ios – 使核心数据线程安全

如果有人感兴趣,我创建了一个线程安全的NSManagedObject子类,通过注入我自己的setter方法来代替Core Data通常会生成的一些方法.这是使用以下代码实现的：

//implement these so that we know what thread our associated context is on
- (void) awakeFromInsert {
    myThread = [NSThread currentThread];
}
- (void) awakeFromFetch {
    myThread = [NSThread currentThread];
}

//helper for re-invoking the dynamic setter method,because the NSInvocation requires a @selector and dynamicSetter() isn't one
- (void) recallDynamicSetter:(SEL)sel withObject:(id)obj {
    dynamicSetter(self,sel,obj);
}

//mapping invocations back to the context thread
- (void) runInvocationOnCorrectThread:(NSInvocation*)call {
    if (! [self myThread] || [NSThread currentThread] == [self myThread]) {
        //okay to invoke
        [call invoke];
    }
    else {
        //remap to the correct thread
        [self performSelector:@selector(runInvocationOnCorrectThread:) onThread:myThread withObject:call waitUntilDone:YES];
    }
}

//magic!  perform the same operations that the Core Data generated setter would,but only after ensuring we are on the correct thread
void dynamicSetter(id self,SEL _cmd,id obj) {
    if (! [self myThread] || [NSThread currentThread] == [self myThread]) {
        //okay to execute
        //XXX:  clunky way to get the property name,but meh...
        NSString* targetSel = NSStringFromSelector(_cmd);
        NSString* propertyNameUpper = [targetSel substringFromIndex:3];  //remove the 'set'
        NSString* firstLetter = [[propertyNameUpper substringToIndex:1] lowercaseString];
        NSString* propertyName = [NSString stringWithFormat:@"%@%@",firstLetter,[propertyNameUpper substringFromIndex:1]];
        propertyName = [propertyName substringToIndex:[propertyName length] - 1];

        //NSLog(@"Setting property:  name=%@",propertyName);

        [self willChangeValueForKey:propertyName];
        [self setPrimitiveValue:obj forKey:propertyName];
        [self didChangeValueForKey:propertyName];

    }
    else {
        //call back on the correct thread
        NSMethodSignature* sig = [self methodSignatureForSelector:@selector(recallDynamicSetter:withObject:)];
        NSInvocation* call = [NSInvocation invocationWithMethodSignature:sig];
        [call retainArguments];
        call.target = self;
        call.selector = @selector(recallDynamicSetter:withObject:);
        [call setArgument:&_cmd atIndex:2];
        [call setArgument:&obj atIndex:3];

        [self runInvocationOnCorrectThread:call];
    }
}

//bootstrapping the magic; watch for setters and override each one we see
+ (BOOL) resolveInstanceMethod:(SEL)sel {
    NSString* targetSel = NSStringFromSelector(sel);
    if ([targetSel startsWith:@"set"] && ! [targetSel contains:@"Primitive"]) {
        NSLog(@"Overriding selector:  %@",targetSel);
        class_addMethod([self class],(IMP)dynamicSetter,"v@:@");
        return YES;
    }

    return [super resolveInstanceMethod:sel];
}

这与我的线程安全上下文实现一起解决了问题,让我想到了;一个线程安全的环境,我可以传递给任何我想要的人,而不必担心后果.

当然这不是一个防弹解决方案,因为我已经确定了至少以下限制：

/* Also note that using this tool carries several small caveats:
 *
 *      1.  All entities in the data model MUST inherit from 'ThreadSafeManagedObject'.  Inheriting directly from 
 *          NSManagedObject is not acceptable and WILL crash the app.  Either every entity is thread-safe,or none 
 *          of them are.
 *
 *      2.  You MUST use 'ThreadSafeContext' instead of 'NSManagedObjectContext'.  If you don't do this then there 
 *          is no point in using 'ThreadSafeManagedObject' (and vice-versa).  You need to use the two classes together,*          or not at all.  Note that to "use" ThreadSafeContext,all you have to do is replace every [[NSManagedObjectContext alloc] init]
 *          with an [[ThreadSafeContext alloc] init].
 *
 *      3.  You SHOULD NOT give any 'ThreadSafeManagedObject' a custom setter implementation.  If you implement a custom 
 *          setter,then ThreadSafeManagedObject will not be able to synchronize it,and the data model will no longer 
 *          be thread-safe.  Note that it is technically possible to work around this,by replicating the synchronization
 *          logic on a one-off basis for each custom setter added.
 *
 *      4.  You SHOULD NOT add any additional @dynamic properties to your object,or any additional custom methods named
 *          like 'set...'.  If you do the 'ThreadSafeManagedObject' superclass may attempt to override and synchronize 
 *          your implementation.
 *
 *      5.  If you implement 'awakeFromInsert' or 'awakeFromFetch' in your data model class(es),thne you MUST call 
 *          the superclass implementation of these methods before you do anything else.
 *
 *      6.  You SHOULD NOT directly invoke 'setPrimitiveValue:forKey:' or any variant thereof.  
 *
 */

然而,对于大多数典型的中小型项目,我认为线程安全数据层的好处明显超过了这些限制.