    }
    
    @Override
    public boolean process(Set<? extends TypeElement> set, RoundEnvironment roundEnvironment) {
        //拿到所有添加BindView注解的成员变量
        Set<? extends Element> elements = roundEnvironment.getElementsAnnotatedWith(BindView.class);
        messager.printMessage(Diagnostic.Kind.NOTE,"111111111111111");
        //如果set的长度为0则不处理返回false,这里这样处理是因为process方法并不是只调用一次，所以要进行过滤
        if (set.size()<=0){
            return false;
        }
        
        //for循环把每个activity中的控件放入统一list中
        for (Element element : elements) {
            messager.printMessage(Diagnostic.Kind.NOTE,element.getKind().toString());
            PackageElement packageElement = elementUtils.getPackageOf(element);
            messager.printMessage(Diagnostic.Kind.NOTE," getQualifiedName = "+packageElement.getQualifiedName().toString());
            VariableElement variableElement = (VariableElement) element;
            TypeElement  typeElement = (TypeElement) variableElement.getEnclosingElement();
            List<VariableElement> list = map.get(typeElement);
            if (list == null){
                list = new ArrayList<>();
                map.put(typeElement,list);
            }
            Name simpleName = variableElement.getSimpleName();
            BindView annotation = variableElement.getAnnotation(BindView.class);
            messager.printMessage(Diagnostic.Kind.NOTE," simpleName = "+simpleName.toString());
            list.add(variableElement);
        }


        //for循环生成每个activity对应的view绑定工具类
        for (TypeElement typeElement : map.keySet()) {
            String className = typeElement.getSimpleName().toString();
            String packageName = elementUtils.getPackageOf(typeElement).getQualifiedName().toString();
            String finalClassName = className+"Binding";
            messager.printMessage(Diagnostic.Kind.NOTE,"类名 = "+finalClassName);

            ClassName classType = ClassName.bestGuess(className);

            String paramName = "activity";
            MethodSpec.Builder builder = MethodSpec.methodBuilder("bind")
                    .addModifiers(Modifier.PUBLIC,Modifier.STATIC)
                    .returns(void.class)
                    .addParameter(classType,paramName);


            List<VariableElement> variableElements = map.get(typeElement);
            for (int i = 0; i < variableElements.size(); i++) {
                VariableElement variableElement = variableElements.get(i);
                String type = variableElement.asType().toString();
                int viewId = variableElement.getAnnotation(BindView.class).value();
                String name = variableElement.getSimpleName().toString();
                builder.addStatement("$L.$L = ($L)$L.findViewById($L)",paramName,name,type,paramName,viewId);
            }
            MethodSpec methodSpec = builder.build();

            TypeSpec typeSpec = TypeSpec.classBuilder(finalClassName)
                    .addModifiers(Modifier.PUBLIC)
                    .addMethod(methodSpec)
                    .build();
            JavaFile javaFile = JavaFile.builder(packageName,typeSpec).build();
            try {
                javaFile.writeTo(filer);
                messager.printMessage(Diagnostic.Kind.NOTE,"3333");
            } catch (IOException e) {
                e.printStackTrace();
            }

        }
        return true;
    }
}

plugins {
    id 'java-library'
}

java {
    sourceCompatibility = JavaVersion.VERSION_1_8
    targetCompatibility = JavaVersion.VERSION_1_8
}


dependencies{
    //自动注册，动态生成 META-INF/...文件
    implementation 'com.google.auto.service:auto-service:1.0-rc6'
    annotationProcessor 'com.google.auto.service:auto-service:1.0-rc6'
    //依赖apt-annotation
    implementation project(':apt-annotation')
    //帮助生成代码的工具
    implementation 'com.squareup:javapoet:1.13.0'
}

3.app中使用

package com.example.apttest;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.widget.TextView;

import com.example.apt_annotation.BindView;

public class MainActivity extends AppCompatActivity {

    @BindView(R.id.contentTv)
    public TextView textView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        MainActivityBinding.bind(this);
        textView.setText("77777777777");
    }
}

dependencies {

    implementation 'androidx.appcompat:appcompat:1.5.1'
    implementation 'com.google.android.material:material:1.4.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
    testImplementation 'junit:junit:4.13.2'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'

    implementation project(':apt-annotation')
    annotationProcessor project(':apt-processor')

}

生成的工具类代码

package com.example.apttest;

public class MainActivityBinding {
  public static void bind(MainActivity activity) {
    activity.textView = (android.widget.TextView)activity.findViewById(2131230834);
  }
}