Objective-C: Subclassing for delegate behavior!

PSTTextView.h

// A demonstration: fixing the UITextView return bug with a subclass
//
// Code for acutally fixing the bug from http://inessential.com/2014/01/07/uitextview_the_solution


#import <UIKit/UIKit.h>

@interface PSTTextView : UITextView
@end

PSTTextView.m

#import "PSTTextView.h"
#import <objc/runtime.h>

@interface PSTTextView () <UITextViewDelegate>
@property (nonatomic, weak) id<UITextViewDelegate> externalDelegate;
@end

@implementation PSTTextView

- (id)initWithCoder:(NSCoder *)coder {
    if (self = [super initWithCoder:coder]) {
        [super setDelegate:self];
    }
    return self;
}

- (instancetype)initWithFrame:(CGRect)frame textContainer:(NSTextContainer *)textContainer {
    if (self = [super initWithFrame:frame textContainer:textContainer]) {
        [super setDelegate:self];
    }
    return self;
}


#pragma mark - Mutator

- (void)setDelegate:(id<UITextViewDelegate>)delegate {
    self.externalDelegate = delegate;
}


#pragma mark - Method forwarding

+ (BOOL)isInstanceMethodSelector:(SEL)selector inProtocol:(Protocol *)protocol {
    struct objc_method_description requiredMethod = protocol_getMethodDescription(protocol, selector, YES, YES);
    struct objc_method_description optionalMethod = protocol_getMethodDescription(protocol, selector,  NO, YES);
    
    return (requiredMethod.name != NULL || optionalMethod.name != NULL);
}

- (id)forwardingTargetForSelector:(SEL)selector {
    BOOL isDelegateSelector = [[self class] isInstanceMethodSelector:selector
                                                          inProtocol:@protocol(UITextViewDelegate)];
    if (isDelegateSelector && [self.externalDelegate respondsToSelector:selector]) {
        return self.externalDelegate;
    }
    
    return [super forwardingTargetForSelector:selector];
}

- (BOOL)respondsToSelector:(SEL)selector {
    BOOL isDelegateSelector = [[self class] isInstanceMethodSelector:selector
                                                          inProtocol:@protocol(UITextViewDelegate)];
    if (isDelegateSelector && [self.externalDelegate respondsToSelector:selector]) {
        return YES;
    }

    return [super respondsToSelector:selector];
}


#pragma mark - Return bug fix

- (BOOL)textView:(UITextView *)textView shouldChangeTextInRange:(NSRange)range replacementText:(NSString *)text {
    if ([text isEqualToString:@"\n"] || [text isEqualToString:@""]) {
        [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(scrollToShowSelection) object:nil];
		[self performSelector:@selector(scrollToShowSelection) withObject:nil afterDelay:0.1]; /*Smaller delays are unreliable.*/
	}
    
    if ([self.externalDelegate respondsToSelector:@selector(textView:shouldChangeTextInRange:replacementText:)]) {
        return [self.externalDelegate textView:textView shouldChangeTextInRange:range replacementText:text];
    }
    else return YES;
}

- (void)scrollToShowSelection {
	if (self.selectedRange.location < self.text.length) {
		return;
	}
    
	CGPoint bottomOffset = CGPointMake(0, MAX(0, self.contentSize.height - self.bounds.size.height));
	[self setContentOffset:bottomOffset animated:YES];
}

@end