MVVM for iOS with objective-c based on KVO.

The ViewModelKit is a framework to support the iOS app development to use the MVVM paradigm. It uses KVO as binding technic to connect the models with view models and view controllers.

Carthage is a decentralized dependency manager that builds your dependencies and provides you with binary frameworks.

You can install Carthage with Homebrew using the following command:

$ brew update
$ brew install carthage

To integrate ViewModelKit into your Xcode project using Carthage, specify it in your Cartfile:

github "Andrenalin/ViewModelKit" ~> 1.0

Run carthage to build the framework and drag the built ViewModelKit.framework into your Xcode project.

The observing works with two instances. The first object is a VMKBindingUpdater and the second is VMKObservable.

VMKBindingUpdater *bu = [[VMKBindingUpdater alloc] initWithObserver:self updateAction:@selector(nameDidChange)];

self.myObservable = [[VMKObservable alloc] initWithObject:self.model forKeyPath:@"name" bindingUpdater:bu];
[self.myObservable startObservation];

First create a VMKBindingUpdater instance that stores the connection between the observer and an update method. In the example the method nameDidChange will be called on self and self is the observer.

Second create the VMKObservable with the model object that will be observed. The keypath must be property on the model object that is KVO compatible. In the example the property name will be observed on the self.model object. Everytime the name has changed the nameDidChange method will be called on the observer self.

Don't forget to call startObservation method on the VMKObservable instance otherwise it will not observe the porperty. The startObservation method does also a check if the property on the model and the updateAction method on the binding object exist.

It is best practise to have a strong reference to the model object (self.model) and the observable (self.myObservable). KVO usually crashes if any of the involved objects are deallocated. The wrapper object VMKBindingUpdater and VMKObservable take care about this common issue.

The VMKObservable has a strong reference on the model object it will not crash but it is good practise to have also a strong reference in the observer instance. The VMKBindingUpdater holds the observer as a weak property so that if you deallocating the observer the VMKBindingUpdater will not call the updateAction method anymore. Also the VMKObservable instance can be deallocate ànytime and disconntects the KVO correctly.

The public header contains usually only readonly properties and can specify the model it belongs to. An init is not necessary if only one model is used.

@interface ExampleViewModel : VMKViewModel<ExampleModel *>
@property (nonatomic, copy, readonly) NSString *name;
@property (nonatomic, assign, readonly) NSInteger someNo;
@end

If the view model needs multiple or different model instances the type might be not specify. For CoreData you can use the generic VMKViewModel class as super class or the specific VMKCoreDataViewModel class. Both classes use only one model.

The private header declares the properties as readwrite to change the properties if the model has changed.

@interface ExampleViewModel ()
@property (nonatomic, copy, readwrite) NSString *name;
@property (nonatomic, assign, readwrite) NSInteger someNo;
@end

The implementation is something like that:

@implementation ExampleViewModel

#pragma mark - binding

- (VMKBindingDictionary *)modelBindings {
    return @{
        @"numberTwo": [self bindingUpdaterOnSelector:@selector(numberDidChange)],
        VMK_BINDING_KEYPATH_PROPERTY(@"numberOne", number),
        // @"numberOne": [self bindingUpdaterOnSelector:@selector(numberDidChange)],
        VMK_BINDING_PROPERTY(name)
        // NSStringFromSelector(@selector(name)): [self bindingUpdaterOnSelector:@selector(nameDidChange)],
    };
}

#pragma mark - update

- (void)nameDidChange {
    self.name = self.model.name;
}

- (void)numberDidChange {
    self.someNo = self.model.numberOne + self.model.numberTwo;
}

@end

The specific view model is binding the model properties to xxxDidChange methods on its own. Everytime the model properties are changing the xxxDidChange methods are calculating the properties of the ViewModel again. In this example the ExampleModel and ExampleViewModel are more or less straight forward but there are more complex relationships possible between a Model and a ViewModel.

The startModelObservation from VMKViewModel class will use the modelBindings to connect the model with the view model. If it is not overwritten no bindings will be setup then the specific class is responsible to do the bindings.

One key feature of the ViewModelKit is the VMKBindingDictionary * type. It is a dictionary that describes how a model is binded to an observer. The key is a keypath to the model and the value is a VMKBindingUpdater instance. A VMKViewModel has a helper method bindingUpdaterOnSelector: to create a VMBindingUpdater instance direct on the view model. The line: @"numberTwo": [self bindingUpdaterOnSelector:@selector(numberDidChange)], means connect the keypath numberTwo to the view model and call numberDidChange at the first connect and then after every change.

VMK_BINDING_KEYPATH_PROPERTY(keypath, name of didChange) is a macro to shorten the typing. The shortest version is VMK_BINDING_PROPERTY(name) and will use the NSStringFromSelector(@selector(name)) for the keypath and create a default binding updater with the naming convention nameDidChange.`

The definition of a custom ViewController looks like this:

@import ViewModelKit;

@interface ExampleViewController : VMKViewController<ExampleViewModel *>
@end

The VMKViewController is a specialized class of the UIViewController and has several helper methods. It has a generic viewModel property and a controllerModel property. The ViewModel can be specify as generic type. In the example the ExampleViewController has a viewModel property from type ExampleViewModel *.

The implementation of a custom ViewController is pretty straight forward like a custom ViewModel:

@interface ExampleViewController ()
@property (weak, nonatomic) IBOutlet UILabel *nameLabel;
@property (weak, nonatomic) IBOutlet UILabel *someNoLabel;
@end

@implementation ExampleViewController

- (VMKBindingDictionary *)viewModelBindings {
    return @{
        VMK_BINDING_PROPERTY(name),
        VMK_BINDING_PROPERTY(seomNo)
    };
}

#pragma mark - binding did changed

- (void)nameDidChanged {
    self.nameLabel.text = self.viewModel.name;
}

- (void)someNoDidChanged {
    self.someNoLabel.text = [[NSString alloc] initWithFormat:@"%d", (int)self.viewModel.someNo];
}

@end

The VMKViewController life cycle viewWillAppear method is calling the bindViewModel method which is using the viewModelBindings method to get the bindings. At viewWillDisappear the unbindViewModel method gets called and disconntect the view model from the view controller.

To use the ExampleViewController it will be instanciate from a previous view controller (or storyborad) and a setup (for example prepareForSegue:forSender: method) sets the view model on it.

The view model class VMKCoreDataViewModel allows any NSManagedObject as model object.

@interface YourCoreDataViewModel : VMKCoreDataViewModel<YourCoreData *>
@end

It uses the same methods as any normal VMKViewModel class.

The VMKTableViewController and VMKCollectionViewController are similar to VMKViewController but they add additional functionality and use a data source to handle cell view models easier.

On the first glance a table view with a table view controller seems to make it more complex but it uses the same principles you have seen already with VMKViewModel and VMKViewController.

A full setup needs a custom VMKTableViewController with a custom VMKViewModel from type VMKDataSourceViewModelType. The VMKDataSourceViewModelType is a protocol and adds a data source to the view model.

The VMKTableViewController and VMKCollectionViewController are working with a VMKDataSource and that is the second part of the setup. The typical data sources are already implemented but it can be customized by implementing an own data source.

Every cell in a table view or cellection view has a cell view model. The VMKDataSource is creating VMKViewModel from type VMKCellType for the requested cell. These cell view models are the view models for the cells like a VMKViewModel is for the VMKViewController.

The VMKTableViewController can customize the view related part with the VMKTableViewDataSourceDelegate. The view models are not defining any view related parts like what kind of cell.

@implementation TableViewController (VMKTableViewDataSourceDelegate)

- (NSString *)dataSource:(VMKTableViewDataSource *)dataSource cellIdentifierAtIndexPath:(NSIndexPath *)indexPath {

    // VMKViewModel<VMKCellType> *vm = [dataSource viewModelAtIndexPath:indexPath];
    return @"CustomCell";
}

@end

It is possible to interact with the cell view model to get the right cell identifier.

There are some more methods in the VMKTableViewDataSourceDelegate that allow you to customize the table view behaivor and they are quite similar to the UITableViewDataSource.

Some of the UITableViewDelegate methods are get passed to the VMKTableViewController. The others are already handled.

The view model for the VMKTableViewController or the VMKCollectionViewController have one important method to implement: the dataSource getter.

@implementation TableViewModel (VMKDataSourceViewModelType)

- (VMKDataSource *)dataSource {
    if (!_dataSource) {
        NSArray<VMKViewModel<VMKCellType> *> *viewModels = [self getViewNodels];
        _dataSource = [[VMKArrayDataSource alloc] initWithViewModels:viewModels];
    }
    return _dataSource;
}

@end

There are several data source possiblities. The typical data soucres like an array, a mutable array or a fetched resultscontroller data source are available to use immediately without subclassing. They called data source provider because they providing cell view models.

It is also possible to write your own data source provider by subclassing the VMKDataSource.

The cell view model is any VMKViewModel that conforms to the VMKCellType protocol. These cell view models are the view models for any cells. The corresponding views are VMKTableViewCell, VMKCollectionViewCell or their subclasses. They are observing the cell view models to display any data like the other VMKViewModel.

In the example the cell view model has a core data entity SomeData and uses the VMKCoreDataViewModel as basis class. It conforms to the VMKCellType to be a cell view model.

@interface CellViewModel : VMKCoreDataViewModel<SomeData *> <VMKCellType>
@end

The private header declares the necessary properties to readwrite. Usually only the viewModel property is necessary for a cell view model, because this is the view model for the detail view controller.

For a default UITableViewCell the property title, subtitle and image can be implemented without having a custom cell view. Here, in the example the image is not used.

@interface CellViewModel ()
// protocol VMKCellViewModelType optional
@property (nonatomic, copy, readwrite, nullable) NSString *title;
@property (nonatomic, copy, readwrite, nullable) NSString *subtitle;
// protocol VMKCellViewModelType
@property (nonatomic, weak, readwrite, nullable) VMKViewModel *viewModel;
@end

The implementation does the binding.

@implementation CellViewModel

#pragma mark - binding

- (VMKBindingDictionary *)modelBindings {
    return @{
        VMK_BINDING_KEYPATH_PROPERTY(@"title", title),
        VMK_BINDING_KEYPATH_PROPERTY(@"subtitle", subtitle)
    }
}

- (void)titleDidChange {
    self.title = self.model.title;
}

- (void)subtitleDidChange {
    self.subtitle = self.model.subtitle;
}

#pragma mark - VMKCellType

- (VMKViewModel *)viewModel {
    if (_viewModel) {
        return _viewModel;
    }

    // the _viewModel is weak therefore use a strong heap varaiable
    DetailViewModel *dvm = [[DetailViewModel alloc] initWithModel:self.model];
    _viewModel = dvm;
    return dvm;
}

The implementation is using the same structure like any other view model. The viewModel is only needed if there is a detail view controller to present from the cell. This viewModel gets passed to the detail view controller.

The Framework is divided into several sections. The sections are:

• Observer
• Model
• CellViewModel
• ViewController
• DataSource
• CoreData
• ChangeSet
• Controller
• UIKit Views

The Observer section encapsulate the KVO in two classes and two managers. The classes are

• VMKObservable
• VMKBindingUpdater
• VMKObservableManager
• VMKObservableNotificationManager

The VMKObservable holds the object, the keypath and an observer to inform about updates. The VMKObservable and VMKBindingUpdater are the heart of the ViewModelKit. They can be used standalone to do bindings between any model object and any observer.

The VMKBindingUpdater holds the observer as a weak property to avoid retain cycle and let the observer deallocate. The class checks if the observer is gone before calling the update method. So KVO will not crash if the observer is already deallocated.

The VMKObservableManager manages all instances of VMKObservables. An instance of this class holds the observables and deallocate everything on deallocation. It manages the bindings between any object, keypath and observers. Every VMKViewModel has its own ``VMKObserverManager`. One reason is if any view model will be released the bindings will be disconnected immediately.

The VMKObservableNotificationManager is inside the Observer section but is has nothing to do with the KVO observing. It is a helper class for NSNotification that uses blocks.

The Model section groups the basic view model classes and model related protocols together.

• VMKViewModel
• VMKControllerModel
• VMKArrayModel
• VMKDataSourceViewModelType
• VMKEditingType
• VMKImageEditingType
• VMKViewModelCache

The VMKViewModel is the basis class of any view model and takes care of the binding mechanism. It has two observable managers. One is internally for the view model itself and primarily aviable for subclassing. The other is for the external bindings. There is a interface with several methods to bind any observer to the view model. This is mostly used from view controllers. Subclasses normally only write the modelBindings method. The bindModel and unbindModel are get called during the setModel method or from startModelObservation. The init is not starting any observation even if a model is passed as argument.

The VMKControllerModel is used inside a view controller to pass view related settings around. For example to change the index path of the selected cell from a detail view controller. Or set the next view controller into an edit mode.

The VMKArrayModel observes an given array. Use the interface to generate KVO changes for the different operation.

The VMKDataSourceViewModelType is declaring a data source and they are used for a table view controller or collection view controller.

The VMKEditingType and VMKImageEditingType are used to specify that a view model can be edited or have an image that can be edited. There are some other controller model implementations that are using the types.

The VMKViewModelCache is a caching class for view model. The class is used for data source provider to reuse view models.

The CellViewModel section is releated to cell view model that can be used for the UITableViewCell or UICollectionViewCell.

• VMKCellType
• VMKViewCellType
• VMKHeaderFooterType
• VMKViewHeaderFooterType
• VMKCellViewModelFactory
• VMKInsertCellViewModel

The VMKCellType is used for any cell view model and define several mostly optional methods. The only non optional method is the viewModel getter. This view model is used for any detail view for that specific cell. Beside that the three getters for title, subtitle and image can be implemented in conjunction with UITableViewCell or UICollectionViewCell. A custom cell may use them or define different ones. Several methods are used to indicate any editing possiblitites like canEdit or canDelete.

The VMKViewCellType is a type for any UITableViewCell or UICollectionViewCell that can be used with the ViewModelKit. Any custom view cell class must be conform to this protocol. The generic VMKTableViewCell and VMKCollectionViewCell do it.

The VMKHeaderFooterType and VMKViewHeaderFooterType are analog to the cell ones. They are defining the view model and the UITableViewHeaderFooterView for ViewModelKit.

The VMKCellViewModelFactory protocol describes how to create a cell view model from a data source with an object. It is used for some data source provider to delegate this task back to a view model.

The VMKInsertCellViewModel is a cell view model for a cell that can insert new data. It is a static cell.

The ViewController section has three view controllers.

• VMKViewController
• VMKTableViewController
• VMKCollectionViewController

The VMKViewController can be used as typical view controller. It has the bin and unbind during the view life cycle process integrated. In addition to a viewModelproperty it has a controllerModel property. Also the view controller is able to show an alert controller and a choose image controller immediately. A subclass should provide the type of the view model and overwrite the viewModelBindings.`

The VMKTableViewController and VMKCollectionViewController are similar to the VMKViewController but have in addition implemented the delegate and necessary data source protocol methods. For example will a table view update all changes from the data source lively via row animations. The view controllers are using several wrapper classes to combine the delegate and data source protocol from UIKit properly with the ViewModelKit.

If you cannot use one of the three view controller classes them you have to write the binding and have to conform to the view model protocols by yourself. The implementation of the classes can give you usefull information how to do.

The DataSource section has classes to abstract the data source for UITableView or UICollectionView. Both have the same data source interface and uses cell view models.

The section is divided into four subsection Abstract, Provider, Multi and Grouped data sources.

• VMKDataSource
• VMKDataSourceDelegate
• VMKDataSourceType

The VMKDataSource is the abstract class of all data source classes. It provides a delegate property and an editing property. There are some inserting and deleting row methods and the reordering methods. These information will be obtained from the cell view model.

The VMKDataSourceDelegate is the reporting system to notify an object about a change. It uses the VMKChangeSet. Subclasses of VMKDataSource can implement this protocol to catch changes from lower data sources to manipluate the changes.

The VMKDataSourceType is basis of any data source for the ViewModelKit. It defines three must have methods. Every VMKDataSourceType must implement sections, rowsInSection: and viewModelAtIndexPath:. Then it defines several section header, section footer and section index methods a la UITableViewDataSource. The setEditing is optional.

• VMKArrayDataSource
• VMKMutableArrayDataSource
• VMKFetchedDataSource

The VMKArrayDataSource uses an array of cell view models and provide them. This data source is static and no change set will be send. It can have a header view model as a section header.

The VMKMutableArrayDataSource uses an VMKArrayModel which contains any model objects. The model object will be converted into cell view model by the VMKCellViewModelFactory delegate. These cell view model are cached with the VMKViewModelCache. Everytime the VMKArrayModel has changed the VMKArrayUpdater get informed and generate a change set.

The VMKFetchedDataSource uses a NSFetchedResultsController to get the models from core data. A VMKCellViewModelFactory delegate creates the cell view model. Some settings allow to configure the data source. The changes will be reported similar like VMKMutableArrayDataSource does it.

The multi data sources are used to build up a tree of data sources. The provider data source are the leaves of the tree the trunk or branches can be any multi data source.

There are two general multi data sources. On divide the data source into sections and the other are rows. Both can add or remove a data source at runtime. Also insert a data source at an index or remove it at an index.

The VMKMultiSectionDataSource can have several data sources. Every data source are handled as part of sections. For example the first data source has 2 sections and the second one has 4 sections. Then the multi data source will return 6 section. Every access at section 0 to 1 will be routed to the first data source. Every access to the section 2 to 5 will be mapped to 0 to 3 and then routed to the second data source.

The VMKMultiRowDataSource can have also several data sources. Every data source are handled as part of rows. It is complete similar to the VMKMultiSectionDataSource except it uses rows.

The VMKEditDataSource is specialized multi data source that has two data sources. In edit mode the edit data source is shown. There is a mode add the edit data source as rows or sections.

The VMKGroupedDataSource is specialized data source that has one data source to get the grouped keys of a set of objects. Then it uses the grouped key to cluster several objects together and generate only one data source of each grouped key.

The CoreData section has three classes.

• VMKCoreDataViewModel
• VMKChangeWatcher
• VMKDeleteWatcher

The VMKCoreDataViewModel is a view model class for NSManagedObject. It has three additional method for editing and saving. This might be changing in the future.

The VMKChangeWatcher is a class that informs a delegate if any core data object has changed. The class is managing which object will be observed.

The VMKDeleteWatcher is a class that informs a delegate if any core data object has deleted. The class is managing which object will be observed.

The ChangeSet section has two classes related to a change set and two classes that emit a change set.

• VMKChangeSet
• VMKSingleChange
• VMKArrayUpdater
• VMKFetchedUpdater

The VMKChangeSet object is used to take all changes in the view models through the data source into the view controller. The change set itself has the changes of any collection of view models. The changes are distinct between section and row changes. The row changes are inserted, deleted, updated and moved. The section changes are inserted, deleted and updated. If for example a fetched resultscontroller got changes from core data every single change will be recorded and stored in the history of a change set. This change set will be transmitted through the data source to the VMKTableViewController or VMKCollectionViewController. The controller itself replay all changes into row anmination.

The VMKSingleChange is one specific change. The type of the change is a VMKSingleChangeType. The class has several convinence getter and a method to manipulate the offsets.

The VMKArrayUpdater takes a VMKArrayModel and observes all changes in the array. The changes will be recorded and send as VMKChangeSet to a delegate.

The VMKFetchedUpdater is a NSFetchedResultsControllerDelegate and observes all changes in the fetch request of the fetched result controller. The changes will be recorded and send as VMKChangeSet to a delegate.

The Controller section has two system controllers and related view models.

• VMKAlertController
• VMKAlertViewModel
• VMKChooseImageController
• VMKChooseImageViewModel

The VMKAlertController is the UIAlertController that uses the VMKAlertViewModel. The three view controllers have already a VMKAlertController and can show immediately one.

The VMKAlertViewModel configures the alert view controller.

The VMKChooseImageController is the UIImagePickerController that uses the VMKChooseImageViewModel. The three view controllers have already a VMKChooseImageController and can show immediately one.

The VMKChooseImageViewModel configures the image picker controller.

The tests are using OCHamcrest and OCMockito. Both frameworks can be installed through Carthage.

Please read CONTRIBUTING.md for details on our code of conduct, and the process for submitting pull requests to us.

We use SemVer for versioning. For the versions available, see the tags on this repository.

• Andre Trettin - Creator - Andrenalin
• Daniel Rinser - CollectionView - danielr
• Daniel Seebach - TableHeaderFooter - DanielSeebach

See also the list of contributors who participated in this project.

This project is licensed under the MIT License - see the LICENSE file for details