在日常开发中，能用到的多线程知识点有NSThread，NSOperationQueue，GCD。至于Pthreads，因为基本用不到，就忽略不计了。

多线程基本知识

进程： 一个具有一定独立功能的程序关于某个数据集合的一次运行活动。可以理解成一个运行中的应用程序。
线程： 程序执行流的最小单元，线程是进程中的一个实体。
同步： 只能在当前线程按先后顺序依次执行，不开启新线程。
异步： 可以在当前线程开启多个新线程执行，可不按顺序执行。
队列： 装载线程任务的队形结构。
并发： 线程执行可以同时一起进行执行。
串行： 线程执行只能依次逐一先后有序的执行。

具体组合如下

多线程比较

NSThread

先看一下NSThread的相关属性方法

@interface NSThread : NSObject
//当前线程
@property (class, readonly, strong) NSThread *currentThread;
//使用类方法创建线程执行任务
+ (void)detachNewThreadWithBlock:(void (^)(void))block API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));
+ (void)detachNewThreadSelector:(SEL)selector toTarget:(id)target withObject:(nullable id)argument;
//判断当前是否为多线程
+ (BOOL)isMultiThreaded;
//指定线程的线程参数，例如设置当前线程的断言处理器。
@property (readonly, retain) NSMutableDictionary *threadDictionary;
//当前线程暂停到某个时间
+ (void)sleepUntilDate:(NSDate *)date;
//当前线程暂停一段时间
+ (void)sleepForTimeInterval:(NSTimeInterval)ti;
//退出当前线程
+ (void)exit;
//当前线程优先级
+ (double)threadPriority;
//设置当前线程优先级
+ (BOOL)setThreadPriority:(double)p;
//指定线程对象优先级 0.0～1.0，默认值为0.5
@property double threadPriority NS_AVAILABLE(10_6, 4_0);
//服务质量
@property NSQualityOfService qualityOfService NS_AVAILABLE(10_10, 8_0);
//线程名称
@property (nullable, copy) NSString *name NS_AVAILABLE(10_5, 2_0);
//栈区大小
@property NSUInteger stackSize NS_AVAILABLE(10_5, 2_0);
//是否为主线程
@property (class, readonly) BOOL isMainThread NS_AVAILABLE(10_5, 2_0);
//获取主线程
@property (class, readonly, strong) NSThread *mainThread NS_AVAILABLE(10_5, 2_0);
//初始化
- (instancetype)init NS_AVAILABLE(10_5, 2_0) NS_DESIGNATED_INITIALIZER;
//实例方法初始化，需要再调用start方法
- (instancetype)initWithTarget:(id)target selector:(SEL)selector object:(nullable id)argument NS_AVAILABLE(10_5, 2_0);
- (instancetype)initWithBlock:(void (^)(void))block API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));
//线程状态，正在执行
@property (readonly, getter=isExecuting) BOOL executing NS_AVAILABLE(10_5, 2_0);
//线程状态，正在完成
@property (readonly, getter=isFinished) BOOL finished NS_AVAILABLE(10_5, 2_0);
//线程状态，已经取消
@property (readonly, getter=isCancelled) BOOL cancelled NS_AVAILABLE(10_5, 2_0);
//取消，仅仅改变线程状态，并不能像exist一样真正的终止线程
- (void)cancel NS_AVAILABLE(10_5, 2_0);
//开始
- (void)start NS_AVAILABLE(10_5, 2_0);
//线程需要执行的代码，一般写子类的时候会用到
- (void)main NS_AVAILABLE(10_5, 2_0);
@end
另外，还有一个NSObject的分类，瞅一眼：
@interface NSObject (NSThreadPerformAdditions)
//隐式的创建并启动线程，并在指定的线程（主线程或子线程）上执行方法。
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray *)array;
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait;
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray *)array NS_AVAILABLE(10_5, 2_0);
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait NS_AVAILABLE(10_5, 2_0);
- (void)performSelectorInBackground:(SEL)aSelector withObject:(nullable id)arg NS_AVAILABLE(10_5, 2_0);
@end

以上有两个带有selector创建线程的方法，selector和target是有对应关系的，应该是target的selector方法，不能一贯的写self

创建线程示例

ViewController.m

#import "ViewController.h"
#import "Person.h"

@interface ViewController ()

@property (nonatomic, strong) Person *person;

@end

@implementation ViewController


- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person = [[Person alloc]init];
        self.person = [[Person alloc]init];
    [NSThread detachNewThreadSelector:@selector(sayHello:) toTarget:self.person withObject:@"Jim"];
    // 崩溃，self没有sayHello()方法
//    [NSThread detachNewThreadSelector:@selector(sayHello:) toTarget:self withObject:@"Jim"];
    
    NSThread *thread = [[NSThread alloc]initWithTarget:self.person selector:@selector(sayHello:) object:@"Sam"];
    // 崩溃，self没有sayHello()方法
//    NSThread *thread = [[NSThread alloc]initWithTarget:self selector:@selector(sayHello:) object:@"Sam"];
    [thread start];

}
@end

Person类

//Person.h
#import <Foundation/Foundation.h>

@interface Person : NSObject

- (void)sayHello:(NSString *)name;

@end

//Person.m
#import "Person.h"

@implementation Person

- (void)sayHello:(NSString *)name{
    NSLog(@"Hello, %@", name);
    NSLog(@"hello %@",[NSThread currentThread]);
}

@end

用NSThread下载图片阻塞示例：

//为开辟线程按钮事件
- (IBAction)blockTest:(id)sender {
    UIAlertView *alert = [[UIAlertView alloc]initWithTitle:@"Hint" message:@"Just For Test" delegate:nil cancelButtonTitle:@"YES" otherButtonTitles: nil];
    [alert show];
}

//开辟新线程按钮时间
- (IBAction)noMultiAction:(id)sender {
    [self loadImage];
}

-(void)loadImage{
    NSURL *imageUrl = [NSURL URLWithString:kImageUrl];
    NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];
    [self updateImageData:imageData];
}
-(void)updateImageData:(NSData*)imageData{
    UIImage *image = [UIImage imageWithData:imageData];
    [self performSelectorOnMainThread:@selector(updateImageOnMainThread:) withObject:image waitUntilDone:NO]; //更新UI的部分应该在主线程
}

- (void)updateImageOnMainThread:(UIImage *)image{
    self.showImageView.image = image;
}

- (IBAction)multiAction:(id)sender {
    [NSThread detachNewThreadSelector:@selector(loadImage) toTarget:self withObject:nil];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

点击左侧按钮效果如下：

QQ20180515-162631-HD.gif

点击右侧按钮效果如下：

QQ20180515-162739-HD.gif

用多线程果然能解决线程阻塞的问题，并且NSThread也比Pthreads好用，只不过实际开发中也许只有[NSThread currentThread];用的最多了。所以仅仅NSThread是不能满足我们的需要的。那就需要另外的手段了，比如GCD。