内存泄漏原因

所有语言的内存泄漏无非就是GC Root（线程，全局或静态数组或引用等）的引用链持有了需要释放的引用，导致无法释放。
解决释放的方法，无非就是2种，如下：

1. 在当前类，或者方法等移除改引用，让其他自动释放，等下一轮GC扫描释放。如

class B{void doSomething(){}
}class A{B? b;
}
A a = A();
void test(){a.b = B();a.b?.doSomething();
}void dispose(){a.b = null;
}void main() {test();dispose();
}

A 这时候时全局的，如果使用完B后，想要B自动释放，只需要调用dispose() 将B置null就好。

然后，现实就是有可能，忘记调用dispose().导致B无法释放。怎么办呢？对，你猜到了，就是弱引用，Dart也有！

来了来了就是那个闪亮的仔， WeakReference(和Java的弱引用名字一样哦)

2. 使用弱引用-----WeakReference，当前使用完，生命周期结束后，自动释放。

在上面代码，B的生命周期，仅仅是在test这个方法里面，所以执行test后B就不需要用了，那么就让他自动等GC移除.
让我们看看他的源码

/// A weak reference to a Dart object.
///
/// A _weak_ reference to the [target] object which may be cleared
/// (set to reference `null` instead) at any time
/// when there is no other way for the program to access the target object.
///
/// _Being the target of a weak reference does not keep an object
/// from being garbage collected._
///
/// There are no guarantees that a weak reference will ever be cleared
/// even if all references to its target are weak references.
///
/// Not all objects are supported as targets for weak references.
/// The [WeakReference] constructor will reject any object that is not
/// supported as an [Expando] key.
///
/// Use-cases like caching can benefit from using weak references. Example:
///
/// ```dart
/// /// [CachedComputation] caches the computation result, weakly holding
/// /// on to the cache.
/// ///
/// /// If nothing else in the program is holding on the result, and the
/// /// garbage collector runs, the cache is purged, freeing the memory.
/// ///
/// /// Until the cache is purged, the computation will not run again on
/// /// a subsequent request.
/// ///
/// /// Example use:
/// /// ```
/// /// final cached = CachedComputation(
/// ///     () => jsonDecode(someJsonSource) as Object);
/// /// print(cached.result); // Executes computation.
/// /// print(cached.result); // Most likely uses cache.
/// /// ```
/// class CachedComputation<R extends Object> {
///   final R Function() computation;
///
///   WeakReference<R>? _cache;
///
///   CachedComputation(this.computation);
///
///   R get result {
///     final cachedResult = _cache?.target;
///     if (cachedResult != null) {
///       return cachedResult;
///     }
///
///     final result = computation();
///
///     // WeakReferences do not support nulls, bools, numbers, and strings.
///     if (result is! bool && result is! num && result is! String) {
///       _cache = WeakReference(result);
///     }
///
///     return result;
///   }
/// }
/// ```
@Since("2.17")
abstract class WeakReference<T extends Object> {/// Creates a [WeakReference] pointing to the given [target].////// The [target] must be an object supported as an [Expando] key,/// which means [target] cannot be a number, a string, a boolean, a record,/// the `null` value, or certain other types of  special objects.external factory WeakReference(T target);/// The current object weakly referenced by [this], if any.////// The value is either the object supplied in the constructor,/// or `null` if the weak reference has been cleared.T? get target;
}

好家伙，上面就用例子叫我们怎么使用,属性target就是获取当前需要的引用。让我们改改上面

class B{void doSomething(){}
}class A{WeakReference<B>? b;
}
A a = A();
void test(){a.b = WeakReference(B());a.b?.target?.doSomething();
}void dispose(){a.b = null;
}void main() {test();print(a.b?.target);
}

让我们执行下，看看结果
控制台：

Instance of ‘B’

？？？ ，有同学就问，怎么还持有，怎么还不释放。
因为当前没触发GC垃圾回收器，所以没必要进行回收。看过我这篇文章Android内存抖动
的同学知道，因为频繁回收会出现内存抖动现象，从而导致App掉帧甚至卡顿现象，GC垃圾回收器在这做了优化。那如果真的想测试验证这个结果，怎么办？
可以2种方案，第一种，老套路啦，强制制造大量无效垃圾引用，让GC触发回收。
如：制造大量的引用，并且需要释放的引用

void main() {test();List.generate(10000, (index) {ByteData byteData = ByteData(10000);});print(a.b?.target);
}

让我们执行下，看看结果
控制台：

null

当当当～，回收了吧

使用vm_service插件触发GC

还有一种办法，就是调用vm_service插件提供的VmService进行回收。注意一点是，这个GC做了优化，回收是单独对线程进行回收的，所以需要筛选对应的线程进行回收，如我们的线程基本是在UI线程也就是main线程中进行的，那么我们就筛选main

class VmServiceHelper {bool debug = false;VmServiceHelper() {assert(() {debug = true;return true;}());}VmService? _vmService;VM? _vm;Uri? _observatoryUri;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;}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);}}Future<Isolate?> getMainIsolate() async {if (!debug) {return null;}IsolateRef? ref;final vm = await getVM();if (vm == null) return null;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;}Future<VM?> getVM() async {if (!debug) {return null;}return _vm ??= await (await getVmService())?.getVM();}
}

在使用之前，我们需要在运行配置上面加入

VsCode

{..."args": ["--disable-dds"],"type": "dart"},

Android Studio

然后我们使用VmServiceHelper().try2GC()，代替上面的语句如下。但是需要运行app来测试。

test();
await VmServiceHelper().try2GC();
print(a.b?.target);

好了，如果本次文章觉得对你有用，请点赞，收藏，关注3连谢谢。
需要VmServiceHelper这个的朋友，可以到 flutter_leak_canary: ^1.0.1这个上面下载哦，如果顺手，给我点个Like哟