前言

在使用ReactiveCocoa的時(shí)候，對(duì)于型號(hào)在主線程上的處理經(jīng)常會(huì)用到observeOn(UIScheduler())或者observeOn(QueueScheduler.main)。從字面意思來(lái)講UI線程和主線程應(yīng)該是同一個(gè)線程，那么這兩個(gè)用法有什么區(qū)別呢？

源碼

要想知道內(nèi)部的區(qū)別，需要從源碼入手。
下面是ReactiveCocoa5.0.0-alpha.3的相關(guān)代碼。

UISchdule的描述：

/// A scheduler that performs all work on the main queue, as soon as possible.
///
/// If the caller is already running on the main queue when an action is
/// scheduled, it may be run synchronously. However, ordering between actions
/// will always be preserved.

盡可能的在主隊(duì)列上執(zhí)行所有工作的調(diào)度器。
如果調(diào)度程序在操作時(shí)已在主隊(duì)列上運(yùn)行，則可以同步運(yùn)行。 但是，操作之間的順序?qū)⑹冀K保留。

UISchdule的代碼：

public final class UIScheduler: SchedulerProtocol {
    private static let dispatchSpecificKey = DispatchSpecificKey<UInt8>()
    private static let dispatchSpecificValue = UInt8.max
    private static var __once: () = {
            DispatchQueue.main.setSpecific(key: UIScheduler.dispatchSpecificKey,
                                           value: dispatchSpecificValue)
    }()
······
    public init() {
        /// This call is to ensure the main queue has been setup appropriately
        /// for `UIScheduler`. It is only called once during the application
        /// lifetime, since Swift has a `dispatch_once` like mechanism to
        /// lazily initialize global variables and static variables.
        _ = UIScheduler.__once
    }
······

代碼中可以看到，在初始化方法中調(diào)用了自己的私有的靜態(tài)函數(shù),在主線程上設(shè)置了一對(duì)鍵值。
而從注釋可以看出，init方式是用來(lái)確保UIScheduler已經(jīng)正確設(shè)置為主線程，而且只會(huì)在應(yīng)用周期里面調(diào)用一次。

QueueScheduler的描述：

A scheduler backed by a serial GCD queue.
由串行GCD支持的調(diào)度器

簡(jiǎn)單明了。

QueueScheduler.main的相關(guān)代碼：

public final class QueueScheduler: DateSchedulerProtocol {
    /// A singleton `QueueScheduler` that always targets the main thread's GCD
    /// queue.
    ///
    /// - note: Unlike `UIScheduler`, this scheduler supports scheduling for a
    ///         future date, and will always schedule asynchronously (even if 
    ///         already running on the main thread).
    public static let main = QueueScheduler(internalQueue: DispatchQueue.main)
    public let queue: DispatchQueue
    
    internal init(internalQueue: DispatchQueue) {
        queue = internalQueue
    }
}

簡(jiǎn)單來(lái)說(shuō)，QueueScheduler.main其實(shí)就是QueueScheduler在主線程上的單例模式，而下面note這句話也最終解釋了UIScheduler與QueueScheduler.main的區(qū)別的區(qū)別。

區(qū)別

UIScheduler與QueueScheduler.main背后都是通過(guò)GCD調(diào)用的主線程，區(qū)別是QueueScheduler.main只能在主線程上異步執(zhí)行，而調(diào)用UIScheduler時(shí)，如果本身已經(jīng)在主線程上了，那么就可以可以在主線程上同步執(zhí)行，