我知道[弱自我]和[无主自我],但我不知道[自我]。
它是如何工作的?
[self]=[无主的self]或[self]=strong参考
通过捕获子句捕获变量:
var closure = { [x] in x.doSomething() }
将创建x在创建闭包时引用的值的强捕获。请参见末尾的示例。
如果您没有像中那样使用捕获子句
var closure = { x.doSomething() }
这将创建一个对x在执行时引用的值的强引用。
也就是说,捕获[self]
会创建一个强引用,但这是完全没有必要的,除非您预计self
在某个时间点发生变化(这将是非常不寻常的(。
示例
class ValueClass {
var value:String
init (_ v:String) { value = v }
func info() { print("(value)") }
deinit { print ("deinit: (value)") }
}
if (1==1) {
print (">> No capture clause")
var a:ValueClass? = ValueClass("a")
let closure = { a != nil ? a!.info() : print("a is nil") }
a = ValueClass("changed a") // "deinit: a"
closure() // "a is nil"
print ("<< done")
}// "deinit: changed a"
print ("------------------------------")
if (1==1) {
print (">> strong capture clause")
var b:ValueClass? = ValueClass("b")
let closure = { [b] in b != nil ? b!.info() : print("b is nil") }
b = ValueClass("changed b")
closure() // "b"
print ("<< done")
} // "deinit: b"; "deinit: changed b"
print ("------------------------------")
if (1==1) {
print (">> weak capture clause")
var c:ValueClass? = ValueClass("c")
let closure = { [weak c] in c != nil ? c!.info() : print("c is nil") }
c = ValueClass("changed c") // "deinit: c"
closure() // "c is nil"
print ("<< done")
} // "deinit: changed c"
print ("------------------------------")
if (1==1) {
print (">> unowned capture clause")
var d:ValueClass? = ValueClass("d")
let closure = { [unowned d] in d != nil ? d!.info() : print("d is nil") }
d = ValueClass("changed d") // "deinit: d"
closure() // error: Execution was interrupted, reason: signal SIGABRT.
print ("<< done")
}
输出为
>> No capture clause
deinit: a
changed a
<< done
deinit: changed a
------------------------------
>> strong capture clause
b
<< done
deinit: b
deinit: changed b
------------------------------
>> weak capture clause
deinit: c
c is nil
<< done
deinit: changed c
------------------------------
>> unowned capture clause
deinit: d
(CRASH)
正如你所看到的,
- 当使用no capture子句时,使用执行时的值("changed a"(
- 当使用
[b]
时;b";由于b是"0";b";在创建时 - 当使用
[weak c]
时;c";创建;由于c已经变为"0";改变c";,旧的";c";因此捕获为零并且输出为"0";c为零"> - 使用
[unowned c]
时相同,只是程序崩溃