MVVM Philosophy

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Philosophy

  • View doesn’t have control flow. View cannot modify the data. View only knows how to map the data.

    Bad

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    viewModel.title
        .map { $0 + "!" } // Bad: View should not modify the data
        .bindTo(self.titleLabel)

    Good

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    viewModel.title
        .bindTo(self.titleLabel)

  • View doesn’t know what ViewModel does. View can only communicate to ViewModel about what View did.

    Bad

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    viewModel.login() // Bad: View should not know what ViewModel does (login)

    Good

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    self.loginButton.rx_tap
        .bindTo(viewModel.loginButtonDidTap) // "Hey I clicked the login button"

    self.usernameInput.rx_controlEvent(.EditingDidEndOnExit)
        .bindTo(viewModel.usernameInputDidReturn) // "Hey I tapped the return on username input"

  • Model is hidden by ViewModel. ViewModel only exposes the minimum data so that View can render.

    Bad

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    struct ProductViewModel {
        let product: Driver<Product> // Bad: ViewModel should hide Model
    }

    Good

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    struct ProductViewModel {
        let productName: Driver<String>
        let formattedPrice: Driver<String>
        let formattedOriginalPrice: Driver<String>
        let originalPriceHidden: Driver<Bool>
    }