多线程之NSThread

开启线程

• 分离主线程创建：
  创建线程后会自动执行，但是线程外部不可获取到该线程对象
  detachNewThreadWithBlock:
detachNewThreadSelector:toTarget:withObject:

+ (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;

[NSThread detachNewThreadSelector:(SEL)aSelector toTarget:(id)aTarget withObject:(id)anArgument];
[NSThread detachNewThreadWithBlock:^{
    // do something
}];

• 构造方法创建：
  可拿到该线程对象，线程执行需手动启动

  - (instancetype)init NS_AVAILABLE(10_5, 2_0) NS_DESIGNATED_INITIALIZER;
  - (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));
  
  NSThread *thread = [[NSThread alloc] initWithTarget:(id)aTarget selector:(SEL)aSelector object:(id)anArgument];
  [thread start];
  

• performSelector方法：NSObject的NSThreadPerformAdditions类别
  任何OC对象均可由下列方法开启子线程或回到主线程

  - (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray<NSString *> *)array;
  - (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait;
  // equivalent to the first method with kCFRunLoopCommonModes
  
  - (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray<NSString *> *)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);
  // equivalent to the first method with kCFRunLoopCommonModes
  - (void)performSelectorInBackground:(SEL)aSelector withObject:(nullable id)arg NS_AVAILABLE(10_5, 2_0);
  

NSThread生命周期

• 创建后开始执行：start

- (void)start NS_AVAILABLE(10_5, 2_0);

• 执行任务:main
  main方法为线程内执行的主要方法，NSThread的子类需重写该方法，加入任务代码

- (void)main NS_AVAILABLE(10_5, 2_0);

• 取消：cancel

- (void)cancel NS_AVAILABLE(10_5, 2_0);

• 终止线程：exit
  exit是类方法，表示终止当前线程，且不可恢复

+ (void)exit;

• 一些属性
  • isExecuting：是否正在执行

    @property (readonly, getter=isExecuting) BOOL executing NS_AVAILABLE(10_5, 2_0);
    

  • isFinished：是否结束

    @property (readonly, getter=isFinished) BOOL finished NS_AVAILABLE(10_5, 2_0);
    

  • isCancelled：是否取消

    @property (readonly, getter=isCancelled) BOOL cancelled NS_AVAILABLE(10_5, 2_0);
    

线程间通讯：通过NSObject的NSThreadPerformAdditions类别方法调用

• 回归主线程
  在主线程中运行方法，wait表示是否阻塞这个方法的调用，如果为YES则等待主线程中运行方法结束。一般可用于在子线程中调用UI方法。

- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray<NSString *> *)array;
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait;

• 在指定线程中执行
  在指定线程中执行，但该线程必须具备runloop（kCFRunLoopCommonModes）

// equivalent to the first method with kCFRunLoopCommonModes
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray<NSString *> *)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);

• Background

// equivalent to the first method with kCFRunLoopCommonModes
- (void)performSelectorInBackground:(SEL)aSelector withObject:(nullable id)arg NS_AVAILABLE(10_5, 2_0);

NSThread的其它一些常用的属性和方法

• 获得当前线程

@property (class, readonly, strong) NSThread *currentThread;

• 线程休眠

+ (void)sleepUntilDate:(NSDate *)date;
+ (void)sleepForTimeInterval:(NSTimeInterval)ti;

3. 获得主线程

@property (class, readonly, strong) NSThread *mainThread NS_AVAILABLE(10_5, 2_0);

• 当前线程是否主线程

@property (readonly) BOOL isMainThread NS_AVAILABLE(10_5, 2_0);
@property (class, readonly) BOOL isMainThread NS_AVAILABLE(10_5, 2_0);

• 线程名

@property (nullable, copy) NSString *name NS_AVAILABLE(10_5, 2_0);

• 线程优先级

+ (double)threadPriority;
+ (BOOL)setThreadPriority:(double)p;
@property double threadPriority NS_AVAILABLE(10_6, 4_0); // To be deprecated; use qualityOfService below

• 服务质量：NSQualityOfService
  从iOS8开始，苹果框架概念基础中将不再过分强调线程这个概念。新的qualityOfService属性替换了ThreadPriority。这些新的语义允许应用程序推迟非关键工作，以确保始终如一的用户体验。

@property NSQualityOfService qualityOfService NS_AVAILABLE(10_10, 8_0); // read-only after the thread is started

typedef NS_ENUM(NSInteger, NSQualityOfService) {
    // 与用户交互的任务，这些任务通常跟UI级别的刷新相关，比如动画，这些任务需要在一瞬间完成
    NSQualityOfServiceUserInteractive = 0x21,
    // 在实现用户精确请求请求相关工作时使用UserInitiated QoS，但不要求精确到毫秒，比如动画。例如，如果用户打开email app马上查看邮件。
    NSQualityOfServiceUserInitiated = 0x19,
    // Utility QoS用于执行已经由用户请求自动发生的任务。例如，电子邮件应用程序可以被配置为每隔5分钟自动检查邮件。如果系统是非常有限的资源，而电子邮件检查被推迟几分钟这也是被允许的。
    NSQualityOfServiceUtility = 0x11,
    // Background QoS用于执行用户可能甚至都没有意识到正在发生的工作，比如email app可能使用它来执行索引搜索
    NSQualityOfServiceBackground = 0x09,
    // 优先级介于user-initiated 和 utility，当没有 QoS信息时默认使用，开发者不应该使用这个值来设置自己的任务
    NSQualityOfServiceDefault = -1
} NS_ENUM_AVAILABLE(10_10, 8_0);

• 线程数据：threadDictionary

@property (readonly, retain) NSMutableDictionary *threadDictionary;

9. 指定线程的栈空间：stackSize属性，值是4KB及其倍数，最大512KB，主线程最大1MB

@property NSUInteger stackSize NS_AVAILABLE(10_5, 2_0);

一些非线程调用（NSObject的NSThreadPerformAdditions类别方法）

• 在当前线程立即执行：注意它们会阻塞当前线程（包括UI线程）：

- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;

• 在当前线程延迟执行：

- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray *)modes;
- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay;

如果当前线程没有显式使用NSRunLoop或已退出就无法执行了，需要注意，它们可以被下列方法终止：

+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget selector:(SEL)aSelector object:(id)anArgument;
+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget;