Explanation of __weak and __strong keywords

WeakifyExplained.mm

void foo()
{
  // __strong guarantees that the object is "alive" until the end of foo() stack frame. The __strong keyword is used implicitly and may be omitted.
  // Unless it's retained by other objects
  __strong SomeObjectClass *someObject = ...
  
  // __weak does not let the block increase the retain count of "someObject"
  // 
  __weak SomeObjectClass *weakSomeObject = someObject;
  
  
  someObject.completionHandler = ^void(void){ // default signature is ``` ^id(void)```
      
      // Ensure that someObject won't be deallocated until the end of the someObject.completionHandler() stack frame 
      // SomeObject becomes retained only when the block is invoked. And it becomes released after the return statement within the someObject.completionHandler()
      SomeObjectClass *strongSomeObject = weakSomeObject;
      
      // "if" statement is not that useful since invoking a selector on a nil objects results to IDLE (no action).
      if (strongSomeObject == nil) {
          // The original someObject doesn't exist anymore.
          // Ignore, notify or otherwise handle this case.
      }
 
      // okay, NOW we can do something with someObject
      [strongSomeObject someMethod];
  };
}

Callback block not being executed

Callback block being executed

---

void foo()
{
 SomeObjectClass *someObject = ...

  someObject.completionHandler = ^void(void){ 
      [someObject someMethod];
  };
}

Retain cycle :

Not using __strong within a callback

void foo()
{
  // __strong guarantees that the object is "alive" until the end of foo() stack frame. The __strong keyword is used implicitly and may be omitted.
  // Unless it's retained by other objects
  __strong SomeObjectClass *someObject = ...

  // __weak does not let the block increase the retain count of "someObject"
  // 
  __weak SomeObjectClass *weakSomeObject = someObject;


  someObject.completionHandler = ^void(void){ // default signature is ``` ^id(void)``` 
      // weakSomeObject may be nil
      [weakSomeObject someMethod]; // but not during the call

      // weakSomeObject may be nil
      [weakSomeObject someOtherMethod];
  };
}