iOS多线程编程

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一、什么是多线程

a) 进程是运行着的程序；

b) 一个进程至少包含一个线程（主线程，界面更新UIKit）;

c) 线程共用程序内存，但每个线程都有自己的stack空间；

d) 多线程就是多个执行流，可以同步或异步，是一种并发执行技术。

二、iOS中实现方法

a) pthread：iOS是某个版本的Mac OS所以可以使用unix中的线程方法；

b) NSThread：轻量级多线程技术，需要自己管理线程的生命周期；

c) NSOperation &NSOperationQueen：相比NSThread技术，使用更加简单，不需要线程管理，直接使用；

d) GCD(Grand Central Dispatch)：Ios4.0版本推出，用于取代NSThread和NSOperation，简单方便，功能强大，推荐使用。

三、NSThread

注：可继承NSThread重写main方法

a) + (void)detachNewThreadSelector:(SEL)selector toTarget:(id)target withObject:(nullableid)argument;

b) - (instancetype)initWithTarget:(id)target selector:(SEL)selector object:(nullableid)argument NS_AVAILABLE(10_5,2_0);

c) - (void)performSelectorInBackground:(SEL)aSelector withObject:(nullableid)arg NS_AVAILABLE(10_5,2_0);//属于NSObject

d) /*更新主线程UI*/- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullableid)arg waitUntilDone:(BOOL)wait;//属于NSObject

e) 加锁：NSLock/NSCondition

四、NSOperation& NSOperationQueen

a) NSInvocationOperation/NSBlockOperation/NSOperation/NSOperationQueen

b) 可继承NSOperation实现main方法

c) 只要把NSOperation的对象放到NSOperationQueen中即会进行多线程的运行

d) 并发执行/非并发执行

e) -(void)start

f) – (BOOL)isConcurrent

g) – (BOOL)isFinishd

h) – (BOOL)isExecuting

i) – (void)main

五、GCD(GrandCentral Dispatch)

a) GCD是iOS在4.0版本后推出的，用于取代以上方式的多线程，其可根据程序的任务队列合理分配核去处理，充分利用硬件多核，效率高。

b) 概念：任务和队列（串行队列/并行队列/同步任务/异步任务）

c) api:dispatch_queue_t/dispatch_queue_create/dispatch_get_main_queue/dispatch_get_global_queue/dispatch_sync/dispatch_async/dispatch_group_t/dispatch_group_create/dispatch_group_async/dispatch_group_notify

示例代码：

////  ViewController.m//  ThreadDemo////  Created by LiLeo on 16/6/24.//  Copyright © 2016年 LeoLi. All rights reserved.//#import "ViewController.h"@interface ViewController ()@property (weak, nonatomic) IBOutlet UIImageView *imageView;@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.    NSString * string = @"http://c.hiphotos.baidu.com/image/pic/item/ae51f3deb48f8c54469d4dc23e292df5e1fe7f95.jpg";    /*NSThread的三种方法*///    [NSThread detachNewThreadSelector:@selector(downloadImageByUrl:) toTarget:self withObject:string];//    NSThread * thread = [[NSThread alloc] initWithTarget:self selector:@selector(downloadImageByUrl:) object:string];//    [thread start];    //[self performSelectorInBackground:@selector(downloadImageByUrl:) withObject:string];        /*NSOperation的实现方法*/    /*     * 若要继承NSOperation有两种方式，只实现main方法的是非同步的，同步的需要实现：start/isConcurrent/isFinished     *///    NSInvocationOperation * operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(downloadImageByUrl:) object:string];//    NSOperationQueue * queue = [[NSOperationQueue alloc] init];//    [queue addOperation:operation];    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);    dispatch_async(queue, ^{        NSURL * url = [NSURL URLWithString:string];        NSData * data = [NSData dataWithContentsOfURL:url];        UIImage * image = [[UIImage alloc] initWithData:data];        dispatch_async(dispatch_get_main_queue(), ^{            _imageView.image = image;        });    });}- (void)didReceiveMemoryWarning {    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}- (void)downloadImageByUrl:(NSString *)string {    NSURL * url = [NSURL URLWithString:string];    NSData * data = [[NSData alloc] initWithContentsOfURL:url];    UIImage * image = [UIImage imageWithData:data];    [self performSelectorOnMainThread:@selector(updateImage:) withObject:image waitUntilDone:YES];}- (void)updateImage:(UIImage *)image {    _imageView.image = image;}@end

六、参考文档

a) http://mobile.51cto.com/iphone-403490.htm

b) http://blog.csdn.net/totogo2010/article/details/8010231

c) http://www.cnblogs.com/wendingding/p/3805088.html

d) http://www.cocoachina.com/ios/20150731/12819.html

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