questions/problems while implementing reusable thread-safe core data pattern

Question

I'm having trouble implementing the thread-safe core data concepts outlined in this tutorial. My goal is to have a reusable portion of code that can take arguments in, do core data operations (adds, updates, deletes) and then callback asynchronously when done.

So heres the block that 'safely' modifies core data objects:

+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock
{
    NSManagedObjectContext *context = [NSManagedObjectContext context];
    [context setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
    [defaultContext setMergePolicy:NSMergeObjectByPropertyStoreTrumpMergePolicy];
    [defaultContext observeContext:context];                    
 
    block(context);                                     
    if ([context hasChanges])                               
    {
        [context save];
    }
} 

From the way I understand it, this executes a block of code? I don't understand how the 'context' being passed in figures in. Is this part of the block's signature?

So here is the wrapper that does the operation in the background and adds a completion call:

+ (void)saveDataInBackgroundWithContext:(void(^)(NSManagedObjectContext *context))saveBlock completion:(void(^)(void))completion
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        [self saveDataInContext:saveBlock];

        dispatch_sync(dispatch_get_main_queue(), ^{
            completion();
        });
    });
}

Here is an example using it:

NSArray *listOfPeople = ...;
[NSManagedObjectHelper saveDataInBackgroundWithContext:^(NSManagedObjectContext *localContext){
    for (NSDictionary *personInfo in listOfPeople)
    {
        PersonEntity *person = [PersonEntity createInContext:localContext];
        [person setValuesForKeysWithDictionary:personInfo];
    }
} completion:^{
    self.people = [PersonEntity findAll];
}];

What is the 'localContext' passed in here? I think most of my issues here revolve around not understanding blocks.

Answer 1

A brief look at that tutorial shows it is talking about magical record. I have never used it, so I can't speak for it.

// This declares a class method that returns void and takes a block as parameter.
// The block returns void and has one parameter, namely, a pointer to an
// NSManagedObjectContext object.
+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock;

You would call that method like this...

[SomeClass saveDataInContext:^(NSManagedObjectContext *context){
    // Some code
}];

That means you are passing in a block of code to the function. At some point it will execute the bock of code you gave it. When it does, it's going to pass a managed object context into the block so it can do something with it.

Now, look at the implementation of that method...

+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock
{
    // Create a MOC - note there is no concurrency type, so it will get
    // NSConfinementConcurrencyType, which means it must be used exclusively
    // from the thread in which it was created.  Since it is a local variable
    // and it gets destroyed after this function is called, that should be cool
    // PROVIDED the using block does not do anything untoward with it.
    NSManagedObjectContext *context = [NSManagedObjectContext context];

    // Set the merge policy
    [context setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];

    // MR must set some default context...
    // Some how the above context needs a persistent store to save...
    [defaultContext setMergePolicy:NSMergeObjectByPropertyStoreTrumpMergePolicy];

    // Probably setting up notification handler for DidSave
    [defaultContext observeContext:context];                    


    // Now, this is where the block you passed in gets called.
    // Note, that the managed object context has already been setup for you.
    // Now that it's setup, the block of code that you passed in is going
    // to be called, and it will be given a context that it can use to execute
    // code in the calling thread.
    block(context);                 

    // If you changed something to the context in your block of code, the save.
    if ([context hasChanges])                               
    {
        [context save];
    }
} 

Let's revisit a our code that called this method...

[SomeClass saveDataInContext:^(NSManagedObjectContext *context){
    // Now, the saveDataInContext method has been called.  However, inside
    // that method, a call was made to the block that was passed in.
    // That would be this here block of code.  So, if you look up in
    // the method, where is calls "block(context)" this block of code will
    // be executed right there.  Mentally, you can cut and paste this code
    // in that spot.
    // The context parameter is the context that was passed to this block.
    // you can use it to do any Core Data stuff...
}];

Now, this code is very similar, but it takes two blocks. One is used to execute some code on the context, and the other is a block that will get executed with the asynchronous save has completed.

saveBlock should be familiar. It's the same concept as in the above example.

completion is a block, that returns void, and takes not parameters. It will get called when all the work has been done.

+ (void)saveDataInBackgroundWithContext:(void(^)(NSManagedObjectContext *context))saveBlock completion:(void(^)(void))completion
{
    // Dispatch some work on one of the global concurrent queues.  It will
    // get done on some thread, nobody knows which one, but it does not matter
    // because the code in this block calls saveDataInContext, and passes the
    // block it was given that does some modifications to the context.
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        [self saveDataInContext:saveBlock];

        // Now, when the above work is done, we are still running in some random
        // thread.  I guess the library wants all callbacks to happen on the main
        // thread, so this block is dispatched on the main thread.  Note that it
        // calls the second bock passed in as the completion block.
        // So the <saveBlock> will be run on some random thread, and then, when
        // it is done, the <completion> block will be called on the main thread.
        dispatch_sync(dispatch_get_main_queue(), ^{
            completion();
        });
    });
}

Like earlier, when you call that method, you can mentally replace the with the first block you pass in, and replace with the second block.

[NSManagedObjectHelper saveDataInBackgroundWithContext:^(NSManagedObjectContext *localContext){
    // This is the first block.  It gets executed where you see <saveBlock>
    // being used in the earlier method.  You are being given the already
    // prepared MOC, and it's name is <localContext>.  Do your managed object
    // context stuff with it.  Note that it will be running in some unknown thread.
    for (NSDictionary *personInfo in listOfPeople)
    {
        PersonEntity *person = [PersonEntity createInContext:localContext];
        [person setValuesForKeysWithDictionary:personInfo];
    }
} completion:^{
    // Now, this is the second block, which is run when all the core data saving
    // has been completed.  It will run on the main thread.
    self.people = [PersonEntity findAll];
}];

Hopefully, that helps you understand what is happening, even though I don't know what magical record is really doing under the covers.

EDIT

In response to this comment...

I don't think I understand how these blocks work. If a block has this method signature "+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock" why is the block NOT using "context" or "saveBlock"? Which is the return value for a block and which is the passed in value? – Mike S

First, the block does not have this signature...

+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock;

That is a class method. Let's break it down piece by piece. First, however, let's forget the block parameter, and use something easy, for comparison.

+ (void)foo:(NSInteger)someInteger;

That is a is a class method, foo:, which returns void and takes one argument. The type of that single argument is NSInteger. If I wanted to call it, I would do so like this:

[SomeClass foo:42];

Likewise...

+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock;

is a class method, saveDataInContext:, which returns void and takes one argument. The type of that single argument is void(^)(NSManagedObjectContext *context).

Now, don't let that gobbly-dee-gook fool you. It's just a type (albeit a somewhat confusing one to parse if you don't understand much C) So, what is void(^)(NSManagedObjectContext *context)

First, it is a block. If that (^) after the void were (*) it would be a function pointer.

Basically, it means that the type of that argument is a block that returns void and has one parameter, namely a pointer to a NSManagedObjectContext (with a name context).

So, if we read it out loud...

+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock;

is a class method, with selector saveDataInContext: which returns void and has one parameter, which has a name saveBlock and is of the type "block that returns void and has one parameter of type NSManagedObjectContext *."

Just like we call the first example like this...

[SomeClass foo:42];

we call the latter example like this...

[SomeClass saveDataInContext:^(NSManagedObjectContext *context){
    // We are creating a bock of code, so stuff some code in here.
}];

Now, just like you passed the integer 42 to foo: you are passing the block in between the {} as the argument to saveDataInContext:.

Now, note that the signature of thesaveDataInContext: method wants a block that itself has a parameter. So, when you provide your block, you are basically saying, "Hey, here is a block of code for you to call, and when you do so, make sure you give me a pointer to a NSManagedObjectContext object that I can use.

What this means is that when your block is called, the calling code will call your block and provide a NSManagedObjectContext * to you with the variable name context.

Think of it like this, as a trivial example of saveDataInContext:.

+ (void)saveDataInContext:(void(^)(NSManagedObjectContext *context))saveBlock {
    // Create a context to give the block we are going to call..
    NSManagedObjectContext *moc = //       
    saveBlock(moc);
}

Now, when your code is called, you will get the moc object as your argument. Bascially, that method creates a managed object context, does all the thread safety stuff, then calls your block of code, and gives you a pointer to the managed object context that it has safely created. Your code is executed within the confines of that safe environment, using the MOC passed to it as a function (block) parameter.

I hope that didn't make it worse...