Which is better to use: closures or delegates?

The choice between closures and delegates depends on the specific situation and requirements. Both mechanisms are used for passing data or messages between objects but have different usage patterns.

Here are the key differences and use cases:

Delegates:

• Pattern: Delegates implement the "Delegation" pattern, where one object (delegating object) hands over responsibility for executing certain tasks to another object (delegate).
• Relationship: Explicit, protocol-based. The delegating object holds a weak reference to the delegate object to avoid retain cycles.
• Multiplicity: One delegating object can have only one delegate per delegation protocol.
• Usage:
  • Sending notifications about events (e.g., UITableViewDelegate, UITextFieldDelegate).
  • Modifying object behavior (e.g., UITableViewDataSource).
  • When there is a clearly defined set of events or actions that the delegating object expects from the delegate.
  • Widely used in standard iOS frameworks (UIKit, AppKit, etc.).

Closures:

• Pattern: Based on the closure mechanism, which allows a block of code to "capture" variables from the surrounding scope.
• Relationship: Implicit. Closure captures variables by reference, which can lead to strong retain cycles if capture lists ([weak self], [unowned self]) are not used.
• Multiplicity: One object can have multiple closures for handling different events or tasks.
• Usage:
  • Callbacks for asynchronous operations (e.g., network requests).
  • Passing data from one screen to another after completing an operation.
  • Configuring behavior of small, self-contained components.
  • When a block of code needs to be executed at a specific moment.
  • When flexibility and dynamic assignment of handlers are required.

Comparison:

Characteristic	Delegates	Closures
Pattern	Delegation	Callbacks, variable capture
Relationship	Explicit, via protocol (weak)	Implicit, via capture
Multiplicity	One delegate per protocol	Multiple closures
Memory management	Easier to avoid cycles (weak)	Requires attention (capture lists)
Readability	Usually more structured	Sometimes less obvious

Conclusion:

• Delegates are better to use when there is a primary recipient of events or data, and this pattern is standard for interacting with Apple frameworks. They are more formal and explicitly define the interaction interface.
• Closures are better to use when flexibility is needed, for implementing callbacks in asynchronous operations, or when an object can have multiple different handlers for various events. They are more concise for simple cases.

Often, both mechanisms are used in the same application depending on the context.