The difference between accessing an attribute and accessing an instance variable

1, because do not pass Objective-c "method Dispatch" step, so direct access to the instance variable speed will be relatively fast. In this case, the code generated by the compiler accesses the block of memory that holds the object instance directly;

2. When accessing an instance variable directly, it does not invoke its "set method", which bypasses the "Memory management semantics" defined for setting the related property;

3, if the instance variable is accessed directly, then the "Key value observation" (Key-value OBSERVING,KVO) notification will not be triggered. Whether this will cause problems and depends on the specific object behavior.

Therefore, a reasonable compromise is that when the instance variable is written, it is done by its "set method", and when the instance variable is read, it is accessed directly. This method can not only increase the speed of the read operation, but also control the write operation to the property. The primary reason for writing instance variables through the set method is to ensure that the memory management semantics of the relevant properties are enforced. However, there are several issues to be aware of when choosing this approach:

1. The instance variable should always be accessed directly in the Init method and Dealloc method, because the subclass may "overwrite" (override) the setting method;

2, "lazy initialization" (lay initialization), in which case the property must be accessed through the Get method, otherwise the instance variable will never be initialized.

The difference between accessing an attribute and accessing an instance variable