How do I retrieve keystrokes from a custom keyboard on an iOS app?

Greg’s approach should work but I have an approach that doesn’t require the keyboard to be told about the text field or text view. In fact, you can create a single instance of the keyboard and assign it to multiple text fields and/or text views. The keyboard handles knowing which one is the first responder.

Here is my approach. I’m not going to show any code for creating the keyboard layout. That’s the easy part. This code shows all of the plumbing.

Edit: This has been updated to properly handle UITextFieldDelegate textField:shouldChangeCharactersInRange:replacementString: and UITextViewDelegate textView:shouldChangeTextInRange:replacementText:.

The header file:

@interface SomeKeyboard : UIView <UIInputViewAudioFeedback>

@end

The implementation file:

@implmentation SomeKeyboard {
    id<UITextInput> _input;
    BOOL _tfShouldChange;
    BOOL _tvShouldChange;
}

- (id)init {
    self = [super init];
    if (self) {
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(checkInput:) name:UITextFieldTextDidBeginEditingNotification object:nil];
    }

    return self;
}

// This is used to obtain the current text field/view that is now the first responder
- (void)checkInput:(NSNotification *)notification {
    UITextField *field = notification.object;

    if (field.inputView && self == field.inputView) {
        _input = field;

        _tvShouldChange = NO;
        _tfShouldChange = NO;
        if ([_input isKindOfClass:[UITextField class]]) {
            id<UITextFieldDelegate> del = [(UITextField *)_input delegate];
            if ([del respondsToSelector:@selector(textField:shouldChangeCharactersInRange:replacementString:)]) {
                _tfShouldChange = YES;
            }
        } else if ([_input isKindOfClass:[UITextView class]]) {
            id<UITextViewDelegate> del = [(UITextView *)_input delegate];
            if ([del respondsToSelector:@selector(textView:shouldChangeTextInRange:replacementText:)]) {
                _tvShouldChange = YES;
            }
        }
    }
}

// Call this for each button press
- (void)click {
    [[UIDevice currentDevice] playInputClick];
}

// Call this when a button on the keyboard is tapped (other than return or backspace)
- (void)keyTapped:(UIButton *)button {
    NSString *text = ???; // determine text for the button that was tapped

    if ([_input respondsToSelector:@selector(shouldChangeTextInRange:replacementText:)]) {
        if ([_input shouldChangeTextInRange:[_input selectedTextRange] replacementText:text]) {
            [_input insertText:text];
        }
    } else if (_tfShouldChange) {
        NSRange range = [(UITextField *)_input selectedRange];
        if ([[(UITextField *)_input delegate] textField:(UITextField *)_input shouldChangeCharactersInRange:range replacementString:text]) {
            [_input insertText:text];
        }
    } else if (_tvShouldChange) {
        NSRange range = [(UITextView *)_input selectedRange];
        if ([[(UITextView *)_input delegate] textView:(UITextView *)_input shouldChangeTextInRange:range replacementText:text]) {
            [_input insertText:text];
        }
    } else {
        [_input insertText:text];
    }
}

// Used for a UITextField to handle the return key button
- (void)returnTapped:(UIButton *)button {
    if ([_input isKindOfClass:[UITextField class]]) {
        id<UITextFieldDelegate> del = [(UITextField *)_input delegate];
        if ([del respondsToSelector:@selector(textFieldShouldReturn:)]) {
            [del textFieldShouldReturn:(UITextField *)_input];
        }
    } else if ([_input isKindOfClass:[UITextView class]]) {
        [_input insertText:@"\n"];
    }
}

// Call this to dismiss the keyboard
- (void)dismissTapped:(UIButton *)button {
    [(UIResponder *)_input resignFirstResponder];
}

// Call this for a delete/backspace key
- (void)backspaceTapped:(UIButton *)button {
    if ([_input respondsToSelector:@selector(shouldChangeTextInRange:replacementText:)]) {
        UITextRange *range = [_input selectedTextRange];
        if ([range.start isEqual:range.end]) {
            UITextPosition *newStart = [_input positionFromPosition:range.start inDirection:UITextLayoutDirectionLeft offset:1];
            range = [_input textRangeFromPosition:newStart toPosition:range.end];
        }
        if ([_input shouldChangeTextInRange:range replacementText:@""]) {
            [_input deleteBackward];
        }
    } else if (_tfShouldChange) {
        NSRange range = [(UITextField *)_input selectedRange];
        if (range.length == 0) {
            if (range.location > 0) {
                range.location--;
                range.length = 1;
            }
        }
        if ([[(UITextField *)_input delegate] textField:(UITextField *)_input shouldChangeCharactersInRange:range replacementString:@""]) {
            [_input deleteBackward];
        }
    } else if (_tvShouldChange) {
        NSRange range = [(UITextView *)_input selectedRange];
        if (range.length == 0) {
            if (range.location > 0) {
                range.location--;
                range.length = 1;
            }
        }
        if ([[(UITextView *)_input delegate] textView:(UITextView *)_input shouldChangeTextInRange:range replacementText:@""]) {
            [_input deleteBackward];
        }
    } else {
        [_input deleteBackward];
    }

    [self updateShift];
}

@end

This class requires a category method for UITextField:

@interface UITextField (CustomKeyboard)

- (NSRange)selectedRange;

@end

@implementation UITextField (CustomKeyboard)

- (NSRange)selectedRange {
    UITextRange *tr = [self selectedTextRange];

    NSInteger spos = [self offsetFromPosition:self.beginningOfDocument toPosition:tr.start];
    NSInteger epos = [self offsetFromPosition:self.beginningOfDocument toPosition:tr.end];

    return NSMakeRange(spos, epos - spos);
}

@end