Grand Central Dispatch可以简化多线程编程。可以将应用程序需要执行的工作拆分为可分散在多个线程和多个cpu上更小的块。多线程还是得了解底层的实现，GCD只是进行了封装。
以下为实例：
#import <UIKit/UIKit.h> @interface ViewController : UIViewController - (IBAction)doWork:(id)sender; @property (weak, nonatomic) IBOutlet UITextView *resultsTextView; @property (weak, nonatomic) IBOutlet UIButton *startButton; @property (weak, nonatomic) IBOutlet UIActivityIndicatorView *spinner; @end
//
// ViewController.m
// Study20130220
//
// Created by Dwen on 13-2-20.
// Copyright (c) 2013年 Dwen. All rights reserved.
//
#import "ViewController.h"
@interface ViewController ()
@end
@implementation ViewController
@synthesize startButton,resultsTextView,spinner;
- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"load view...");
// //定义程序块
// void(^loggerBlock)(void);
// //实现程序块
// loggerBlock = ^{NSLog(@"i am test block.");};
// //执行程序块
// loggerBlock();
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
- (NSString *)fetchSomethingFromServer{
[NSThread sleepForTimeInterval:1];
return @"Hi there";
}
- (NSString *)processData:(NSString *)data{
[NSThread sleepForTimeInterval:2];
return [data uppercaseString];
}
- (NSString *)calculateFirstResult:(NSString *)data{
[NSThread sleepForTimeInterval:3];
return [NSString stringWithFormat:@"Number of chars: %d",[data length]];
}
- (NSString *)calculateSecondResult:(NSString *)data{
[NSThread sleepForTimeInterval:4];
return [data stringByReplacingOccurrencesOfString:@"E" withString:@"e"];
}
- (IBAction)doWork:(id)sender {
//并发程序块
startButton.enabled = NO;
startButton.alpha = 0.5;
[spinner startAnimating];
NSDate *startTime = [NSDate date];
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSString *fetchedData = [self fetchSomethingFromServer];
NSString *processData = [self processData:fetchedData];
__block NSString *firstResult;
__block NSString *secondResult;
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
NSLog(@"calculateFirstResult...");
firstResult = [self calculateFirstResult:processData];
});
dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
NSLog(@"calculateSecondResult...");
secondResult = [self calculateSecondResult:processData];
});
//dispatch_group_notify指定一个额外的程序块，该程序块将在组中的所有程序块运行完成时执行
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
NSString *resultsSummary = [NSString stringWithFormat:@"First: [%@]\nSecond: [%@]",firstResult,secondResult];
NSLog(@"resultsSummary :%@",resultsSummary);
//dispatch_get_main_queue主线程
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(@"change value.");
startButton.enabled = YES;
startButton.alpha = 1.0;
[spinner stopAnimating];
resultsTextView.text = resultsSummary;
});
NSDate *endTime = [NSDate date];
NSLog(@"Completed in %f seconds",[endTime timeIntervalSinceDate:startTime]);
});
});
//异步执行
// startButton.enabled = NO;
// startButton.alpha = 0.5;
// [spinner startAnimating];
// NSDate *startTime = [NSDate date];
// dispatch_async(dispatch_get_global_queue(0, 0), ^{
// NSString *fetchedData = [self fetchSomethingFromServer];
// NSString *processData = [self processData:fetchedData];
// NSString *firstResult = [self calculateFirstResult:processData];
// NSString *secondResult = [self calculateSecondResult:processData];
// NSString *resultsSummary = [NSString stringWithFormat:@"First: [%@]\nSecond: [%@]",firstResult,secondResult];
// NSLog(@"resultsSummary :%@",resultsSummary);
// dispatch_async(dispatch_get_main_queue(), ^{
// NSLog(@"change value.");
// startButton.enabled = YES;
// startButton.alpha = 1.0;
// [spinner stopAnimating];
// resultsTextView.text = resultsSummary;
// });
// NSDate *endTime = [NSDate date];
// NSLog(@"Completed in %f seconds",[endTime timeIntervalSinceDate:startTime]);
// });
//同步执行
// NSDate *startTime = [NSDate date];
// NSString *fetchedData = [self fetchSomethingFromServer];
// NSString *processData = [self processData:fetchedData];
// NSString *firstResult = [self calculateFirstResult:processData];
// NSString *secondResult = [self calculateSecondResult:processData];
// NSString *resultsSummary = [NSString stringWithFormat:@"First: [%@]\nSecond: [%@]",firstResult,secondResult];
// resultsTextView.text = resultsSummary;
// NSDate *endTime = [NSDate date];
// NSLog(@"Completed in %f seconds",[endTime timeIntervalSinceDate:startTime]);
}
@end
异步执行需花10秒，而并发程序块只花了7秒，大大提高了效率。在真实的应用程序中，加速程度将取决于所执行的工作和可用的资源。只有在多个cpu核心可用时，执行cpu资源密集型的计算才能受益于此技术。
=================================================================================
NSThread和GCD是多线程的两种实现方式，NSThread实现起来比GCD稍微复杂些，GCD要比较简单些，GCD是C语言提供的API。GCD支持多核CPU处理。
在GCD中，有3种类型的派发队列：
1、串行队列。串行队列通常用于同步访问一个特定的资源，每次只能执行一个任务。使用函数dispatch_queue_create,可以创建串行队列。
2、并发队列。也叫全局派发队列，可以并发地执行一个或多个任务。并发队列分为高、中、低3个优先级队列，中级为默认级别。可以使用调用dispatch_get_global_queue函数设定优先级来访问队列。
扫描二维码关注公众号，回复：
652183 查看本文章
3、主队列。它在应用程序的主线程中，用于更新UI。使用dispatch_get_main_queue函数，可以获得主队列。
以下两种图片下载方式，效率完全不一样。
同步下载图片：
NSString *picPath = [NSString stringWithFormat:@"%@?imageView2/0/w/%d",_mdExhibitVo.exhibitPic,(int)(([UIScreen mainScreen].bounds.size.width-8*2)*2)];
picPath = [picPath stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSData * picData = [NSData dataWithContentsOfURL:[NSURL URLWithString:picPath]];
self.imageView.image = [UIImage imageWithData:picData]];
异步下载图片：
NSString *smallImgStr = [NSString stringWithFormat:@"%@?imageView/0/w/200/q/20",_mdExhibitWorkVo.worksPic];
smallImgStr = [smallImgStr stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
dispatch_queue_t downloadQueue = dispatch_queue_create("com.test.works.image", NULL);
dispatch_async(downloadQueue, ^{
NSData *imgData = [NSData dataWithContentsOfURL:[NSURL URLWithString:smallImgStr]];
UIImage *img = [UIImage imageWithData:imgData];
dispatch_async(dispatch_get_main_queue(), ^{
_headerImageView.image = img;
[[ImageProgressView sharedASIImageViewAppearance] setPlaceholderImage:img];
});
});