读写操作一般都是比较耗时的操作,为了保证读写安全,同时提升性能,一般采取“多读单写”模式.即对同一资源,同一时间,保证以下状态:
只能有一个写操作
或者多个读操作
读、写两种操作不能同时进行
iOS大概有三种方式可以实现读写安全:
1. automic
automic 用于保证属性setter(写)、getter(读)的原子性操作, 相当于在setter和getter方法中添加了线程同步锁.
参考objc4的objc-accessors.mm源码
-
setter方法:
static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy)
{
if (offset == 0) {
object_setClass(self, newValue);
return;
}
id oldValue;
id *slot = (id*) ((char*)self + offset);
if (copy) {
newValue = [newValue copyWithZone:nil];
} else if (mutableCopy) {
newValue = [newValue mutableCopyWithZone:nil];
} else {
if (*slot == newValue) return;
newValue = objc_retain(newValue);
}
if (!atomic) {
oldValue = *slot;
*slot = newValue;
} else {
spinlock_t& slotlock = PropertyLocks[slot];
slotlock.lock();
oldValue = *slot;
*slot = newValue;
slotlock.unlock();
}
objc_release(oldValue);
}
-
getter方法:
id objc_getProperty(id self, SEL _cmd, ptrdiff_t offset, BOOL atomic) {
if (offset == 0) {
return object_getClass(self);
}
// Retain release world
id *slot = (id*) ((char*)self + offset);
if (!atomic) return *slot;
// Atomic retain release world
spinlock_t& slotlock = PropertyLocks[slot];
slotlock.lock();
id value = objc_retain(*slot);
slotlock.unlock();
// for performance, we (safely) issue the autorelease OUTSIDE of the spinlock.
return objc_autoreleaseReturnValue(value);
}
note: automic 原子操作,保证setter和getter操作的原子性,内部通过加锁来实现
2.读写锁
iOS中pthread_rwlock(读写锁)刚好可以满足这种场景. 读写锁有两种模式,阻塞式(xxx_xxlock. 加锁不成功,本线程阻塞等待)和非阻塞式(xxx_tryxxlock. 加锁不成功,本函数失败返回)
等待锁的线程会进入休眠
//初始化锁 - 初始化完成后, 是非锁定状态
pthread_rwlock_t lock;
//动态初始化
pthread_rwlock_init(&lock, NULL);
//静态初始化
PTHREASD_RWLOCK_INITIALIZER
//读-加锁 返回0表示成功
pthread_rwlock_rdlock(&lock);
//读-尝试加锁 返回0表示成功
pthread_rwlock_tryrdlock(&lock);
//写-加锁 返回0表示成功
pthread_rwlock_wrdlock(&lock);
//写-尝试加锁 返回0表示成功
pthread_rwlock_trywrlock(&lock);
//解锁
pthread_rwlock_unlock(&lock);
//销毁
pthread_rwlock_destroy(&lock);
example:
#import
@property (assign, nonatomic) pthread_rwlock_t lock;
- (void)viewDidLoad {
[super viewDidLoad];
// 初始化锁
pthread_rwlock_init(&_lock, NULL);
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
for (int i = 0; i < 10; i++) {
dispatch_async(queue, ^{
[self read];
});
dispatch_async(queue, ^{
[self write];
});
}
}
- (void)read {
pthread_rwlock_rdlock(&_lock);
sleep(1);
NSLog(@"%s", __func__);
pthread_rwlock_unlock(&_lock);
}
- (void)write {
pthread_rwlock_wrlock(&_lock);
sleep(1);
NSLog(@"%s", __func__);
pthread_rwlock_unlock(&_lock);
}
- (void)dealloc {
pthread_rwlock_destroy(&_lock);
}
3.线程+栅栏
- GCD
使用dispatch_barrier_async, 传入的队列必须是dispatch_queue_cretate创建的并发队列 (FMDB使用串行队列保证读写安全,但是读操作也是串行执行,似乎...)
example:
// 初始化队列
dispatch_queue_t queue = dispatch_queue_create("rw_queue", DISPATCH_QUEUE_CONCURRENT);
// 读
dispatch_async(self.queue, ^{
[self read];
});
// 写
dispatch_barrier_async(self.queue, ^{
[self write];
});
- NSOperationQueue
串行队列可以保证写操作的安全-串行执行,并发队列可以实现读操作的高效-并行执行.
一个NSOperationQueue可以满足这两种要求,只需切换maxConcurrentOperationCount即可切换这两种模式.
同时为了满足读写互斥,还需添加栅栏或依赖. 具体,NSOperationQueue iOS13开始添加了栅栏操作:- (void)addBarrierBlock:(void (^)(void))barrier API_AVAILABLE(macos(10.15), ios(13.0), tvos(13.0), watchos(6.0));,使用栅栏比较简单一些, iOS13以下系统需要通过添加依赖实现该功能.
example:
@interface DataModel : NSObject
@property(nonatomic,copy)NSString *name;
@property(nonatomic)NSInteger count;
@end
@implementation DataModel
- (NSString *)description
{
return [NSString stringWithFormat:@"You Have %ld %@ !", (long)self.count, self.name];
}
@end
/*NSOperation+RW.h*/
typedef enum : NSUInteger {
OperationUnKnow = 0,
OperationRead = 1,
OperationWrite = 2,
OperationToRead = 3,
OperationToWrite = 4,
} OpereationFlag;
@interface NSOperation (RW)
@property(nonatomic)OpereationFlag flag;
@end
/*NSOperation+RW.m*/
static void *OperationReadWriteFlagKey = &OperationReadWriteFlagKey;
@implementation NSOperation (RW)
-(void)setFlag:(OpereationFlag)flag{
objc_setAssociatedObject(self, OperationReadWriteFlagKey, @(flag), OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (OpereationFlag)flag{
id flag = objc_getAssociatedObject(self, OperationReadWriteFlagKey);
return (OpereationFlag)[flag integerValue];
}
@end
/*DataBaseHandler.m*/
- (void)readDataWithQuery:(nonnull NSString *)query parameter:(id _Nullable)param result:(nonnull DBResultBlock)result {
[self appendTaskWith:query parameter:param result:result read:YES];
}
- (void)updateDataWithQuery:(nonnull NSString *)query parameter:(id _Nullable)param result:(nonnull DBResultBlock)result {
[self appendTaskWith:query parameter:param result:result read:NO];
}
-(void)appendTaskWith:(nonnull NSString *)query parameter:(id _Nullable)param result:(nonnull DBResultBlock)result read:(BOOL)read{
__weak typeof(self) weakSelf = self;
NSOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
if (read) {
sleep(2);
}else{
if ([param isKindOfClass:[NSDictionary class]]) {
[weakSelf.model setValuesForKeysWithDictionary:param];
}
sleep(1);
}
result(weakSelf.model.description);
}];
if (read) {
operation.flag = OperationRead;
}else{
operation.flag = OperationWrite;
}
NSLog(@"**********************");
static BOOL log = NO;
NSOperation *lastOperation = [[self.readWriteQueue operations] lastObject];
if (@available(iOS 13.0, *)) {
//使用栅栏操作比较简单
if (read) {
if (lastOperation) {
if (lastOperation.flag==OperationRead) {
[self.readWriteQueue addOperation: operation];
}else{
[self.readWriteQueue addBarrierBlock:^{
weakSelf.readWriteQueue.maxConcurrentOperationCount = NSOperationQueueDefaultMaxConcurrentOperationCount;
}];
[self.readWriteQueue addOperation:operation];
}
}else{
self.readWriteQueue.maxConcurrentOperationCount = NSOperationQueueDefaultMaxConcurrentOperationCount;
[self.readWriteQueue addOperation:operation];
}
}else{
if (lastOperation) {
if (lastOperation.flag==OperationWrite) {
[self.readWriteQueue addOperation: operation];
}else{
[self.readWriteQueue addBarrierBlock:^{
weakSelf.readWriteQueue.maxConcurrentOperationCount = 1;
}];
[self.readWriteQueue addOperation:operation];
}
}else{
self.readWriteQueue.maxConcurrentOperationCount = 1;
[self.readWriteQueue addOperation:operation];
}
}
}else{
/*
连续的读操都需要依赖前边最接近的to-read-barrier,
to-read-barrier 依赖前边最后一个写操作
写操作前后依赖, 第一个依赖前边最近的to-write-barrier,
to-write-barrier 依赖前边最近的连续的读操作
**/
if (read) {
if (lastOperation) {
if (lastOperation.flag==OperationRead) {
NSOperation *operatonObj = nil;
NSArray *operations = [self.readWriteQueue.operations copy];
NSInteger count = self.readWriteQueue.operations.count;
for (int i = 0; i < count -1; i++) {
NSOperation *obj = [operations objectAtIndex:count - 1 - i];
if (obj.flag==OperationToRead&&!obj.finished&&!obj.cancelled) {
operatonObj = obj;
break;
}
}
if (operatonObj) {
if (log) {
NSLog(@"R依赖TR");
}
[operation addDependency:operatonObj];
}
if (log) {
NSLog(@"添加了R");
}
[self.readWriteQueue addOperation: operation];
}else if(lastOperation.flag==OperationWrite){
NSOperation *toReadBarrier = [NSBlockOperation blockOperationWithBlock:^{
}];
[toReadBarrier setCompletionBlock:^{
weakSelf.readWriteQueue.maxConcurrentOperationCount = NSOperationQueueDefaultMaxConcurrentOperationCount;
}];
toReadBarrier.flag = OperationToRead;
[toReadBarrier addDependency:lastOperation];
[operation addDependency:toReadBarrier];
if (log) {
NSLog(@"TR依赖W");
NSLog(@"R依赖TR");
NSLog(@"添加了TR");
NSLog(@"添加了R");
}
[self.readWriteQueue addOperation:toReadBarrier];
[self.readWriteQueue addOperation:operation];
}
}else{
if (log) {
NSLog(@"添加了R");
}
self.readWriteQueue.maxConcurrentOperationCount = NSOperationQueueDefaultMaxConcurrentOperationCount;
[self.readWriteQueue addOperation:operation];
}
}else{
if (lastOperation) {
if (lastOperation.flag==OperationWrite) {
if (log) {
NSLog(@"W依赖W");
NSLog(@"添加了W");
}
[operation addDependency:lastOperation];
[self.readWriteQueue addOperation:operation];
}else if(lastOperation.flag==OperationRead){
NSOperation *toWriteBarrier = [NSBlockOperation blockOperationWithBlock:^{
// NSLog(@"toWriteBarrier");
}];
[toWriteBarrier setCompletionBlock:^{
weakSelf.readWriteQueue.maxConcurrentOperationCount = 1;
}];
toWriteBarrier.flag = OperationToWrite;
NSArray *operations = [self.readWriteQueue.operations copy];
NSInteger count = self.readWriteQueue.operations.count;
for (int i = 0; i < count - 1; i++) {
NSOperation *operatonObj = [operations objectAtIndex:count - 1 - i];
if (operatonObj.flag==OperationRead) {
if (!operation.cancelled&&!operation.isFinished) {
[toWriteBarrier addDependency:operatonObj];
if (log) {
NSLog(@"TW依赖R");
}
}
}else{
break;
}
}
if (log) {
NSLog(@"W依赖TW");
NSLog(@"添加了TW");
NSLog(@"添加了W");
}
[operation addDependency:toWriteBarrier];
[self.readWriteQueue addOperation:toWriteBarrier];
[self.readWriteQueue addOperation:operation];
}
}else{
self.readWriteQueue.maxConcurrentOperationCount = 1;
[self.readWriteQueue addOperation:operation];
if (log) {
NSLog(@"添加了W");
}
}
}
}
}
@end
测试:
/*in test place*/
-(void)testReadWriteSingleQueue{
id db = [DataBaseRWQueueHandler shareHandler];
id apple = @{@"name": @"Apple", @"count": @2};
id orange = @{@"name": @"Orange", @"count":@10};
id peach = @{@"name": @"Peach", @"count":@10};
id grape = @{@"name": @"Grape", @"count":@20};
BOOL log = YES;
[db updateDataWithQuery:@"" parameter:apple result:^(id _Nonnull result) {
if (log) {NSLog(@"write1: %@", result);}
}];
[db readDataWithQuery:@"" parameter:nil result:^(id _Nonnull result) {
if (log) {NSLog(@"read1: %@", result);}
}];
[db updateDataWithQuery:@"" parameter:orange result:^(id _Nonnull result) {
if (log) {NSLog(@"write2: %@", result);}
}];
[db readDataWithQuery:@"" parameter:nil result:^(id _Nonnull result) {
if (log) {NSLog(@"read2: %@", result);}
}];
[db updateDataWithQuery:@"" parameter:peach result:^(id _Nonnull result) {
if (log) {NSLog(@"write3: %@", result);}
}];
[db updateDataWithQuery:@"" parameter:grape result:^(id _Nonnull result) {
if (log) {NSLog(@"write4: %@", result);}
}];
[db readDataWithQuery:@"" parameter:nil result:^(id _Nonnull result) {
if (log) {NSLog(@"read3: %@", result);}
}];
[db readDataWithQuery:@"" parameter:nil result:^(id _Nonnull result) {
if (log) {NSLog(@"read4: %@", result);}
}];
}
/*
打印:
2020-03-14 17:27:09.101971+0800 Basic_iOS[48718:1193248] write1: You Have 2 Apple !
2020-03-14 17:27:11.103866+0800 Basic_iOS[48718:1193248] read1: You Have 2 Apple !
2020-03-14 17:27:12.107587+0800 Basic_iOS[48718:1193248] write2: You Have 10 Orange !
2020-03-14 17:27:14.109110+0800 Basic_iOS[48718:1193247] read2: You Have 10 Orange !
2020-03-14 17:27:15.110935+0800 Basic_iOS[48718:1193247] write3: You Have 10 Peach !
2020-03-14 17:27:16.111248+0800 Basic_iOS[48718:1193248] write4: You Have 20 Grape !
2020-03-14 17:27:18.115692+0800 Basic_iOS[48718:1193247] read3: You Have 20 Grape !
2020-03-14 17:27:18.115692+0800 Basic_iOS[48718:1193248] read4: You Have 20 Grape !
*/
共同学习,欢迎指正...
参考文章:
https://www.jianshu.com/p/a459ea2e7fec