所以，我要写这个系列，尽量把官网文档中GCD的强大功能完整的表达出来。方便自己，也方便别人，如果发现有问题，欢迎提出
的一篇文章会进行总结，总结出GCD的经典使用场景。源代码只提供Swift版本。因为要上班，计划一个月内完成。每周两篇。
原创Blog，转载请注明出处
这个专栏地址
http://blog.csdn.net/column/details/swift-gcd.html
#define DISPATCH_QUEUE_PRIORITY_HIGH 2 #define DISPATCH_QUEUE_PRIORITY_DEFAULT 0 #define DISPATCH_QUEUE_PRIORITY_LOW (-2) #define DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN
typealias dispatch_queue_t = NSObject //轻量级的用来描述执行任务的队列 typealias dispatch_block_t = () -> Void //队列执行的闭包(Objective C中的block)
func dispatch_async(_ queue: dispatch_queue_t!,
_ block: dispatch_block_t!)参数：func dispatch_async_f(_ queue: dispatch_queue_t!,
_ context: UnsafeMutablePointer<Void>,
_ work: dispatch_function_t)参数work 执行的函数(C语言函数)
dispatch_sync 和 dispatch_sync的参数和上述对应一致，所以不再列出
总得来说带有后缀_f(比如dispatch_sync_f，dispatch_after_f)就是提交给队列一个C语言函数，因为极少用到这种形式，这里仅给出一个简单例子，后面的涉及到_f的都略过。func dispatch_get_global_queue(_ identifier: Int,
_ flags: UInt) -> dispatch_queue_t!这个函数的第一个参数是队列的优先级，第二个参数尚没有意义，直接写0就可以了。class ViewController: UIViewController{
var imageview = UIImageView(frame: CGRectMake(40,40,200,200))
override func viewDidLoad(){
super.viewDidLoad()
imageview.contentMode = UIViewContentMode.ScaleAspectFit
self.view.addSubview(imageview)
let url = "http://f.hiphotos.baidu.com/image/pic/item/e1fe9925bc315c60191d32308fb1cb1348547760.jpg"
let imageURL = NSURL(string:url)
var globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)
dispatch_async(globalQueueDefault){
var imageData = NSData(contentsOfURL:imageURL!)
var image = UIImage(data:imageData!)
if let successfulImage = image{
sleep(2)
dispatch_async(dispatch_get_main_queue()){
self.imageview.image = successfulImage
}
}
}
}
override func didReceiveMemoryWarning(){
super.didReceiveMemoryWarning()
}
} 执行，观察下效果：view立刻载入，然后过一段时间，图片下载完了，UI改变dispatch_sync(globalQueueDefault,0){#import "hwcTest.h"
#include <stdio.h> #include <unistd.h> typedef void (*hwcTestForGCD)(void*); hwcTestForGCD getFuncPointer();
#include "hwcTest.h"
void realFunction(void *input){
for(int i = 0;i < 5;i++){
printf("%d\n",i);
sleep(1);
}
}
hwcTestForGCD getFuncPointer(){
return realFunction;
}ViewController.swiftclass ViewController: UIViewController{
var imageview = UIImageView(frame: CGRectMake(40,40,200,200))
override func viewDidLoad(){
super.viewDidLoad()
var globalQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)
dispatch_async_f(globalQueueDefault,nil,getFuncPointer())
println("dispatch is over")
}
override func didReceiveMemoryWarning(){
super.didReceiveMemoryWarning()
}
} 然后执行，会发现输出0 dispatch is over 1 2 3 4
0 1 2 3 4 5 dispatch is over这里更能体会到了，什么是同步，什么是异步了吧。
func dispatch_queue_create(_ label: UnsafePointer<Int8>,
_ attr: dispatch_queue_attr_t!) -> dispatch_queue_t!参数class ViewController: UIViewController{
var imageview = UIImageView(frame: CGRectMake(40,40,200,200))
override func viewDidLoad(){
super.viewDidLoad()
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)){
for var i = 0;i < 5;i++ {
NSLog("First task:%d",i)
sleep(1)
}
}
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0)){
for var j = 0;j < 5;j++ {
NSLog("Second task:%d",j)
sleep(1)
}
}
println("dispatch is over")
}
override func didReceiveMemoryWarning(){
super.didReceiveMemoryWarning()
}
}
这里执行输出为：
First task:0 Second task:0 First task:1 Second task:1 First task:2 Second task:2 First task:3 Second task:3 First task:4 Second task:4这段代码执行时间4.03s
class ViewController: UIViewController{
var imageview = UIImageView(frame: CGRectMake(40,40,200,200))
override func viewDidLoad(){
super.viewDidLoad()
var serialQueue = dispatch_queue_create(label:
"com.test.helloHwc",attr:DISPATCH_QUEUE_SERIAL)
dispatch_async(serialQueue){
for var i = 0;i < 5;i++ {
NSLog("First task:%d",i)
sleep(1)
}
}
dispatch_async(serialQueue){
for var j = 0;j < 5;j++ {
NSLog("Second task:%d",j)
sleep(1)
}
}
println("dispatch is over")
}
override func didReceiveMemoryWarning(){
super.didReceiveMemoryWarning()
}
}
这里输出为
First task:0 First task:1 First task:2 First task:3 First task:4 Second task:0 Second task:1 Second task:2 Second task:3 Second task:4这段代码执行时间8.06秒
看出来并行和串行执行的差别了吧。
所以，记住一点，把过程不相关的任务，提交到并行的队列中会显著提高效率
下一篇预计更新时间，本周末
BTY：如果图片下载失败，可能因为时间的原因，那张图片从服务器上删除了，自己找一个URL即可
原图
完整详解GCD系列（一）dispatch_async;dispatch_sync;dispatch_async_f;dispatch_sync_f
原文：http://blog.csdn.net/hello_hwc/article/details/41073287
