详解flutter engine 那些没被释放的东西

时间:2021-04-03 人气:2

由于flutter一直存在内存泄漏的问题，导致很多开发者不胜困扰，博主在0.9.4就开始对其代码内部内存问题在engine层面修改代码，得到解决，但是对于每个版本都需要跟随官方打包,对于开发者并不是很友好。

然而喜出望外的是，在后来的几个版本中，官方内置开发了手动释放内存的方式:smile_cat:

/**
 * Destroy running context for an engine.
 *
 * This method can be used to force the FlutterEngine object to release all resources.
 * After sending this message, the object will be in an unusable state until it is deallocated.
 * Accessing properties or sending messages to it will result in undefined behavior or runtime
 * errors.
 */
- (void)destroyContext;

翻译如下：

销毁引擎的运行上下文。此方法可用于强制FlutterEngine对象释放所有资源。发送此消息后，对象将处于不可用状态，直到解除分配为止。访问属性或向其发送消息将导致未定义的行为或运行时错误。

但是 , 但是 , 但是 ，(重要的事说三遍) 在Flutter engine开发群里面，有群友反馈还有很多问题

无法完全释放内存
偶现崩溃

偶现崩溃的是什么鬼，暂时没有遇到，不好说。之前博主遇到的崩溃是自己使用方式的问题，在fluttervc关闭之后还有任务在执行methodchannel，即还在调用plugin，这个可以在开发上避免。值得注意的是，flutter中使用c++实现，自己对于内存管理并不是很好

内存问题自测如下

确实存在问题，还有将近30M没有被释放，查看一下当前内存对象，如下图

一个一个看还有那些没有被释放吧

android:LruCache

Least Recently Used 近期最少使用算法。内存管理的一种页面置换算法，对于在内存中但又不用的数据块（内存块）叫做LRU，flutter engine 会根据哪些数据属于LRU而将其移出内存而腾出空间来加载另外的数据。

dart::BackgroundComplier 对isolate编译优化的类

BackgroundCompiler 在后台线程中运行优化编译的类。实现：每个隔离一个任务，它与拥有isolate一起消失，后台编译器中没有OSR编译。

dart::bin::socket

vm和开发平台通信的机制，比如jit即时编译的dill文件，通过socket传递给dart vm，vm通过rpc加载文件，重置线程，从而实现hotreload热重载

dart::BoolPrameter

dart::EnumParameter
dart::IdParameter
dart::IdParameter
dart::xxxPrameter

定义在dart vm，service.cc中，都继承自MethodParameter，做对应参数校验，参数解析用。编译dart文件用的

dart::OSThread

在dart 运行时负责操作系统线程，创建线程，移除线程，线程查找与管理。如下图

FlutterEngineRegistrar 注册使用key注册plugin的地方，所有plugin调用dart底层的方法都会通过 handlemethodcall 回调给使用者，其初始化的地方是引起内存泄漏的地方

- (instancetype)initWithPlugin:(NSString*)pluginKey flutterEngine:(FlutterEngine*)flutterEngine {
 self = [super init];
 NSAssert(self, @"Super init cannot be nil");
 _pluginKey = pluginKey;// [pluginKey retain];
 _flutterEngine = flutterEngine;// [flutterEngine retain];
 return self;
}

此处有一篇文章介绍，解决engine的循环引用文章

FlutterStandardMethodCodec 标准方法编解码

FlutterStringCodec string编解码 FlutterJsonMessageCodec json编解码

不看不知道，一看吓一跳，也竟然是个单例，当然不会被释放了，也能理解，在flutter中用到jsonmssage的地方很多，用不着每次都初始化

代码实现的地方

@implementation FlutterJSONMessageCodec
+ (instancetype)sharedInstance {
 static id _sharedInstance = nil;
 if (!_sharedInstance) {
 _sharedInstance = [FlutterJSONMessageCodec new];
 }
 return _sharedInstance;
}

std:share_ptrxxx 共享指针

指针获取 flutter isolate service dartvm symbolmapping

~~ 文章完 ~~

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完整测试代码如下

#import "FlutterTesterViewController.h"
#import <Flutter/Flutter.h>
#import "GeneratedPluginRegistrant.h" 
 
@interface FlutterTesterViewController ()
@property (nonatomic, weak) FlutterViewController * ctr;
@property (nonatomic, weak) FlutterEngine * engine;
@end

@implementation FlutterTesterViewController
- (void)viewDidLoad {
 [super viewDidLoad];
 
 float Y = 210;
 [self createButton:@"加载boundle资源" frame:CGRectMake(80.0, Y, 160.0, 40.0) action:@selector(handleBoundleResource )];
 
 Y += 40.0 + 10;
 [self createButton:@"autorelease" frame:CGRectMake(80.0, Y, 160.0, 40.0) action:@selector(handleAutoRelase)];
 
 NSArray * paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
 NSString * path = [[paths objectAtIndex:0] stringByAppendingPathComponent:@"flutter_assets"] ;
 NSLog(@"path: %@",path);
 
}

-(void)handleNetWorkResource:(UIButton *)button{
 
}

/** 
* 加载boundle资源 
*/
- (void)handleBoundleResource {
 
 FlutterDartProject * dart = [[FlutterDartProject alloc] init];
 FlutterEngine * engine = [[FlutterEngine alloc] initWithName:@"ios.dart.flutter"
               project:dart];
 [engine runWithEntrypoint:nil];
 FlutterViewController* flutterViewController = [[FlutterViewController alloc] initWithEngine:engine nibName:nil bundle:nil];
 [GeneratedPluginRegistrant registerWithRegistry:flutterViewController];
 [self addBackButton:flutterViewController];
  [flutterViewController setInitialRoute:@"route1"];
 [self presentViewController:flutterViewController animated:YES completion:nil];
 self.engine = engine;
}

-(void)handleAutoRelase{
 
  FlutterBasicMessageChannel* channel;
 FlutterEngine * engine;
 @autoreleasepool {
  FlutterViewController* flutterViewController =
  [[FlutterViewController alloc] init];
  channel = flutterViewController.engine.systemChannel;
  engine = flutterViewController.engine;
  NSLog(@"engine111:%@",engine);
 }
 NSLog(@"engine222:%@",engine);
 [channel sendMessage:@"Hello!"];
 [channel setMessageHandler:^(id _Nullable message, FlutterReply _Nonnull callback) { }];
}

-(void)addBackButton:(UIViewController *)flutterViewController{
 dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
  UIButton * btn = [UIButton buttonWithType:UIButtonTypeSystem];
  [btn setTitle:@"关闭" forState:UIControlStateNormal];
  btn.frame = CGRectMake(10, 100, 50, 30);
  [btn addTarget:self action:@selector(buttonTap:) forControlEvents:UIControlEventTouchUpInside];
  [flutterViewController.view addSubview:btn];
  self.ctr = flutterViewController;
 });
}

-(void)buttonTap:(id)sender{
// [self.navigationController popViewControllerAnimated:YES];
 
 __weak __typeof(self)weakSelf = self;
 [self.ctr dismissViewControllerAnimated:YES completion:^{
  
  [weakSelf.engine destroyContext];
 }];

}

- (void)didReceiveMemoryWarning {
 [super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.
}



-(UIButton *)createButton:(NSString *)title frame:(CGRect)frame action:(SEL)selector{
 
 UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
 [button addTarget:self
    action:selector
  forControlEvents:UIControlEventTouchUpInside];
 [button setTitle:title forState:UIControlStateNormal];
 UIColor * bgcolor = [UIColor colorWithRed:arc4random()%256/255. green:arc4random()%256/255. blue:arc4random()%256/255. alpha:1];
 [button setBackgroundColor:bgcolor];
 button.frame = frame;
 [self.view addSubview:button];
 return button;
}

@end

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