我如何查找Objective-C的属性@synthesize getter setter代码 [英] how can I look up objective-c's property @synthesize getter setter code

问题描述

在采访中有一个问题"NSMutableArray属性是否可以使用副本，为什么不使用副本"，我已经搜索了答案，但是我想看看自动生成的getter setter代码怎么办?

in interview has a question "Could NSMutableArray property use copy why and why not " I have searched the answer but I want to see the auto generate's getter setter code how can I do?

推荐答案

您只能看到汇编程序的安装程序的Obj-C代码.为此，首先选择包含Xcode中的属性的源文件，然后选择菜单项Produce > Perform Action > Assemble "File.m".这将打开一个包含汇编代码的窗口.搜索setPropertyName:，其中PropertyName是资产的大写名称.

You cannot see the Obj-C code for the setter only the assembler. To do this first select the source file containing the property in Xcode and then choose the menu item Produce > Perform Action > Assemble "File.m". This will open up a window containing the assembly code. Search for setPropertyName: where PropertyName is the capitalised name of your property.

这将向您显示设置程序调用_objc_setProperty_nonatomic_copy或_objc_setProperty_atomic_copy.要查看那些代码，您将需要使用调试器并逐步使用它们.他们实际上只是调用copyWithZone:.

This will show you the setter calls _objc_setProperty_nonatomic_copy or _objc_setProperty_atomic_copy. To see the code for those you will need to use the debugger and step into them. They essentially just call copyWithZone:.

更重要的是面试问题背后的原因.通过简单的实验或如上所述的汇编程序挖掘，都表明copy属性属性始终执行copy而不是mutableCopy.所以声明:

More important is the reason behind the interview question. Either simple experimentation, or digging through the assembler as above, shows that the copy property attribute always does a copy and not a mutableCopy. So the declaration:

@property (nonatomic, copy) NSMutableArray *shouldBeInvalid;

在理想情况下会生成编译器错误.如果您将可变数组值分配给属性:

would in an ideal world generate a compiler error. If you assign a mutable array value to the property:

self.shouldBeInvalid = @[ @24 ].mutableCopy;

然后由于copy实际分配的值是不可变数组(NSArray)，与NSMutableArray声明的类型相反.稍后尝试将属性值用作可变数组:

then due to the copy the value actually assigned is an immutable array (NSArray), contradicting the NSMutableArray declared type. Trying to use the property value later as a mutable array:

[self.shouldBeInvalid addObject:@42];

会产生运行时错误，因为该属性的值是一个与其声明的类型相反的不可变对象...

will produce a runtime error as the property's value is an immutable object contrary to its declared type...

您还将发现编译器很高兴允许您将一个不变数组分配给该属性:

You'll also find that the compiler happily allows you to assign an immutable array to the property:

self.shouldBeInvalid = @[ @24 ];

没有警告.

面试官可能正在寻找的是让您解释的是，对于属性copy +可变类型，在Objective-C中没有意义，因为副本将产生不可变的对象值.但是，具有copy和不可变对象类型(NSArray，NSDictionary等)的属性确实有意义，因此，如果分配了可变对象值，则会生成该对象的不可变副本，从而防止了属性值的更改意外地，例如:

What the interviewer was probably seeking was for you to explain that for a property copy + mutable type makes no sense in Objective-C as the copy will produce an immutable object value. However a property with copy and an immutable object type (NSArray, NSDictionary, etc.) does make sense so that if a mutable object value is assigned an immutable copy of it is made, which prevents the property's value changing unexpectedly, e.g.:

@property (nonatomic) NSArray *shouldHaveCopy;

NSMutableArray *sample = @[ @"oops" ].mutableCopy;

self.shouldHaveCopy = sample;

[sample addObject:@"'immutable' property changed"];

// self.shouldHaveCopy now references a two element array despite
// its type being `NSArray` 

所以一般规则是:

• 旨在具有可变对象值(NSMutableDictionary 等)的属性永远不应指定copy；和
• 当具有对应的可变对象子类(例如，NSArray具有NSMutableArray等)时，旨在具有不变对象值的属性应始终指定copy.

• A property intended to have a mutable object value (NSMutableDictionary et al) should never specify copy; and
• A property intended to have an immutable object value, when there is a corresponding mutable object subclass (e.g. NSArray has NSMutableArray etc.), should always specify copy.

HTH

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