虽然使用惰性初始化全局变量对于某些一次性初始化有意义,但对于其他类型则没有意义。对于单例之类的事情使用惰性初始化全局变量很有意义,对于保护 swizzle 设置之类的事情没有多大意义。
这是dispatch_once 的 Swift 3 风格实现:
public extension DispatchQueue {
private static var _onceTracker = [String]()
/**
Executes a block of code, associated with a unique token, only once. The code is thread safe and will
only execute the code once even in the presence of multithreaded calls.
- parameter token: A unique reverse DNS style name such as com.vectorform.<name> or a GUID
- parameter block: Block to execute once
*/
public class func once(token: String, block:@noescape(Void)->Void) {
objc_sync_enter(self); defer { objc_sync_exit(self) }
if _onceTracker.contains(token) {
return
}
_onceTracker.append(token)
block()
}
}
这是一个示例用法:
DispatchQueue.once(token: "com.vectorform.test") {
print( "Do This Once!" )
}
或使用 UUID
private let _onceToken = NSUUID().uuidString
DispatchQueue.once(token: _onceToken) {
print( "Do This Once!" )
}
由于我们目前正处于从 swift 2 到 3 的过渡时期,以下是 swift 2 实现的示例:
public class Dispatch
{
private static var _onceTokenTracker = [String]()
/**
Executes a block of code, associated with a unique token, only once. The code is thread safe and will
only execute the code once even in the presence of multithreaded calls.
- parameter token: A unique reverse DNS style name such as com.vectorform.<name> or a GUID
- parameter block: Block to execute once
*/
public class func once(token token: String, @noescape block:dispatch_block_t) {
objc_sync_enter(self); defer { objc_sync_exit(self) }
if _onceTokenTracker.contains(token) {
return
}
_onceTokenTracker.append(token)
block()
}
}