GitBucket源码阅读笔记

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学习函数式编程已经有一个半月了，学习了RxSwift和RAC2.X，尝试过写一些小的demo，但是一直没有了解过MVVM中是如何使用RAC进行通信的，由于公司项目准备使用MVVM+RAC模式重构，在github找到了使用MVVM+RAC构建的Github开源app：GitBucket，本文主要记录该项目中RAC在MVVM模式中的应用。

项目类图


项目中主要有两大继承体系：MRCViewModel和MRCViewController，分别对应ViewModel层和Controller层，实现代码复用

在MRCViewModel父类中存放了一些需发送给Controller层的信号，如标题变化，错误信号，控制器隐藏信号等；
MRCViewController中实现了监听ViewModel信号的方法，如监听ViewModel的标题变化

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RAC(self.navigationItem, titleView) = [RACObserve(self.viewModel, titleViewType).distinctUntilChanged map:^(NSNumber *value) {
    MRCTitleViewType titleViewType = value.unsignedIntegerValue;
    switch (titleViewType) {
        case MRCTitleViewTypeDefault:
            return titleView;
        case MRCTitleViewTypeDoubleTitle:
            return (UIView *)doubleTitleView;
        case MRCTitleViewTypeLoadingTitle:
            return (UIView *)loadingTitleView;
    }
}];

另外项目中使用了Service层提供ViewModel所需的各种服务
OCTClient：Github的三方库api，提供了RAC的支持
MRCRepositoryService/MRCRepositoryServiceImpl：应用自有的服务类
MRCViewModelServices：提供界面跳转的实现

界面跳转

作者没有使用系统提供的push/present操作实现界面跳转，而是在MRCViewModelServices中实现了一系列空操作

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- (void)pushViewModel:(MRCViewModel *)viewModel animated:(BOOL)animated {}

- (void)popViewModelAnimated:(BOOL)animated {}

- (void)popToRootViewModelAnimated:(BOOL)animated {}

- (void)presentViewModel:(MRCViewModel *)viewModel animated:(BOOL)animated completion:(VoidBlock)completion {}

- (void)dismissViewModelAnimated:(BOOL)animated completion:(VoidBlock)completion {}

- (void)resetRootViewModel:(MRCViewModel *)viewModel {}

之后在视图层维护了一个MRCNavigationControllerStack，通过RAC订阅Service的跳转方法的调用，提供真正的界面跳转的代码实现

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@weakify(self)
   [[(NSObject *)self.services
       rac_signalForSelector:@selector(pushViewModel:animated:)]
       subscribeNext:^(RACTuple *tuple) {
           @strongify(self)
          
           MRCViewController *topViewController = (MRCViewController *)[self.navigationControllers.lastObject topViewController];
           if (topViewController.tabBarController) {
               topViewController.snapshot = [topViewController.tabBarController.view snapshotViewAfterScreenUpdates:NO];
           } else {
               topViewController.snapshot = [[self.navigationControllers.lastObject view] snapshotViewAfterScreenUpdates:NO];
           }
           
           UIViewController *viewController = (UIViewController *)[MRCRouter.sharedInstance viewControllerForViewModel:tuple.first];
           viewController.hidesBottomBarWhenPushed = YES;
           [self.navigationControllers.lastObject pushViewController:viewController animated:[tuple.second boolValue]];
       }];

这样Controller只需要做数据绑定即可，需要跳转界面时，ViewModel层调用service的方法，MRCNavigationControllerStack监听到方法调用后做界面跳转
[self.services pushViewModel:viewModel animated:YES];

另外由于VM层不可引入View层的类，所以跳转的一系列方法的传参都为ViewModel，通过维护一个ViewModel与Controller层的映射字典，实现了视图与界面的分离
@property (nonatomic, copy) NSDictionary *viewModelViewMappings; // viewModel到view的映射

#RAC用法

###通过RAC提供代理的函数实现

使用rac_signalForSelector:fromProtocol方法订阅键盘的回车键

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@weakify(self)
[[self
	rac_signalForSelector:@selector(textFieldShouldReturn:)
    fromProtocol:@protocol(UITextFieldDelegate)]
	subscribeNext:^(RACTuple *tuple) {
        @strongify(self)
        if (tuple.first == self.passwordTextField) [self.viewModel.loginCommand execute:nil];
    }];

###RAC监听UI事件
使用rac_signalForControlEvents方法订阅按钮的点击信号，当按钮被点击时，执行登录的命令

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[[self.loginButton
    rac_signalForControlEvents:UIControlEventTouchUpInside]
    subscribeNext:^(id x) {
        @strongify(self)
        [self.viewModel.loginCommand execute:nil];
    }];

###RAC监听通知
使用rac_addObserverForName方法订阅app从后台进入前台的通知，takeUntil保证在对象销毁后取消订阅

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[[[[NSNotificationCenter defaultCenter]
    rac_addObserverForName:UIApplicationWillEnterForegroundNotification object:nil]
    takeUntil:self.rac_willDeallocSignal]
    subscribeNext:^(id x) {
        @strongify(self)
        [self.viewModel.requestRemoteDataCommand execute:nil];
    }];

###RAC在TableView中的应用
监听ViewModel中dataSource数组的变化，dataSource发生改变时重新加载tableView

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@weakify(self)
[[[RACObserve(self.viewModel, dataSource)
    distinctUntilChanged]
    deliverOnMainThread]
    subscribeNext:^(id x) {
        @strongify(self)
        [self reloadData];
    }];

刷新列表时显示loading菊花,刷新完成时隐藏

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[[[RACSignal
    combineLatest:@[ self.viewModel.requestRemoteDataCommand.executing, RACObserve(self.viewModel, dataSource) ]
    reduce:^(NSNumber *executing, NSArray *dataSource) {
        return @(executing.boolValue && dataSource.count == 0);
    }]
    deliverOnMainThread]
    subscribeNext:^(NSNumber *showHUD) {
        @strongify(self)
        if (showHUD.boolValue) {
            [MBProgressHUD showHUDAddedTo:self.view animated:YES].labelText = MBPROGRESSHUD_LABEL_TEXT;
        } else {
            [MBProgressHUD hideHUDForView:self.view animated:YES];
        }
    }];

使用rac_sequence和map将model转化为viewmodel数据源

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- (NSArray *)dataSourceWithEvents:(NSArray *)events {
    if (events.count == 0) return nil;
    
    @weakify(self)
    NSArray *viewModels = [events.rac_sequence map:^(OCTEvent *event) {
        @strongify(self)
        MRCNewsItemViewModel *viewModel = [[MRCNewsItemViewModel alloc] initWithEvent:event];
        viewModel.didClickLinkCommand = self.didClickLinkCommand;
        return viewModel;
    }].array;
    
    return @[ viewModels ];
}