泛型类中的访问函数协议

Question

我想访问协议中的函数，但是 XCode 投诉

实例成员“createColumns”不能用于类型“T”；你是否 意思是改用这种类型的值？

我做了什么：

创建协议：

protocol StorageModelDelegate {
   func createColumns(for tableBuilder: TableBuilder)
}

创建接收 StorageModelDelegate 的类泛型：

class SQLiteStorage<T: StorageModelDelegate> {
     func createTable(tableName: TableKey) -> Bool {
           let table = Table(tableName.rawValue)

           let query = table.create(ifNotExists: true) { (builder: TableBuilder) in
                T.createColumns(for: builder) // -> this is the error comes up.
           }
     }
}

创建实现 SQLiteStorage 的类：

final class InfoStorageModel {
    private let sqlite: SQLiteStorage = SQLiteStorage<Info>()
}

那么，如何修复 SQLiteStorage 类中的错误？

Answer 1

你想调用静态方法而不是实例方法。

为了修复，你应该添加实例参数：

首先，使用弱 var 委托来防止循环保留。

protocol StorageModelDelegate: class {
   func createColumns(for tableBuilder: TableBuilder)
}

final class SQLiteStorage<T: StorageModelDelegate> {
weak var delegate: T?
     func createTable(tableName: TableKey) -> Bool {
           let table = Table(tableName.rawValue)

           let query = table.create(ifNotExists: true) { (builder: TableBuilder) in
                delegate?.createColumns(for: builder)
           }
     }
}

或者使用静态协议方法：

protocol StorageModelDelegate {
   static func createColumns(for tableBuilder: TableBuilder)
}


final class SQLiteStorage<T: StorageModelDelegate> {
weak var delegate: T?
     func createTable(tableName: TableKey) -> Bool {
           let table = Table(tableName.rawValue)

           let query = table.create(ifNotExists: true) { (builder: TableBuilder) in
                T.createColumns(for: builder)
           }
     }
}

Answer 2

错误表明您需要T 的实例，而不是类型 本身。

所以你需要这样的东西：

class SQLiteStorage<T: StorageModelDelegate> {
    var delegate:T

    init (delegate:T) {
      self.delegate = delegate
    }

    func createTable(tableName: TableKey) -> Bool {
          let table = Table(tableName.rawValue)

          let query = table.create(ifNotExists: true) { (builder: TableBuilder) in
                self.delegate.createColumns(for: builder) // -> this is the error comes up.
          }
    }
}