> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Swift ObjC Members Attribute
The `@objcMembers` attribute is a class-level modifier in Swift that implicitly applies the `@objc` attribute to eligible Objective-C compatible members of a class, its extensions, its subclasses, and all of their extensions. It instructs the compiler to expose these declarations to the Objective-C runtime by generating Objective-C thunks, enabling Objective-C message passing without requiring explicit `@objc` annotations on every individual property, method, or initializer. Crucially, this implicit behavior explicitly excludes members with `private` or `fileprivate` access control.
```swift theme={"dark"}
@objcMembers
class Entity: NSObject {
var name: String = "" // Implicitly @objc
func update() {} // Implicitly @objc
private func internalUpdate() {} // NOT @objc due to access control
@objc private func tapHandler() {} // Explicitly @objc
}
```
## Dispatch Mechanics
Applying `@objcMembers` (or `@objc`) exposes members to the Objective-C runtime but **does not** alter how Swift dispatches calls to those members. Swift continues to use standard vtable (virtual) or static dispatch for Swift-to-Swift calls. To force dynamic dispatch via Objective-C message passing within Swift (which is required for features like Key-Value Observing or method swizzling), the `dynamic` modifier must be explicitly applied to the specific member.
## Technical Mechanics
When `@objcMembers` is applied to a class, the Swift compiler evaluates every member to determine if it receives the implicit `@objc` attribute based on the following rules:
* **Objective-C Compatibility:** The class itself must inherit from an Objective-C class (typically `NSObject`).
* **Implicit Bridging:** Properties, methods, and initializers utilizing Swift types that bridge to Objective-C (e.g., `String`, `Int`, `Array`, `@objc` protocols) automatically receive the `@objc` attribute.
* **Access Control Exceptions:** The `@objcMembers` attribute explicitly ignores `private` and `fileprivate` members. Even if a private member is Objective-C compatible, it will not receive the implicit `@objc` attribute. If a private member requires Objective-C runtime visibility (such as a target-action method referenced via `#selector`), it must be explicitly annotated with `@objc`.
* **Swift-Exclusive Types:** Members utilizing Swift-only features—such as tuples, generics, structs, or enums with associated values—are strictly incompatible with the Objective-C runtime. The compiler silently ignores these members; they do not receive the implicit `@objc` attribute and remain hidden from Objective-C.
## Inheritance and Extensions
The `@objcMembers` attribute propagates down the inheritance hierarchy and across extensions:
1. **Subclasses:** Any class that inherits from an `@objcMembers` class automatically inherits the `@objcMembers` behavior. All eligible members of the subclass are implicitly exposed to Objective-C.
2. **Extensions:** Members defined within extensions of the annotated class, or extensions of its subclasses, are also implicitly marked as `@objc`.
## Opting Out with `@nonobjc`
Because `@objcMembers` applies a blanket exposure to the Objective-C runtime, it generates Objective-C thunks for all eligible members, which increases the compiled binary size. To override this implicit behavior for specific members, you must use the `@nonobjc` attribute. This explicitly hides the member from the Objective-C runtime.
```swift theme={"dark"}
@objcMembers
class Configuration: NSObject {
// Implicitly @objc (Exposed to Objective-C runtime)
var timeout: Double = 30.0
// Implicitly @objc
func reset() {}
// Ignored by @objcMembers (Tuple is a Swift-only type)
var metadata: (Int, String) = (0, "none")
// Ignored by @objcMembers due to 'private' access control
private func calculateHash() {}
// Explicitly @objc despite 'private' access control (e.g., for #selector)
@objc private func handleTimeout() {}
// Explicitly excluded from Objective-C runtime
@nonobjc func performInternalRouting() {}
// Explicitly marked dynamic to force Objective-C message passing in Swift
dynamic func observableMethod() {}
}
// Inherits @objcMembers behavior
class NetworkConfiguration: Configuration {
// Implicitly @objc
var endpoint: String = ""
}
```
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