allow communication between our controllers, without the need to have excessive coupling.
NSNullis used throughout Foundation and other frameworks to skirt around the limitations of collections like
NSDictionarynot being able to contain
nilvalues. You can think of
NSNullas effectively boxing the
nilvalue so that it can be used in collections:
It is also possible to execute an operation object manually by calling its
startmethod, but doing so does not guarantee that the operation runs concurrently with the rest of your code.
In an operation object, you specify the thread priority as a floating-point value in the range 0.0 to 1.0, with 0.0 being the lowest priority and 1.0 being the highest priority. If you do not specify an explicit thread priority, the operation runs with the default thread priority of 0.5.
This new priority remains in effect for the duration of your operation’s
If you want to reference any object from outside in the block, remember that you should pass in a weak reference.
if you want to do something that is related to the UI in the block, you must do it on the main thread