思路

检测

通过借鉴Android的内存泄漏检测工具LeakCanary的原理，使用弱引用持有引用，当这个引用执行释放动作的时候，执行Full GC后，如果弱引用的持有还在，那么就代表这个引用泄漏了。

Dart 也有弱引用-----WeakReference

关于Dart弱引用WeakReference怎么使用，我的这篇文章2分钟教你Flutter怎么避免引用内存泄漏>>会对你有帮助.

如何执行Full GC？

通过使用vm_service这个插件,在Dev可以执行Full GC请求，通过获取VmService的引用后，调用执行

vms.getAllocationProfile(isolate!.id!, gc: true)

就可以请求Full GC

如何知道一个引用他的文件路径以及类名？

vm_service这个插件里面有Api支持反射获取ClassRef读取引用里面的属性名，类名，以及路径等。

代码实践

有了以上的思路，我们就可以通过代码方式来实现检测内存泄漏，然后把泄漏的引用通知到UI展示出来。
代码我已经写好在 flutter_leak_canary: ^1.0.1，可做参考修改

第一步，实现Full GC

1. 添加vm_service插件，获取VmService引用

 Future<VmService?> getVmService() async {if (_vmService == null && debug) {ServiceProtocolInfo serviceProtocolInfo = await Service.getInfo();_observatoryUri = serviceProtocolInfo.serverUri;if (_observatoryUri != null) {Uri url = convertToWebSocketUrl(serviceProtocolUrl: _observatoryUri!);try {_vmService = await vmServiceConnectUri(url.toString());} catch (error, stack) {print(stack);}}}return _vmService;}

2. 执行GC的时候,flutter的无效引用回收是每个Isolate线程独立的，因为内存独立，相互不受影响。由于我们几乎所有代码都在UI线程执行的，所以我们需要进行筛选出UI线程，也就是’main’线程。

Future<VM?> getVM() async {if (!debug) {return null;}return _vm ??= await (await getVmService())?.getVM();
}//获取ui线程
Future<Isolate?> getMainIsolate() async {if (!debug) {return null;}IsolateRef? ref;final vm = await getVM();if (vm == null) return null;//筛选出ui线程的索引var index = vm.isolates?.indexWhere((element) => element.name == 'main');if (index != -1) {ref = vm.isolates![index!];}final vms = await getVmService();if (ref?.id != null) {return vms?.getIsolate(ref!.id!);}return null;
}

3.根据上面方法，落实Full GC

//请求执行Full GC
Future try2GC() async {if (!debug) {return;}final vms = await getVmService();if (vms == null) return null;final isolate = await getMainIsolate();if (isolate?.id != null) {await vms.getAllocationProfile(isolate!.id!, gc: true);}
}

第二步，如何根据对象引用，获取出他的类名，路径等信息。

1. 思路大概是这样，通过一个文件的路径能获取当前LibraryRef对象，通过这个LibraryRef对象可以调用这个文件里面的顶级函数，返回值可以加工得到刚才提过的ClassRef。
2. 利用这个特性，我们可以先把需要检测的对象，丢到一个Map里面，然后写一个高级函数返回这个map保存的对象。然后通过api获取这个对象id后，可以得到Obj， 根据Obj可以得到对应Instance，这个Instance里面就有ClassRef

具体实现如下：

const String vmServiceHelperLiraryPath ='package:flutter_leak_canary/vm_service_helper.dart';
//dont remove this method, it's invoked by getObjectId
String getLiraryResponse() {return "Hello LeakCanary";
}
//dont remove this method, it's invoked by getObjectId
dynamic popSnapObject(String objectKey) {final object = _snapWeakReferenceMap[objectKey];return object?.target;
}//
class VmServiceHelper {
//....    //根据文件获取getLiraryByPath
Future<LibraryRef?> getLiraryByPath(String libraryPath) async {if (!debug) {return null;}Isolate? mainIsolate = await getMainIsolate();if (mainIsolate != null) {final libraries = mainIsolate.libraries;if (libraries != null) {final index =libraries.indexWhere((element) => element.uri == libraryPath);if (index != -1) {return libraries[index];}}}return null;
}//通过顶部函数间接获取这个对象的objectId
Future<String?> getObjectId(WeakReference obj) async {if (!debug) {return null;}final library = await getLiraryByPath(vmServiceHelperLiraryPath);if (library == null || library.id == null) return null;final vms = await getVmService();if (vms == null) return null;final mainIsolate = await getMainIsolate();if (mainIsolate == null || mainIsolate.id == null) return null;Response libRsp =await vms.invoke(mainIsolate.id!, library.id!, 'getLiraryResponse', []);final libRspRef = InstanceRef.parse(libRsp.json);String? libRspRefVs = libRspRef?.valueAsString;if (libRspRefVs == null) return null;_snapWeakReferenceMap[libRspRefVs] = obj;try {Response popSnapObjectRsp = await vms.invoke(mainIsolate.id!, library.id!, "popSnapObject", [libRspRef!.id!]);final instanceRef = InstanceRef.parse(popSnapObjectRsp.json);return instanceRef?.id;} catch (e, stack) {print('getObjectId $stack');} finally {_snapWeakReferenceMap.remove(libRspRefVs);}return null;
}//根据objectId获取Obj
Future<Obj?> getObjById(String objectId) async if (!debug) {return null;}final vms = await getVmService();if (vms == null) return null;final mainIsolate = await getMainIsolate();if (mainIsolate?.id != null) {try {Obj obj = await vms.getObject(mainIsolatereturn obj;} catch (e, stack) {print('getObjById>>$stack');}}return null;
}//根据objectId获取Instance.  
Future<Instance?> getInstanceByObjectId(String objectId) async {if (!debug) {return null;}Obj? obj = await getObjById(objectId);if (obj != null) {var instance = Instance.parse(obj.json);return instance;}return null;
}//根据objectId获取出具体的类名，文件名，类在文件的第几行，第几列
//顶级函数>objectId>Obj>Instance
Future<LeakCanaryWeakModel?> _runQuery(objectId) async {final vmsh = VmServiceHelper();Instance? instance = await vmsh.getInstanceByObjectId(objectId!);if (instance != null &&instance.id != 'objects/null' &&instance.classRef is ClassRef) {ClassRef? targetClassRef = instance.classRef;final wm = LeakCanaryWeakModel(className: targetClassRef!.name,line: targetClassRef.location?.line,column: targetClassRef.location?.column,classFileName: targetClassRef.library?.uri);print(wm.className);return wm;}return null;
}}//泄漏信息模型
class LeakCanaryWeakModel {//泄漏时间late int createTime;//类名final String? className;
//所在文件名final String? classFileName;//所在列final int? line;//所在行数final int? column;LeakCanaryWeakModel({required this.className,required this.classFileName,required this.column,required this.line,}) {createTime = DateTime.now().millisecondsSinceEpoch;}
}

第三步，定义工具接口

定义一个接口，里面有添加监听，检测是否泄漏，获取当前泄漏的引用列表，通知当前有泄漏的引用

abstract class LeakCanaryMananger {//具体实现管理类，这个后面会介绍factory LeakCanaryMananger() => _LeakCanaryMananger();//监听当前引用，初始化时候调用void watch(WeakReference obj);//生命周期结束的以后，检测引用有没有泄漏void try2Check(WeakReference wr);//当前的泄漏列表List<LeakCanaryWeakModel> get canaryModels;//当前内存有新泄漏引用通知ValueNotifier get leakCanaryModelNotifier;
}

第四步，添加代理类，隔离实现类

class FlutterLeakCanary implements LeakCanaryMananger {final _helper = LeakCanaryMananger();static final _instance = FlutterLeakCanary._();FlutterLeakCanary._();factory() => _instance;static FlutterLeakCanary get() {return _instance;}@overridevoid watch(obj) {_helper.watch(obj);}@overridevoid try2Check(WeakReference wr) {_helper.try2Check(wr);}void addListener(VoidCallback listener) {_helper.leakCanaryModelNotifier.addListener(listener);}void removeListener(VoidCallback listener) {_helper.leakCanaryModelNotifier.removeListener(listener);}@overrideList<LeakCanaryWeakModel> get canaryModels => List.unmodifiable(_helper.canaryModels);@overrideValueNotifier get leakCanaryModelNotifier => _helper.leakCanaryModelNotifier;
}

第五步， 提供State的mixin监听类

我们最不希望看到的泄漏类，一定是state。他泄漏后，他的context,也就是element无法回收，然后它里面持有所有的渲染相关的引用都无法回收，这个泄漏非常严重。
通过WeakReference来持有这个对象以来可以用来检测，二来避免自己写的工具导致内存泄漏。
initState的时候，把它放到检测队列，dispose以后进行检测

mixin LeakCanaryStateMixin<T extends StatefulWidget> on State<T> {late WeakReference _wr;String? objId;@mustCallSuper@overridevoid initState() {super.initState();_wr = WeakReference(this);FlutterLeakCanary.get().watch(_wr);}@mustCallSuper@overridevoid dispose() {super.dispose();FlutterLeakCanary.get().try2Check(_wr);}
}

第六步，提供其他类的mixin监听类

mixin LeakCanarySimpleMixin {late WeakReference _wr;String? objId;void watch()  {_wr = WeakReference(this);FlutterLeakCanary.get().watch(_wr);}void try2Check() {FlutterLeakCanary.get().try2Check(_wr);}
}

第七步，实现具体的管理类

对于引用的检测，是把引用包装到GCRunnable，使用消费者设计模型来做，3秒轮询检测一次。尽量用线程去分担检测，避免影响UI线程性能开销的统计。

class _LeakCanaryMananger implements LeakCanaryMananger {static final vmsh = VmServiceHelper();//objId:instancefinal _objectWeakReferenceMap = HashMap<int, WeakReference?>();List<GCRunnable> runnables = [];Timer? timer;bool isDetecting = false;//3秒轮训loopRunnables() {timer ??= Timer.periodic(Duration(seconds: 3), (timer) {if (isDetecting) {return;}if (runnables.isNotEmpty) {isDetecting = true;final trunnables = List<GCRunnable>.unmodifiable(runnables);runnables.clear();//使用线程去GCcompute(runGc, null).then((value) async {await Future.forEach<GCRunnable>(trunnables, (runnable) async {if (runnable.objectId == "objects/null") {return;}try {final LeakCanaryWeakModel? wm = await runnable.run();//如果非空，就是泄漏了，然后对泄漏的进行class信息获取，发送到订阅的地方，一般是ui，进行刷新if (wm != null) {canaryModels.add(wm);leakCanaryModelNotifier.value = wm;}} catch (e, s) {print(s);} finally {_objectWeakReferenceMap.remove(runnable.wkObj.hashCode);}});isDetecting = false;});}});}@overridevoid watch(WeakReference wr) async {bool isDebug = false;assert(() {isDebug = true;return true;}());if (!isDebug) {return;}_objectWeakReferenceMap[wr.hashCode] = wr;loopRunnables();}@overrideValueNotifier leakCanaryModelNotifier = ValueNotifier(null);//添加到待检测执行队列里，轮询扫描的时候执行，这样可以避免检测瓶颈void _check(WeakReference? wr) {assert(() {WeakReference? wkObj = _objectWeakReferenceMap[wr.hashCode];runnables.add(GCRunnable(wkObj: wkObj));return true;}());}@overridevoid try2Check(WeakReference wr) async {bool isDebug = false;assert(() {isDebug = true;return true;}());if (!isDebug) {return;}if (wr.target != null) {_check(wr);}}@overrideList<LeakCanaryWeakModel> canaryModels = [];
}class GCRunnable {String? objectId;final WeakReference? wkObj;GCRunnable({required this.wkObj});Future<LeakCanaryWeakModel?> run() async {if (wkObj?.target != null) {final vmsh = VmServiceHelper();//cant quary objectId with isolate, but quary instanceobjectId = await vmsh.getObjectId(wkObj!);LeakCanaryWeakModel? weakModel = await compute(_runQuery, objectreturn weakModel;}}
}

运行测试

环境配置 --disable-dds

VsCode需要配置.vscode

“configurations”: [
{
…
“args”: [
“–disable-dds”
],
“type”: “dart”
},

]

Android Studio

检验成果

读下面的代码，看看那些会泄漏，然后在看看结果。

class WeakPage extends StatefulWidget {const WeakPage({super.key});@overrideState<WeakPage> createState() => _WeakPageState();
}class TestModel with LeakCanarySimpleMixin {Timer? timer;int count = 0;init() {watch();timer = Timer.periodic(Duration(seconds: 1), (timer) {count++;print("TestModel $count");});}void dispose() {// timer?.cancel();try2Check();}
}class TestModel2 with LeakCanarySimpleMixin {Timer? timer;int count = 0;init() {watch();}void dispose() {timer?.cancel();timer = null;try2Check();}
}class _WeakPageState extends State<WeakPage> with LeakCanaryStaTestModel? test = TestModel();TestModel2? test2 = TestModel2();Timer? timer;int count = 0;@overridevoid initState() {super.initState();test?.init();test2?.init();timer = Timer.periodic(Duration(seconds: 1), (timer) {count++;print("_WeakPageState ${count}");});}@overridevoid dispose() {// TODO: implement disposesuper.dispose();//timer.cancel();test?.dispose();test2?.dispose();test = null;test2 = null;}@overrideWidget build(BuildContext context) {return Material(child: Center(child: Container(child: InkWell(onTap: () {Navigator.of(context).pop();},child: Text('back')),),),);}

泄漏结果：

需要获取源码的同学，到这里获取，点击>>flutter_leak_canary: ^1.0.1<<

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