Reader Submissions - New Year's 2016

Written by Nate Cook January 4^th, 2016

With 2015 behind us and the new year begun, it’s time again for an NSHipster tradition: reader submissions! As in year’s past, this installment is chock full of tips and tricks that can help ease your days working with Xcode, Swift, and Objective-C.

Many thanks to Cédric Luthi, Josip Ćavar, Juraj Hilje, Kyle Van Essen, Luo Jie, Mathew Huusko V, Nicolás Jakubowski, Nolan O’Brien, Orta Therox, Ray Fix, Stephen Celis, Taylor Franklin, Ursu Dan, Matthew Flint, @biggercoffee, and @vlat456 for their contributions!

Swift’s `defer` in Objective-C

From Nolan O’Brien:

With the excellent addition of defer to Swift we Objective-C holdouts can’t help but feel envious of the improvements happening so rapidly to the Swift language. Until Apple officially adds @defer devs can actually implement defer support simply enough with a macro in Objective-C. Below I’ve outlined how one can go about doing a defer today in Objective-C. Personally, having Apple add @defer seem like an easy win for Objective-C, but we’ll see what happens. :)

Nolan’s macro uses the GCC (cleanup()) attribute to execute a block when scope exits:

// some helper declarations
#define _nob_macro_concat(a, b) a##b
#define nob_macro_concat(a, b) _nob_macro_concat(a, b)
typedef void(^nob_defer_block_t)();
NS_INLINE void nob_deferFunc(__strong nob_defer_block_t *blockRef)
{
    nob_defer_block_t actualBlock = *blockRef;
    actualBlock();
}

// the core macro
#define nob_defer(deferBlock) \
__strong nob_defer_block_t nob_macro_concat(__nob_stack_defer_block_, __LINE__) __attribute__((cleanup(nob_deferFunc), unused)) = deferBlock

Blocks used with nob_defer are executed in reverse order, just like Swift defer statements:

#include <nob_defer.h>

- (void)dealWithFile
{
    FILE *file = fopen(…);
    nob_defer(^{
        if (file) {
            fclose(file);
        }
    });

    // continue code where any scope exit will
    // lead to the defer being executed
}

- (void)dealWithError
{
    __block NSError *scopeError = nil;
    nob_defer(^{
        if (scopeError) {
            [self performCustomErrorHandling: scopeError];
        }
    });

    // assign any errors to "scopeError" to handle the error
    // on exit, no matter how you exit
}

#define NOBDeferRelease(cfTypeRef) nob_defer(^{ if (cfTypeRef) { CFRelease(cfTypeRef); } })

- (void)cleanUpCFTypeRef
{
    CFStringRef stringRef = ... some code to create a CFStringRef ...;
    NOBDeferRelease(stringRef);

    // continue working without having to worry
    // about the CFTypeRef needing to be released
}

I’ve been using my custom defer macro in production code since June and it is really the Bee’s Knees!

Swift Protocol Extensions

From Juraj Hilje:

Keep inheritance trees shallow and use protocol composition:

protocol Hello {
    func sayHello() -> String
}

extension Hello {
    func sayHello() -> String {
        return "Hello, stranger"
    }
}

class MyClass: Hello {
}

let c = MyClass()
c.sayHello() // "Hello, stranger"

Public Read-only Variables

From Stephen Celis:

Classes commonly have public, read-only properties but need the ability privately modify them. I’ve come across the following pattern a few times:

public class Person {
    public var name: String {
        return _name
    }
    private var _name: String
    …
}

Luckily, there’s a better, oft-overlooked way that avoids the extra variable:

public class Person {
    public private(set) var name: String
    …
}

Swift `where` Everywhere

From Taylor Franklin:

The addition of the where clause has made my code simple and compact while remaining readable. In addition, it has a broad application in Swift, so that it can be applied in nearly any kind of control-flow statement, such as for loop, while loop, if, guard, switch, and even in extension declarations. One simple way I like to use it is in my prepareForSegue method:

if let segueID = segue.identifier where segueID == "mySegue" {
    ...
}

The combo of unwrapping and performing a condition check is most commonly where I use the where clause. The where clause is not going to change your life, but it should be an easy and useful addition to your Swift skills.

Improved Optional Binding

From Ursu Dan:

The improved optional binding in Swift is amazing and I use it virtually everywhere now and avoid the pyramid of doom:

if let
    url          = NSBundle.mainBundle().URLForResource("users", withExtension: "json"),
    data         = NSData(contentsOfURL: url),
    deserialized = try? NSJSONSerialization.JSONObjectWithData(data, options: []),
    userList     = deserialized as? [ [String: AnyObject] ]
{

    for userDict in userList {
        if let
            id          = userDict["id"] as? Int,
            name        = userDict["name"] as? String,
            username    = userDict["username"] as? String,
            email       = userDict["email"] as? String,
            phone       = userDict["phone"] as? String,
            address     = userDict["address"] as? [String: AnyObject]
        {
            users.append(User(id: id, name: name, ...))
        }
    }
}

Unbuffered `xcodebuild` Output

From Cédric Luthi:

Using xcpretty because the output of xcodebuild test is unreadable? Unfortunately, the output of the test results becomes buffered when piped. Solution: set the NSUnbufferedIO environment variable for a smooth experience. 😎

env NSUnbufferedIO=YES xcodebuild [flags] | xcpretty

Multiline Labels in a Table View

From Ray Fix:

Using autolayout to toggle a label in a table view from one line to many:

tableView.beginUpdates()
label.numberOfLines = label.numberOfLines == 0 ? 1 : 0
tableView.endUpdates()

You can see Ray’s technique in action in an example project:

Multiline demo

`AmIRunningAsAnExtension`

Another from Nolan O’Brien:

With extensions in iOS, it is critical that frameworks that can be linked to both extensions and apps be cognizant of their uses so they don’t call any APIs that might not be available to an extension (like UIApplication). Here’s a function to help determine if you are running in an extension at runtime:

(Per Apple, an extension will have a top level “NSExtension” dictionary in the info.plist.)

let NOBAmIRunningAsAnExtension: Bool = {
    let extensionDictionary: AnyObject? = NSBundle.mainBundle().infoDictionary?["NSExtension"]
    return extensionDictionary?.isKindOfClass(NSDictionary.self) ?? false
}()

FOUNDATION_EXTERN BOOL AmIRunningAsAnExtension() __attribute__((const));

BOOL NOBAmIRunningAsAnExtension()
{

    static BOOL sIsExtension;
    static dispatch_once_t sOnceToken;
    dispatch_once(& sOnceToken, ^{
        NSDictionary *extensionDictionary = [[NSBundle mainBundle] infoDictionary][@"NSExtension"];
        sIsExtension = [extensionDictionary isKindOfClass:[NSDictionary class]];
    });
    return sIsExtension;
}

That frees you to do things like this:

- (void)startBackgroundTask
{
#if TARGET_OS_IPHONE
    if (!NOBAmIRunningAsAnExtension()) {
        Class UIApplicationClass = NSClassFromString(@"UIApplication");
        id sharedApplication = [UIApplicationClass sharedApplication];
        self.backgroundTaskIdentifier = [sharedApplication beginBackgroundTaskWithExpirationHandler:^{
            if (self.backgroundTaskIdentifier != UIBackgroundTaskInvalid) {
                [sharedApplication endBackgroundTask:self.backgroundTaskIdentifier];
                self.backgroundTaskIdentifier = UIBackgroundTaskInvalid;
            }
        }];
    }
#endif
}

- (void)endBackgroundTask
{
#if TARGET_OS_IPHONE
    if (self.backgroundTaskIdentifier != UIBackgroundTaskInvalid) {
        NSAssert(!NOBAmIRunningAsAnExtension());
        Class UIApplicationClass = NSClassFromString(@"UIApplication");
        id sharedApplication = [UIApplicationClass sharedApplication];
        [sharedApplication endBackgroundTask:self.backgroundTaskIdentifier];
        self.backgroundTaskIdentifier = UIBackgroundTaskInvalid;
    }
#endif
}

Beyond Breakpoints

From Matthew Flint:

This has been my year of truly appreciating Xcode breakpoints, beyond the standard “break at this line” type. It breaks my heart to see other developers not using them to their potential.

Right click on a breakpoint and choose Edit Breakpoint… for access to advanced features:

Breakpoint Options Popup

Particularly the ones with actions (such as logging to the console) that continue automatically without pausing any threads, because you can add/edit them without recompiling. I’ll never accidentally commit NSLogs again. :)

Fix Console `po frame` Printing

GitHub user @biggercoffee reminds us that po frame printing fails in the LLDB console by default:

Broken po frame

Fix it mid-debugging session with expr @import UIKit, or fix it once and for all by adding a couple lines to your “.lldbinit”. From the command line:

touch ~/.lldbinit
echo display @import UIKit >> ~/.lldbinit
echo target stop-hook add -o \"target stop-hook disable\" >> ~/.lldbinit

Fixed po frame

Avoiding `-DDEBUG` in Swift

From GitHub user @vlat456:

For those, who like me, are trying to avoid the mess with -DDEBUG in Swift, but have to know which version of executable is running, Debug or Release.

// PreProcessorMacros.m:
#include "PreProcessorMacros.h"
#ifdef DEBUG
BOOL const DEBUG_BUILD = YES;
#else
BOOL const DEBUG_BUILD = NO;
#endif

// PreProcessorMacros.h:
#ifndef PreProcessorMacros_h
#define PreProcessorMacros_h
#include

extern BOOL const DEBUG_BUILD;

#endif /* PreProcessorMacros_h */

// in Bridged header:
#import "PreProcessorMacros.h"

And then from Swift:

if DEBUG_BUILD {
debugPrint("It's Debug build")
} else {
debugPrint("It's Release build")
}

Checking For Null Blocks

From Nicolás Jakubowski:

This macro for checking block nullability before executing them:

#define BLOCK_EXEC(block, ...) if (block) { block(__VA_ARGS__); };

Old and busted:

if (completionBlock)
{
    completionBlock(arg1, arg2);
}

New and shiny:

BLOCK_EXEC(completionBlock, arg1, arg2);

Swiftier GCD

From Luo Jie:

You can use enums and protocol extensions to provide a GCD convenience API:

protocol ExcutableQueue {
    var queue: dispatch_queue_t { get }
}

extension ExcutableQueue {
    func execute(closure: () -> Void) {
        dispatch_async(queue, closure)
    }
}

enum Queue: ExcutableQueue {
    case Main
    case UserInteractive
    case UserInitiated
    case Utility
    case Background

    var queue: dispatch_queue_t {
        switch self {
        case .Main:
            return dispatch_get_main_queue()
        case .UserInteractive:
            return dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0)
        case .UserInitiated:
            return dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)
        case .Utility:
            return dispatch_get_global_queue(QOS_CLASS_UTILITY, 0)
        case .Background:
            return dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0)
        }
    }
}

enum SerialQueue: String, ExcutableQueue {
    case DownLoadImage = "myApp.SerialQueue.DownLoadImage"
    case UpLoadFile = "myApp.SerialQueue.UpLoadFile"

    var queue: dispatch_queue_t {
        return dispatch_queue_create(rawValue, DISPATCH_QUEUE_SERIAL)
    }
}

Downloading something then could be written like this:

Queue.UserInitiated.execute {
    let url = NSURL(string: "http://image.jpg")!
    let data = NSData(contentsOfURL: url)!
    let image = UIImage(data: data)

    Queue.Main.execute {
        imageView.image = image
    }
}

`_ObjectiveCBridgeable`

From Mathew Huusko V:

Using Swift’s _ObjectiveCBridgeable (implicit castability between types) to create a generic protocol for Obj-C compatible objects that wrap pure Swift structs (keeping Swift framework API clean, but Obj-C compatible for as long as desired).

This first part defines an extension with default implementations for the bridging bookkeeping methods:

public protocol BackedObjectType: AnyObject {
    typealias Backing

    var backingObject: Backing { get }

    init(_ backingObject: Backing)
}

public protocol ObjectBackable: _ObjectiveCBridgeable {
    typealias Backed: BackedObjectType
}

public extension ObjectBackable where Backed.Backing == Self {
    static func _isBridgedToObjectiveC() -> Bool {
        return true
    }

    static func _getObjectiveCType() -> Any.Type {
        return Backed.self
    }

    func _bridgeToObjectiveC() -> Backed {
        return Backed(self)
    }

    static func _forceBridgeFromObjectiveC(source: Backed, inout result: Self?) {
        result = source.backingObject
    }

    static func _conditionallyBridgeFromObjectiveC(source: Backed, inout result: Self?) -> Bool {
        _forceBridgeFromObjectiveC(source, result: &result)
        return true
    }

    func toBridgedObject() -> Backed {
        return _bridgeToObjectiveC()
    }
}

Here the Swift struct SomeModel and Objective-C class M5SomeModel are declared and bridged. Bridging between them is accomplished with an as cast:

public struct SomeModel {
    public let ID: Int
    public let name: String
    public let category: String
}

extension SomeModel: ObjectBackable {
    public typealias Backed = M5SomeModel
}

@objc public final class M5SomeModel: NSObject, BackedObjectType {
    public let backingObject: SomeModel
    public init(_ backingObject: SomeModel) {
        self.backingObject = backingObject
    }

    public var ID: Int { return backingObject.ID }
    public var name: String { return backingObject.name }
    public var category: String { return backingObject.category }
}

// Usage:
let model = SomeModel(ID: 2, name: "awesome", category: "music")
let objcCompatibleModel = model as M5SomeModel
let originalModel = objcCompatibleModel as SomeModel

Phantom Types

Josip Ćavar writes in about getting additional type safety with phantom types. In the example below, Kilometer and Meter are used to constrain what kinds of DistanceT instances can be added together:

struct Kilometer {}
struct Meter {}

struct DistanceT<T> {
    private let value: Int

    init(value: Int) {
        self.value = value
    }
}

func +<T>(left: DistanceT<T>, right: DistanceT<T>) -> DistanceT<T> {
    return DistanceT(value: left.value + right.value)
}

extension Int {
    var km: DistanceT<Kilometer> {
        return DistanceT<Kilometer>(value: self)
    }
    var m: DistanceT<Meter> {
        return DistanceT<Meter>(value: self)
    }
}

let distanceKilometers = 5.km
let distanceMeters = 15.m
let newDistance = distanceKilometers + distanceKilometers // Ok
let newDistance = distanceKilometers + distanceMeters // Compiler error

Easier Configuration

From Kyle Van Essen by way of Orta Therox comes a function that streamlines multi-step initialization and configuration processes.

@warn_unused_result
public func Init<Type>(value : Type, @noescape block: (object: Type) -> Void) -> Type
{
    block(object: value)
    return value
}

func example()
{
    let label = Init(UILabel()) {
        $0.font = UIFont.boldSystemFontOfSize(13.0)

        $0.text = "Hello, World"
        $0.textAlignment = .Center
    }
}

Well, that rounds out this year’s list—thanks again to all who contributed!

Happy New Year! May your code continue to compile and inspire.

Swift’s defer in Objective-C