Except for the last article NSThread Threads , One more GCD(Grand Central Dispath), yes Apple A newly developed solution for multi-core programming , Make full use of CPU resources , Put all the tasks , Put it on a task queue for scheduling . GCD The global concurrent queue is provided by default , For the entire application , No need to create .

  • The difference between synchronous and asynchronous

    • dispatch_sync Sync : Does not have the ability to open threads
    • dispatch_async asynchronous : Have the ability to open threads
  • How the task is performed

    • Concurrent queues : Multiple tasks at the same time , Concurrency can only be performed in one step function “ asynchronous “
    • Serial queue : Let the tasks run one by one

Be careful : In general, the most commonly used in actual development is dispatch_async .

Here are some common queue uses

One : The most common global concurrency queue :

 // Get global concurrent queue 
let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, )
// Add tasks to the queue Concurrent execution
dispatch_async(queue) { () -> Void in
//doSomething... Mission 1
}
dispatch_async(queue) { () -> Void in
//doSomething... Mission 2
}
dispatch_async(queue) { () -> Void in
//doSomething... Mission 3
} // Pause a queue
dispatch_suspend(queue)
// Continue queue
dispatch_resume(queue)

Two : Serial queue , Tasks in the queue are not executed concurrently , One task will be executed before another

 // Create a serial queue 
let serialQueue = dispatch_queue_create("dispatch_queue_name", nil)
// Add tasks to the queue After one task is completed, another task is executed
dispatch_async(serialQueue) { () -> Void in
//doSomething... Mission 1
}
dispatch_async(serialQueue) { () -> Void in
//doSomething... Mission 2
}
dispatch_async(serialQueue) { () -> Void in
//doSomething... Mission 3
}

3、 ... and :GCD Communication between threads , It is usually used when the child thread requests the data , Back to the main thread to update UI

 // Inter thread communication 
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, )) { () -> Void in
// Do something in the subthread
dispatch_async(dispatch_get_main_queue(), { () -> Void in
// Update in main thread UI...
})
}

Four : Thread delay execution

 // Thread delay execution 
let second:Double = 3.0
let dtime = dispatch_time(DISPATCH_TIME_NOW, Int64(second * Double(NSEC_PER_SEC))) //3s Then go back to the main thread to execute the task
dispatch_after(dtime, dispatch_get_main_queue()) { () -> Void in }
//3s After that, a new thread will be started to execute the task in the queue
dispatch_after(dtime, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, )) { () -> Void in }

5、 ... and : One time code , seeing the name of a thing one thinks of its function , In the application life cycle ,block The code block is executed only once .

 var once:dispatch_once_t =
dispatch_once(&once, { () -> Void in
// Only once in the application life cycle
})

6、 ... and : Queue group : Put some tasks into a queue group , When all tasks are completed , I'll run the team notify.

 /********************************** Queue group ******************************************/
let group = dispatch_group_create()
let groupQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, )
dispatch_group_async(group, groupQueue){ () -> Void in
// Mission 1
}
dispatch_group_async(group, groupQueue){ () -> Void in
// Mission 2
}
dispatch_group_async(group, groupQueue){ () -> Void in
// Mission 3
}
dispatch_group_notify(group, groupQueue) { () -> Void in
// I'll wait for the mission 1、 Mission 2、 Mission 3 Execute after execution
}

End !

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