本文主要探究：
- 当一个类遵循了协议，且协议和类都有方法实现时，实例对象调用方法的优先顺序
代码示例1：
转为SIL源码查看：swiftc -emit-sil main.swift | xcrun swift-demangle >> ./main.sil
main函数
代码语言：javascript复制protocol YRProtocol {
func teach()
}
extension YRProtocol {
func teach()
}
class YRTeach : YRProtocol {
func teach()
@objc deinit
init()
}
@_hasStorage @_hasInitialValue var object1: YRProtocol { get set }
@_hasStorage @_hasInitialValue var object2: YRTeach { get set }
// object1
sil_global hidden @main.object1 : main.YRProtocol : $YRProtocol
// object2
sil_global hidden @main.object2 : main.YRTeach : $YRTeach
// main
sil @main : $@convention(c) (Int32, UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>) -> Int32 {
bb0(%0 : $Int32, %1 : $UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>):
alloc_global @main.object1 : main.YRProtocol // id: %2
%3 = global_addr @main.object1 : main.YRProtocol : $*YRProtocol // users: %9, %7
%4 = metatype $@thick YRTeach.Type // user: %6
// function_ref YRTeach.__allocating_init()
%5 = function_ref @main.YRTeach.__allocating_init() -> main.YRTeach : $@convention(method) (@thick YRTeach.Type) -> @owned YRTeach // user: %6
%6 = apply %5(%4) : $@convention(method) (@thick YRTeach.Type) -> @owned YRTeach // user: %8
%7 = init_existential_addr %3 : $*YRProtocol, $YRTeach // user: %8
store %6 to %7 : $*YRTeach // id: %8
%9 = begin_access [read] [dynamic] %3 : $*YRProtocol // users: ,
= alloc_stack $YRProtocol // users: , , ,
copy_addr %9 to [initialization] : $*YRProtocol // id:
end_access %9 : $*YRProtocol // id:
= open_existential_addr immutable_access : $*YRProtocol to $*@opened("AE485D14-5A5E-11EC-81A7-ACDE48001122") YRProtocol // users: , ,
= witness_method $@opened("AE485D14-5A5E-11EC-81A7-ACDE48001122") YRProtocol, #YRProtocol.teach : <Self where Self : YRProtocol> (Self) -> () -> (), : $*@opened("AE485D14-5A5E-11EC-81A7-ACDE48001122") YRProtocol : $@convention(witness_method: YRProtocol) <τ_0_0 where τ_0_0 : YRProtocol> (@in_guaranteed τ_0_0) -> () // type-defs: ; user:
= apply <@opened("AE485D14-5A5E-11EC-81A7-ACDE48001122") YRProtocol>() : $@convention(witness_method: YRProtocol) <τ_0_0 where τ_0_0 : YRProtocol> (@in_guaranteed τ_0_0) -> () // type-defs:
destroy_addr : $*YRProtocol // id:
dealloc_stack : $*YRProtocol // id:
alloc_global @main.object2 : main.YRTeach // id:
= global_addr @main.object2 : main.YRTeach : $*YRTeach // users: #, $
= metatype $@thick YRTeach.Type // user: "
// function_ref YRTeach.__allocating_init()
! = function_ref @main.YRTeach.__allocating_init() -> main.YRTeach : $@convention(method) (@thick YRTeach.Type) -> @owned YRTeach // user: "
" = apply !( ) : $@convention(method) (@thick YRTeach.Type) -> @owned YRTeach // user: #
store " to : $*YRTeach // id: #
$ = begin_access [read] [dynamic] : $*YRTeach // users: %, '
% = load $ : $*YRTeach // users: 0, (, ), &
strong_retain % : $YRTeach // id: &
end_access $ : $*YRTeach // id: '
( = class_method % : $YRTeach, #YRTeach.teach : (YRTeach) -> () -> (), $@convention(method) (@guaranteed YRTeach) -> () // user: )
) = apply ((%) : $@convention(method) (@guaranteed YRTeach) -> ()
strong_release % : $YRTeach // id: 0
1 = integer_literal $Builtin.Word, 1 // user: 3
// function_ref _allocateUninitializedArray<A>(_:)
2 = function_ref @Swift._allocateUninitializedArray<A>(Builtin.Word) -> ([A], Builtin.RawPointer) : $@convention(thin) <τ_0_0> (Builtin.Word) -> (@owned Array<τ_0_0>, Builtin.RawPointer) // user: 3
3 = apply 2<Any>(1) : $@convention(thin) <τ_0_0> (Builtin.Word) -> (@owned Array<τ_0_0>, Builtin.RawPointer) // users: 5, 4
4 = tuple_extract 3 : $(Array<Any>, Builtin.RawPointer), 0 // user: F
5 = tuple_extract 3 : $(Array<Any>, Builtin.RawPointer), 1 // user: 6
6 = pointer_to_address 5 : $Builtin.RawPointer to [strict] $*Any // user: C
7 = string_literal utf8 "end" // user: B
8 = integer_literal $Builtin.Word, 3 // user: B
9 = integer_literal $Builtin.Int1, -1 // user: B
@ = metatype $@thin String.Type // user: B
// function_ref String.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:)
A = function_ref @Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String : $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1, @thin String.Type) -> @owned String // user: B
B = apply A(7, 8, 9, @) : $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1, @thin String.Type) -> @owned String // user: D
C = init_existential_addr 6 : $*Any, $String // user: D
store B to C : $*String // id: D
// function_ref _finalizeUninitializedArray<A>(_:)
E = function_ref @Swift._finalizeUninitializedArray<A>(__owned [A]) -> [A] : $@convention(thin) <τ_0_0> (@owned Array<τ_0_0>) -> @owned Array<τ_0_0> // user: F
F = apply E<Any>(4) : $@convention(thin) <τ_0_0> (@owned Array<τ_0_0>) -> @owned Array<τ_0_0> // users: U, R
// function_ref default argument 1 of print(_:separator:terminator:)
G = function_ref @default argument 1 of Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) () -> @owned String // user: H
H = apply G() : $@convention(thin) () -> @owned String // users: T, R
// function_ref default argument 2 of print(_:separator:terminator:)
I = function_ref @default argument 2 of Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) () -> @owned String // user: P
P = apply I() : $@convention(thin) () -> @owned String // users: S, R
// function_ref print(_:separator:terminator:)
Q = function_ref @Swift.print(_: Any..., separator: Swift.String, terminator: Swift.String) -> () : $@convention(thin) (@guaranteed Array<Any>, @guaranteed String, @guaranteed String) -> () // user: R
R = apply Q(F, H, P) : $@convention(thin) (@guaranteed Array<Any>, @guaranteed String, @guaranteed String) -> ()
release_value P : $String // id: S
release_value H : $String // id: T
release_value F : $Array<Any> // id: U
V = integer_literal $Builtin.Int32, 0 // user: W
W = struct $Int32 (V : $Builtin.Int32) // user: X
return W : $Int32 // id: X
} // end sil function 'main'
复制代码查看main源码知道，生成的object1通过witness_method调用了YRProtocol.Teach方法
witness_method的官方说明:
witness_method 表示在witness_table中查询协议方法的实现并调用
此处我们在SIL源码中查找YRProtocol协议的witness_table方法
sil_vtable YRTeach {
#YRTeach.teach: (YRTeach) -> () -> () : @main.YRTeach.teach() -> () // YRTeach.teach()
#YRTeach.init!allocator: (YRTeach.Type) -> () -> YRTeach : @main.YRTeach.__allocating_init() -> main.YRTeach // YRTeach.__allocating_init()
#YRTeach.deinit!deallocator: @main.YRTeach.__deallocating_deinit // YRTeach.__deallocating_deinit
}
sil_witness_table hidden YRTeach: YRProtocol module main {
method #YRProtocol.teach: <Self where Self : YRProtocol> (Self) -> () -> () : @protocol witness for main.YRProtocol.teach() -> () in conformance main.YRTeach : main.YRProtocol in main // protocol witness for YRProtocol.teach() in conformance YRTeach
}
复制代码从witness_table中我们发现确实有定义YRProtocol.teach()方法
然后查看YRProtocol.teach()的具体实现
总结1. 当protocol中声明了方法时，源码做了如下操作：
- 在
witness_table中存放协议方法 - 实例对象调用协议方法时，编译器会通过
witness_method这个方法去witness_m ethod表中查找对应协议方法并调用 - 协议方法的实现，本质是调用遵循了协议的类里协议方法的实现(通过class_method在
sil_vtable中查询)，而并不会调用协议拓展里的方法实现
2. 代码示例2：
此时发现打印变了，打印了YRProtocol extension里的方法,查看SIL源码：
同时，源码中也没有witness_table中也没有方法声明和实现
总结2： 当protocol中没有声明方法时，witness_table中没有协议方法的声明，编译器会调用协议 extension中的方法实现
总结：
- 当协议中有方法声明时(witness_table有声明)，
- 类中有方法实现: 实例对象优先调用类中的方法实现
- 类中没有方法实现: 实例对象调用协议拓展中的方法实现
- 当协议中没有方法声明时(witness_table没有声明)：
实例对象总是调用协议拓展中的方法实现
参考：
官方文档：witness_method
官方文档：witness_table
