为什么我收到错误“无法将类型（类）的值分配给类型 UICollectionViewDelegate、UICollectionViewDataSource？”

Question

When I declare my collection view, I got error "cannot assign value of type (class) to type UICollectionViewDelegate, UICollectionViewDataSource":

let collectionView: UICollectionView = {
    let layout = UICollectionViewFlowLayout()
    let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
    collectionView.delegate = self
    collectionView.dataSource = self
    return collectionView
}()

But when I add "lazy var" the error is gone. I don't know why? Can someone explain for me?

lazy var collectionView: UICollectionView = {
    let layout = UICollectionViewFlowLayout()
    let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
    collectionView.delegate = self
    collectionView.dataSource = self
    return collectionView
}()

Answer 1

That the closure is called during initialization so you cannot use self to access any properties or methods of the instance yet. If you need to access self you must replace the let with lazy var.

let collectionView: UICollectionView = {
   let layout = UICollectionViewFlowLayout()
   let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
   collectionView.delegate = self // You cannot use
   collectionView.dataSource = self // You cannot use
   return collectionView 
}()

Answer 2

adding lazy forces iOS to check for instantiation of collectionView only when it is needed for the first time. It therefore not gives you an error at compilation. Prior to that, it was giving an error because the initialization was yet to be completed and you were setting properties on the same.

Answer 3

   let collectionView: UICollectionView = {
    let layout = UICollectionViewFlowLayout()
    let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
    collectionView.delegate = self
    collectionView.dataSource = self
    return collectionView
}()

You can not access self until its initialized. As there is no object of your class till now. Instance method(s) and variable belong to the Object of the class not to the class ie they can be called after creating the Object of the class. So it gives you error.

lazy var collectionView: UICollectionView = {
    let layout = UICollectionViewFlowLayout()
    let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
    collectionView.delegate = self
    collectionView.dataSource = self
    return collectionView
}()

lazy var indicate to skip this variable on the time of intialization. If any variable marked as lazy it will not allocated until it is used for first time. You have marked this computed variable as lazy. so, whenever it's going to be used by any of the function in class, it will always get the object of class (self) allocated.