flutter处理图片压缩效率不高，如果同时压缩多张图片，那就更慢了，有时候甚至要等10几秒甚至几十秒，使用了flutter_luban这个框架压缩，仍然不能达到比较理想的效果，想到原生的压缩效率很高，于是便考虑使用iOS/Android原生来处理压缩，然后通过消息传递的方式返回到flutter端显示和使用
压缩框架就使用luban压缩 压缩效果好，清晰度高还尺寸小，据说微信图片算法就是用的这个
iOS https://github.com/GuoZhiQiang/Luban_iOS
Android https://github.com/Curzibn/Luban

压缩比如下表.Compare：

方案确定了下面就开始flutter与原生的通讯代码
1、确定好通讯方式这里使用MethodChannel，定义好通讯channelName 和 methodName
2、定义好通讯方式编写flutter代码，发送消息传递List图片列表

import 'package:flutter/services.dart';

class ChannelUtils {
  static const MethodChannel platform =
      const MethodChannel('kzmall.flutter.io');
  static const String METHOD_COMPRESS_FILE = 'method_compress_file';

  // 工厂模式
  factory ChannelUtils() => _getShared();

  static ChannelUtils get shared => _getShared();
  static ChannelUtils _shared;

  ChannelUtils._internal();

  static ChannelUtils _getShared() {
    if (_shared == null) {
      _shared = ChannelUtils._internal();
    }
    return _shared;
  }

  Future<List<dynamic>> compressFile(List<dynamic> files) async{
    try {
      print('flutter发送图片路径');
      final res = await platform.invokeMethod<List<dynamic>>(METHOD_COMPRESS_FILE, files);
       print('flutter接收到消息');
      return Future.value(res);
    } catch (e) {
      print(e);
      return null;
    }
  }
}

3、iOS端定义一个页面继承FlutterViewController，引入Luban-iOS压缩分类，编写通讯代码，处理多图片压缩的逻辑,由于luban压缩只支持单张图片压缩，所有这里通过dispatch_group开启多线程实现多图片压缩功能，将压缩后的图片存储在沙盒中，并返回图片的沙盒路径Path

//
//  KZFlutterViewController.m
//  Runner
//
//  Created by chenhao on 2021/3/31.
//

#import "KZFlutterViewController.h"
#import <Foundation/Foundation.h>
#import "UIImage+Luban_iOS_Extension_h.h"
#import <UIKit/UIKit.h>

///通道名
#define MSG_CHANNEL_NAME    @"kzmall.flutter.io"
///方法名
#define MSG_METHOD_NAME     @"method_compress_file"

#define MSG_METHOD_CLEAR_NAME     @"method_clear_compress_file"


#define FILE_PATH [NSTemporaryDirectory() stringByAppendingPathComponent:@"compress_imagefile_ios"]


@interface KZFlutterViewController ()

@property (nonatomic,strong) FlutterMethodChannel *methodChannel;

@end

@implementation KZFlutterViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    /// 创建消息对象
    self.methodChannel = [FlutterMethodChannel methodChannelWithName:MSG_CHANNEL_NAME binaryMessenger:self.binaryMessenger];
    
    __weak __typeof__(self) weakSelf = self;
    /// 监听消息
    [self.methodChannel setMethodCallHandler:^(FlutterMethodCall* call, FlutterResult result) {
        if([call.method isEqualToString:MSG_METHOD_NAME]){
            NSArray *list = call.arguments;
            if([list isKindOfClass:[NSArray class]]){
                [weakSelf handleMsgData:list result:result];
            }
        }else if([call.method isEqualToString:MSG_METHOD_CLEAR_NAME]){
            //清空缓存
            [weakSelf clearTmpDirectory:FILE_PATH];
        }
        else{
            result(FlutterMethodNotImplemented);
        }
    }];
}

-(void)handleMsgData:(NSArray *)list result:(FlutterResult)result{
    NSMutableArray *newList = [NSMutableArray array];
    //取到数组
    //    NSLog(@"------------------start");
    //    __block NSInteger stepIndex = 0;
    //    for (NSString *imgPath in list) {
    //        dispatch_async(dispatch_get_global_queue(0, 0), ^{
    //            NSLog(@"当前线程：%@",[NSThread currentThread]);
    //            NSString *name = [imgPath componentsSeparatedByString:@"/"].lastObject;
    //            if([[NSFileManager defaultManager] fileExistsAtPath:imgPath]){
    //                UIImage *img = [UIImage imageWithContentsOfFile:imgPath];
    //                NSData *newImgData = [UIImage lubanCompressImage:img];
    //                UIImage *newImg = [[UIImage alloc] initWithData:newImgData];
    //                //拿到新的路径
    //                NSString *newPath = [self imageFilePathWithName:name];
    //                [UIImagePNGRepresentation(newImg) writeToFile:newPath atomically:YES];
    //                [newList addObject:newPath];
    //                stepIndex++;
    //                if(stepIndex == list.count){
    //                    dispatch_async(dispatch_get_main_queue(), ^{
    //                        NSLog(@"------------------end");
    //                        result(newList);
    //                    });
    //                }
    //            }
    //        });
    //    }
    
    //创建group
    dispatch_group_t group = dispatch_group_create();
    
    //    NSString *imgPath;
    for (NSString *imgPath in list) {
        //创建GCD组
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //创建信号量 并初始化信号量值为0
            dispatch_semaphore_t  sema = dispatch_semaphore_create(0);
            //由于信号量初始化值为0，所有当第一次执行到这里的时候信号量值仍然为0，程序就开始在这里等待信号量变化，前面异步的网络请求结束后，执行了dispatch_semaphore_signal(sema)方法，信号量+1，这样，dispatch_semaphore_wait监听到信号量由0变为1后，跳出等待，并且结束当前group队列
            NSString *name = [imgPath componentsSeparatedByString:@"/"].lastObject;
            if([[NSFileManager defaultManager] fileExistsAtPath:imgPath]){
                UIImage *img = [UIImage imageWithContentsOfFile:imgPath];
                NSData *newImgData = [UIImage lubanCompressImage:img];
                UIImage *newImg = [[UIImage alloc] initWithData:newImgData];
                //拿到新的路径
                NSString *newPath = [self imageFilePathWithName:name];
                [UIImagePNGRepresentation(newImg) writeToFile:newPath atomically:YES];
                [newList addObject:newPath];
                dispatch_semaphore_signal(sema);
            }
            dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
        });
    }
    
    //把group_notify放在主队列中，这样可以在这个队列中刷新UI
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        //排序
        result(newList);
    });
}

-(NSString *)imageFilePathWithName:(NSString *)name{
    
    NSString *path = FILE_PATH;
    NSFileManager *fileManager = [NSFileManager defaultManager];
    // fileExistsAtPath 判断一个文件或目录是否有效，isDirectory判断是否一个目录
    
    BOOL isDir = NO;
    BOOL existed = [fileManager fileExistsAtPath:path isDirectory:&isDir];
    if (!(isDir && existed)) {
        // 在Document目录下创建一个archiver目录
        [fileManager createDirectoryAtPath:path withIntermediateDirectories:YES attributes:nil error:nil];
    }
    path = [path stringByAppendingPathComponent:[NSString stringWithFormat:@"%@",name]];
    return path;
}

-(void)clearTmpDirectory:(NSString *)directoryPath{
    NSFileManager *mgr = [NSFileManager defaultManager];
    if([mgr fileExistsAtPath:directoryPath]){
        NSError *error;
        [mgr removeItemAtPath:directoryPath error:&error];
        if(error){
            NSLog(@"%@",error);
        }
    }
}


-(void)sendEvent:(NSString *)eventName
       arguments:(NSDictionary *)arguments
          result:(FlutterResult _Nullable)callback{
    if(!eventName) return;
    NSMutableDictionary *msg = [NSMutableDictionary new];
    msg[@"name"] = eventName;
    msg[@"arguments"] = arguments;
    [self.methodChannel invokeMethod:MSG_METHOD_NAME
                           arguments:msg
                              result:callback];
}

3、android端MainActivity继承FlutterActivity，导入Luban implementation 'top.zibin:Luban:1.1.8' 库，并引入import top.zibin.luban.Luban import top.zibin.luban.OnCompressListener，编写通讯和压缩的业务代码，android的luban框架更完善 支持同时压缩多张图片和自动缓存图片返回路径功能

class MainActivity : FlutterActivity() {

    private val CHANNEL_NAME = "kzmall.flutter.io"

    private val METHOD_COMPRESS_FILE = "method_compress_file"

    override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
//        super.configureFlutterEngine(flutterEngine)
        GeneratedPluginRegistrant.registerWith(flutterEngine)
        MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL_NAME).setMethodCallHandler { methodCall: MethodCall, result: MethodChannel.Result ->
            if (methodCall.method == METHOD_COMPRESS_FILE) {
                if (methodCall.arguments != null && methodCall.arguments is List<*>) {
                    var handlerNum = 0
                    var fileList: List<String> = methodCall.arguments as List<String>
                    var compressFiles: ArrayList<String> = arrayListOf()
                    Luban.with(this)
                            .load<String>(fileList)
                            .ignoreBy(100)
                            .filter { path -> !(TextUtils.isEmpty(path) || path.toLowerCase().endsWith(".gif")) }
                            .setCompressListener(object : OnCompressListener {
                                override fun onStart() {
                                }

                                override fun onSuccess(file: File?) {
                                    handlerNum++
                                    if (file != null) {
                                        compressFiles.add(file.absolutePath)
                                    }
                                    if (handlerNum == fileList.size) {
                                        result.success(compressFiles)
                                    }
                                }

                                override fun onError(e: Throwable) {
                                    result.error("-1", "图片压缩失败", null)
                                }
                            }).launch()
                } else {
                    result.error("-2", "参数类型错误", null)
                }


            } else {
                result.notImplemented()
            }
        }
    }
}

4、flutter端在使用的地方调用即可返回压缩后的图片

List<dynamic> imges = [图片列表]
final res = await ChannelUtils.shared.compressFile(imges)