Objective C - Passing data between threads

Question

So I have a chunk of code that does some magic with a query and gives me a variable called JSONresult which has been told to run in it's own thread. Once I get this result, I would like take that variable from that thread and make it so that when an IBAction method is called on (triggerd by a button press) if data in the variable JSONresult exists that it will display it in an NSTextView.

So far I've been able to get the data to appear in an NSTextView if I specify the data manually within the IBAction method, but trying to use the JSONresult variable doesn't work for obvious reasons.

Here is what I've got so far.

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    [NSThread detachNewThreadSelector:@selector(getJSONquery) toTarget:self withObject:nil];
}

-(void)getJSONquery {
    [[NSThread currentThread] setName:@"Get JSON Query"];

    //Some fancy stuff here

    NSArray *JSONresult = [string componentsSeparatedByString:@"<|...|>"];
}

After all this has happened and I've now got my JSONresult I have an IBAction setup so that if the button is pushed, the JSONresult will be taken and displayed in a NSTextView

-(IBAction)showContent:(id)sender {
    [content setString:JSONresult];
}

How would I go about passing the variable JSONresult between the two threads/methods?

Thanks in advance! :)

Also if I should update this post with the header file please let me know. I'm new to the world of Objective C and have a very bad understanding of the general how-to so I don't know whether it's relevant to this situation or not

possible duplicate of [Passing Data Between Threads](http://stackoverflow.com/questions/4646264/passing-data-between-threads) — JensG, May 12 '14 at 11:50

score 1 · Answer 1 · answered May 12 '14 at 10:17

1

You don't really pass data between threads, as all threads have access to the same data within a process, however you need synchronize access to the data so that multiple threads don't trample over the shared data, which can lead to its corruption.

In your case, however I don't think their is much synchronization required so your question isn't about passing data between threads, it's more to do with how to make the JSON result available to the button method.

As has already been answered by @Leandros, you would normally do this using a private property:

YourClass.m:

@interface YourClass ()
@property NSArray *jsonResult;     // Note: strong, atomic
@end

...

-(void)getJSONquery {
    [[NSThread currentThread] setName:@"Get JSON Query"];

    //Some fancy stuff here

    self.jsonResult = [string componentsSeparatedByString:@"<|...|>"];
}

answered May 12 '14 at 10:17

trojanfoe

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Why do you use a atomic property? Does it really have to be thread safe? – Leandros May 12 '14 at 10:20
@Leandros Yes, as multiple threads are using the property, however given the setter/getter is merely assigning a pointer it won't make much difference at all. – trojanfoe May 12 '14 at 10:22
Sure, but since I can check if the pointer is set and only one thread is modifying it, I don't think it matters. But I agree, it isn't bad either. – Leandros May 12 '14 at 10:26

Leandros · Answer 2 · 2014-05-12T10:30:52.887

0

You can create a property.

// You can add this to the .h file.
@property (atomic, strong) NSArray *JSONresult;

// Or this above the @implementation in your .m file.
@interface YourClass()
@property (atomic, strong) NSArray *JSONresult;
@end

- (void)getJSONquery {
    [[NSThread currentThread] setName:@"Get JSON Query"];

    //Some fancy stuff here

    self.JSONresult = [string componentsSeparatedByString:@"<|...|>"];
}

- (IBAction)showContent:(id)sender {
    if (self.JSONresult) {
        [content setString:self.JSONresult];
    }
}

edited May 12 '14 at 10:30

answered May 12 '14 at 10:13

Leandros

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Does this go in the .m or the .h file? – SteppingHat May 12 '14 at 10:14
Yes, you can't set a `NSArray` as text to a `UILabel`. – Leandros May 12 '14 at 10:19
When I press the button I get this in the console `2014-05-12 20:17:47.963 JaxWire[13454:303] *** -[NSTextView replaceCharactersInRange:withString:]: nil NSString given.` – SteppingHat May 12 '14 at 10:20
How would I go about converting it? – SteppingHat May 12 '14 at 10:20
You can't really convert an array to a string, since an array holds a number of strings. You can add all strings in the array up, but this is another question. – Leandros May 12 '14 at 10:22
If I were to specify a single key in the array, say `JSONresult[0]` would that work? – SteppingHat May 12 '14 at 10:23
I tried doing within the `(IBOutlet)` method `NSLog(@"%@", self,JSONresult)` and it returns `(null`) – SteppingHat May 12 '14 at 10:27
Well, than your `NSArray` isn't set yet. – Leandros May 12 '14 at 10:28
Check if your property is named `JSONResult` or `JSONresult` (capital R). – Leandros May 12 '14 at 10:29
Inside the `getJSONquery` method if I do `NSLog(@"%@", JSONresult)` it returns some data. It seems that the data isn't being passed between the methods – SteppingHat May 12 '14 at 10:30
There must be an error on your side. Check your variable naming. – Leandros May 12 '14 at 10:31
Since this about sharing values between threads I recommend using `atomic` instead of `nonatomic` for the property. – DarkDust May 12 '14 at 10:32
All the variable names seem to be in line. Perhaps would there be a better way to go about this rather than using separate methods? Essentially all I want is to just display the result into an `NSTextView` once it is generated. – SteppingHat May 12 '14 at 10:40
Ahh. That's way easier to achieve. And something completely different. – Leandros May 12 '14 at 10:41
@SteppingHat Refer to this answer: http://stackoverflow.com/questions/23489830/return-data-after-afnetworking-is-done/23489940#23489940 – Leandros May 12 '14 at 10:42
Awesome I'll have a look and see how I go. Thanks for all your help! :) – SteppingHat May 12 '14 at 10:43

Objective C - Passing data between threads

2 Answers2