littleWhale

中介者模式（Mediator Pattern）是用来降低多个对象和类之间的通信复杂性。这种模式提供了一个中介类，该类通常处理不同类之间的通信，并支持松耦合，使代码易于维护。中介者模式属于行为型模式。
在MVC 框架中，其中C（控制器）就是 M（模型）和 V（视图）的中介者。我们接下来用一个搜索视图来展示中介者模式:

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protocol HistoryView {
    var isHidden: Bool { get set }
    func setHistory(_ history: [String])
}

protocol HistoryRepository {
    var history: [String] { get }
    func addSearchTerm(_ term: String)
}

class SearchHistoryMediator: NSObject {
    private let searchBar: UISearchBar
    private var historyView: HistoryView
    private var observasion: NSKeyValueObservation?
    private let historyRepository: HistoryRepository

    init(searchBar: UISearchBar, historyView: HistoryView, historyRepository: HistoryRepository) {
        self.searchBar = searchBar
        self.historyView = historyView
        self.historyRepository = historyRepository
        super.init()

        self.historyView.isHidden = true
        self.historyView.setHistory(historyRepository.history)

        searchBar.delegate = self
        observasion = searchBar.observe(\.text) { [weak self] (searchBar, _) in
            self?.historyView.isHidden = searchBar.text?.isEmpty ?? false
        }
    }
}

extension SearchHistoryMediator: UISearchBarDelegate {

    func searchBarTextDidBeginEditing(_ searchBar: UISearchBar) {
        historyView.isHidden = false
    }

    func searchBarTextDidEndEditing(_ searchBar: UISearchBar) {
        historyView.isHidden = true
    }

    func searchBarSearchButtonClicked(_ searchBar: UISearchBar) {
        historyView.isHidden = true
        if let text = searchBar.text, !text.isEmpty {
            historyRepository.addSearchTerm(text)
            historyView.setHistory(historyRepository.history)
            searchBar.text = nil
        }
        searchBar.resignFirstResponder()
    }

    func searchBarCancelButtonClicked(_ searchBar: UISearchBar) {
        historyView.isHidden = true
        searchBar.resignFirstResponder()
    }
}

//在控制器中使用中介者
private(set) lazy var mediator: SearchHistoryMediator = {
    return SearchHistoryMediator(searchBar: searchBar, historyView: historyView, historyRepository: historyRepository)
}()

override func viewDidLoad() {
    super.viewDidLoad()
    _ = mediator
}

优点：降低了类的复杂度，将一对多转化成了一对一。各个类之间的解耦，减少对象之间的关联性，让每一个对象都能够独立

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