Swift Closure: demystifying @escaping and @autoclosure attributes

In this post I will talk about Swift closure and the potential of the @escaping and @autoclosure attributes.

As reported in the official Swift documentation and as we saw in in one of my previous post, closures are:

self-contained blocks of functionality that can be passed around and used in your code. They can capture and store references to any constants and variables from the context in which they are defined.

In this post I will show you two interesting closure features: @autoclosure and @escaping.

An @escaping closure is passed as a parameter to a function, but it is not executed inside it. So, basically the closure is executed after the function returns. The classical example is a closure being stored in a variable outside that function.

An @autoclosure attribute can be applied to a closure parameter for a function, and automatically creates a closure from an expression you pass in.
This two attributes combined have great potential. Let's see an example where you can avoid multiple if/switch with the use of closure and these two attributes.

You could start "abusing" closures and use them everywhere after mastering these two attributes!! 😜 (Maybe it's better to keep calm and don't abuse closures even after seeing this attributes 😌).

For example we can have a UITableView and we want to execute different action for each cell displayed.
If we don't use closure and the attributes @autoclosure and @escaping, we need to distinguish the cells using the position or eventually casting some specialization of a class used to represent the cell data.
Suppose instead that each cell shows an instance of an Operation class, defined in this way:

class Operation {
    let name: String
    let action: () -> ()

    init(name: String, action: @autoclosure @escaping () -> ()) {
        self.name = name
        self.action = action
    }
}

So, basically in the constructor we are expecting something that will be enclosed in a closure, thanks to the @autoclosure attribute,
and we store it as an instance variable of our class. We can store it because we are using also the @escaping attribute.
Now in our controller we can define an array of operation that will be the datasource to our UITableViewController.
We can pass in the constructor of each Operation instance the function that corresponds to the operation that we want to execute.
This function will be executed in the table view delegate method public func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) by accessing the corresponding element in the data source array, without the need to identify the exact cell type selected.
Here you can find the complete OperationsViewController:

class OperationsViewController: UITableViewController {
    var operations: [Operation] = []

    override func viewDidLoad() {
        super.viewDidLoad()
        self.operations.append(Operation(name: "Operation 1", action: self.showOrangeDetail()))
        self.operations.append(Operation(name: "Operation 2", action: self.showGreenDetail()))
    }

    //MARK: TableView Datasource

    public override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return self.operations.count
    }

    public override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell: UITableViewCell = tableView.dequeueReusableCell(withIdentifier: "OperationCell")!
        cell.textLabel?.text = self.operations[indexPath.row].name
        return cell
    }

    //MARK: TableView Delegate

    public override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        self.operations[indexPath.row].action()
    }

    //MARK: Actions

    private func showOrangeDetail() {
        self.performSegue(withIdentifier: "OrangeSegue", sender: nil)
    }

    private func showGreenDetail() {
        self.performSegue(withIdentifier: "GreenSegue", sender: nil)
    }
}

You can download the complete example here.

So basically: no if, no switch, only love ❤️ for @autoclosure and @escaping 😍.

Originally published at https://www.fabrizioduroni.it on June 14, 2017.