Question

I recently added threading to an app so that network requests are not blocking the UI. On doing this, I discovered that I could no longer set my instance variable the same way as I had before implementing threading. My instance variable is a property declared as follows:

@property (nonatomic, strong) NSMutableArray *currentTopPlaces;

Here is how I incorrectly set my instance variable self.currentTopPlaces:

dispatch_queue_t downloadQueue = dispatch_queue_create("Flickr Top Places Downloader", NULL);
dispatch_async(downloadQueue, ^{
    __block NSArray *topPlaces = [FlickrFetcher topPlaces];
    dispatch_async(dispatch_get_main_queue(), ^{
        self.tableRowCount = [topPlaces count];
        [[self currentTopPlaces] setArray:topPlaces];
    });

Using [self currentTopPlace] setArray:topPlaces] worked fine in the blocking version, before I started using GCD.

Now, I must set it like so for things to work correctly:

dispatch_queue_t downloadQueue = dispatch_queue_create("Flickr Top Places Downloader", NULL);
dispatch_async(downloadQueue, ^{
    __block NSArray *topPlaces = [FlickrFetcher topPlaces];
    dispatch_async(dispatch_get_main_queue(), ^{
        self.tableRowCount = [topPlaces count];
        self.currentTopPlaces = topPlaces;
    });

Can someone explain to me the difference between using:

[[self currentTopPlaces] setArray:topPlaces];

and:

self.currentTopPlaces = topPlaces;

Specifically, why the “setArray” call did not work in a threaded block?

I thought dot notation in Objective-C is syntactic sugar and not mandatory. I would like to know the “non-sugared” way to achieve the same behavior.

Answer 1

[self currentTopPlaces] and self.currentTopPlaces are in fact identical, but

[self.currentTopPlaces setArray:topPlaces]; // (1)
self.currentTopPlaces = topPlaces; // (2)

are not. (1) replaces all elements of self.currentTopPlaces with those from topPlaces. (2) assigns a new value to self.currentTopPlaces (releasing the old value if it was not nil).

A difference occurs if self.currentTopPlaces is nil: (1) does nothing, because the setArray: method is sent to nil. (2) assigns a new value to self.currentTopPlaces.

Btw: The __block modifier is not necessary in your code, because the block will not change the value of topPlaces.