Question

//////////////////// Mutation.h

#import <Foundation/Foundation.h>
@interface Mutation : NSObject
@property (assign) NSString *inputString;
@property (assign) NSString *outputString;    
@end

//////////////////// Mutation.m

#import "Mutation.h"
@implementation Mutation
@synthesize inputString;
@synthesize outputString;
@end

//////////////////// NTAppDelegate.h

#import <Cocoa/Cocoa.h>
#import "Mutation.h"
@interface NTAppDelegate : NSObject <NSApplicationDelegate>
@property (assign) IBOutlet NSWindow *window;
@property (weak) IBOutlet NSTextField *dataField;
@property (weak) IBOutlet NSTextField *outputField;
@property (assign) Mutation *mutation;
- (IBAction)receiveUserTextFromTextField:(NSTextField *)sender;
@end

//////////////////// NTAppDelegate.m

#import "NTAppDelegate.h"
#import "Mutation.h"
@implementation NTAppDelegate
@synthesize window = _window;
@synthesize dataField = _dataField;
@synthesize outputField = _outputField;
@synthesize mutation = mutation;    //statement #1

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification //#3 (block)
{
Mutation *aMutation = [[Mutation alloc] init];
[self setMutation:aMutation];
[aMutation setInputString:@"new"];
[aMutation setOutputString:@"old"];
NSLog(@"Mutation inputString is %@; outputString is %@", [aMutation inputString], [aMutation outputString]);    
}

- (IBAction)receiveUserTextFromTextField:(NSTextField *)sender   //#2 (block)
{
// assign the user's entered text to Mutation's inputString
NSString* newText = [sender stringValue];   // -stringValue inherited from NSControl
NSLog (@"%@ was entered", newText);         //  <-THIS WORKS
[mutation setInputString:newText];           //  <-CRASH  statement #4 (crashes)
NSLog(@"Mutation(2) inputString is %@; outputString is %@", [aMutation inputString], [aMutation outputString]);    
}

@end

/////// I am using ARC. Trying to get a handle inductively on objC fundamentals. This is a continuation of my first question…

My specific questions refer to the numbered code lines/blocks (#1-#4) above.

1 is this the creation of an instance of Mutation that is global?

2 do I need to pass a ref to this mutation here? and if so,
by putting another argument in function?

3 I do not understand why when built and run, logging occurs from first from block #3, then block #2

4 why does this line crash (log says unrecog selector)

Log:

when program loads, log records, in order:

a) user data goes here… was entered

b) Mutation(2) inputString is (null); outputString is (null)

c) Mutation inputString is new; outputString is old

then, if user enters data (statement #4):
bad things happen.

Answer 1

1. @synthesize will generate ivar mutation, -mutation, and -setMutation: for you, nothing more than that. The creation of an instance is done in alloc and init.
2. Yes and No. You should make your code clear which mutation you were referring to, by ivar/accessor method, rather than another argument. Once signature changed, the method no longer exists like before.
3. You made a mistake defining your @property (assign) Mutation *mutation. The app delegate bears the ownership of ivar mutation and therefore is responsible for retaining it. What you need is @property (strong) Mutation *mutation.
4. You defined mutation to be a weak ivar, so the setter -setMutation: would not retain it, and it was released when block #3 ends. When the #4 is reached, mutation has been released, and the address the ivar pointing at is probably allocated to other object instances, which is obviously not a kind of Mutation, therefore the error occurred.