blocks|key|1227720|text|基本上，performSelector允许您动态确定在给定对象上调用哪个选择器。换句话说，不需要在运行前确定选择器。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1227721|因此，即使它们是等价的：|1227722|[anObject+aMethod];+
[anObject+performSelector:@selector(aMethod)];+|code-block|syntax|javascript|1227723|第二个表单允许您这样做：|1227724|SEL+aSelector+=+findTheAppropriateSelectorForTheCurrentSituation();
[anObject+performSelector:+aSelector];|1227725|在你发送信息之前。|1227726|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Basically performSelector allows you to dynamically determine which selector to call a selector on the given object. In other words the selector need not be determined before runtime. 

Thus even though these are equivalent:

<pre><code>[anObject aMethod]; 
[anObject performSelector:@selector(aMethod)]; 
</code></pre>

The second form allows you to do this:

<pre><code>SEL aSelector = findTheAppropriateSelectorForTheCurrentSituation();
[anObject performSelector: aSelector];
</code></pre>

before you send the message.

blocks|key|1442699|text|@乏味杀手说得很对。基本上，当您不知道(并且通常不可能)在编译代码时要调用的方法的名称时，动态生成的选择器很有用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1442700|一个关键的区别是-performSelector:和朋友(包括multi-threaded+and+delayed+variants)有一些限制，因为它们被设计为使用带有0-2个参数的方法。例如，使用6个参数调用-outlineView:toolTipForCell:rect:tableColumn:item:mouseLocation:并返回NSString是非常笨拙的，而且所提供的方法也不支持它。|offset|length|style|CODE|1442701|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intfm/NSObject/performSelector:|1|http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/instm/NSObject/performSelector:onThread:withObject:waitUntilDone:|2|http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Reference/NSOutlineViewDelegate_Protocol/Reference/Reference.html#//apple_ref/occ/intfm/NSOutlineViewDelegate/outlineView:toolTipForCell:rect:tableColumn:item:mouseLocation:^0|0|8|H|2Y|1S|4T|8|8|H|0|V|Z|1|2Y|1S|2|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@$D|V|E|W|F|G]|$D|X|E|Y|F|G]|$D|Z|E|10|F|G]]|9|@$D|11|E|12|1|13]|$D|14|E|15|1|16]|$D|17|E|18|1|19]]|A|$]]|$1|H|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|O]]|P|$5|K|L|M|A|$N|Q]]|R|$5|K|L|M|A|$N|S]]]]

@ennuikiller is spot on. Basically, dynamically-generated selectors are useful for when you don't (and usually can't possibly) know the name of the method you'll be calling when you compile the code.

One key difference is that <a href="http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intfm/NSObject/performSelector:" rel="noreferrer"><code>-performSelector:</code></a> and friends (including the <a href="http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/instm/NSObject/performSelector:onThread:withObject:waitUntilDone:" rel="noreferrer">multi-threaded and delayed variants</a>) are somewhat limited in that they are designed for use with methods with 0-2 parameters. For example, calling <a href="http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Reference/NSOutlineViewDelegate_Protocol/Reference/Reference.html#//apple_ref/occ/intfm/NSOutlineViewDelegate/outlineView:toolTipForCell:rect:tableColumn:item:mouseLocation:" rel="noreferrer"><code>-outlineView:toolTipForCell:rect:tableColumn:item:mouseLocation:</code></a> with 6 parameters and returning the <code>NSString</code> is pretty unwieldy, and not supported by the provided methods.

blocks|key|1442772|text|选择器有点像其他语言中的函数指针。当您在编译时不知道要在运行时调用哪个方法时，可以使用它们。而且，像函数指针一样，它们只封装了调用的动词部分。如果该方法有参数，您也需要传递它们。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1442773|NSInvocation的作用与此类似，只是它将更多的信息绑定在一起。它不仅包括动词部分，还包括目标对象和参数。当您希望使用特定参数调用特定对象上的方法时，这很有用，不是现在，而是将来。您可以构建一个适当的NSInvocation，并在以后启动它。|offset|length|style|CODE|1442774|entityMap^0|0|0|C|2V|C|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]|$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|I|$]]

Selectors are a bit like function pointers in other languages. You use them when you don't know at compile time which method you want to call at runtime. Also, like function pointers, they only encapsulate the verb part of invocation. If the method has parameters, you will need to pass them as well.

An <code>NSInvocation</code> serves a similar purpose, except that it binds together more information. Not only does it include the verb part, it also includes the target object and the parameters. This is useful when you want to call a method on a particular object with particular parameters, not now but in the future. You can build an appropriate <code>NSInvocation</code> and fire it later.

blocks|key|1227966|text|对于问题中最基本的例子，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1227967|[object+doSomething];
[object+performSelector:@selector(doSomething)];+|code-block|syntax|javascript|1227968|在将要发生的事情上没有区别。doSomething将由object同步执行。只有"doSomething“是一个非常简单的方法，它不返回任何东西，也不需要任何参数。|1227969|是不是有点复杂，比如：|1227970|(void)doSomethingWithMyAge:(NSUInteger)age;|1227971|事情会变得复杂，因为对象用things做了一些事情:42；|1227972|不能再与"performSelector“的任何变体一起调用，因为所有带参数的变体都只接受对象参数。|1227973|这里的选择器应该是"doSomethingWithMyAge:“，但任何尝试|1227974|[object+performSelector:@selector(doSomethingWithMyAge:)+withObject:42];++|1227975|根本不会编译。传递一个NSNumber：@(+42+)而不是42也没有什么帮助，因为该方法需要一个基本的C类型，而不是一个对象。|1227976|此外，还有多达2个参数的performSelector变体，不能再多了。而很多时候，方法有更多的参数。|1227977|我发现，尽管performSelector的同步变体：|1227978|-+(id)performSelector:(SEL)aSelector;
-+(id)performSelector:(SEL)aSelector+withObject:(id)object;
-+(id)performSelector:(SEL)aSelector+withObject:(id)object1+withObject:(id)object2;|1227979|总是返回一个对象，我也能够返回一个简单的BOOL或NSUInteger，并且它起作用了。|1227980|performSelector的两个主要用途之一是动态编写要执行的方法的名称，如前面的答案所述。例如|1227981|+SEL+method+=+NSSelectorFromString([NSString+stringWithFormat:@"doSomethingWithMy%25@:",+@"Age");
[object+performSelector:method];|1227982|另一个用途是向object异步分派一条消息，该消息稍后将在当前runloop上执行。对于这一点，还有其他几种performSelector变体。|1227983|-+(void)performSelector:(SEL)aSelector+withObject:(id)anArgument+afterDelay:(NSTimeInterval)delay+inModes:(NSArray+*)modes;
-+(void)performSelector:(SEL)aSelector+withObject:(id)anArgument+afterDelay:(NSTimeInterval)delay;
-+(void)performSelector:(SEL)aSelector+target:(id)target+argument:(id)arg+order:(NSUInteger)order+modes:(NSArray+*)modes;
-+(void)performSelectorOnMainThread:(SEL)aSelector+withObject:(id)arg+waitUntilDone:(BOOL)wait+modes:(NSArray+*)array;
-+(void)performSelectorOnMainThread:(SEL)aSelector+withObject:(id)arg+waitUntilDone:(BOOL)wait;
-+(void)performSelector:(SEL)aSelector+onThread:(NSThread+*)thr+withObject:(id)arg+waitUntilDone:(BOOL)wait+modes:(NSArray+*)array;
-+(void)performSelector:(SEL)aSelector+onThread:(NSThread+*)thr+withObject:(id)arg+waitUntilDone:(BOOL)wait;
-+(void)performSelectorInBackground:(SEL)aSelector+withObject:(id)arg;|1227984|(是的，我从几个基础类类别中收集了它们，如NSThread、NSRunLoop和NSObject)|1227985|每种变体都有自己的特殊行为，但都有一些共同之处(至少当waitUntilDone设置为NO时是这样)。"performSelector“调用将立即返回，对象的消息只会在一段时间后被放到当前的运行循环中。|1227986|由于延迟执行--选择器的方法自然没有返回值可用，因此在所有这些异步变量中都有-(void)返回值。|1227987|我希望我以某种方式涵盖了这一点。|1227988|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|1P|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|1Q|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|1R|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1S|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1T|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|1U|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|1V|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1W|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1X|8|@]|9|@]|A|$]]|$1|10|3|11|5|D|7|1Y|8|@]|9|@]|A|$E|F]]|$1|12|3|13|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|14|3|15|5|6|7|20|8|@]|9|@]|A|$]]|$1|16|3|17|5|D|7|21|8|@]|9|@]|A|$E|F]]|$1|18|3|19|5|6|7|22|8|@]|9|@]|A|$]]|$1|1A|3|1B|5|D|7|23|8|@]|9|@]|A|$E|F]]|$1|1C|3|1D|5|6|7|24|8|@]|9|@]|A|$]]|$1|1E|3|1F|5|6|7|25|8|@]|9|@]|A|$]]|$1|1G|3|1H|5|6|7|26|8|@]|9|@]|A|$]]|$1|1I|3|1J|5|6|7|27|8|@]|9|@]|A|$]]|$1|1K|3|-4|5|6|7|28|8|@]|9|@]|A|$]]]|1L|$]]

For this very basic example in the question,

<pre><code>[object doSomething];
[object performSelector:@selector(doSomething)]; 
</code></pre>

there is no difference in what is going to happen. doSomething will be synchronously executed by object. Only "doSomething" is a very simple method, that does not return anything, and does not require any parameters.

were it something a little more complicated, like:

<pre><code>(void)doSomethingWithMyAge:(NSUInteger)age;
</code></pre>

things would get complicated, because 
 [object doSomethingWithMyAge:42];

can no longer be called with any variant of "performSelector", because all variants with parameters only accept object parameters. 

The selector here would be "doSomethingWithMyAge:" but any attempt to 

<pre><code>[object performSelector:@selector(doSomethingWithMyAge:) withObject:42]; 
</code></pre>

simply won't compile. passing an NSNumber: @(42) instead of 42, wouldn't help either, because the method expects a basic C type - not an object.

In addition, there are performSelector variants up to 2 parameters, no more. While methods many times have many more parameters.

I have found out that although synchronous variants of performSelector:

<pre><code>- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;
</code></pre>

always return an object, I was able to return a simple BOOL or NSUInteger too, and it worked.

One of the two main uses of performSelector is to compose dynamically the name of the method you want to execute, as explained in a previous answer. For example

<pre><code> SEL method = NSSelectorFromString([NSString stringWithFormat:@"doSomethingWithMy%@:", @"Age");
[object performSelector:method];
</code></pre>

The other use, is to asynchronously dispatch a message to object, that will be executed later on the current runloop. For this, there are several other performSelector variants.

<pre><code>- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray *)modes;
- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay;
- (void)performSelector:(SEL)aSelector target:(id)target argument:(id)arg order:(NSUInteger)order modes:(NSArray *)modes;
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait modes:(NSArray *)array;
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait;
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(id)arg waitUntilDone:(BOOL)wait modes:(NSArray *)array;
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(id)arg waitUntilDone:(BOOL)wait;
- (void)performSelectorInBackground:(SEL)aSelector withObject:(id)arg;
</code></pre>

(yes I gathered them from several Foundation class categories, like NSThread, NSRunLoop and NSObject)

Each of the variants has its own special behavior, but all share something in common (at least when waitUntilDone is set to NO). The "performSelector" call would return immediately, and the message to object will only be put on the current runloop after some time.

Because of the delayed execution - naturally no return value is available form the method of the selector, hence the -(void) return value in all these asynchronous variants.

I hope I covered this somehow...

I'm still kind of new to Objective-C and I'm wondering what is the difference between the following two statements?

<pre><code>[object performSelector:@selector(doSomething)]; 

[object doSomething];
</code></pre>

Using -performSelector: vs. just calling the method

Objective-C

我仍然是Objective-C的新手，我想知道下面两个语句之间的区别是什么？[object performSelector:@selector(doSomething)]; [object doSomething];

问使用-performSelector:与仅仅调用方法
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