Wait unitil a loop with asynchronous tasks finishes executing

A completion Handler in iOS takes a thread to asynchronously get it done. So it means that we can’t know an exact point which the tasks finish. For example, if we use a for loop to execute several tasks in the background, how could we know all the task are done?

In this case you can use dispatch_group.

1. Create a dispatch group which graps all the tasks in a for loop with dispatch_group_create().
2. You point out a position which your code enters the dispatch group with dispatch_group_enter(groupname).
3. You point out a position which your code leave the dispatch group with dispatch_group_leave(groupname).
4. Lastly you can get a notification when all the tasks you assigned to a dispatch group is done with dispatch_group_notify.

If you want to see a concrete example for it, please refer this.

Read More

The latest Swift's Regular Expression Tutorial

Recently I needed to extract a data from Apple’s HealthKit. They provide various queries in HealthKit framework. But it didns’t meet my intention.
So I started to use NSString substring function to extract the data. But, it wasn’t good to match the data that I want.
Finally I got the exact result by using NSRegularExpression and during the process this tutorial was really helpful.

And I think that this is the latest NSRegularExpression tutorial on the web.

So if you want to learn how to use a regular expression in Swift, I recommend the link above.

Read More

Let's learn Set in Swift language

Set is different from Array. Array is ordered. By the way Set isn’t. Another different thing is Array can have same elements.

For example,

let ar1 = [1, 1, 1, 1] 

The code above is possible in Array. But you can’t do it in Set.

Set could be similar to Dictionary. Because each element in Set must be unique like Dictionary’s key value. Set conforms Hashable protocol in order to have unique elements.

The different thing between Set and Dictionary is that Set doesn’t have Key value.

Let me organize everything above.

Collection Type	Ordered	Unique	Stores
Array	Yes	No	Elements
Dictionary	No	Keys	Key-value pairs
Set	No	Elements	Elements

Read More