Question

我的方法在私有队列中运行其代码，在完成后，将调用传入的回调。是否需要检查传入的回调是否打算从主队列运行？

e.g。

- (void)doSomethingWithCalback:(void(^)())callback {
    dispatch_async(self.privateQueue, ^{
    // Should I make sure this gets dispatched 
    // to a main thread if it was passed in from a main thread?
        if (callback) callback();
    });
}

我应该做以下事情：

    - (void)doSomethingWithCalback:(void(^)())callback {
        BOOL isMainThread = [NSThread isMainThread];
        dispatch_async(self.privateQueue, ^{
            if (callback) {
               if (isMainThread) {
                 dispatch_async(dispatch_get_main_thread, callback); 
               }
               else {
                 callback();
               }
            }
        });
    }

Answer 1

虽然在任何地方都没有规定，但如果你看一下Cocoa API，你会看到三种常见的模式：

主线程：完成处理程序，明确指定将使用主队列。例如，请参阅CLGeocoder asynchronous query methods，其中＆＃34;您的完成处理程序块将在主线程上执行。＆＃34;
任意队列：完成处理程序，您无法保证代码将在哪个队列中运行。例如，如果您使用requestAccessForEntityType CNContactStore，the documentation says＆＃34;则会在任意队列上调用完成处理程序。＆＃34;
指定的队列：完成处理程序，您可以在其中指定要使用的队列。例如，如果使用[NSURLSession sessionWithConfiguration:delegate:queue:]，则可以指定将用于委托方法和回调块/闭包的队列。（但是，顺便说一句，如果你没有指定一个队列，它会使用它自己设计的一个，而不是默认为主队列。）

您提出的模式并不遵循任何这些非正式约定，而是有时使用主线程（如果碰巧从主线程调用它），但有时使用一些任意队列。在这种情况下，我没有看到引入新公约的迫切需要。

我建议选择上述方法之一，然后在已发布的API中明确说明。例如，如果您要使用privateQueue：

@interface MyAPI : NSObject

/// Private queue for callback methods

@property (nonatomic, strong, readonly) dispatch_queue_t privateQueue;

/// Do something asynchronously
///
/// @param callback  The block that will be called asynchronously.
///                  This will be called on the privateQueue.

- (void)doSomethingWithCallback:(void(^)())callback;

@end

@implementation MyAPI

- (void)doSomethingWithCallback:(void(^)())callback {
    dispatch_async(self.privateQueue, ^{
        if (callback) callback();
    });
}

@end

或者

@interface MyAPI : NSObject

/// Private queue used internally for background processing

@property (nonatomic, strong, readonly) dispatch_queue_t privateQueue;

/// Do something asynchronously
///
/// @param callback  The block that will be called asynchronously.
///                  This will be called on the main thread.

- (void)doSomethingWithCallback:(void(^)())callback;

@end

@implementation MyAPI

- (void)doSomethingWithCallback:(void(^)())callback {
    dispatch_async(self.privateQueue, ^{
        if (callback) { 
            dispatch_async(dispatch_get_main_queue(), ^{
                callback();
            });
        }
    });
}

@end

注意，无论您使用哪种约定，我都建议您在标题中使用///条评论或/** ... */条评论，这样当您在代码中使用该方法时，您就可以看到该队列行为显示在右侧的快速帮助面板中。

Answer 2

您应该指定API方法/// callback will be executed on the queue passed in to onQueueOrNil. /// If onQueueOrNil is nil, it will be executed on the main queue - (void)doSomethingWithCallback:(void(^)())callback onQueue:(dispatch_queue_t)onQueueOrNil { dispatch_async(self.privateQueue, ^{ if (callback){ if(!onQueueOrNil) { onQueueOrNil = dispatch_get_main_queue(); } dispatch_async(onQueueOrNil, callback); } }); }在执行回调时将使用的队列，或者允许方法的使用者传入他们希望执行的队列，如下所示：

<!DOCTYPE html>
<html lang="es">
    <head>
        <meta charset="utf-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="initial-scale=1, maximum-scale=1, user-scalable=no" />
        <link rel="stylesheet" href="../bower_components/bootstrap/dist/css/bootstrap.min.css">
        <link rel="stylesheet" href="../bower_components/alertify.js/dist/css/alertify.css">
    </head>
    <body ng-app="gameApp">

        <div ng-view=""></div>

        <!-- COMPONENTS -->
        <script src="../bower_components/angular/angular.js"></script>
        <script src="../bower_components/react/react.js"></script>
        <script src="../bower_components/react/react-dom.js"></script>
        <script src="../bower_components/ngReact/ngReact.min.js"></script>
        <script src="../bower_components/angular-route/angular-route.js"></script>
        <script src="../bower_components/angular-resource/angular-resource.js"></script>
        <script src="../bower_components/jquery/dist/jquery.min.js"></script>
        <script src="../bower_components/bootstrap/dist/js/bootstrap.min.js"></script>
        <script src="../bower_components/alertify.js/dist/js/alertify.js"></script>
        <script src="../bower_components/babel/browser.js"></script>
        <!-- SOCKET.IO -->
        <script src="../node_modules/socket.io-client/socket.io.js"></script>

        <!-- REACT COMPONENTS -->
        <script type="text/babel" src="js/components/LoginComponent.js"></script>
        <!-- DIRECTIVES -->
        <script src="js/directives/login.js"></script>
        <!-- JAVASCRIPT FILES -->
        <script src="js/app.js"></script>

        <!-- CONTROLLERS -->
        <script src="js/controllers/login.js"></script>

        <!-- SERVICES -->
        <script src="js/services/commonServices.js"></script>



    </body>
</html>

在专用队列上运行任务并返回回调

2 个答案: