以我从未见过的方式保留财产？

发布于 2024-10-24 04:34:40 字数 285 浏览 3 评论 0原文

我看到了一个带有以下代码的示例代码：

在 .h 文件中：

  CALayer *_animationLayer;
  @property (nonatomic, retain) CALayer *animationLayer;

在 .m 文件中：

  @synthesize animationLayer = _animationLayer;

我猜它与保留计数有关？

有人可以解释一下吗？

原文

I saw a sample code with this code:

in the .h file:

  CALayer *_animationLayer;
  @property (nonatomic, retain) CALayer *animationLayer;

And in the .m file:

  @synthesize animationLayer = _animationLayer;

I am guessing it is related to the retain count?

Can someone explain that?

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偷得浮生 2024-10-31 04:34:40

将其视为变量名的别名。

来自 Cocoa 基础指南：

@synthesize 的语法也是
包括一个扩展，允许您
使用不同的名称
属性及其实例变量
贮存。例如，考虑
以下声明：

@synthesize title, directReports, role = jobDescrip;

这告诉计算机合成访问器方法
属性 title、directReports 和
角色，并使用jobDesrip
支持角色的实例变量
财产。

Consider it like an alias for the variable name.

From the Cocoa Fundamentals Guide:

The syntax for @synthesize also
includes an extension that allows you
to use different names for the
property and its instance-variable
storage. Consider, for example, the
following statement:

@synthesize title, directReports, role = jobDescrip;

This tells the computer to synthesize accessor methods for
properties title, directReports, and
role, and to use the jobDescrip
instance variable to back the role
property.

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つ低調成傷 2024-10-31 04:34:40

.h 文件中的代码声明了两件事，一个名为 _animationLayer 的变量，其类型为 CALayer*，以及一个名为 animationLayer 的属性，也是CALayer*类型。 .m 文件中的代码指示 Objective-C 编译器自动为 animationLayer 属性生成 getter 和 setter，使用 _animationLayer 变量来保存实际值放。

例如：

_animationLayer = nil;  //initialize the variable to nil
self.animationLayer = [[CALayer alloc] init];  //assign a value to the property
NSLog("_animationLayer is:  %@", _animationLayer);  //it's not set to nil anymore
_animationLayer = nil;  //set it to nil again
NSLog("self.animationLayer is:  %@", self.animationLayer);  //now the property is nil

你是对的，这确实与对象的retainCount有一些关系（因此将变量视为属性的直接别名并不完全正确）。本质上，直接使用_animationLayer变量设置值不会保留新值或释放旧值。使用属性访问器设置值即可。例如：

_animationLayer = [[CALayer alloc] init];  //retain count is 1
self.animationLayer = nil;                 //retain count is 0

self.animationLayer = [[CALayer alloc] init];  //oops, retain count is 2
self.animationLayer = nil;  //retain count is 1, the object leaked

_animationLayer = [[CALayer alloc] init];  //retain count is 1
[_animationLayer release];               //retain count is 0
self.animationLayer = nil;               //oops, just released it again

The code in the .h file is declaring two things, a variable called _animationLayer which is of type CALayer*, and a property called animationLayer which is also of type CALayer*. The code in the .m file is instructing the objective-c compiler to automatically generate getters and setters for the animationLayer property, using the _animationLayer variable to hold the actual value that is set.

So for instance:

_animationLayer = nil;  //initialize the variable to nil
self.animationLayer = [[CALayer alloc] init];  //assign a value to the property
NSLog("_animationLayer is:  %@", _animationLayer);  //it's not set to nil anymore
_animationLayer = nil;  //set it to nil again
NSLog("self.animationLayer is:  %@", self.animationLayer);  //now the property is nil

And you're correct, this does have some relationship to the object's retainCount (so it's not quite correct to think of the variable as a direct alias for the property). In essence, setting the value directly using the _animationLayer variable does not retain the new value or release the old value. Setting the value using the property accessor does. For example:

_animationLayer = [[CALayer alloc] init];  //retain count is 1
self.animationLayer = nil;                 //retain count is 0

self.animationLayer = [[CALayer alloc] init];  //oops, retain count is 2
self.animationLayer = nil;  //retain count is 1, the object leaked

_animationLayer = [[CALayer alloc] init];  //retain count is 1
[_animationLayer release];               //retain count is 0
self.animationLayer = nil;               //oops, just released it again

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