iOS常見的多線程方案
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GCD源碼：https://github.com/apple/swift-corelibs-libdispatch
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死鎖
- (void)viewDidLoad {
[super viewDidLoad];
NSLog(@"1");
dispatch_queue_t queue = dispatch_get_main_queue();
dispatch_sync(queue, ^{
NSLog(@"2");
});
NSLog(@"3");
}
當前隊列需要等待viewDidLoad執(zhí)行完,才能執(zhí)行block里面的內容,而viewDidLoad又需要等待block執(zhí)行完.雙方都在等待對方完成,而造成的死鎖.
另一個死鎖
- (void)interView02
{
NSLog(@"執(zhí)行任務1");
dispatch_queue_t queue = dispatch_queue_create("myqueue", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
NSLog(@"執(zhí)行任務2");
dispatch_sync(queue, ^{
NSLog(@"執(zhí)行任務3");
});
NSLog(@"執(zhí)行任務4");
});
NSLog(@"執(zhí)行任務5");
}
網(wǎng)上一道面試題
- (void)test
{
NSLog(@"3");
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
dispatch_queue_t queque = dispatch_get_global_queue(0, 0);
dispatch_async(queque, ^{
NSLog(@"1");
[self performSelector:@selector(test) withObject:nil
afterDelay:0];
NSLog(@"2");
[[NSRunLoop currentRunLoop]addPort:[NSPort new] forMode:NSDefaultRunLoopMode];
[[NSRunLoop currentRunLoop]runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
});
}
[self performSelector:@selector(test) withObject:nil afterDelay:0];的本質是往runloop中添加定時器,子線程默認沒有開啟runloop.
GNUStep
是GNU計劃的項目之一,它將cocoa的oc庫開源實現(xiàn)了一遍,非常接近蘋果的源碼.
源碼地址:http://www.gnustep.org/resources/downloads.php 這里可以看到runloop的相關實現(xiàn)
多線程的隱患
當多個線程訪問同一塊資源時抛猫，很容易引發(fā)數(shù)據(jù)錯亂和數(shù)據(jù)安全問題
eg.存錢取錢問題.
@interface ViewController ()
@property (nonatomic,assign) int moeny;
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.moeny = 100;
// Do any additional setup after loading the view.
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_async(queue, ^{
for (int i = 0; i < 10; i++) {
[self saveMoeny];
}
});
dispatch_async(queue, ^{
for (int i = 0; i < 10; i++) {
[self drawMoney];
}
});
}
//取錢
- (void)drawMoney
{
int oldMoeny = self.moeny;
sleep(0.2);
oldMoeny = oldMoeny - 20;
self.moeny = oldMoeny;
NSLog(@"取20, 還剩%d--%@",oldMoeny,[NSThread currentThread]);
}
//存錢
- (void)saveMoeny
{
int newMoney = self.moeny;
sleep(.2);
newMoney = newMoney + 50;
self.moeny = newMoney;
NSLog(@"存50, 還剩%d--%@",newMoney,[NSThread currentThread]);
}
線程同步技術
OSSpinLockos_unfair_lockpthread_mutexdispatch_semaphoredispatch_queue(DISPATCH_QUEUE_SERIAL)NSLockNSRecursiveLockNSConditionNSConditionLock@synchronized
OSSpinLock
- OSSpinLock叫做”自旋鎖”浩蓉，等待鎖的線程會處于忙等（busy-wait）狀態(tài)峦失，一直占用著CPU資源
- 目前已經(jīng)不再安全置侍，可能會出現(xiàn)優(yōu)先級反轉問題
- 如果等待鎖的線程優(yōu)先級較高厉颤，它會一直占用著CPU資源转唉，優(yōu)先級低的線程就無法釋放鎖
#import <libkern/OSAtomic.h>
//取錢
- (void)drawMoney
{
OSSpinLockLock(&_lock);
int oldMoeny = self.moeny;
sleep(0.2);
oldMoeny = oldMoeny - 20;
self.moeny = oldMoeny;
NSLog(@"取20, 還剩%d--%@",oldMoeny,[NSThread currentThread]);
OSSpinLockUnlock(&_lock);
}
//存錢
- (void)saveMoeny
{
OSSpinLockLock(&_lock);
int newMoney = self.moeny;
sleep(.2);
newMoney = newMoney + 50;
self.moeny = newMoney;
NSLog(@"存50, 還剩%d--%@",newMoney,[NSThread currentThread]);
OSSpinLockUnlock(&_lock);
}
新建基類lockDemo,以后每個種類的lock繼承于它便于演示.
@interface lockDemo : NSObject
- (void)drawAndSaveMoneyTest;
- (void)ticketTest;
- (void)drawMoney;
- (void)saveMoeny;
- (void)saleTicket;
@end
@interface lockDemo ()
@property (nonatomic,assign) int moeny;
@property (nonatomic,assign) int ticketsCount;
@end
@implementation lockDemo
- (void)drawAndSaveMoneyTest
{
self.moeny = 100;
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_async(queue, ^{
for (int i = 0; i < 10; i++) {
[self saveMoeny];
}
});
dispatch_async(queue, ^{
for (int i = 0; i < 10; i++) {
[self drawMoney];
}
});
}
//取錢
- (void)drawMoney
{
int oldMoeny = self.moeny;
sleep(0.2);
oldMoeny = oldMoeny - 20;
self.moeny = oldMoeny;
NSLog(@"取20, 還剩%d--%@",oldMoeny,[NSThread currentThread]);
}
//存錢
- (void)saveMoeny
{
int newMoney = self.moeny;
sleep(.2);
newMoney = newMoney + 50;
self.moeny = newMoney;
NSLog(@"存50, 還剩%d--%@",newMoney,[NSThread currentThread]);
}
- (void)saleTicket{
int oldTicketsCount = self.ticketsCount;
sleep(.2);
oldTicketsCount--;
self.ticketsCount = oldTicketsCount;
NSLog(@"還剩%d張票 - %@", oldTicketsCount, [NSThread currentThread]);
}
/**
賣票演示
*/
- (void)ticketTest
{
self.ticketsCount = 15;
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_async(queue, ^{
for (int i = 0; i < 5; i++) {
[self saleTicket];
}
});
dispatch_async(queue, ^{
for (int i = 0; i < 5; i++) {
[self saleTicket];
}
});
dispatch_async(queue, ^{
for (int i = 0; i < 5; i++) {
[self saleTicket];
}
});
}
@end
os_unfair_lock
- os_unfair_lock用于取代不安全的OSSpinLock 孽惰，從iOS10開始才支持
- 從底層調用看耕肩，等待os_unfair_lock鎖的線程會處于休眠狀態(tài)，并非忙等
- 需要導入頭文件#import <os/lock.h>
@interface os_unfair_locDemo()
@property (nonatomic,assign) os_unfair_lock moneyLock;
@property (nonatomic,assign) os_unfair_lock ticketLock;
@end
@implementation os_unfair_locDemo
- (instancetype)init
{
self = [super init];
if (self) {
self.moneyLock = OS_UNFAIR_LOCK_INIT;
self.ticketLock = OS_UNFAIR_LOCK_INIT;
}
return self;
}
- (void)saleTicket
{
os_unfair_lock_lock(&_ticketLock);
[super saleTicket];
os_unfair_lock_unlock(&_ticketLock);
}
- (void)saveMoeny
{
os_unfair_lock_lock(&_moneyLock);
[super saveMoeny];
os_unfair_lock_unlock(&_moneyLock);
}
- (void)drawMoney
{
os_unfair_lock_lock(&_moneyLock);
[super drawMoney];
os_unfair_lock_unlock(&_moneyLock);
}
@end
pthread_mutex
- mutex叫做”互斥鎖”欲逃，等待鎖的線程會處于休眠狀態(tài)
- 需要導入頭文件#import <pthread.h>
@interface mutexDemo()
@property (nonatomic,assign) pthread_mutex_t moneyMutex;
@property (nonatomic,assign) pthread_mutex_t ticketMutex;
@end
@implementation mutexDemo
- (instancetype)init
{
self = [super init];
if (self) {
[self __initMutex:&_moneyMutex];
[self __initMutex:&_ticketMutex];
}
return self;
}
//這里必須要傳指針 否則傳進來都是同一個pthread_mutex_t
- (void)__initMutex:(pthread_mutex_t *)mutex
{
//定義屬性
pthread_mutexattr_t attr;
//初始化屬性
pthread_mutexattr_init(&attr);
//設置屬性
/*
#define PTHREAD_MUTEX_NORMAL 0
#define PTHREAD_MUTEX_ERRORCHECK 1
#define PTHREAD_MUTEX_RECURSIVE 2 //遞歸鎖
#define PTHREAD_MUTEX_DEFAULT PTHREAD_MUTEX_NORMAL
*/
pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
pthread_mutex_init(mutex, &attr);
//銷毀屬性
pthread_mutexattr_destroy(&attr);
}
- (void)saleTicket
{
pthread_mutex_lock(&_ticketMutex);
[super saleTicket];
pthread_mutex_unlock(&_ticketMutex);
}
- (void)saveMoeny
{
pthread_mutex_lock(&_moneyMutex);
[super saveMoeny];
pthread_mutex_unlock(&_moneyMutex);
}
- (void)drawMoney
{
pthread_mutex_lock(&_moneyMutex);
[super drawMoney];
pthread_mutex_unlock(&_moneyMutex);
}
- (void)dealloc
{
pthread_mutex_destroy(&_moneyMutex);
pthread_mutex_destroy(&_ticketMutex);
}
@end
遞歸鎖
- (instancetype)init
{
if (self = [super init]) {
[self __initMutex:&_mutex];
}
return self;
}
/**
線程1：otherTest（+-）
otherTest（+-）
otherTest（+-）
線程2：otherTest（等待）
*/
- (void)otherTest
{
pthread_mutex_lock(&_mutex);
NSLog(@"%s", __func__);
static int count = 0;
if (count < 10) {
count++;
[self otherTest];
}
pthread_mutex_unlock(&_mutex);
}
NS的鎖基本上是上述鎖的封裝.
dispatch_semaphore_t
- semaphore叫做”信號量”
- 信號量的初始值找蜜，可以用來控制線程并發(fā)訪問的最大數(shù)量
- 信號量的初始值為1饼暑，代表同時只允許1條線程訪問資源稳析，保證線程同步
@interface semaphore()
@property (nonatomic,strong) dispatch_semaphore_t semaphore;
@end
@implementation semaphore
- (instancetype)init
{
self = [super init];
if (self) {
self.semaphore = dispatch_semaphore_create(5);
}
return self;
}
- (void)otherTest
{
for (int i = 0; i<100; i++) {
[[[NSThread alloc]initWithTarget:self selector:@selector(test) object:nil] start];
}
}
- (void)test
{
dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
sleep(2);
NSLog(@"12345");
dispatch_semaphore_signal(self.semaphore);
}
@end
synchronized
- @synchronized是對mutex遞歸鎖的封裝
- 源碼查看：objc4中的objc-sync.mm文件
- @synchronized(obj)內部會生成obj對應的遞歸鎖洗做，然后進行加鎖、解鎖操作
什么情況使用自旋鎖比較劃算彰居？
- 預計線程等待鎖的時間很短
- 加鎖的代碼（臨界區(qū)）經(jīng)常被調用诚纸，但競爭情況很少發(fā)生
- CPU資源不緊張
- 多核處理器
什么情況使用互斥鎖比較劃算？
- 預計線程等待鎖的時間較長
- 單核處理器
- 臨界區(qū)有IO操作
- 臨界區(qū)代碼復雜或者循環(huán)量大
- 臨界區(qū)競爭非常激烈