Do Swift-based applications work on OS X 10.9/iOS 7 and lower?

I just tested it for you, Swift applications compile into standard binaries and can be run on OS X 10.9 and iOS 7.

Simple Swift application used for testing:

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: NSDictionary?) -> Bool {
    self.window = UIWindow(frame: UIScreen.mainScreen().bounds)

    var controller = UIViewController()
    var view = UIView(frame: CGRectMake(0, 0, 320, 568))
    view.backgroundColor = UIColor.redColor()
    controller.view = view

    var label = UILabel(frame: CGRectMake(0, 0, 200, 21))
    label.center = CGPointMake(160, 284)
    label.textAlignment = NSTextAlignment.Center
    label.text = "I'am a test label"
    controller.view.addSubview(label)

    self.window!.rootViewController = controller
    self.window!.makeKeyAndVisible()
    return true
}

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Swift code can be deployed to OS X 10.9 and iOS 7.0. It will usually crash at launch on older OS versions.

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Apple has announced that Swift apps will be backward compatible with iOS 7 and OS X Mavericks. The WWDC app is written in Swift.

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Update - As per Xcode 6 Beta 4

iOS 7 and OS X 10.9 minimum deployment target The Swift compiler and Xcode now enforce a minimum deployment target of iOS 7 or OS X Mavericks. Setting an earlier deployment target results in a build failure.

From Xcode 6 release note

So my previous answer(Shown below) will not be applicable to any further development. Swift will no longer available for iOS6 and below

A Swift application can be run on iOS 6. Even though many people are saying that Swift will support only iOS 7+ and OS X 10.9+, from my experience it's not.

I have tested a simple application written completely in Swift in an iOS 6 device. It works perfectly fine. As Apple says, Swift code is binary compatible with Objective-C code. It uses the same compiler and runtime to create the binary.

Here is the code I have tested:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {

        super.viewDidLoad()

        let button   = UIButton.buttonWithType(UIButtonType.System) as UIButton
        button.frame = CGRectMake(100, 100, 100, 50)
        button.backgroundColor = UIColor.greenColor()
        button.setTitle("Test Button", forState: UIControlState.Normal)
        button.addTarget(self, action: "buttonTapped:", forControlEvents: UIControlEvents.TouchUpInside)

        self.view.addSubview(button)
    }

    func buttonTapped(sender: UIButton!) {
        println("buttonTapped")
    }
}

It is a simple application, just adding a button programmatically. My application contains only two files, AppDelegate.swift and ViewController.swift.

So if you are not using any new APIs added as part of the iOS 8 SDK or some Swift specific APIs (corresponding API is not available for Objective-C) your application will seamlessly work on iOS 6 or later (tested and working), even on iOS 5 (not tested). Most of the APIs in Swift are just the replacement of the existing Objective-C APIs. In fact they are the same in binary.

Note: As per Xcode 6 beta 4 for swift apps deployment target should be iOS 7 or OS X 10.9(see the above update). So swift will no longer available for iOS6 and below

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In brief:

Swift based applications can target back to OS X Mavericks or iOS 7 with that same app.

How is it possible ?

Xcode embeds a small Swift runtime library within your app’s bundle. Because the library is embedded, your app uses a consistent version of Swift that runs on past, present, and future OS releases.

Why should I trust this answer ?

Because I am not saying this answer as one apple guy told me in twitter or I wrote hello world and tested it.

I took it from apple developer blog.

so you can trust this.

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Swift uses the same runtime as Objective-C and can even live side-by-side Objective-C in the same application (as per the WWDC 2014 keynote).

This will need to be checked/verified using Xcode 6 and the new SDK to find a final answer.

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I read all answers that said: No, Swift does not work with less than iOS 7. But I said YES, I just created a Swift project that does run in Xcode 5 with 6.0 deployment target.

I just create a demo project in Xcode 6 BETA with the Swift programming language selected.

Close Xcode 6 beta, and I open this demo project in Xcode 5 with deployment target 6.0

And also select simulator 6.1.

Then that project runs well in simulator 6.1. My MacOS X is 10.9.3, so I said yes, that runs in lower than iOS 7. with 10.9.3 Mac OS X.

Here it is a screenshot of the simulator:

https://i.stack.imgur.com/jRymf.png

Here is a demo as well

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While code confirms it, Apple itself has said that Swift will be compatible on iOS 7 and Mavericks in their technical keynote (State of the platforms, session 102, around the 34 min 00 sec mark) at WWDC 2014.

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Swift applications are supported on iOS 7 and above as stated in Beta 4 release notes. iOS 6.0, 6.1, 7.0, 7.1, 8.0 in Xcode 6 Beta

Swift applications are supported on platforms OS X 10.9 and above. OS X 10.4 to 10.10 in Deployment Target. I have tested on targeting 10.5 to 10.10, and running on 10.9.3

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It appears Swift applications don't run on OS X 10.7. I just created a simple GUI application (a view, a lable, a button) which runs fine on Mavericks. The base SDK is set to 10.9 and the deployment target to 10.7. I copied that application from the DerivedData folder over to my 10.7 virtual machine, and it crashs when starting, show this error:

Crashed Thread:  0

Exception Type:  EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000002, 0x0000000000000000

Application Specific Information:
dyld: launch, loading dependent libraries

    Dyld Error Message:

  Library not loaded: /System/Library/Frameworks/CoreGraphics.framework/Versions/A/CoreGraphics
  Referenced from: /Users/USER/Desktop/Swift-Test.app/Contents/MacOS/../Frameworks/libswiftAppKit.dylib
  Reason: image not found

Binary Images:
       0x109c65000 -        0x109c6afff +private.Swift-Test (1.0 - 1) <649695D0-58FD-3D02-9176-2D40D4E711F2> /Users/USER/Desktop/Swift-Test.app/Contents/MacOS/Swift-Test
       0x109c83000 -        0x109dbffff +libswift_stdlib_core.dylib (1.0 - 600.0.34.4.5) <10AAC369-9404-321D-A892-49F65856D7AF> /Users/USER/Desktop/Swift-Test.app/Contents/Frameworks/libswift_stdlib_core.dylib
...

However this message is irritating, since there's certainly a CoreGraphics library in this VM. Other applications that make heavy use of CoreGraphics work just fine.

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This is the post I read from apple Swift blog, might be helpful:

App Compatibility:

If you write a Swift app you can trust that your app will work well into the future. In fact, you can target back to OS X Mavericks or iOS 7 with that same app. This is possible because Xcode embeds a small Swift runtime library within your app's bundle. Because the library is embedded, your app uses a consistent version of Swift that runs on past, present, and future OS releases.

Binary Compatibility and Frameworks:

While your app's runtime compatibility is ensured, the Swift language itself will continue to evolve, and the binary interface will also change. To be safe, all components of your app should be built with the same version of Xcode and the Swift compiler to ensure that they work together.

This means that frameworks need to be managed carefully. For instance, if your project uses frameworks to share code with an embedded extension, you will want to build the frameworks, app, and extensions together. It would be dangerous to rely upon binary frameworks that use Swift — especially from third parties. As Swift changes, those frameworks will be incompatible with the rest of your app. When the binary interface stabilizes in a year or two, the Swift runtime will become part of the host OS and this limitation will no longer exist.

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I tested running a bare-bone, Swift-based app on an iPod Touch (3rd gen) device. It appears Swift-based apps don't work with iOS 5.x but do work with iOS 6.x.

Here's what shows up in the debug log when I tried to launch the test app with iOS 5.0.1:

dyld: F_ADDFILESIGS failed for /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswift_stdlib_core.dylib with errno=1
dyld: F_ADDFILESIGS failed for /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswiftCoreGraphics.dylib with errno=1
dyld: F_ADDFILESIGS failed for /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswiftDarwin.dylib with errno=1
dyld: F_ADDFILESIGS failed for /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswiftDispatch.dylib with errno=1
dyld: F_ADDFILESIGS failed for /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswiftFoundation.dylib with errno=1
dyld: F_ADDFILESIGS failed for /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswiftObjectiveC.dylib with errno=1
dyld: F_ADDFILESIGS failed for /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswiftUIKit.dylib with errno=1
dyld: Symbol not found: _OBJC_CLASS_$_NSObject
  Referenced from: /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswift_stdlib_core.dylib
  Expected in: /usr/lib/libobjc.A.dylib
 in /private/var/mobile/Applications/59E31E79-9525-43B0-9DF6-8FEF3C0080F1/SwiftTestApp.app/Frameworks/libswift_stdlib_core.dylib

For iOS 6.1.6, the app runs fine without displaying those error messages.

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There seems to be a lot of old answers here so I just wanted to post the official response from the Swift team. Swift is backwards compatible with OS X Mavericks and iOS 7

Apple developer swift blog : Objective-C id as Swift Any

Jul 11, 2014

Compatibility

One of the most common questions we heard at WWDC was, “What is the compatibility story for Swift?”. This seems like a great first topic.

App Compatibility Simply put, if you write a Swift app today and submit it to the App Store this Fall when iOS 8 and OS X Yosemite are released, you can trust that your app will work well into the future. In fact, you can target back to OS X Mavericks or iOS 7 with that same app. This is possible because Xcode embeds a small Swift runtime library within your app’s bundle. Because the library is embedded, your app uses a consistent version of Swift that runs on past, present, and future OS releases.

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I also tried a very simple app on 10.8 (one button, sets text on a label). It crashed at startup, as Greg Parker stated:

Dyld Error Message:
  Symbol not found: __dispatch_source_type_memorypressure
  Referenced from: /Volumes/*/SwifTest.app/Contents/MacOS/../Frameworks/libswiftDispatch.dylib
  Expected in: /usr/lib/libSystem.B.dylib
in /Volumes/*/SwifTest.app/Contents/MacOS/../Frameworks/libswiftDispatch.dylib

(This was using a deployment target of 10.7)

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Try the following code:

It's working without StoryBoard:

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: NSDictionary?) -> Bool {
    self.window = UIWindow(frame: UIScreen.mainScreen().bounds)
    self.window!.backgroundColor = UIColor.whiteColor()

    // Create a nav/vc pair using the custom ViewController class

    let nav = UINavigationController()
    let vc = ViewController(nibName: "ViewController", bundle: nil)

    // Push the vc onto the nav
    nav.pushViewController(vc, animated: false)

    // Set the window’s root view controller
    self.window!.rootViewController = nav

    // Present the window
    self.window!.makeKeyAndVisible()
    return true
}

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Yes, in fact Apple has announced that Swift apps will be backward compatible with iOS 7 and OS X Mavericks. Furthermore the WWDC app is written in the Swift programming language.

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When it comes to Swift Frameworks. As for today, with Xcode version 6.1.1 (6A2008a), if the Swift framework is targeted to iOS 7.1, linker report warning

ld: warning: embedded dylibs/frameworks only run on iOS 8 or later.

and application can't be submitted to AppStore. Check this issue : Lint to prevent dynamic libraries and frameworks from passing with iOS 7

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Quick Update, effective from February 15th, 2015, we cannot submit apps to the store that were developed using an SDK prior to iOS 8. So, keeping that in mind , its better to not to worry about this issue as many people have suggested that apps made in Swift can be deployed to OS X 10.9 and iOS 7.0 as well.

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Answered code-snippet posted by Leandros seems bit old. I have fixed and made it compilable in Swift 5.

Swift 5

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: NSDictionary?) -> Bool {
    self.window = UIWindow(frame: UIScreen.main.bounds)
    let controller = UIViewController()
    let view = UIView(frame: CGRect(x: 0, y: 0, width: 320, height: 568))
    view.backgroundColor = UIColor.red
    controller.view = view

    let label = UILabel(frame: CGRect(x: 0, y: 0, width: 200, height: 21))
    label.center = CGPoint(x: 160, y: 284)
    label.textAlignment = NSTextAlignment.center
    label.text = "I'am a test label"
    controller.view.addSubview(label)

    self.window!.rootViewController = controller
    self.window!.makeKeyAndVisible()
    return true
}

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