__weak NSString *text = self.textField.text has inconsistent behaviour

Question

I think I may made a silly mistake, but I can't figure out why:

I have a method and Block to handle some network API like:

-(IBAction)confirm:(id)sender {         
    __weak typeof(self) weakSelf = self;
    __weak NSString *anotherNumber = self.nextPhoneTextField.text;
    [SharedInstance bindNewPhoneNumber:self.nextPhoneTextField.text pinCode:self.verifyCodeTextField.text sucess:^(id result) {

      // update phone number
      SharedInstance.phoneNumber = anotherNumber;

    }]; 
}

before the block, I can see newNumber has value correctly,

However when the block is invoked, the newNumber is nil, instead of the text. But I was able to print weakSelf.nextPhoneTextField.text, which is not changed.

Any explainations is appreciated!

UPDATE:

After creating a sample project, I found it's not reproducible. the weak string pointer has valid text. Then I start debugging it, and I found that,

In order to avoid the new keyword, I changed the pointer name to anotherNumber

In my real project, when calling __weak NSString *anotherNumber = self.nextPhoneTextField.text; the anotherNumber has a new address, rather than the self.nextPhoneTextField.text; address:

(lldb) p anotherNumber
(__NSCFString *) $2 = 0x00007f88b3ff2960 @"12345678901"
(lldb) p self.nextPhoneTextField.text
(__NSCFString *) $3 = 0x00007f88b15f8690 @"12345678901"

However in the sample project, I have the similar function,

- (void)clickBlock:(void (^)(NSString * string))block {
    if (block) {
        block(@"haha");
    }
}

- (IBAction)clicked:(id)sender {
    __weak typeof(self) weakSelf = self;
    __weak NSString *text = self.textField.text;

    [self clickBlock:^(NSString *string) {
        NSLog(text);
        NSLog(string);
    }];
}

it is the same address:

(lldb) p text
(NSTaggedPointerString *) $2 = 0xa000000747365744 @"test"
(lldb) p self.textField.text
(NSTaggedPointerString *) $3 = 0xa000000747365744 @"test"

and the class type changed also... Looking for answers!!!

Another update:

I delete the block, simply create two weak pointers with some strings like "hello" and "12345678901", the formmer one has the same address and marked as NSTaggedPointerString, however the latter one has different address and marked as NSCFString

It seems to me that once the text reach a specific length, it will have the NSCFString and different address, and after some tests, the bounty is 9. once more than 9 words, it will be NSCFString, tested on iOS 9.1 iPhone 6S simulator.

on iOS 8.4 simulator, all the strings with different length result in different mem adress and NSCFString

sample project:https://github.com/liuxuan30/WeakStringPointer

Answer 1

__weak NSString *anotherNumber = self.nextPhoneTextField.text;

with this line , NSString is copied by value and NOT by reference, so after assigning a NSString to another NSString , it creates new copy of it and creates a reference to newly created copy and not the original one, and because the reference is weak , the object will be nil after the current function goes out of context ,

If you try to change the textfield's text it will only change the textFields text and not anotherNumber object.

NSString *test = self.nextPhoneTextField.text;
self.nextPhoneTextField.text = @"Something else";
NSSLog(@"Test object contains %@ , the textField contains %@ ",test,self.nextPhoneTextField.text);

Your code does following:

1. Creates a new Copy NSString from self.nextPhoneTextField.text
2. Asigns a new Copy NSString to anotherNumber
3. Since anotherNumber is __weak , it will not retain the object (NSString), it will hold __weak reference to this object and after this function goes out of context and it becomes nil.

To confirm this behavior, you can directly log anotherNumber after setting its value and in different context

__weak NSString *anotherNumber = self.nextPhoneTextField.text;
NSString *strongAnotherNumber = self.nextPhoneTextField.text;
NSLog(@"Weak number - %@ , strong - %@",anotherNumber,strongAnotherNumber);
dispatch_async(dispatch_get_main_queue(), ^{
    NSLog(@"Block Weak number - %@ , strong - %@",anotherNumber,strongAnotherNumber);
});

please also have a look at Why do weak NSString properties not get released in iOS?