objective-c 在 Objective C 中，你能检查一个对象是否有特定的属性或消息吗？
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In Objective C, can you check if an object has a particular property or message?
提问by Andrew Johnson
I wat to do something like this:
我想做这样的事情：
if (viewController.mapView) [viewController.mapView someMethod];
However, if mapView is not a class variable, this crashes. How do I check if mapView exists?
但是，如果 mapView 不是类变量，则会崩溃。如何检查 mapView 是否存在？
采纳答案by dcrosta
For ordinary selectors, you can use respondsToSelector:. I'm not certain if this will work for new-style property access (as it appears you are using in this example). To test if a class responds to a given selector, use instancesRespondToSelector:.
对于普通选择器，您可以使用respondsToSelector:. 我不确定这是否适用于新式属性访问（正如您在本示例中使用的那样）。要测试类是否响应给定选择器，请使用instancesRespondToSelector:.
回答by Robert
回答by Andrew Johnson
Oops, found it:
哎呀，找到了：
if ([vc respondsToSelector:@selector(mapView)]) {
[[vc mapView] viewWillAppear:YES];
}
回答by SpareTime
Here is more than you asked for but a category I have found useful to generically handle NSObject properties:
这比你要求的要多，但我发现一个类别对于一般处理 NSObject 属性很有用：
http://www.whynotsometime.com/Why_Not_Sometime/Category_Enhancing_NSObject.html
http://www.whynotsometime.com/Why_Not_Sometime/Category_Enhancing_NSObject.html
