CODEWITHSUNDEEP
iosobjective-cblocking
DevilInDisguise
DevilInDisguise

Reputation: 360

logic behind blocks in method parameter

I have a little trouble understanding how this works. I am currently working with a sync table which uses calls like the following:

-(void)addItem:(NSDictionary *)item completion:(CompletionBlock)completion{
[self.syncTable insert:item completion:^(NSDictionary *result, NSError *error) {
    [self LogErrorIfNotNil:error];


}];

I understand how you can use a block as a parameter to execute some extra code within your function, for example with dispatch_async. but when it comes to this line

[self.syncTable insert:item completion:^(NSDictionary *result, NSError *error) {

result is here a dictionary including all extra columns that follows after "item" has been added to the table. Thinking about it,it seems that "result" is more like the resulting type of addItem: instead of being a parameter of the method (due to being the result of the executed method)

EDIT: Basically, I don't understand where the NSDictionary *result variable comes from. To me it seems it should be return type of addItem:item

Upvotes: 0

Views: 57

Answers (1)

timgcarlson
timgcarlson

Reputation: 3146

Here is an explanation of how the above blocks work and in what order.

Completions aren't really return values, perhaps that is what is confusing you. A completion block is a way for you to send code to a method that can be run within the scope of that method.

So when you send a block to a method, this is a possible order of events...

- (void)addItem:(NSDictionary *)item completion:(CompletionBlock)completion {
    NSLog(@"1");
    [self.syncTable insert:item 
                completion:^(NSDictionary *result, NSError *error) {
                    NSLog(@"2");
                }
    ];
    NSLog(@"3"); 
}

// SyncTable.m?
- (void)insert:(NSDictionary *)item completion:(CompletionBlock)completion {
    NSLog(@"4");
    NSDictionary *result = ...;  // Prepare the result dictionary to be used in the completion block.
    NSError *error = nil
    completion(result, &error);
}

The order of these logs will output is as follows...

1
3
4
2

Notice that 2 is not logged until the completion block is called in insert:completion:? Does this clarify the use of parameters in a block?

Upvotes: 1

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