Android Metaprogramming (Droidcon NYC 2015)

Do you like clean, dry, reusable code? Sure, we all do. For some use cases the cleanest and most efficient way to build and maintain a feature is to get meta. Are you ready to level up your developer skills? Metaprogramming is working with code that works with your code. Aspects, annotations, bytecode manipulation, runtime wizardry. What is possible? What unique limitations and opportunities exist when working within Android code? This is a practical introduction to Android Metaprogramming from a cross-platform native SDK and app developer.

Transcript

1. @auditty
   +AudreyTroutt
   Android
   Metaprogramming
   Audrey Troutt
   

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2. @auditty
   +AudreyTroutt
   What is metaprogramming?
   Writing code that reads, generates,
   analyzes, or transforms other code.
   

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3. @auditty
   +AudreyTroutt
   Why use metaprogramming?
   // BEFORE
   class AndroidWay extends Activity {
   TextView name;
   ImageView thumbnail;
   LocationManager loc;
   Drawable icon;
   String myName;
   public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.main);
   name = (TextView) findViewById(R.id.name);
   thumbnail = (ImageView) findViewById(R.id.thumbnail);
   loc = (LocationManager) getSystemService(Activity.
   LOCATION_SERVICE);
   icon = getResources().getDrawable(R.drawable.icon);
   myName = getString(R.string.app_name);
   name.setText( "Hello, " + myName );
   }
   }
   // AFTER
   @ContentView(R.layout.main)
   class RoboWay extends RoboActivity {
   @InjectView(R.id.name) TextView name;
   @InjectView(R.id.thumbnail) ImageView thumbnail;
   @InjectResource(R.drawable.icon) Drawable icon;
   @InjectResource(R.string.app_name) String myName;
   @Inject LocationManager loc;
   public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   name.setText( "Hello, " + myName );
   }
   }
   // example borrowed from RoboGuice :) https://github.
   com/roboguice/roboguice/wiki
   

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4. @auditty
   +AudreyTroutt
   Why use metaprogramming?
   // BEFORE
   class AndroidWay extends Activity {
   TextView name;
   ImageView thumbnail;
   LocationManager loc;
   Drawable icon;
   String myName;
   public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.main);
   name = (TextView) findViewById(R.id.name);
   thumbnail = (ImageView) findViewById(R.id.thumbnail);
   loc = (LocationManager) getSystemService(Activity.
   LOCATION_SERVICE);
   icon = getResources().getDrawable(R.drawable.icon);
   myName = getString(R.string.app_name);
   name.setText( "Hello, " + myName );
   }
   }
   // AFTER
   @ContentView(R.layout.main)
   class RoboWay extends RoboActivity {
   @InjectView(R.id.name) TextView name;
   @InjectView(R.id.thumbnail) ImageView thumbnail;
   @InjectResource(R.drawable.icon) Drawable icon;
   @InjectResource(R.string.app_name) String myName;
   @Inject LocationManager loc;
   public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   name.setText( "Hello, " + myName );
   }
   }
   // example borrowed from RoboGuice :) https://github.
   com/roboguice/roboguice/wiki
   

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5. @auditty
   +AudreyTroutt
   Why use metaprogramming?
   ● You are a library developer, probably
   

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6. @auditty
   +AudreyTroutt
   #import "UILabel+TextMagic.h"
   // This is a category
   @implementation UILabel (TextMagic)
   - (CGFloat)bottomYCoordinate {
   return self.frame.origin.y + self.frame.size.height;
   }
   - (CGFloat)rightXCoordinate {
   return self.frame.origin.x + self.frame.size.width;
   }
   @end
   // from https://github.com/ArtisanMobile/ArtisanCategoryMagic
   Metaprogramming is easy* on iOS
   

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7. @auditty
   +AudreyTroutt
   #import
   // This is method swizzling
   + (void)replaceOriginalInstanceMethod:(SEL)originalMethod
   from:(id)original
   withInstanceMethod:(SEL)replacementMethod
   from:(id)replacementObject {
   Method originalMethod = class_getInstanceMethod([original class], originalMethod);
   Method mockMethod = class_getInstanceMethod([replacementObject class], replacementMethod);
   method_exchangeImplementations(originalMethod, mockMethod);
   }
   // Don’t do this in production code! It’s more polite to always call the original code.
   Metaprogramming is easy* on iOS
   

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8. @auditty
   +AudreyTroutt
   // You can perform any selector, even hidden ones.
   id myClone = [anObject performSelector:@selector(copy)];
   Metaprogramming is easy* on iOS
   

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9. @auditty
   +AudreyTroutt
   Reflection
   Useful parts:
   ● Runtime tool
   ● Inspect classes, their annotations,
   fields and methods
   ● Invoke methods
   ● Change the values of fields
   

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10. @auditty
    +AudreyTroutt
    import java.lang.reflect.Method;
    private void logPropertyValueForObject(Object t, String accessorMethodName) {
    try {
    Method m = t.getClass().getDeclaredMethod(accessorMethodName); // (1)
    m.setAccessible(true); // (2) just in case this is private/not accessible
    Object o = m.invoke(t); // (3) using reflection to invoke an arbitrary method
    Log.d("META", accessorMethodName + " value is " + o);
    } catch (Exception e) {
    Log.e("META", "uh oh! Something went wrong!", e);
    }
    }
    protected void onStop() {
    super.onStop();
    logPropertyValueForObject(this, "timeOnScreen"); // (4)
    }
    private long timeOnScreen() {
    return new Date().getTime() - activityStartTime;
    }
    Reflection
    

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11. @auditty
    +AudreyTroutt
    import java.lang.reflect.Method;
    private void logPropertyValueForObject(Object t, String accessorMethodName) {
    try {
    Method m = t.getClass().getDeclaredMethod(accessorMethodName); // (1)
    m.setAccessible(true); // (2) just in case this is private/not accessible
    Object o = m.invoke(t); // (3) using reflection to invoke an arbitrary method
    Log.d("META", accessorMethodName + " value is " + o);
    } catch (Exception e) {
    Log.e("META", "uh oh! Something went wrong!", e);
    }
    }
    protected void onStop() {
    super.onStop();
    logPropertyValueForObject(this, "timeOnScreen"); // (4)
    }
    private long timeOnScreen() {
    return new Date().getTime() - activityStartTime;
    }
    Reflection
    

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12. @auditty
    +AudreyTroutt
    import java.lang.reflect.Method;
    private void logPropertyValueForObject(Object t, String accessorMethodName) {
    try {
    Method m = t.getClass().getDeclaredMethod(accessorMethodName); // (1)
    m.setAccessible(true); // (2) just in case this is private/not accessible
    Object o = m.invoke(t); // (3) using reflection to invoke an arbitrary method
    Log.d("META", accessorMethodName + " value is " + o);
    } catch (Exception e) {
    Log.e("META", "uh oh! Something went wrong!", e);
    }
    }
    protected void onStop() {
    super.onStop();
    logPropertyValueForObject(this, "timeOnScreen"); // (4)
    }
    private long timeOnScreen() {
    return new Date().getTime() - activityStartTime;
    }
    Reflection
    

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13. @auditty
    +AudreyTroutt
    import java.lang.reflect.Method;
    private void logPropertyValueForObject(Object t, String accessorMethodName) {
    try {
    Method m = t.getClass().getDeclaredMethod(accessorMethodName); // (1)
    m.setAccessible(true); // (2) just in case this is private/not accessible
    Object o = m.invoke(t); // (3) using reflection to invoke an arbitrary method
    Log.d("META", accessorMethodName + " value is " + o);
    } catch (Exception e) {
    Log.e("META", "uh oh! Something went wrong!", e);
    }
    }
    protected void onStop() {
    super.onStop();
    logPropertyValueForObject(this, "timeOnScreen"); // (4)
    }
    private long timeOnScreen() {
    return new Date().getTime() - activityStartTime;
    }
    Reflection
    

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14. @auditty
    +AudreyTroutt
    import java.lang.reflect.Method;
    private void logPropertyValueForObject(Object t, String accessorMethodName) {
    try {
    Method m = t.getClass().getDeclaredMethod(accessorMethodName); // (1)
    m.setAccessible(true); // (2) just in case this is private/not accessible
    Object o = m.invoke(t); // (3) using reflection to invoke an arbitrary method
    Log.d("META", accessorMethodName + " value is " + o);
    } catch (Exception e) {
    Log.e("META", "uh oh! Something went wrong!", e);
    }
    }
    protected void onStop() {
    super.onStop();
    logPropertyValueForObject(this, "timeOnScreen"); // (4)
    }
    private long timeOnScreen() {
    return new Date().getTime() - activityStartTime;
    }
    Reflection
    

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15. @auditty
    +AudreyTroutt
    Reflection
    Limitations:
    ● is slow
    ● does not change code behavior
    

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16. @auditty
    +AudreyTroutt
    Annotations
    Useful parts:
    ● Define metadata about code
    

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17. @auditty
    +AudreyTroutt
    @Background // An annotation!
    void translateInBackground(String textToTranslate) {
    String translatedText = callGoogleTranslate(textToTranslate);
    showResult(translatedText);
    }
    // example from AndroidAnnotations https://github.com/excilys/androidannotations
    Annotations
    

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18. @auditty
    +AudreyTroutt
    import java.lang.annotation.*;
    @Target(ElementType.METHOD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface WatchedMethod {
    String eventName();
    String logProperty() default "";
    }
    // SAMPLE USAGE IN MY ACTIVITY
    @Override
    @WatchedMethod(eventName = "disappear", logProperty = "timeOnScreen")
    protected void onStop() {
    super.onStop();
    }
    Annotations
    

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19. @auditty
    +AudreyTroutt
    import java.lang.annotation.*;
    @Target(ElementType.METHOD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface WatchedMethod {
    String eventName();
    String logProperty() default "";
    }
    // SAMPLE USAGE IN MY ACTIVITY
    @Override
    @WatchedMethod(eventName = "disappear", logProperty = "timeOnScreen")
    protected void onStop() {
    super.onStop();
    }
    Annotations
    

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20. @auditty
    +AudreyTroutt
    import java.lang.annotation.*;
    @Target(ElementType.METHOD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface WatchedMethod {
    String eventName();
    String logProperty() default "";
    }
    // SAMPLE USAGE IN MY ACTIVITY
    @Override
    @WatchedMethod(eventName = "disappear", logProperty = "timeOnScreen")
    protected void onStop() {
    super.onStop();
    }
    Annotations
    

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21. @auditty
    +AudreyTroutt
    import java.lang.annotation.*;
    @Target(ElementType.METHOD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface WatchedMethod {
    String eventName();
    String logProperty() default "";
    }
    // SAMPLE USAGE IN MY ACTIVITY
    @Override
    @WatchedMethod(eventName = "disappear", logProperty = "timeOnScreen")
    protected void onStop() {
    super.onStop();
    }
    Annotations
    

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22. @auditty
    +AudreyTroutt
    import java.lang.annotation.*;
    @Target(ElementType.METHOD)
    @Retention(RetentionPolicy.RUNTIME)
    public @interface WatchedMethod {
    String eventName();
    String logProperty() default "";
    }
    // SAMPLE USAGE IN MY ACTIVITY
    @Override
    @WatchedMethod(eventName = "disappear", logProperty = "timeOnScreen")
    protected void onStop() {
    super.onStop();
    }
    Annotations
    

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23. @auditty
    +AudreyTroutt
    ● Before compilation
    ● Inspect annotations, generate source
    Annotation Processors
    

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24. @auditty
    +AudreyTroutt
    Limitations:
    ● Can’t modify existing
    implementations
    ● Requires developer binding
    ● Method counts
    Annotation Processors
    

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25. @auditty
    +AudreyTroutt
    Bytecode Manipulation
    Useful parts:
    ● Can both modify and add methods
    and classes
    

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26. @auditty
    +AudreyTroutt
    Android build process
    

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27. @auditty
    +AudreyTroutt
    Libraries:
    ● ASM
    ○ Working directly with bytecode
    ○ http://asm.ow2.org/
    ● DexMaker
    ○ Enables runtime manipulation on android
    ○ https://github.com/crittercism/dexmaker
    ● Javaassist
    ○ higher-level coding
    ○ http://jboss-javassist.github.io/javassist
    ● AspectJ
    ○ the easiest to use solution*
    Bytecode Manipulation
    

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28. @auditty
    +AudreyTroutt
    // Examples of using javaassist
    // (1) add superclass
    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("test.Child");
    cc.setSuperclass(pool.get("test.Parent"));
    // (2) insert code into a method body before existing code
    CtMethod m = cc.getDeclaredMethod("say");
    m.insertBefore("{ Log.d(\"META\",\"So META!\"); }");
    // (3) wrap the existing method body with a try/catch
    CtClass etype = ClassPool.getDefault().get("java.io.IOException");
    m.addCatch("{ Log.e(\"META\",\"There was an error\", $e); }", etype);
    // (4) write the modified class back out
    cc.writeFile();
    Bytecode Manipulation
    

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29. @auditty
    +AudreyTroutt
    // Examples of using javaassist
    // (1) add superclass
    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("test.Child");
    cc.setSuperclass(pool.get("test.Parent"));
    // (2) insert code into a method body before existing code
    CtMethod m = cc.getDeclaredMethod("say");
    m.insertBefore("{ Log.d(\"META\",\"So META!\"); }");
    // (3) wrap the existing method body with a try/catch
    CtClass etype = ClassPool.getDefault().get("java.io.IOException");
    m.addCatch("{ Log.e(\"META\",\"There was an error\", $e); }", etype);
    // (4) write the modified class back out
    cc.writeFile();
    Bytecode Manipulation
    

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30. @auditty
    +AudreyTroutt
    // Examples of using javaassist
    // (1) add superclass
    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("test.Child");
    cc.setSuperclass(pool.get("test.Parent"));
    // (2) insert code into a method body before existing code
    CtMethod m = cc.getDeclaredMethod("say");
    m.insertBefore("{ Log.d(\"META\",\"So META!\"); }");
    // (3) wrap the existing method body with a try/catch
    CtClass etype = ClassPool.getDefault().get("java.io.IOException");
    m.addCatch("{ Log.e(\"META\",\"There was an error\", $e); }", etype);
    // (4) write the modified class back out
    cc.writeFile();
    Bytecode Manipulation
    

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31. @auditty
    +AudreyTroutt
    // Examples of using javaassist
    // (1) add superclass
    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("test.Child");
    cc.setSuperclass(pool.get("test.Parent"));
    // (2) insert code into a method body before existing code
    CtMethod m = cc.getDeclaredMethod("say");
    m.insertBefore("{ Log.d(\"META\",\"So META!\"); }");
    // (3) wrap the existing method body with a try/catch
    CtClass etype = ClassPool.getDefault().get("java.io.IOException");
    m.addCatch("{ Log.e(\"META\",\"There was an error\", $e); }", etype);
    // (4) write the modified class back out
    cc.writeFile();
    Bytecode Manipulation
    

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32. @auditty
    +AudreyTroutt
    // Examples of using javaassist
    // (1) add superclass
    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("test.Child");
    cc.setSuperclass(pool.get("test.Parent"));
    // (2) insert code into a method body before existing code
    CtMethod m = cc.getDeclaredMethod("say");
    m.insertBefore("{ Log.d(\"META\",\"So META!\"); }");
    // (3) wrap the existing method body with a try/catch
    CtClass etype = ClassPool.getDefault().get("java.io.IOException");
    m.addCatch("{ Log.e(\"META\",\"There was an error\", $e); }", etype);
    // (4) write the modified class back out
    cc.writeFile();
    Bytecode Manipulation
    

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33. @auditty
    +AudreyTroutt
    // Examples of using javaassist
    // (1) add superclass
    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("test.Child");
    cc.setSuperclass(pool.get("test.Parent"));
    // (2) insert code into a method body before existing code
    CtMethod m = cc.getDeclaredMethod("say");
    m.insertBefore("{ Log.d(\"META\",\"So META!\"); }");
    // (3) wrap the existing method body with a try/catch
    CtClass etype = ClassPool.getDefault().get("java.io.IOException");
    m.addCatch("{ Log.e(\"META\",\"There was an error\", $e); }", etype);
    // (4) write the modified class back out
    cc.writeFile();
    Bytecode Manipulation
    

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34. @auditty
    +AudreyTroutt
    // Examples of using javaassist
    // (1) add superclass
    ClassPool pool = ClassPool.getDefault();
    CtClass cc = pool.get("test.Child");
    cc.setSuperclass(pool.get("test.Parent"));
    // (2) insert code into a method body before existing code
    CtMethod m = cc.getDeclaredMethod("say");
    m.insertBefore("{ Log.d(\"META\",\"So META!\"); }");
    // (3) wrap the existing method body with a try/catch
    CtClass etype = ClassPool.getDefault().get("java.io.IOException");
    m.addCatch("{ Log.e(\"META\",\"There was an error\", $e); }", etype);
    // (4) write the modified class back out
    cc.writeFile();
    Bytecode Manipulation
    

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35. @auditty
    +AudreyTroutt
    Limitations:
    ● Danger!
    ● Jack & Jill?
    ● Long developer ramp-up
    Bytecode Manipulation
    

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36. @auditty
    +AudreyTroutt
    AspectJ
    Useful parts:
    ● Can both modify and add methods
    and classes
    ● Easy to use*
    

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37. @auditty
    +AudreyTroutt
    AspectJ
    ● aspect
    ● pointcut
    ● join point
    ● advice
    

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38. @auditty
    +AudreyTroutt
    // (1) define pointcut
    pointcut onViewClicked(View view) : execution(* *.*(View)) &&
    args(view) && (within(Activity+ || View.OnClickListener+));
    // (2) Inject code before/after/around the existing method(s)
    before(View view) : onViewClicked(view) {
    TrackEvent.onViewClicked(view);
    }
    // inspired by Artisan's TrackEvent.aj
    AspectJ
    

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39. @auditty
    +AudreyTroutt
    // (1) define pointcut
    pointcut onViewClicked(View view) : execution(* *.*(View)) &&
    args(view) && (within(Activity+ || View.OnClickListener+));
    // (2) Inject code before/after/around the existing method(s)
    before(View view) : onViewClicked(view) {
    TrackEvent.onViewClicked(view);
    }
    // inspired by Artisan's TrackEvent.aj
    AspectJ
    

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40. @auditty
    +AudreyTroutt
    // (1) define pointcut
    pointcut onViewClicked(View view) : execution(* *.*(View)) &&
    args(view) && (within(Activity+ || View.OnClickListener+));
    // (2) Inject code before/after/around the existing method(s)
    before(View view) : onViewClicked(view) {
    TrackEvent.onViewClicked(view);
    }
    // inspired by Artisan's TrackEvent.aj
    AspectJ
    

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41. @auditty
    +AudreyTroutt
    // (1) define pointcut
    pointcut onViewClicked(View view) : execution(* *.*(View)) &&
    args(view) && (within(Activity+ || View.OnClickListener+));
    // (2) Inject code before/after/around the existing method(s)
    before(View view) : onViewClicked(view) {
    TrackEvent.onViewClicked(view);
    }
    // inspired by Artisan's TrackEvent.aj
    AspectJ
    

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42. @auditty
    +AudreyTroutt
    // (1) define pointcut
    pointcut onViewClicked(View view) : execution(* *.*(View)) &&
    args(view) && (within(Activity+ || View.OnClickListener+));
    // (2) Inject code before/after/around the existing method(s)
    before(View view) : onViewClicked(view) {
    TrackEvent.onViewClicked(view);
    }
    // inspired by Artisan's TrackEvent.aj
    AspectJ
    

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43. @auditty
    +AudreyTroutt
    // (3) set superclass with inter-type declarations
    declare parents: com.audreytroutt.demo.activity.MainActivity extends
    AudreyInjectedActivity;
    // (4) add a method
    public void com.audreytroutt.demo.activity.MainActivity.
    injectedMethodInYourActivity() {
    Log.d("META", "I'm in your code, changing your bytes!");
    }
    AspectJ
    

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44. @auditty
    +AudreyTroutt
    // (3) set superclass with inter-type declarations
    declare parents: com.audreytroutt.demo.activity.MainActivity extends
    AudreyInjectedActivity;
    // (4) add a method
    public void com.audreytroutt.demo.activity.MainActivity.
    injectedMethodInYourActivity() {
    Log.d("META", "I'm in your code, changing your bytes!");
    }
    AspectJ
    

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45. @auditty
    +AudreyTroutt
    // (3) set superclass with inter-type declarations
    declare parents: com.audreytroutt.demo.activity.MainActivity extends
    AudreyInjectedActivity;
    // (4) add a method
    public void com.audreytroutt.demo.activity.MainActivity.
    injectedMethodInYourActivity() {
    Log.d("META", "I'm in your code, changing your bytes!");
    }
    AspectJ
    

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46. @auditty
    +AudreyTroutt
    AspectJ
    Limitations:
    ● Can’t modify Android base classes
    ● Android developers hated it!
    

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47. @auditty
    +AudreyTroutt
    First Metaprogramming Steps
    ● Leverage existing libraries
    ● Peek inside and see how they work
    ● Thank a library developer
    

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48. @auditty
    +AudreyTroutt
    Android
    Metaprogramming
    Audrey Troutt
    

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