blocks|key|1334226|text|为什么不分解所有这些调用呢？这样，您可以检查两次调用之间的值：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1334227|Class+myclass+=+NSClassFromString(@"Class_from_String");
id+obj+=+[[myclass+alloc]+init];
[obj+method_from_class];|code-block|syntax|javascript|1334228|顺便问一下，method_from_class是实例方法还是类方法？如果是后者，则可以直接对myclass值调用method_from_class：|offset|length|style|CODE|1334229|[myclass+method_from_class];|1334230|entityMap^0|0|0|6|H|1A|7|1K|H|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]|$I|V|J|W|K|L]|$I|X|J|Y|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|P|$]]

Why not decomposing all these calls ? This way, you can check the values between the calls:

<pre><code>Class myclass = NSClassFromString(@"Class_from_String");
id obj = [[myclass alloc] init];
[obj method_from_class];
</code></pre>

By the way, is <code>method_from_class</code> an instance method or a class method ? If it is the later, then you can directly call <code>method_from_class</code> on the <code>myclass</code> value:

<pre><code>[myclass method_from_class];
</code></pre>

blocks|key|1119187|text|如果尝试从静态库实例化一个类，则必须将"-ObjC“标志添加到"Other+Linker+Flags”生成设置中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1119188|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

If you are trying to instantiate a class from a static library, you must add the "-ObjC" flag to the "Other Linker Flags" build setting.

blocks|key|1119292|text|我还看到了一个奇怪的地方，添加苹果支持的标准单例代码会阻止类被加载。代码正常工作，然后我添加了单例，突然NSClassFromString开始返回nil。注释掉单例代码会导致NSClassFromString正确解析类。我不理解交互，但我认为单例静态变量不知何故被破坏了，以隐藏类名...？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1119293|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I also saw an oddity where adding the standard singleton code espoused by apple prevented the class from being loaded. The code was working as expected, then I added the singleton, and suddenly the NSClassFromString started returning nil. Commenting out the singleton code resulted in the NSClassFromString resolving the class correctly. I don't understand the interaction, but I think the singleton static var was somehow getting mangled to hide the class name...?

blocks|key|1334453|text|您还需要确保您尝试实例化的类包含在项目中。如果是稍后添加的，则需要单击正在构建的Target旁边的复选框。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1334454|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You also need to make sure the class you are trying to instantiate is included in the project. If you added it later, you made need to click the checkbox next to the Target you are building.

blocks|key|1334519|text|如果这是对类的唯一引用，则可能没有链接到它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1334520|我有一个工厂方法来实例化各种类型的子类。工厂有一个switch语句，它转到适当的子类，并分配和初始化它。我注意到除了类名之外，所有的alloc/init语句都是完全相同的。因此，我能够使用NSClassFromString()函数消除整个开关块。|1334521|我遇到了同样的问题--回报为零。这是因为这个类没有在程序的其他地方使用，所以它没有被链接，所以在运行时找不到它。|1334522|您可以通过包含以下语句来解决此问题：|1334523|[MyClass+class];|code-block|syntax|javascript|1334524|这违背了我试图实现的全部目标，但这可能就是您需要的全部。|1334525|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|S|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|T|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|U|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

It is possible that your class is not getting linked if this is the only reference to it.

I had a factory method to instantiate various types of subclass. The factory had a switch statement that went to the appropriate subclass and alloc'd and init'ed it. I noticed that all of the alloc/init statements were exactly the same, except for the name of the class. So I was able to eliminate the entire switch block using the NSClassFromString() function.

I ran into the same problem - the return was nil. This was because the class was not used elsewhere in the program, so it wasn't getting linked, so it could not be found at runtime.

You can solve this by including the following statement:

<pre><code>[MyClass class];
</code></pre>

That defeats the whole purpose of what I was trying to accomplish, but it might be all you need.

Why does <code>NSClassFromString</code> return <code>nil</code> ? As per the definition it has to return class name.
How should I take care to rectify this problem? I need to instantiate a class from string and call the method, which is in the class, using the instance created.

This is how my code looks like:

<pre><code>id myclass = [[NSClassFromString(@"Class_from_String") alloc] init];
[myclass method_from_class];
</code></pre>

But the <code>method_from_class</code> function is not being called, control is not going into it. And my code is error free. Any idea how to solve this in Objective-C?

NSClassFromString returns nil

Objective-C

为什么NSClassFromString会返回nil？根据定义，它必须返回类名。我应该如何注意纠正这个问题？我需要从string实例化一个类，并使用创建的实例调用类中的方法。我的代码是这样的：id myclass = [[NSClassFromString(@"Class_from_String") alloc] in...

问NSClassFromString返回nil
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