[英]Why does [weak self] work but [unowned self] break in a Swift closure?
[英]How dose [self] work on Swift ? It work like [unowned self] or strong reference
我知道 [weak self] 和 [unowned self] 但我不知道 [self]。
它的作用如何?
[self] = [unowned self]或[self] = 強引用
通過捕獲子句捕獲變量:
var closure = { [x] in x.doSomething() }
將創建對 x 在創建閉包時引用的值的強捕獲。 見最后的例子。
如果您不使用捕獲子句,如
var closure = { x.doSomething() }
這將創建對執行閉包時 x 所引用的值的強引用。
話雖如此,捕獲[self]
會創建一個強引用,但完全沒有必要,除非您期望self
在某個時間點發生變化(這將是非常不尋常的)。
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")
}
output 是
>> 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)
如你看到的,
[b]
時,會創建並輸出對“b”的強引用,因為 b 在創建時是“b”[weak c]
時,會創建對 "c" 的弱引用; 由於 c 已更改為“更改 c”,舊的“c”被取消,因此捕獲被取消,output 為“c 為 nil”[unowned c]
時相同,除了您的程序崩潰
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