如何在Swift中返回Realm列表？

Question

I have 2 Realm List

and I'm trying to make healper function so I can choose between them to display one of them on UITableview

my model looks like this

public class RealmModel : Object {

    public var arrayList1 = List<realmList1>()
    public var arrayList2 = List<realmList2>()


    func ChooseList<T> (cehck : Bool) ->  List<T> {

        if cehck == true {
            return arrayList1 as! List<T>
        } else if cehck == false {
            return arrayList2 as! List<T>

        }
        return arrayList1 as! List<T>
    }


}

I'm trying using Generic Type but with no luck

when I try to use the function ChooseList I got error

Generic parameter 'T' could not be inferred

I don't know how to do it , and if there is another way to archive what i'm trying to archive

Answer 1

In this case,arrayList1 is SomeOtherType. List is not that type, with the as! cast is dangerous.Always Use as?

Try this one,

public var arrayList1 : DataType = List<realmList1>()
public var arrayList2 : DataType = List<realmList2>()


func ChooseList<T> (cehck : Bool) ->  List<T>? {

    if cehck == true {
        return arrayList1 as? List<T>
    } else if cehck == false {
        return arrayList2 as? List<T>

    }
    return nil
}

Answer 2

i dont think that you need generics here if your realmList1 and realmList2 are inherited from Object you can write your chooseList method like this

    func ChooseList(cehck : Bool) ->  List<Object> {

    if cehck == true {
        return arrayList1 as! List<realmList1> // or just return arrayList1
    } else if cehck == false {
        return arrayList2 as! List<realmList2> // or just return arrayList2

    }
    return arrayList1 as! List<realmList1>
}

Answer 3

I'll suggest an alternative solution. This is macOS but very similar to iOS and it's really just to provide another option.

Suppose we have a tableView and want to switch what's listed in the tableView depending on what the user selects.

Give two Realm objects RedGrape, WhiteGrape

and then a class that contains lists of both of those called Grape. This is similar to what's in the original question but I wanted to use a more well defined item for use in our lists.

class RedGrape: Object {
    @objc dynamic var grapeName = ""
}

class WhiteGrape: Object {
    @objc dynamic var grapeName = ""
}

class Grape: Object {
    let reds = List<RedGrape>()
    let whites = List<WhiteGrape>()
}

and then a class that handles the tableView within a viewController, note this is just conceptual:

class ViewController: NSViewController, NSTableViewDelegate, NSTableViewDataSource {
    @IBOutlet weak var myTableView: NSTableView!

    var self.userSelection = 0 //changed by user: 0 for red, 1 for white
    var myGrape = //set up the Grape Object var from Realm

    override func viewDidLoad() {
        super.viewDidLoad()

        self.userSelection = 0

        self.myTableView.delegate = self
        self.myTableView.dataSource = self

        self.myTableView.reloadData()
    }


    func numberOfRows(in tableView: NSTableView) -> Int {
        if self.userSelection == 0 {
            return self.myGrape.reds.count
        } else {
            return self.myGrape.white.count 
        }
    }

    func tableView(_ tableView: NSTableView, 
                   viewFor tableColumn: NSTableColumn?,
                   row: Int) -> NSView? {
        var name = ""
        if self.userSelection == 0 {
            name = self.myGrape.reds[row].grapeName
        } else {
            name = self.myGrape.whites[row].grapeName
        }

        let cellIdentifier = NSUserInterfaceItemIdentifier(rawValue:"MyCellView")

        if let cellView = self.myTableView.makeView(withIdentifier: cellIdentifier, 
                                                 owner: nil) as? NSTableCellView {
            cellView.textField?.stringValue = name
            return cellView
        }

        return nil
    }
}

from here you could implement more decision making code within the Grape class:

class Grape: Object {
    let reds = List<RedGrape>()
    let whites = List<WhiteGrape>()

    func rowCountFor(selection: Int) -> Int {
      if selection == 0 {
         return self.Grape.reds.count
      } else {
         return self.Grape.white.count 
      }

    func nameFor(selection: Int, forRow: Int) -> String {
       if selection == 0 {
         return self.reds[forRow]
      } else {
         return self.whites[forRow]
      }
    }
}

then your tableView numberOfRows becomes

    func numberOfRows(in tableView: NSTableView) -> Int {
        self.myGrape.rowCountFor(selection: self.userSelection)
    }