条件语句的优化

要求判断一个NSArray有值,严谨的写法是

[objc]  view plain copy

1. NSString *object = @"";  
2.   
3. if (object && [object isKindOfClass:[NSArray class]] && ((NSArray *)object).count > 0)  
4. {  
5.     NSLogDD  
6. }  

先判断这个object非空,再判断是NSArray类型,再判断count>0,如果顺序写成

[objc]  view plain copy

1. NSString *object = @"";  
2.   
3. if (object && ((NSArray *)object).count > 0 && [object isKindOfClass:[NSArray class]])  
4. {  
5.     NSLogDD  
6. }  

则会运行错误,因为if里的代码是串行.

因此,对于多条件的if语句,尤其是大量运行的,我们可以调整 (条件) 的顺序,来改善代码效率.如:

[objc]  view plain copy

1. [scores enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOLBOOL *stop) {  
2.     if (obj > 59 && obj != 100)  
3.     {  
4.         NSLogDD  
5.     }  
6. }];  

当我们取考试及格,但是又不是满分的人,通常情况下就可以把 > 59 放到前面(通常情况下满分的人少).

当一段逻辑有多个if出现的情况,如:

[objc]  view plain copy

1. +(UIControlEvents)eventWithName:(NSString *)name  
2. {  
3.     if([name isEqualToString:@"UIControlEventTouchDown"])           return UIControlEventTouchDown;  
4.     if([name isEqualToString:@"UIControlEventTouchDownRepeat"])     return UIControlEventTouchDownRepeat;  
5.     if([name isEqualToString:@"UIControlEventTouchDragInside"])     return UIControlEventTouchDragInside;  
6.     if([name isEqualToString:@"UIControlEventTouchDragOutside"])    return UIControlEventTouchDragOutside;  
7.     if([name isEqualToString:@"UIControlEventTouchDragEnter"])      return UIControlEventTouchDragEnter;  
8.     if([name isEqualToString:@"UIControlEventTouchDragExit"])       return UIControlEventTouchDragExit;  
9.     if([name isEqualToString:@"UIControlEventTouchUpInside"])       return UIControlEventTouchUpInside;  
10.     if([name isEqualToString:@"UIControlEventTouchUpOutside"])      return UIControlEventTouchUpOutside;  
11.     if([name isEqualToString:@"UIControlEventTouchCancel"])         return UIControlEventTouchCancel;  
12.     if([name isEqualToString:@"UIControlEventTouchDown"])           return UIControlEventTouchDown;  
13.     if([name isEqualToString:@"UIControlEventValueChanged"])        return UIControlEventValueChanged;  
14.     if([name isEqualToString:@"UIControlEventEditingDidBegin"])     return UIControlEventEditingDidBegin;  
15.     if([name isEqualToString:@"UIControlEventEditingChanged"])      return UIControlEventEditingChanged;  
16.     if([name isEqualToString:@"UIControlEventEditingDidEnd"])       return UIControlEventEditingDidEnd;  
17.     if([name isEqualToString:@"UIControlEventEditingDidEndOnExit"]) return UIControlEventEditingDidEndOnExit;  
18.     if([name isEqualToString:@"UIControlEventAllTouchEvents"])      return UIControlEventAllTouchEvents;  
19.     if([name isEqualToString:@"UIControlEventAllEditingEvents"])    return UIControlEventAllEditingEvents;  
20.     if([name isEqualToString:@"UIControlEventApplicationReserved"]) return UIControlEventApplicationReserved;  
21.     if([name isEqualToString:@"UIControlEventSystemReserved"])      return UIControlEventSystemReserved;  
22.     if([name isEqualToString:@"UIControlEventAllEvents"])           return UIControlEventAllEvents;  
23.     return UIControlEventAllEvents;  
24. }  

我们可以通过 把多个if 改成if () else if... 来优化逻辑,同样可以合理的安排条件的顺序.

还有一种更加优美的方法,用字典来代替if, 如

[objc]  view plain copy

1. XY_DicControlStringEvent = [@{@"UIControlEventTouchDown": @(UIControlEventTouchDown),  
2.                              @"UIControlEventTouchDownRepeat": @(UIControlEventTouchDownRepeat),  
3.                              @"UIControlEventTouchDragInside": @(UIControlEventTouchDragInside),  
4.                              @"UIControlEventTouchDragOutside": @(UIControlEventTouchDragOutside),  
5.                              @"UIControlEventTouchDragEnter": @(UIControlEventTouchDragEnter),  
6.                              @"UIControlEventTouchDragExit": @(UIControlEventTouchDragExit),  
7.                              @"UIControlEventTouchUpInside": @(UIControlEventTouchUpInside),  
8.                              @"UIControlEventTouchUpOutside": @(UIControlEventTouchUpOutside),  
9.                              @"UIControlEventTouchCancel": @(UIControlEventTouchCancel),  
10.                              @"UIControlEventValueChanged": @(UIControlEventValueChanged),  
11.                              @"UIControlEventEditingDidBegin": @(UIControlEventEditingDidBegin),  
12.                              @"UIControlEventEditingChanged": @(UIControlEventEditingChanged),  
13.                              @"UIControlEventEditingDidEnd": @(UIControlEventEditingDidEnd),  
14.                              @"UIControlEventEditingDidEndOnExit": @(UIControlEventEditingDidEndOnExit),  
15.                              @"UIControlEventAllTouchEvents": @(UIControlEventAllTouchEvents),  
16.                              @"UIControlEventAllEditingEvents": @(UIControlEventAllEditingEvents),  
17.                              @"UIControlEventApplicationReserved": @(UIControlEventApplicationReserved),  
18.                              @"UIControlEventSystemReserved": @(UIControlEventSystemReserved),  
19.                              @"UIControlEventAllEvents": @(UIControlEventAllEvents)  
20.                              }  retain];  

[objc]  view plain copy

1. +(UIControlEvents)eventWithName:(NSString *)name  
2. {  
3.     return [[XY_DicControlStringEvent objectForKey:name] integerValue];  
4. }  

这里虽然有个装箱拆箱的过程,但是逻辑比大量的 if 清晰多了