How to call segue programmatic?

Ans : 

For segue first, We have to set identifier in Class Inspector right side in XCode. Through that identifier, we can call like

performSegue(withIdentifier: "identifier", sender: nil) 

What is Content Hugging and Content Compression Resistance Priority?

Ans : 

The priority really come in to play only if two different constraints conflict. The system will give importance to the one with higher priority. So, Priority is the tie-breaker in the autolayout world.

1. Content Hugging Priority : 

Larger the content hugging priority , the views bound will hug to the intrinsic content more tightly preventing the view to grow beyond its intrinsic content size. Setting a larger value to this priority indicates that we don’t want the view to grow larger than its content.

In above example, we set both label leading, trailing, top and bottom but not set width constraint. So here conflicts will occur. You can see red line between two label showing conflict.

Solution : If we have label's content hugging priority higher than there will be no conflicts as hight priority label will not grow its size more than its content size. See below image :

Blue label has more content hugging priority (251) than green label(250). Blue label's width will be set fixed as its content size.

2. Content Compression Resistance : 

Setting a higher value means that we don’t want the view to shrink smaller than the intrinsic content size. It's simple : Higher the priority means larger the resistance to get shrunk.

Example . One button having large name and auto layout is set on button as its width is become 40. So, button's content is not readable. See image :

Button horizontal compression resistance is 750 and width constraint priority is 1000.

For Solution, let's change horizontal compression resistance to 1000 and width constraint priority less than 1000 i.e 999 . Now see effect on below image :

Button horizontal compression resistance is 1000 and width constraint priority is 999.

If you have any comment, question, or recommendation, feel free to post them in the comment section below!

What is closure? Why we use closure instead of function sometime?

Ans : 

The two most used cases are completion blocks and higher order functions in Swift. 

Completion blocks: for example, when you have some time consuming task, you want to be notified when that task is finished. You can use closures for that, instead of a delegate (or many other things)

func longAction(completion: () -> ()) {

    for index in veryLargeArray {

        // do something with veryLargeArray, which is extremely time-consuming

    }

    completion() // notify the caller that the longAction is finished

}

//Or asynch version

func longAction(completion: () -> ()) {

    

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {

        

        for elem in veryLargeArray {

            // do something with veryLargeArray, which is extremely time-consuming

        }

        dispatch_async(dispatch_get_main_queue(), {

            completion() // notify the caller that the longAction is finished

        })

    }

}

longAction { print("work done") }

In the example above, when you have a time consuming task, you want to know when the for loop finishes iterating through the very large array. You put the closure { println("work done") } as an input parameter for the function which will be executed after the for loop finishes its work, and print "work done". And what happened is that you gave a function (closure) to longAction and name it to completion, and that function will be executed when you call completion in longAction.

Sorted method works using closure.

About how sorted (probably) works: So the idea is, that sorted will go through the array, and compare two consecutive elements (i, i + 1) with each other, and swap them, if needed. What does it mean "if needed"? You provided the closure { (s1: String, s2: String) -> Bool in return s1 > s2 }, which will return true if s1 is greater than s2. And if that closure returned true, the sorted algorithm will swap those two elements, and continues this with the next two elements (i + 1, i + 2, if the end of the array is not reached). So basically you have to provide a closure for sorted which will tell "when" to swap to elements.

Understand Closure

Differences between internal testers and external testers in test-flight?

Ans : 

Both internal and external testers will install your app from the TestFlight app. Once invited, they will be sent an email asking them to install the TestFlight app. Once they have done so, they'll be able to install your beta app...

Internal Testers: Think of these users as employees who receive instant updates to your app without approval/review

• Must be added manually via iTC
• 25 Max allowed
• Once your app is uploaded it's available immediately for internal testers (before it has been reviewed)
• All internal testers must be added as a user in your iTC "Users and Roles" settings, which gives them certain permissions (review other answers and the docs for this). You wouldn't want to give just anyone permissions here.
• Do not have a 60-day time limit

External Testers

• Will only be able to use your uploaded build for up to 60 days. If you add additional builds, they can update, and the 60 days starts over again.
• Will be able to test your app after
  1. You have submitted it for review
  2. It gets approved in TestFlight review and
  3. You set it to be available for testing. The review process us usually instant for new builds with the same version number. If you add a new version number, the review process can take up to 48hrs as of 10/2016.
• Up to 2000 email addresses can be added. Each email address will allow a user to install the app on multiple devices. The email addresses do not need to match their Apple IDs.
• They receive an invite to install your app once your first build is available for testing. If you add a new user after making a build available for testing, they'll immediately receive an invite. All users will receive notifications to install newer versions of the app if you upload additional builds.
• Will be disallowed from using your app after you have pushed it to the official app store (which promptly ends the beta) or 60 days have passed since you started the beta, whichever comes first. If you end the beta without launching in the app store, and they try to open it, it will crash. Yay, Apple UX! If you do push a version to the app store with the same bundleName, version, and bundleID (build number doesn't matter), then your beta testers will automatically receive the app-store version of the app when it goes live.

What are certificates and provisioning profile for uploading app to app store?

Ans : 

Certificates - This is a cryptographic certificate granted to you by Apple.  It works just like SSL where you get a certificate signed by an authority.  Apple signs the private key that you use to sign different pieces of your application.  Different certificates create different types of trust.  Some allow you to sign and submit your application for the App Store, while others allow your application's web server to send push notifications to users via APNS.  In the latter case, for instance, Apple uses this certificate to trust the web server sending the push notification.  Otherwise, it would be easy for an attacker to spoof a valid push notification and spam users.  The most common certificate you would create signs the key you use to deploy your application to a device or submit it to the App Store.

When you create a certificate through Apple's developer portal, you have to create your key pair and send a "Certificate Signing Request," which at first is likely pretty confusing to developers just trying to see their application run on a device.
If you visit the developer portal, you'll find you can create certificates for Development or Distribution.  These certificates are rooted to different authorities, so that the two worlds are never confused (though all iOS devices trust both in a seemingly equal fashion).
Provisioning Profiles - Probably the most confusing component in the system, a provisioning profile indicates the devices for which an application is correctly signed.  If you visit the developer portal, you'll notice you can create two types (again called Development and Distribution).  Provisioning profiles say "applications with this Identifier signed with this Certificate's private key are okay to run on these devices."  Now that you know a provisioning profile is tied to a certificate, you can see why you have to decide whether to create a Development or Distribution profile.  Development profiles are limited to 100 devices.  Distribution profiles can either be Ad-Hoc or App Store distribution profiles.  I am not sure whether Ad Hoc profiles have device limits.
You might ask, then, why not always use a Distribution profile?  It can deploy to an unlimited number of devices, and is still attached to a certificate owned by the developer.  Another piece of Apple's security puzzle are Entitlements.  In an iOS application's bundle, you'll find Entitlements.plist, which is a list of capabilities that an application wants.  When signing your application using a certificate intended for distribution, Xcode (really the signing utility) will not allow an entitlement with get-task-allow set to YES.  This is because get-task-allow is what allows a debugger to connect to a process, and Apple doesn't want that happening on apps meant for distribution.

Short Ans : 

1. A Certificate authenticates you as an entity. It can represent you as an individual, or your company.
2. The Identifier is a unique ID for your mobile app.
3. A provisioning profile associates your certificate with the App ID. It is the link between #1 and #2 above.