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lilyball
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Type-Safe Associated Objects in Swift

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Type-Safe Associated Objects in Swift

Version Platforms Languages License Carthage compatible CocoaPods

TSAO is an implementation of type-safe associated objects in Swift. Objective-C associated objects are useful, but they are also untyped; every associated object is only known to be

id
at compile-time and clients must either test the class at runtime or rely on it being the expected type.

Swift allows us to do better. We can associate the value type with the key used to reference the value, and this lets us provide a strongly-typed value at compile-time with no runtime overhead¹. What's more, it allows us to store value types as associated objects, not just object types, by transparently boxing the value (although this involves a heap allocation). We can also invert the normal way associated objects work and present this type-safe adaptor using the semantics of a global map from

AnyObject
to
ValueType
.

It's also possible to specify the association policy. For all values, atomic/nonatomic retain is supported. For class values, assign is also supported. And for

NSCopying
values, atomic/nonatomic copy is supported.

To properly use this library, the

AssocMap
values you create should be static or global values (they should live for the lifetime of the program). You aren't required to follow this rule, but any
AssocMap
s you discard will end up leaking an object (this is the only way to ensure safety without a runtime penalty).

¹ It does require a type-check, but the optimizer should in theory be able to remove this check.

Usage example

import TSAO

// create a new map that stores the value type Int // note how this is a global value, so it lives for the whole program let intMap = AssocMap()

// fetch the associated object from obj using intMap func lookup_int_object(obj: AnyObject) -> Int? { // The subscript getter returns a value of type Int? so no casting is necessary return intMap[obj] }

// set the associated object for intMap on obj func set_int_object(obj: AnyObject, val: Int?) { // The subscript setter takes an Int? directly, trying to pass // a value of any other type would be a compile-time error intMap[obj] = val }

// This map stores values of type NSString with the nonatomic copy policy let strMap = AssocMap(copyAtomic: false)

// fetch the associated object from obj using strMap func lookup_str_object(obj: AnyObject) -> NSString? { // The subscrip getter returns a value of type NSString? return strMap[obj] }

// set the associated object for strMap on obj func set_str_object(obj: AnyObject, val: NSString?) { // The subscript setter takes an NSString? directly, trying to pass // an Int? like we did with intMap would be a compile-time error strMap[obj] = val }

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