如何使用if语句将变量类型设置为自定义tableCell的特定类型？

Question

I have three different types of custom UITableCells. I have an if statement that sets them up:

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {                
    if somePosts[indexPath.row].typeOfPost == .linkPost {
        let cell: LinkTableViewCell = self.tableView.dequeueReusableCell(withIdentifier: "linkTableViewCell") as! LinkTableViewCell

    } else if somePosts[indexPath.row].typeOfPost == .picturePost {
        let cell: PictureTableViewCell = self.tableView.dequeueReusableCell(withIdentifier: "pictureTableViewCell") as! PictureTableViewCell

    } else if somePosts[indexPath.row].typeOfPost == .textPost {
        let cell: TextTableViewCell = self.tableView.dequeueReusableCell(withIdentifier: "textTableViewCell") as! TextTableViewCell


    } else {
        print("Type of post is not link, picture, or text")
    }
}

Each of the custom cells has similar labels such as title and time. I would like to set these labels using the same line of code, such as:

cell.titleLabel.text = "Some title here"

However, in this example, I get an error saying I am using an unresolved identifier "cell," obviously because my variables are being declared non-globally. Is there a way around this since swift is strongly typed? Thanks!

Answer 1

Make a protocol that your TableViewCell classes extend, and store cell as a variable of that type.

protocol MyTableViewCell {
    var titleLabel: UILabel { get }
    // ...
}

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

    let identifier: String

    switch somePosts[indexPath.row].typeOfPost {
        case .linkPost: identifier = "linkTableViewCell"
        case .picturePost: identifier = "pictureTableViewCell"
        case .textPost: identifier = "textTableViewCell"
        default: fatalError("Type of post is not link, picture, or text")
    }

    guard let cell = self.tableView.dequeueReusableCell(withIdentifier: identifier) as? MyTableViewCell else {
        fatalError("Cell isn't castable to MyTableViewCell")
    }

    cell.titleLabel.text = "Some title here" 
    // ...
}

Answer 2

You have three basic solutions.

1. Repeat cell.text = ... inside each block. But this isn't what you really want as stated in your question.
2. Have your three custom cell classes all extend a common base class. Have this base class define any common properties.
3. Define a protocol with the common properties and have each of your custom cell classes conform to the protocol.

For options 2 and 3 you would declare a variable of the base/protocol type before the first if statement. Then after the whole if/else block, you can assign any of the common properties.

If you need to update any cell type specific properties, you can do that inside the appropriate block as well.

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    var cell: BaseTableViewCell?
    if somePosts[indexPath.row].typeOfPost == .linkPost {
        cell = self.tableView.dequeueReusableCell(withIdentifier: "linkTableViewCell") as! LinkTableViewCell
    } else if somePosts[indexPath.row].typeOfPost == .picturePost {
        cell = self.tableView.dequeueReusableCell(withIdentifier: "pictureTableViewCell") as! PictureTableViewCell
    } else if somePosts[indexPath.row].typeOfPost == .textPost {
        cell = self.tableView.dequeueReusableCell(withIdentifier: "textTableViewCell") as! TextTableViewCell
    } else {
        print("Type of post is not link, picture, or text")
    }

    if let cell = cell {
        cell.commonProperty = ...

        return cell
    } else {
        return nil // this shouldn't happen but if it does, you have a bug to fix
    }
}

Answer 3

If the subclasses each have their own titleLabel property, you will need to make them all conform to a protocol. Let's call it ConfigurableCell .

protocol ConfigurableCell {
    var titleLabel: UILabel { get set }
}

Then, you can initialize your cells all the same way, but declare them as a ConfigurableCell :

var cell: ConfigurableCell? = nil    // not set yet
if somePosts[indexPath.row].typeOfPost == .linkPost {
    cell = self.tableView.dequeueReusableCell(withIdentifier: "linkTableViewCell") as! LinkTableViewCell
} else if somePosts[indexPath.row].typeOfPost == .picturePost {
    cell = self.tableView.dequeueReusableCell(withIdentifier: "pictureTableViewCell") as! PictureTableViewCell
} else if somePosts[indexPath.row].typeOfPost == .textPost {
    cell = self.tableView.dequeueReusableCell(withIdentifier: "textTableViewCell") as! TextTableViewCell
} 

guard let cell = cell else {
    // how to handle this error case is up to you
    print("Type of post is not link, picture, or text")
    return UITableViewCell()    
}

// now, cell is a ConfigurableCell with a titleLabel property, regardless of class
cell.titleLabel.text = "Some title"

Of course, UITableViewCell does have a built-in textLabel property, which you could try to utilize in your cell classes, and then a protocol wouldn't be necessary, because the property is in UITableViewCell .