> On Jun 8, 2017, at 11:11 AM, Joel Loeshelle via swift-users
> <swift-users@swift.org> wrote:
>
> Hi all,
>
> Is there any support for having an optional type in an initializer? My use
> case is I want to have a struct SectionWrapper that includes information
> about a Collection View Section. So you can pass the CellType, Cell Data
> Source, and an Optional HeaderType and HeaderDataSource if there is a Section
> Header. Here's the code. I first have a protocol ViewModelConfigurable that
> the Cell and Header subclasses conform to:
>
> protocol ViewModelConfigurable
> {
> associatedtype ViewModelType
>
> func configure(with viewModel : ViewModelType);
> }
>
> class TestCell : UICollectionViewCell, ViewModelConfigurable
> {
> typealias ViewModelType = String
> func configure(with viewModel: String) {
> //todo
> }
> }
>
> class HeaderFooter : UICollectionReusableView, ViewModelConfigurable
> {
> typealias ViewModelType = String
> func configure(with viewModel: String) {
> //todo
> }
> }
>
> I then have a SectionWrapper struct where I want to initialize with a
> concrete UICollectionViewCell type that conforms to ViewModelType and pass
> the associated dataSource which must be an array of ViewModelType. I also
> want to have the ability to pass an optional UICollectionReusableView type
> that conforms to ViewModelType with the associated dataSource. You could then
> pass nil if this section does not have a Header. See below:
>
> struct SectionWrapper
> {
> let cellConfigure: (UICollectionViewCell, Int) -> ()
> let headerConfigure: ((UICollectionReusableView) -> ())?
>
> init<ActualCellType : UICollectionViewCell & ViewModelConfigurable,
> HeaderType : UICollectionReusableView & ViewModelConfigurable>(cellType :
> ActualCellType.Type, cellDataSource : [ActualCellType.ViewModelType],
> headerType : (HeaderType.Type)?, headerDataSource :
> (HeaderType.ViewModelType)?)
> {
> self.cellConfigure = { cell, index in
> (cell as! ActualCellType).configure(with: cellDataSource[index])
> }
>
> if let headerDataSource = headerDataSource
> {
> self.headerConfigure = { header in
> (header as! HeaderType).configure(with: headerDataSource);
> }
> }
> else
> {
> self.headerConfigure = nil
> }
> }
> }
>
> If I pass nil for headerType and headerDataSource, I get the following error:
> error: Swift4Test.playground:86:31: error: generic parameter 'ActualCellType'
> could not be inferred
> let noHeader = SectionWrapper.init(cellType: TestCell.self, cellDataSource:
> ["Test"], headerType: nil, headerDataSource: nil) //Gives error
>
>
> let header = SectionWrapper.init(cellType: TestCell.self, cellDataSource:
> ["Test"], headerType: HeaderFooter.self, headerDataSource: "Header") //No
> error
>
> let noHeader = SectionWrapper.init(cellType: TestCell.self, cellDataSource:
> ["Test"], headerType: nil, headerDataSource: nil) //Gives error
>
>
> I have also attached the Playground that I'm working through. Please let me
> know if you have any questions and thanks in advance for your help!
You could overload SectionWrapper.init(), providing two versions: one takes
non-optional headerType and headerDataSource parameters, and the other does not
take them at all, and only has a single generic parameter for ActualCellType.
Unfortunately we don’t support explicit specialization on function calls right
now; if we did, you could write SectionWrapper.init<T, U> to explicitly bind
the generic arguments.
Slava
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