// 先创建后启动
NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];
[thread start];
// 创建完自动启动
[NSThread detachNewThreadSelector:@selector(run) toTarget:self withObject:nil];
// 隐式创建自动启动
[self performSelectorInBackground:@selector(run) withObject:nil];

+ (NSThread *)currentThread;
+ (NSThread *)mainThread;
// 睡眠、暂停当前线程
+ (void)sleepUntilDate:(NSDate *)date;
+ (void)sleepForTimeInterval:(NSTimeInterval)ti;
// 线程的名字
@property (copy) NSString *name

@synchronized(self) {
	// code
}

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // 执行耗时的操作...
      dispatch_async(dispatch_get_main_queue(), ^{
        // 回到主线程刷新UI
        });
});

perform..
[self performSelector:@selector(download:) withObject:@"http://555.jpg" afterDelay:3];
dispatch_after..
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), queue, ^{
    延迟执行的代码
});

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    // 此处的代码程序整个运行过程只执行一次
});

// 懒汉式
// GCD方式
@interface HMDataTool : NSObject
+ (instancetype)sharedDataTool;
+ 
@end
#import "HMDataTool.h"
@implementation HMDataTool
static id instace;
+ (id)allocWithZone:(struct _NSZone *)zone {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instace = [super allocWithZone:zone];
    });
    return instace;
}
+ (instancetype)sharedDataTool {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instace = [[self alloc] init];
    });
    return instace;
}
- (id)copyWithZone:(NSZone *)zone {
    return instace;
}
@end

// 主队列: 添加到主队列中的操作在主线程中执行
[NSOperationQueue mainQueue]    
// 非主队列: 添加到非主队列中的操作在子线程中执行
[[NSOperationQueue alloc] init]

- (void)addOperation:(NSOperation *)operation;   
- (void)addOperationWithBlock:(void(^)(void))block;

// 最大并发数
@property NSInteger maxConcurrentOperationCount;
// 取消所有操作
[queue cancelAllOperations];
// 暂停所有操作
[queue setSuspended:YES];
// 恢复所有操作
[queue setSuspended:NO];

// NSOperation 之间可以设置依赖关系来保证执行顺序
// 操作B依赖操作A(A执行完毕之后才会执行B)
[operationB addDependency:operationA];
// 注意: 不能相互依赖, 但可以在不同队列的操作之间设置依赖关系.

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[queue addOperationWithBlock:^{ 
    // 异步线程执行耗时操作
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        // 回到主线程刷新界面
    }];
}];

// perform…
[self performSelectorOnMainThread:(SEL) withObject:(id) waitUntilDone:(BOOL)];
// GCD
dispatch_async(dispatch_get_main_queue(), ^{
	// code  
});
// NSOperationQueue
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
	// code
}];

#if _has_feature(objc_arc)
// ARC环境
#else
// MRC环境
#endif

// 启动程序时类第一次加载到内存时调用(绝对只执行一次)
+ (void)load
+ 
// 第一次使用类(调用alloc方法等)时调用(一般只执行一次)
+ (void)initialize

@42  == [NSNumber numberWithInteger:42]  
@YES == [NSNumber numberWithBool:YES]  
@42U == [NSNumber numberWithUnsignedInt:42U]

@[@"A", @NO, @2.718] == [NSArray arrayWithObjects:@"A", @NO, @2.718, nil]

@{@"someKey": @"theValue"}

const char * 返回 NSString  
int 返回 NSNumber  等等

// 推荐
UIColor *myColor = [UIColor whiteColor];
// 不推荐
UIColor *myColour = [UIColor whiteColor];

#pragma mark - Lifecycle
- (instancetype)init {}
- (void)dealloc {}
- (void)viewDidLoad {}
- (void)viewWillAppear:(BOOL)animated {}
- (void)viewDidAppear:(BOOL)animated {}
- (void)didReceiveMemoryWarning {}
#pragma mark - Custom Accessors
- (void)setCustomProperty:(id)value {}
- (id)customProperty {}
#pragma mark - IBActions
- (IBAction)submitData:(id)sender {}
#pragma mark - Public
- (void)publicMethod {}
#pragma mark - Private
- (void)privateMethod {}
#pragma mark - UITableViewDataSource
//...
#pragma mark - UITableViewDelegate
//...
#pragma mark - NSCopying
- (id)copyWithZone:(NSZone *)zone {}
#pragma mark - NSObject
- (NSString *)description {}

+ (instancetype)arrayWithObjects:(ObjectType)firstObj, ... NS_REQUIRES_NIL_TERMINATION;
- (instancetype)initWithTitle:(nullable NSString *)title message:(nullable NSString *)message delegate:(nullable id /*<UIAlertViewDelegate>*/)delegate cancelButtonTitle:(nullable NSString *)cancelButtonTitle otherButtonTitles:(nullable NSString *)otherButtonTitles, ... NS_REQUIRES_NIL_TERMINATION NS_EXTENSION_UNAVAILABLE_IOS("Use UIAlertController instead.");
- (instancetype)initWithTitle:(nullable NSString *)title delegate:(nullable id<UIActionSheetDelegate>)delegate cancelButtonTitle:(nullable NSString *)cancelButtonTitle destructiveButtonTitle:(nullable NSString *)destructiveButtonTitle otherButtonTitles:(nullable NSString *)otherButtonTitles, ... NS_REQUIRES_NIL_TERMINATION NS_EXTENSION_UNAVAILABLE_IOS("Use UIAlertController instead.");

int add(int firstNum, ...)
{
    int sum = 0;
    int number = firstNum;
    va_list argumentList;
    va_start(argumentList, firstNum);
    while(1)
    {
        printf("number = %d\n", number);
        sum += firstNum;
        number = va_arg(argumentList, int);
        if (number == 0) // 0作为哨兵参数
        {
            break;
        }
    }
    va_end(argumentList);
    return sum;
}
int main(int argc, char * argv[]) {
    @autoreleasepool {
        printf("sum is %d\n", add(1, 2, 3, 4, 5, 0)); // 调用add函数传入可变参数必须以0结尾
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

// 注意: 可变参数只能是所有参数中的最后一个. 
// NS_REQUIRES_NIL_TERMINATION 用于编译时检查结尾是否为nil.
- (void)variableArgumentsMethod:(NSString *)arg1, ... NS_REQUIRES_NIL_TERMINATION {
    NSMutableArray *stringArray = [NSMutableArray array];
    va_list args;
    va_start(args, arg1);
    if(arg1) {
        NSLog(@"%@", arg1);
        [stringArray addObject:arg1];
        
        NSString *nextArg;
        while((nextArg = va_arg(args, NSString *))) {
            [stringArray addObject:nextArg];
            NSLog(@"%@", nextArg);
        }
    }
    NSLog(@"%@", stringArray);
    va_end(args);
}
- (void)viewDidLoad {
    
    [super viewDidLoad];
    
    [self variableArgumentsMethod:@"Hello", @"World", @"!", nil];
}

+ (void)sr_showActionSheetViewWithTitle:(NSString *)title
                               delegate:(id<SRActionSheetDelegate>)delegate
                      cancelButtonTitle:(NSString *)cancelButtonTitle
                 destructiveButtonTitle:(NSString *)destructiveButtonTitle
                      otherButtonTitles:(NSString *)firstOtherButtonTitle, ... NS_REQUIRES_NIL_TERMINATION;

+ (void)sr_showActionSheetViewWithTitle:(NSString *)title
                               delegate:(id<SRActionSheetDelegate>)delegate
                      cancelButtonTitle:(NSString *)cancelButtonTitle
                 destructiveButtonTitle:(NSString *)destructiveButtonTitle
                      otherButtonTitles:(NSString *)firstOtherButtonTitle, ... NS_REQUIRES_NIL_TERMINATION
{
    NSMutableArray *otherButtonTitles = [NSMutableArray array];
    va_list argumentList;
    if (firstOtherButtonTitle) {
        [otherButtonTitles addObject:firstOtherButtonTitle];
        va_start(argumentList, firstOtherButtonTitle);
        NSString *nextOtherButtonTitle = nil;
        while ((nextOtherButtonTitle = va_arg(argumentList, NSString *))) {
            [otherButtonTitles addObject:nextOtherButtonTitle];
        }
        va_end(argumentList);
    }
    
    [[[self alloc] initWithTitle:title
               cancelButtonTitle:cancelButtonTitle
          destructiveButtonTitle:destructiveButtonTitle
               otherButtonTitles:otherButtonTitles
                        delegate:delegate] show];
}