objective-c Cocoa @selector 用法说明
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Explanation of Cocoa @selector usage
提问by mshafrir
I'm new to Cocoa/Cocoa Touch, and working through a development book. I've come across situations where the @selector() operator is used. I'm a bit lost on how and when the @selector() operator should be used. Can someone provide a short and sweet explanation and example of why it's used and what benefit it gives the developer?
我是 Cocoa/Cocoa Touch 的新手，正在阅读一本开发书籍。我遇到过使用 @selector() 运算符的情况。我对如何以及何时使用 @selector() 运算符有点迷茫。有人可以提供一个简短而甜蜜的解释和示例，说明为什么使用它以及它给开发人员带来什么好处？
By the way, here is sample code taken from Apple's iPhone development site that uses @selector()
顺便说一下，这里是从使用@selector() 的 Apple 的 iPhone 开发站点获取的示例代码
if ([elementName isEqualToString:@"entry"])
{
parsedEarthquakesCounter++;
// An entry in the RSS feed represents an earthquake, so create an instance of it.
self.currentEarthquakeObject = [[Earthquake alloc] init];
// Add the new Earthquake object to the application's array of earthquakes.
[(id)[[UIApplication sharedApplication] delegate]
performSelectorOnMainThread:@selector(addToEarthquakeList:)
withObject:self.currentEarthquakeObject waitUntilDone:YES];
return;
}
回答by Andrew Grant
The selector operator provides a way to refer to a method provided by an object, somewhat similar to a function pointer in C. It is useful because it allows you to decouple the process of calling methods on an object. For example one piece of code could provide a method, and another piece of code could apply that method to a given set of objects.
选择器运算符提供了一种引用对象提供的方法的方法，有点类似于 C 中的函数指针。它很有用，因为它允许您分离调用对象方法的过程。例如，一段代码可以提供一个方法，另一段代码可以将该方法应用于给定的一组对象。
Examples:
例子：
Test to see if an object implements a certain method:
测试一个对象是否实现了某个方法：
[object respondsToSelector:@selector(methodName)]
Store a method to later call on an object;
存储一个方法以供以后调用对象；
SEL method = @selector(methodName);
[object performSelector:method];
Call a method on a different thread (useful for GUI work).
在不同的线程上调用方法（对 GUI 工作很有用）。
[object performSelectorOnMainThread:@selector(methodName)]
回答by Brad Larson
In addition to what's been said, you can also wrap up the @selector in an NSInvocation for later use. You can set the arguments to the NSInvocation a long time after it's created, and activate it when you need the message to be fired. This gives you a lot of power.
除了上面所说的，您还可以将 @selector 包装在 NSInvocation 中以供以后使用。您可以在创建 NSInvocation 很长时间后为其设置参数，并在需要触发消息时激活它。这给了你很大的力量。
For an introduction to the concept, Scott Stevenson has a great post entitled "Dynamic Objective-C with NSInvocation".
关于这个概念的介绍，Scott Stevenson 有一篇很棒的文章，标题为“Dynamic Objective-C with NSInvocation”。
回答by Petteri Hietavirta
One practical example is validateMenuItemmethod where menu items are identified with their target actions.
一个实际的例子是validateMenuItem方法，其中菜单项用它们的目标操作来标识。
Simplified example:
简化示例：
- (BOOL)validateMenuItem:(NSMenuItem *)item {
if ([item action] == @selector(selectFiles:) && otherCondition) {
return YES;
} else {
return NO;
}
}
回答by mouviciel
@selector()is used each time you need to pass the name of a method as an argument to another method, a function or as a variable value. Passing directly the name doesn't work in objective-C.
@selector()每次需要将方法名称作为参数传递给另一个方法、函数或作为变量值时，都会使用它。直接传递名称在 Objective-C 中不起作用。
回答by mouviciel
回答by morroko
You can use a selector to invoke a method on an object—this provides the basis for the implementation of the target-action design pattern in Cocoa.
您可以使用选择器来调用对象上的方法——这为在 Cocoa 中实现目标-动作设计模式提供了基础。
[myObject performSelector:@selector(runMYmethod:) withObject:parameters];
is equivalent to:
相当于：
[myObject runMYmethod:parameters];
