Add a polyfill for Objective-C to select TZStackView only when UIStackView is Unavailable

Polyfill/UIStackView+TZStackView.h

@@ -0,0 +1,100 @@
+//
+//  UIStackView+TZStackView.h
+//
+//  Created by Frederic Barthelemy on 9/9/15.
+//
+
+#import <UIKit/UIKit.h>
+
+// You probably want the real UIStackView!
+#import <UIKit/UIStackView.h>
+
+// Define a preprocessor macro to coopt all your references to UIStackView with this class.
+#define UIStackView _polyfill_UIStackView
+
+/**
+ This class is a polyfill. Go look at UIStackView.h or TZStackView.swift for implementation details.
+
+ Usage: Include this in your Prefix.pch, and use UIStackView * everywhere as if you were on iOS 9.0
+
+ This class works by overriding +(instancetype)alloc to dynamically alloc the appropriate class.
+ Additionally it defines a PreProcessor macro that will replace all your uses of UIStackView with this class.
+
+ @warning: Dynamic tricks like NSStringFromClass([UIStackView class]) will probably not do what you want. Also, the only supported class method is `alloc`.
+
+ All methods defined here are simply stubs to silence the compiler, since you can't ever alloc an instance of this.
+ */
+@interface _polyfill_UIStackView : UIView
+
+#pragma mark - Everything below here is copied in from UIStackView.h -
+
+/* UIStackView enforces that all views in the arrangedSubviews list
+ must be subviews of the UIStackView.
+ Thus, when a view is added to the arrangedSubviews, UIStackView
+ adds it as a subview if it isn't already. And when a view in a
+ UIStackView's arrangedSubviews list receives -removeFromSuperview
+ it is also removed from the arrangedSubviews.
+ */
+- (instancetype)initWithArrangedSubviews:(NSArray<__kindof UIView *> *)views; // Adds views as subviews of the receiver.
+@property(nonatomic,readonly,copy) NSArray<__kindof UIView *> *arrangedSubviews;
+
+/* Add a view to the end of the arrangedSubviews list.
+ Maintains the rule that the arrangedSubviews list is a subset of the
+ subviews list by adding the view as a subview of the receiver if
+ necessary.
+ Does not affect the subview ordering if view is already a subview
+ of the receiver.
+ */
+- (void)addArrangedSubview:(UIView *)view;
+
+/* Removes a subview from the list of arranged subviews without removing it as
+ a subview of the receiver.
+ To remove the view as a subview, send it -removeFromSuperview as usual;
+ the relevant UIStackView will remove it from its arrangedSubviews list
+ automatically.
+ */
+- (void)removeArrangedSubview:(UIView *)view;
+/*
+ Adds the view as a subview of the container if it isn't already.
+ Updates the stack index (but not the subview index) of the
+ arranged subview if it's already in the arrangedSubviews list.
+ */
+- (void)insertArrangedSubview:(UIView *)view atIndex:(NSUInteger)stackIndex;
+
+/* A stack with a horizontal axis is a row of arrangedSubviews,
+ and a stack with a vertical axis is a column of arrangedSubviews.
+ */
+@property(nonatomic) UILayoutConstraintAxis axis;
+
+/* The layout of the arrangedSubviews along the axis
+ */
+@property(nonatomic) UIStackViewDistribution distribution;
+
+/* The layout of the arrangedSubviews transverse to the axis;
+ e.g., leading/trailing edges in a vertical stack
+ */
+@property(nonatomic) UIStackViewAlignment alignment;
+
+/* Spacing between adjacent edges of arrangedSubviews.
+ Used as a strict spacing for the Fill distributions, and
+ as a minimum spacing for the EqualCentering and EqualSpacing
+ distributions. Use negative values to allow overlap.
+ */
+@property(nonatomic) CGFloat spacing;
+
+/* Baseline-to-baseline spacing in vertical stacks.
+ The baselineRelativeArrangement property supports specifications of vertical
+ space from the last baseline of one text-based view to the first baseline of a
+ text-based view below, or from the  top (or bottom) of a container to the first
+ (or last) baseline of a contained text-based view.
+ This property is ignored in horizontal stacks. Use the alignment property
+ to specify baseline alignment in horizontal stacks.
+ Defaults to NO.
+ */
+@property(nonatomic,getter=isBaselineRelativeArrangement) BOOL baselineRelativeArrangement;
+
+/* Uses margin layout attributes for edge constraints where applicable.
+ Defaults to NO.
+ */
+@property(nonatomic,getter=isLayoutMarginsRelativeArrangement) BOOL layoutMarginsRelativeArrangement;
+@end

Polyfill/UIStackView+TZStackView.m

@@ -0,0 +1,28 @@
+//
+//  UIStackView+TZStackView.m
+//
+//  Created by Frederic Barthelemy on 9/9/15.
+//
+
+#import <objc/runtime.h>
+
+#import "FitStar-Swift.h"
+#import "UIStackView+TZStackView.h"
+
+#if __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_9_0
+#error UIStackView+TZStackView Polyfill is no longer required! Remove these files. Congratulations!
+#endif
+
+@implementation _polyfill_UIStackView
+
++ (instancetype)alloc {
+	// Actually pick the best StackView implementation:
+	return [NSClassFromString(@"UIStackView") alloc] ?: [TZStackView alloc];
+}
+
+#pragma mark - Polyfill Stubs
+- (instancetype)initWithArrangedSubviews:(NSArray<__kindof UIView *> *)views FIT_UNAVAILABLE(@"Polyfill Stub",@"You can't actually have an instance of this object.");
+- (void)addArrangedSubview:(UIView *)view FIT_UNAVAILABLE(@"Polyfill Stub",@"You can't actually have an instance of this object.");
+- (void)removeArrangedSubview:(UIView *)view FIT_UNAVAILABLE(@"Polyfill Stub",@"You can't actually have an instance of this object.");
+- (void)insertArrangedSubview:(UIView *)view atIndex:(NSUInteger)stackIndex FIT_UNAVAILABLE(@"Polyfill Stub",@"You can't actually have an instance of this object.");
+@end

README.md

@@ -7,6 +7,7 @@ A wonderful layout component called the [`UIStackView` was introduced with *iOS
 - ✅ Compatible with **iOS 7.x** and **iOS 8.x**
 - ✅ Supports the complete API of `UIStackView` including **all** *distribution* and *alignment* options
 - ✅ Supports animating the `hidden` property of the *arranged subviews*
+- ✅ Optional [Polyfill](#polyfill) for Objective-C code to use the name UIStackView, while dynamically selecting TZStackView when on earlier iOS versions.
 - ❌ Supports *Storyboard*
 
 So this implementation does **not** support Storyboard. It is meant for iOS developers who, like me, want to use the `UIStackView` in our existing apps and like to layout their components in code as opposed to using *Storyboard*.
@@ -67,6 +68,22 @@ UIView.animateWithDuration(0.5, animations: {
 ```
 ![TZStackView hidden animation example](/assets/TZStackView-hide-animation.gif)
 
+## Polyfill
+(Objective-C only)
+
+If you're writing code with Objective-C, and you want to dynamically select UIStackView over TZStackView when on iOS versions that provide it, check out [UIStackView+TZStackView](./Polyfill/)
+
+If you include these files in your project, and include the header in your Prefix.pch, along with TZStackView [see Setup](#setup) you to will be able to type `UIStackView` and have the code dynamically select the right class to fulfill your needs.
+
+If you go down this route, you don't have to do anything to migrate to UIStackView, but to remove TZStackView.
+
+Example:
+```objc
+UIStackView * stack = [[UIStackView alloc] initWithArrangedSubviews:@[/*…*/]];
+````
+
+More documentation in [UIStackView+TZStackView.h](./Polyfill/TZStackView.h)
+
 ## Migrating to UIStackView
 If at a later point you decide to make *iOS 9* the minimum requirement of your app (it will happen sooner or later), you will want to migrate to the real `UIStackView` instead of using this implementation. Because the `TZStackView` is a drop-in replacement for `UIStackView`, you simply replace: