iOS development details about property settings readwrite, readonly, retain, copy, assign, nonatomic

For detailed introduction, please refer to the text description below. This article is very detailed.

1. Readability: readonly, readwrite

@property(readwrite,....) valueType value;

This property is the default property of a variable. That is, if you (readwrite and readonly are not used, then your variable is the readwrite property). By adding the readwrite property, your variable will have the get and set methods.

property(readonly,...) valueType value;

This attribute variable indicates that the variable has only readable methods, that is, you can only use its get method.

2. Assign , setter methods directly assign values ​​and do not perform any retain operations. In order to solve the problem of original type and loop reference

3. The retain and setter method releases the old value and then retains the new value. All implementations are in this order.

4. The copy and setter methods perform Copy operations. Just like the retain processing process, first release the old value, then copy the new object, and retainCount is 1. This is a mechanism introduced to reduce dependence on the context.

5.nonatomic, non-atomic access, unsynchronized, multi-threaded concurrent access will improve performance.

Note that if this property is not added, the default is that both access methods are atomic transaction access. The lock is added to the instance level of the object to which it belongs. So nonatomic is safe with multi-threading.

6 . retain vs. Copy

copy  : Create an object with an index count of 1, and then release the old object
retain: Release the old object, assign the value of the old object to the input object, and then increase the index count of the input object to 1

What's the damn meaning above?

copy actually creates the same object, while retain is not:

For example, define the following properties:

How to use it is as follows:

You can see that testStr and str3 addresses are different, retainCount are both 1

If copy is changed to retain, then they point to the same address, retainCount is 2.
I understand, retain is pointer copy, pointing to the same address, counting is increased by 1, and copy is copying the content.

Objective-C attribute attributes (assign , retain , copy , readonly , readwrite , atomic , nonatomic)

assign: Specify the setter method to use simple assignment, this is the default operation. You can use this property for scalar types (such as int). You can imagine a float, it is not an object, so it cannot retain or copy.

retain: Specifies that retain should be called on the subsequent object, and the previous value sends a release message. You can imagine an NSString instance, which is an object, and you may want to retain it.

copy: Specifies that the copy of the object should be used (deep copy), and the previous value sends a release message. Basically like retain, but without increasing the reference count, is allocating a new piece of memory to place it.

readonly: will only generate getter methods without setter methods (getter methods have no get prefix).

readwrite: default property, will generate getter and setter methods without extra parameters (the setter method has only one parameter).

atomic: For the default attribute of an object, the method generated by setter/getter is an atomic operation. If multiple threads call setters at the same time, there will be no situation where one thread starts executing all the setter statements before another thread starts executing the setter, as if the method is locked at the beginning and end.

nonatomic: The atomicity of setter/getter is not guaranteed, and data may have problems in multithreading situations.

The above content is the detailed discussion of iOS development in detail about the setting of attributes readwrite, readonly, retain, copy, assign, and nonatomic that the editor introduced to you. I hope you like it.