Mixins or Multiple Inheritance in Objective-C?

Question

Let's say for example that I have MyUITextViewSubclass which inherits from UITextView and MyUITextFieldSubclass which inherits from UITextField and both of those subclasses contain a lot of the same methods and properties to add similar behavior to those UI controls.

Since UITextView and UITextField inherit from different classes, is there an easy way to create an abstract class to combine all of that repeated code? In other words, is it possible to create an abstract class that I could inherit from for both of those subclasses and then just override the methods that are different between the two?

What I know so far:

• I know Objective-C doesn't support multiple inheritance (inheritance from two or more classes)
• I know I could add common methods using Categories, but I don't think that solves overriding init methods or adding private properties

Aubarta

could you be more specific about what the common behavior is? – Laurellaurella

Answer 1

Building on Amin's answer, this is how you could do it:

Step 1: Create a TextSurrogateHosting protocol that will contain all the methods of your UITextField and UITextView subclasses that you need to access from the methods that you want to add to both subclasses. This might for example be a text and setText: method, so that your methods can access and set the text of either a text field or a text view. It might look like this:

SPWKTextSurrogateHosting.h

#import <Foundation/Foundation.h>

@protocol SPWKTextSurrogateHosting <NSObject>

- (NSString *)text;
- (void)setText:(NSString *)text;

@end

Step 2: Create a TextSurrogate class that contains all the methods that you want to share between both the UITextField and the UITextView subclasses. Add those methods to a protocol so that we can use code completion in Xcode and avoid compiler warnings/errors.

SPWKTextSurrogate.h

#import <Foundation/Foundation.h>
#import "SPWKTextSurrogateHosting.h"

@protocol SPWKTextSurrogating <NSObject>
@optional
- (void)appendQuestionMark;
- (void)appendWord:(NSString *)aWord;
- (NSInteger)characterCount;
- (void)capitalize;
@end

@interface SPWKTextSurrogate : NSObject <SPWKTextSurrogating>

/* We need to init with a "host", either a UITextField or UITextView subclass */
- (id)initWithHost:(id<SPWKTextSurrogateHosting>)aHost;

@end

SPWKTextSurrogate.m

#import "SPWKTextSurrogate.h"

@implementation SPWKTextSurrogate {
    id<SPWKTextSurrogateHosting> _host;
}

- (id)initWithHost:(id<SPWKTextSurrogateHosting>)aHost
{
    self = [super init];

    if (self) {
        _host = aHost;
    }

    return self;
}

- (void)appendQuestionMark
{
    _host.text = [_host.text stringByAppendingString:@"?"];
}

- (void)appendWord:(NSString *)aWord
{
    _host.text = [NSString stringWithFormat:@"%@ %@", _host.text, aWord];
}

- (NSInteger)characterCount
{
    return [_host.text length];
}

- (void)capitalize
{
    _host.text = [_host.text capitalizedString];
}

@end

Step 3: Create your UITextField subclass. It will contain three necessary boilerplate methods to forward unrecognized method invocations to your SPWKTextSurrogate.

SPWKTextField.h

#import <UIKit/UIKit.h>
#import "SPWKTextSurrogateHosting.h"
#import "SPWKTextSurrogate.h"

@interface SPWKTextField : UITextField <SPWKTextSurrogateHosting, SPWKTextSurrogating>

@end

SPWKTextField.m

#import "SPWKTextField.h"

@implementation SPWKTextField {
    SPWKTextSurrogate *_surrogate;
}

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        _surrogate = [[SPWKTextSurrogate alloc] initWithHost:self];
    }
    return self;
}

#pragma mark Invocation Forwarding
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    if ([_surrogate respondsToSelector:[anInvocation selector]]) {
        [anInvocation invokeWithTarget:_surrogate];
    } else {
        [super forwardInvocation:anInvocation];
    }
}

- (NSMethodSignature*)methodSignatureForSelector:(SEL)selector
{
    NSMethodSignature* signature = [super methodSignatureForSelector:selector];
    if (!signature) {
        signature = [_surrogate methodSignatureForSelector:selector];
    }
    return signature;
}

- (BOOL)respondsToSelector:(SEL)aSelector
{
    if ([super respondsToSelector:aSelector] ||
        [_surrogate respondsToSelector:aSelector])
    {
        return YES;
    }
    return NO;
}

@end

Step 4: Create your UITextView subclass.

SPWKTextView.h

#import <UIKit/UIKit.h>
#import "SPWKTextSurrogateHosting.h"
#import "SPWKTextSurrogate.h"

@interface SPWKTextView : UITextView <SPWKTextSurrogateHosting, SPWKTextSurrogating>

@end

SPWKTextView.m

#import "SPWKTextView.h"
#import "SPWKTextSurrogate.h"

@implementation SPWKTextView {
    SPWKTextSurrogate *_surrogate;
}

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        _surrogate = [[SPWKTextSurrogate alloc] initWithHost:self];
    }
    return self;
}

#pragma mark Invocation Forwarding
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    if ([_surrogate respondsToSelector:[anInvocation selector]]) {
        [anInvocation invokeWithTarget:_surrogate];
    } else {
        [super forwardInvocation:anInvocation];
    }
}

- (NSMethodSignature*)methodSignatureForSelector:(SEL)selector
{
    NSMethodSignature* signature = [super methodSignatureForSelector:selector];
    if (!signature) {
        signature = [_surrogate methodSignatureForSelector:selector];
    }
    return signature;
}

- (BOOL)respondsToSelector:(SEL)aSelector
{
    if ([super respondsToSelector:aSelector] ||
        [_surrogate respondsToSelector:aSelector])
    {
        return YES;
    }
    return NO;
}

@end

Step 5: Use it:

SPWKTextField *textField = [[SPWKTextField alloc] initWithFrame:CGRectZero];
SPWKTextView *textView = [[SPWKTextView alloc] initWithFrame:CGRectZero];

textField.text = @"The green fields";
textView.text = @"What a wonderful view";

[textField capitalize];
[textField appendWord:@"are"];
[textField appendWord:@"green"];
[textField appendQuestionMark];

NSLog(@"textField.text: %@", textField.text);
// Output: The Green Fields are green?

[textView capitalize];
[textView appendWord:@"this"];
[textView appendWord:@"is"];

NSLog(@"textView.text: %@", textView.text);
// Output: What A Wonderful View this is

This pattern should solve your problem. Hopefully :)

Some more background information is available here: https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtForwarding.html#//apple_ref/doc/uid/TP40008048-CH105

Lakieshalakin

Definitely an interesting approach! This solves most of the issues with my original approach. Thanks for sharing :) – Aubarta

Answer 2

What you want is a mixin. This is not supported in Objective-C. Categories are no mixins, because they add an api to one class not to many (>1) classes. Using categories, what is not possible for many reasons as you said, would not help you.

The usual way to solve that problem is to create a helper class containing the additional code and use it in both classes.

Then you will find yourself typing

[myUITextViewSubclass.helper doSomething]

instead of

[myUITextViewSubclass doSomething]

If this is really a problem, you can solve this with forward invocations. Just write a comment.

Jurel

Answer 3

No. It is not possible.

The closest thing you could achieve would be to manually add functionality to UITextView to make it mimic UITextField. The obvious downside is that you must do this all manually, with your own code.

Smetana

Answer 4

You could use a preprocessor macro, but that is error-prone.

Grit

Answer 5

Traits or Mixins are not supported by Objective-C, you only have built-in option of Categories. But fortunately Objective-C Runtime has almost all tools for implementing own idea if mixing or traits with adding methods and properties to your class at runtime. You can read more about opportunities which Objective-C Runtime provides for you on Apple's documentation website Objective-C Runtime Docs

The idea is:

1) You can create an Objective-C protocol (Mixin), in which you will declare properties and methods.

2) Then you create a class (Mixin implementation), which will implement methods from this protocol.

3) You make your some class, in which you want to provide the possibility of composition with mixins, to conform that protocol (Mixin).

4) When your application launches, you add with Objective-C runtime all implementations from (Mixin implementation) class and properties declared in (Mixin) into your class.

5) voilà :)

Or you can use some ready open source projects such as "Alchemiq"

Bernetta