有一个带有以下声明的协议(protocol):
typealias SuggestionSourceCallback = ([Suggestion]) -> ()
protocol SuggestionSource {
func suggest(_ query: SuggestionQuery, callback: @escaping SuggestionSourceCallback)
}
有两个类实现了这个协议(protocol)。第一类异步获取建议(通过 GCD )
final class FisrtClass: SuggestionSource {
private let queue = DispatchQueue(label: "my.app.queue", attributes: [])
private var lastQuery: SuggestionQuery?
// ...
func suggest(_ query: SuggestionQuery, callback: @escaping SuggestionSourceCallback) {
self.queue.async { [weak self] in
// capturing strong self
guard let strongSelf = self else {
return
}
// referencing self here, for example
guard self.lastQuery == query else {
return
}
// suggestions is a local variable
var suggestions: [Suggestion] = []
// ...
DispatchQueue.main.async {
callback(suggestions)
}
}
}
}
...而第二类同步进行
final class SecondClass: SuggestionSource {
// ...
func suggest(_ query: SuggestionQuery, callback: @escaping SuggestionSourceCallback) {
// ...
callback(self.suggestions[query])
}
}
我的问题是:
- 我应该在
FirstClass
的实现中捕获strongSelf
吗? - 我应该在
SecondsClass
的实现中捕获strongSelf
吗?
更新
附加问题。假设 SecondClass
有它的 suggestions
作为一个 static let
,在这种情况下会是什么模式?
final class SecondClass: SuggestionSource {
static let suggestions: [String: [SuggestionQuery]] = {
// ...
}()
// ...
func suggest(_ query: SuggestionQuery, callback: @escaping SuggestionSourceCallback) {
// ...
callback(self.suggestions[query])
}
}
最佳答案
在SecondClass
中,不需要创建strongSelf
变量。你会把它放在哪里?关键是 self
无论如何都保证不会是 nil
,因为您正在其方法之一的范围内运行。
您的附加问题也是如此,但出于不同的原因。 suggestions
现在是 static
,所以用 self
作为前缀是一个语法问题,(我假设你也想为 suggest 添加前缀
方法与 static
)。
但是,在 FirstClass
中,捕获 strongSelf
和不捕获它之间存在细微差别。
因为您使用的是[weak self]
,所以当您进入该 block 时,self
可能是nil
,所以无论如何您都需要检查一下.一种方法是重复使用可选链接,即:
self?.doSomething()
self?.doSomethingElse()
这是在说:
If I have a reference to self, do something. If I still have a reference to self, do something else.
通过添加一个 strongSelf
变量:
guard let strongSelf = self else {
return
}
strongSelf.doSomething()
strongSelf.doSomethingElse()
...你是说:
do something and do something else if you have a reference to
self
, otherwise do nothing.
因此,您保证如果第一件事发生,第二件事也会发生。您采用的方法将取决于您的应用。
关于 swift 3 : capture strong self in @escaping closure without asynchronous work,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/40844049/