在開發(fā)中遇到的一些多線程問題

本文節(jié)選自作者 2018 年的《我所理解的 iOS 并發(fā)編程》，讀者小伙伴么在開發(fā)中遇到的并發(fā)問題，歡迎在留言區(qū)留言

1. NSNotification 與多線程問題

@implementation BLPostNotification

- (void)postNotification {
    dispatch_queue_t queue = dispatch_queue_create("com.bool.post.notification", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        // 從非主線程發(fā)送通知 （通知名字最好定義成一個(gè)常量）
        [[NSNotificationCenter defaultCenter] postNotificationName:@"downloadImage" object:nil];
    });
}
@end

@implementation ImageViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(show) name:@"downloadImage" object:nil];
}

- (void)showImage {
    // 需要 dispatch 到主線程更新 UI
    dispatch_async(dispatch_get_main_queue(), ^{
        // update UI
    });
}
@end

2. NSTimer 與多線程問題

However, for a repeating timer, you must invalidate the timer object yourself by calling its?invalidate?method. Calling this method requests the removal of the timer from the current run loop; as a result, you should always call the?invalidate?method from the same thread on which the timer was installed.

@interface BLTimerTest ()
@property (nonatomic, strong) dispatch_queue_t queue;
@property (nonatomic, strong) NSTimer *timer;
@end

@implementation BLTimerTest
- (instancetype)init {
    self = [super init];
    if (self) {
        _queue = dispatch_queue_create("com.bool.timer.test", DISPATCH_QUEUE_SERIAL);
    }
    return self;
}

- (void)installTimer {
    dispatch_async(self.queue, ^{
        self.timer = [NSTimer scheduledTimerWithTimeInterval:3.0f repeats:YES block:^(NSTimer * _Nonnull timer) {
            NSLog(@"test timer");
        }];
    });
}

- (void)clearTimer {
    dispatch_async(self.queue, ^{
        if ([self.timer isValid]) {
            [self.timer invalidate];
            self.timer = nil;
        }
    });
}
@end

3. Dispatch Once 死鎖問題

- (void)dispatchOnceTest {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        [self dispatchOnceTest];
    });
}

4. Dispatch Group 問題

- (void)testDispatchGroup {
    NSString *path = @"";
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSArray *folderList = [fileManager contentsOfDirectoryAtPath:path error:nil];
    dispatch_group_t taskGroup = dispatch_group_create();
    
    for (NSString *folderName in folderList) {
        dispatch_group_enter(taskGroup);
        NSString *folderPath = [@"path" stringByAppendingPathComponent:folderName];
        NSArray *fileList = [fileManager contentsOfDirectoryAtPath:folderPath error:nil];
        for (NSString *fileName in fileList) {
            dispatch_async(_queue, ^{
                // 異步任務(wù)
                dispatch_group_leave(taskGroup);
            });
        }
    }
}

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