NSNotification的坑

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转载自: http://yangjunsss.github.io/ios/2014/06/23/Notification%E7%9A%84%E5%9D%91.html

在事件驱动的消息处理中，Notification用起来很方便。坑一：NSNotificationQueue的addObserver方式是[NSNotificationCenter defaultCenter] addObserver，而不是[NSNotificationQueue defaultQueue] addObserver Notification提供异步post方式NSNotificationQueue，通过enqueueNotification的接口把Notification入队列，并且提供NSPostASAP, NSPostWhenIdle, and NSPostNow这3种时刻来执行，意思分别为在runloop结束时，在线程idle时和立刻。比如：

...[[NSNotificationQueue defaultQueue] addObserver:self selector:@selector(doIdleTask) name:@"idleTask" object:nil]; // 错误，但编译正确不报错- -[[NSNotificationQueue defaultQueue] enqueueNotification:[NSNotification notificationWithName:@"idleTask" object:self] postingStyle:NSPostWhenIdle];...- (void)doIdleTask{    NSLog(@"do task in idle");    [[NSNotificationQueue defaultQueue] enqueueNotification:[NSNotification notificationWithName:@"idleTask" object:self] postingStyle:NSPostWhenIdle];}

可以重复利用当前线程Idle来碎片化执行任务。

在addObserver的时候很容易写成：[[NSNotificationQueue defaultQueue] addObserver:self selector:@selector(doIdleTask) name:@"idleTask" object:nil];，而且这样写并不报错，因为[NSNotificationQueue defaultQueue]返回的是id,在编译期它能是任意的对象，所以能关联上任意子类的方法，从而都没有编译错误，但事实上NSNotificationQueue并没有addObserver方法，所有Notification的addObserver都使用[NSNotificationCenter defaultCenter]。正确的写法是：

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doIdleTask) name:@"idleTask" object:nil];

坑二：Notification原则上是不支持多线程的，post在哪个线程，observer就在哪个线程接收，所以别以为网络线程要刷新UI的时候去post，然后执行UI操作，这个坑会导致crash。

Apple提供了一种比较粗糙的实现方式来支持多线程，实质上是进行了传递，“传递者”实现如下：

#import <Foundation/Foundation.h>@interface NotificationTransfer : NSObject <NSMachPortDelegate>@property (nonatomic) NSMutableArray *notifications;@property (nonatomic) NSThread *thread;@property (nonatomic) NSLock *lock;@property (nonatomic) NSMachPort *port;- (id) init;- (void) setUpThreadingSupport;- (void) handleMachMessage:(void *)msg;- (void) processNotification:(NSNotification *)notification;@end#import "NotificationTransfer.h"@implementation NotificationTransfer- (id) init{    if (self = [super init]) {            }    return self;}- (void) setUpThreadingSupport{    if (self.notifications) {        return;    }    self.notifications = [NSMutableArray new];    self.lock = [NSLock new];    self.thread = [NSThread currentThread];    self.port = [NSMachPort new];    [self.port setDelegate:self];    [[NSRunLoop currentRunLoop] addPort:self.port forMode:(__bridge NSString *) kCFRunLoopCommonModes];}- (void) handleMachMessage:(void *)msg{    [self.lock lock];        while ([self.notifications count]) {        NSNotification *notification = [self.notifications objectAtIndex:0];        [self.notifications removeObjectAtIndex:0];        [self.lock unlock];        [self processNotification:notification];        [self.lock lock];    }    [self.lock unlock];}- (void) processNotification:(NSNotification *)notification{    NSThread *ct = [NSThread currentThread];    if (ct != _thread) {        [self.lock lock];        [self.notifications addObject:notification];        [self.lock unlock];        [self.port sendBeforeDate:[NSDate date] components:nil from:nil reserved:0];    }else{        NSLog(@"process notification %@,is main %zd",[NSThread currentThread],[NSThread isMainThread]);    }}@end

processNotification会接收post子线程的notification，然后发现不是当前注册的thread就通过schedule一个port并缓存Notification，最后会执行handleMachMessage从而调用逻辑函数，所以这样做需要注册一个NSThread对象，需要统一定义好事件的接收selector。

调用的code：

- (void)viewDidLoad{    [super viewDidLoad];    _transfer = [NotificationTransfer new];    [_transfer setUpThreadingSupport]; // 在主线程定义了_transfer    [[NSNotificationCenter defaultCenter] addObserver:_transfer selector:@selector(processNotification:) name:@"notifi" object:nil]; // 添加了selector    dispatch_queue_t queue = dispatch_queue_create("queue1", DISPATCH_QUEUE_CONCURRENT);    dispatch_async(queue, ^{        [[NSNotificationCenter defaultCenter] postNotificationName:@"notifi" object:nil]; // 子线程post，最终Notification会转发到主线程执行processNotification    });}

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