AspectD

Salute to AspectJ.

AspectD is an AOP(aspect oriented programming) framework for dart. Like other traditional aop framework, AspectD provides call&execute grammar. Besides, as we can't use dart:mirrors in flutter, AspectD also provides a way named inject enhancing the dart code manipulation.

Design

Suppose you have a flutter project named example located in hf_dir.

Installation

1. Create a dart package named aop in hf_dir/example

flutter create --template=package aop

2. Add aspectd&example dependency to aop package

dependencies:
  flutter:
    sdk: flutter
  aspectd:
    git:
      url: git@github.com:alibaba-flutter/aspectd.git
      ref: master
  example:
    path: ../

Fetch package dependency in aop package

flutter packages get

3. Modify aop package

aop.dart(entrypoint)

import 'package:example/main.dart' as app;
import 'aop_impl.dart';

void main()=> app.main();

aop_impl.dart(aop implementation)

import 'package:aspectd/aspectd.dart';

@Aspect()
@pragma("vm:entry-point")
class ExecuteDemo {
  @pragma("vm:entry-point")
  ExecuteDemo();

  @Execute("package:example/main.dart", "_MyHomePageState", "-_incrementCounter")
  @pragma("vm:entry-point")
  void _incrementCounter(PointCut pointcut) {
    pointcut.proceed();
    print('KWLM called!');
  }
}

4. Patch flutter_tools to apply aspectd.dart.snapshot

cd path-for-flutter-git-repo
git apply --3way path-for-aspectd-package/0001-aspectd.patch
rm bin/cache/flutter_tools.stamp

As flutter_tools doesn't support hooks now, the aspectd.patch is necessary currently. As flutter is evolving, this patch might fail sometimes. However, It would be simple to resolve the conflicts as AspectD only adds two hooks when building dill. See XianyuTech#5 for more.

If you want to customize the aop package, edit aopPackageRelPath(aop package relative path to the example's pubspec.yaml) and aopPackageName(aop package folder name and main entry file name) defined in path-for-flutter-git-repo/flutter/packages/flutter_tools/lib/src/aspectd.dart.

const String aopPackageRelPath = '.';
const String aopPackageName = 'aop';

Step 1~3 are expected to run each time you want to add aop to a flutter(dart) package. 4 is expected to run only once unless the dart-sdk changes. For example, If you upgrade flutter, you need to check if to rerun 4.

If you're using example with an aop package not generated locally, remember to run flutter packages get in aop package to get aspectd and check 4.

Tutorial

Now AspectD provides three ways to do AOP programming.

call

Every callsites for a specific function would be manipulated.

import 'package:aspectd/aspectd.dart';

@Aspect()
@pragma("vm:entry-point")
class CallDemo{
  @Call("package:app/calculator.dart","Calculator","-getCurTime")
  @pragma("vm:entry-point")
  Future<String> getCurTime(PointCut pointcut) async{
    print('Aspectd:KWLM02');
    print('${pointcut.sourceInfos.toString()}');
    Future<String> result = pointcut.proceed();
    String test = await result;
    print('Aspectd:KWLM03');
    print('${test}');
    return result;
  }

  @Call("package:app/calculator.dart","Calculator","+getCurTemporature")
  @pragma("vm:entry-point")
  String getCurTemporature(PointCut pointcut) {
    print('Aspectd:KWLM04');
    print('${pointcut.sourceInfos.toString()}');
    try{
      String res = pointcut.proceed();
    } catch (error, trace){
      print('Aspectd:KWLM05');
    }
    return null;
  }

  @Call("package:flutter/src/widgets/binding.dart","","+runApp")
  @pragma("vm:entry-point")
  static void runAppKWLM(PointCut pointcut){
    print('Aspectd:KWLM07');
    print('${pointcut.sourceInfos.toString()}');
    pointcut.proceed();
  }
}

In this case, notice that @Aspect() is needed to mark a class so that the aspectd will know that this class contains AspectD annotation informations. @pragma("vm:entry-point") is needed so that the class/function will not be removed by tree-shaking. For @Call("package:app/calculator.dart","Calculator","-getCurTime"), there are several things to know. Now call/execute/inject accept three positional parameters, package name, class name(If the procedure is a library method, this part is empty string), and function name. The function name may have a prefix('-' or '+'), '-' refers to instance method while '+' refers to library static method(like main) and class method. There is also a named parameter lineNum for inject so that AspectD know which line to inject a code snippet. The lineNum parameter is 1 based and code snippet would be injected before that line.

execute

Every implementation for a specific function would be manipulated.

import 'package:aspectd/aspectd.dart';

@Aspect()
@pragma("vm:entry-point")
class ExecuteDemo{
  @Execute("package:app/calculator.dart","Calculator","-getCurTime")
  @pragma("vm:entry-point")
  Future<String> getCurTime(PointCut pointcut) async{
    print('Aspectd:KWLM12');
    print('${pointcut.sourceInfos.toString()}');
    Future<String> result = pointcut.proceed();
    String test = await result;
    print('Aspectd:KWLM13');
    print('${test}');
    return result;
  }

  @Execute("package:app/calculator.dart","Calculator","+getCurTemporature")
  @pragma("vm:entry-point")
  String getCurTemporature(PointCut pointcut) {
    print('Aspectd:KWLM14');
    print('${pointcut.sourceInfos.toString()}');
    try{
      String res = pointcut.proceed();
    } catch (error, trace){
      print('Aspectd:KWLM15');
    }
    return null;
  }

  @Execute("package:flutter/src/widgets/binding.dart","","+runApp")
  @pragma("vm:entry-point")
  static void runAppKWLM(PointCut pointcut){
    print('Aspectd:KWLM17');
    print('${pointcut.sourceInfos.toString()}');
    pointcut.proceed();
  }
}

inject

For a original function like below:(package:flutter/src/widgets/gesture_detector.dart)

  @override
  Widget build(BuildContext context) {
    final Map<Type, GestureRecognizerFactory> gestures = <Type, GestureRecognizerFactory>{};

    if (onTapDown != null || onTapUp != null || onTap != null || onTapCancel != null) {
      gestures[TapGestureRecognizer] = GestureRecognizerFactoryWithHandlers<TapGestureRecognizer>(
        () => TapGestureRecognizer(debugOwner: this),
        (TapGestureRecognizer instance) {
          instance
            ..onTapDown = onTapDown
            ..onTapUp = onTapUp
            ..onTap = onTap
            ..onTapCancel = onTapCancel;
        },
      );
    }
...
}

import 'package:aspectd/aspectd.dart';
import 'package:flutter/services.dart';

@Aspect()
@pragma("vm:entry-point")
class InjectDemo{
  @Inject("package:flutter/src/widgets/gesture_detector.dart","GestureDetector","-build", lineNum:452)
  @pragma("vm:entry-point")
  static void onTapBuild() {
    Object instance; //Aspectd Ignore
    Object context; //Aspectd Ignore
    print(instance);
    print(context);
    print('Aspectd:KWLM25');
  }
}

After that, the original build function will look like below:

  @override
  Widget build(BuildContext context) {
    final Map<Type, GestureRecognizerFactory> gestures = <Type, GestureRecognizerFactory>{};

    if (onTapDown != null || onTapUp != null || onTap != null || onTapCancel != null) {
      gestures[TapGestureRecognizer] = GestureRecognizerFactoryWithHandlers<TapGestureRecognizer>(
        () => TapGestureRecognizer(debugOwner: this),
        (TapGestureRecognizer instance) {
          instance
            ..onTapDown = onTapDown
            ..onTapUp = onTapUp
            ..onTap = onTap
            ..onTapCancel = onTapCancel;
        },
    	print(instance);
    	print(context);
    	print('Aspectd:KWLM25');
      );
    }
...
}

Notice that //Aspectd Ignore part when using injection, we need to compile the aop package successfully so we need to declare the instance/context variable. However, when injecting to origin function (build in this case), variable declaration

Object instance; //Aspectd Ignore 
Object context; //Aspectd Ignore

would be discarded to avoid overring the original one.

Compatibility

Flutter 1.0 and above.

Notice

Because of the dart compilation implementation, there are several points to pay attention to:

package:aop/aop.dart should contains the main entry for aop package and contains a app.main call.
Every aop implementation file should be imported by aop.dart so that it will work in debug mode.
@pragma("vm:entry-point") is needed to mark class/function to avoid been trimmed by tree-shaking.
inject might fail in some cases while call&execute are expected to be more stable.
If you want to disable AOP, remove the aspectd.dart.snapshot located in aspectd or change the name of aop package, or remove the @Aspect() annotation. Anyone will be fine.

Contact

If you meet any problem when using AspectD, file a issue or contact me directly.

Contact Author

jbg168/aspectd