respondsToSelector 和 instancesRespondToSelector是兩個常用的方法,我們經(jīng)常使用 someObject respondsToSelector,但是對于它和instancesRespondToSelector之間有什么區(qū)別?

首先先看兩個方法的聲明

@interface NSObject <NSObject>
+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
@end

@protocol NSObject
- (BOOL)respondsToSelector:(SEL)aSelector;
@end

我們可以以UIScreen來進行說明

@interface UIScreen : NSObject <UITraitEnvironment>
+ (UIScreen *)mainScreen;
@property(nullable,nonatomic,strong) UIScreenMode *currentMode
@end

測試代碼

    UIScreen *screens = [UIScreen mainScreen];
    if ([screens.class instancesRespondToSelector:@selector(currentMode)]){
        NSLog(@"[screens.class instancesRespondToSelector:@selector(currentMode)]能響應");
    }

    if ([screens respondsToSelector:@selector(currentMode)]){
        NSLog(@"[screens respondsToSelector:@selector(currentMode)]能響應");
    }

    if ([screens.class respondsToSelector:@selector(currentMode)]){
        NSLog(@"[screens.class respondsToSelector:@selector(currentMode)]能響應");
    }


    if ([UIScreen instancesRespondToSelector:@selector(currentMode)]){
        NSLog(@"[UIScreen instancesRespondToSelector:@selector(currentMode)]能響應");
    }

    if ([UIScreen respondsToSelector:@selector(currentMode)]){
        NSLog(@"[UIScreen respondsToSelector:@selector(currentMode)]能響應");
    }

輸出

2016-06-30 16:17:24.501 TestNSlog[84989:2019395] [screens.class instancesRespondToSelector:@selector(currentMode)]能響應
2016-06-30 16:17:24.501 TestNSlog[84989:2019395] [screens respondsToSelector:@selector(currentMode)]能響應
2016-06-30 16:17:24.502 TestNSlog[84989:2019395] [screens.class respondsToSelector:@selector(currentMode)]不能響應
2016-06-30 16:17:26.181 TestNSlog[84989:2019395] [UIScreen instancesRespondToSelector:@selector(currentMode)]能響應
2016-06-30 16:17:26.181 TestNSlog[84989:2019395] [UIScreen respondsToSelector:@selector(currentMode)]不能響應

說明: instancesRespondToSelector是一個類方法,用來判斷該類的的實例(即對象)是否響應某個方法
respondsToSelector 是一個實例(對象)方法,用來判斷該實例(對象)是否響應某個方法

從定義看,UIScreenMode *currentMode是一個實例屬性
所以[screens.class instancesRespondToSelector:@selector(currentMode)] 是能夠響應的!!
[screens respondsToSelector:@selector(currentMode)] 能響應

[screens.class respondsToSelector:@selector(currentMode)],由于screens.class得到的是 類本身,所以當然不能響應實例方法了
同理[UIScreen respondsToSelector:@selector(currentMode)]不能響應!!!

測試代碼

    if ([screens.class instancesRespondToSelector:@selector(mainScreen)]){
        NSLog(@"[screens.class instancesRespondToSelector:@selector(mainScreen)]能響應");
    }
    else{
        NSLog(@"[screens.class instancesRespondToSelector:@selector(mainScreen)]不能響應");
    }

    if ([screens respondsToSelector:@selector(mainScreen)]){
        NSLog(@"[screens respondsToSelector:@selector(mainScreen)]能響應");
    }
    else{
        NSLog(@"[screens respondsToSelector:@selector(mainScreen)]不能響應");
    }

    if ([screens.class respondsToSelector:@selector(mainScreen)]){
        NSLog(@"[screens.class respondsToSelector:@selector(mainScreen)]能響應");
    }
    else{
        NSLog(@"[screens.class respondsToSelector:@selector(mainScreen)]不能響應");
    }


    if ([UIScreen instancesRespondToSelector:@selector(mainScreen)]){
        NSLog(@"[UIScreen instancesRespondToSelector:@selector(mainScreen)]能響應");
    }
    else{
        NSLog(@"[UIScreen instancesRespondToSelector:@selector(mainScreen)]不能響應");
    }

    if ([UIScreen respondsToSelector:@selector(mainScreen)]){
        NSLog(@"[UIScreen respondsToSelector:@selector(mainScreen)]能響應");
    }
    else{
        NSLog(@"[UIScreen respondsToSelector:@selector(mainScreen)]不能響應");
    }

輸出

2016-06-30 16:27:44.244 TestNSlog[85085:2029436] [screens.class instancesRespondToSelector:@selector(mainScreen)]不能響應
2016-06-30 16:27:45.876 TestNSlog[85085:2029436] [screens respondsToSelector:@selector(mainScreen)]不能響應
2016-06-30 16:27:47.364 TestNSlog[85085:2029436] [screens.class respondsToSelector:@selector(mainScreen)]能響應
2016-06-30 16:27:51.156 TestNSlog[85085:2029436] [UIScreen instancesRespondToSelector:@selector(mainScreen)]不能響應
2016-06-30 16:27:53.372 TestNSlog[85085:2029436] [UIScreen respondsToSelector:@selector(mainScreen)]能響應

[screens.class instancesRespondToSelector:@selector(mainScreen)],由于mainScreen是類方法,這里判斷實例是否響應,當然是不能響應!!
[screens respondsToSelector:@selector(mainScreen)],screens是一個實例(對象),這里判斷它是否響應一個類方法,當然不能響應
[screens.class respondsToSelector:@selector(mainScreen)],screens.class是類本身,當然可以響應

我們從中應該知道, 類 其實 也是一個兌現(xiàn)哥只是,是一個特殊對象!