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Anna
Anna

Reputation: 955

Display UI, from a background thread without breaking the flow

I have a code executing in the background thread, which is performing some kind of computation and is within a do-while loop. Due to some changes in the requirements, I have to display a UI to prompt user for input. This UI code will have to be done in the main thread, and after the prompt is entered, the logic needs to continue. Using a dispatch_async on main thread, I can display the UI, but Step -2 should not continue, until the UI is done. What is the best way to accomplish this, without breaking the flow of the code and moving units into blocks?

For example:

-(void) compute
{
    do 
    {
        //calculate some data
        // Step -1...
        ...
        // Step -2
         ...
         ...
     } while(flag)
}

Between Step 1 and Step 2, I want to display a prompt. What is the best way to do so? Is it okay, to block this background thread using a mutex, which will get fired, by the main thread after the UI is done?

Upvotes: 0

Views: 167

Answers (2)

Ben Baron
Ben Baron

Reputation: 14815

For this I would use GCD (Grand Central Dispatch). You can easily execute a block synchronously on the main thread (or asynchronously if you prefer), using dispatch_sync (or dispatch_async). I personally use my wrapper class EX2Dispatch in my EX2Kit library (https://github.com/einsteinx2/EX2Kit), but it's the same thing.

As an example, you would do something like this:

dispatch_sync(dispatch_get_main_queue(), ^{
    // Do some stuff to the UI
};

EDIT:

I was reading it as needing to display information to the user based on the earlier calculation, but if you need a response from the user before continuing, then the loop needs to break after showing the alert, then be called again.

You could use an instance variable to track how far into the loop you are, so that it can be resumed at the same point in the UIAlertView's button clicked delegate method.

Upvotes: 1

iAppDeveloper
iAppDeveloper

Reputation: 1088

Try this

dispatch_queue_t mainQueue = dispatch_get_main_queue(); 
dispatch_async(mainQueue, ^(void) {

    //do stuff here

    });

Upvotes: 0

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