本文会不定期更新,转载请注明出处http://blog.csdn.net/uxyheaven/article/details/47780957

iOS中条件语句的优化

要求判断一个NSArray有值,严谨的写法是

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

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

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

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:

    [scores enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {        if (obj > 59 && obj != 100)        {            NSLogDD        }    }];

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:

+(UIControlEvents)eventWithName:(NSString *)name{    if([name isEqualToString:@"UIControlEventTouchDown"])           return UIControlEventTouchDown;    if([name isEqualToString:@"UIControlEventTouchDownRepeat"])     return UIControlEventTouchDownRepeat;    if([name isEqualToString:@"UIControlEventTouchDragInside"])     return UIControlEventTouchDragInside;    if([name isEqualToString:@"UIControlEventTouchDragOutside"])    return UIControlEventTouchDragOutside;    if([name isEqualToString:@"UIControlEventTouchDragEnter"])      return UIControlEventTouchDragEnter;    if([name isEqualToString:@"UIControlEventTouchDragExit"])       return UIControlEventTouchDragExit;    if([name isEqualToString:@"UIControlEventTouchUpInside"])       return UIControlEventTouchUpInside;    if([name isEqualToString:@"UIControlEventTouchUpOutside"])      return UIControlEventTouchUpOutside;    if([name isEqualToString:@"UIControlEventTouchCancel"])         return UIControlEventTouchCancel;    if([name isEqualToString:@"UIControlEventTouchDown"])           return UIControlEventTouchDown;    if([name isEqualToString:@"UIControlEventValueChanged"])        return UIControlEventValueChanged;    if([name isEqualToString:@"UIControlEventEditingDidBegin"])     return UIControlEventEditingDidBegin;    if([name isEqualToString:@"UIControlEventEditingChanged"])      return UIControlEventEditingChanged;    if([name isEqualToString:@"UIControlEventEditingDidEnd"])       return UIControlEventEditingDidEnd;    if([name isEqualToString:@"UIControlEventEditingDidEndOnExit"]) return UIControlEventEditingDidEndOnExit;    if([name isEqualToString:@"UIControlEventAllTouchEvents"])      return UIControlEventAllTouchEvents;    if([name isEqualToString:@"UIControlEventAllEditingEvents"])    return UIControlEventAllEditingEvents;    if([name isEqualToString:@"UIControlEventApplicationReserved"]) return UIControlEventApplicationReserved;    if([name isEqualToString:@"UIControlEventSystemReserved"])      return UIControlEventSystemReserved;    if([name isEqualToString:@"UIControlEventAllEvents"])           return UIControlEventAllEvents;    return UIControlEventAllEvents;}

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

    XY_DicControlStringEvent = [@{@"UIControlEventTouchDown": @(UIControlEventTouchDown),                                 @"UIControlEventTouchDownRepeat": @(UIControlEventTouchDownRepeat),                                 @"UIControlEventTouchDragInside": @(UIControlEventTouchDragInside),                                 @"UIControlEventTouchDragOutside": @(UIControlEventTouchDragOutside),                                 @"UIControlEventTouchDragEnter": @(UIControlEventTouchDragEnter),                                 @"UIControlEventTouchDragExit": @(UIControlEventTouchDragExit),                                 @"UIControlEventTouchUpInside": @(UIControlEventTouchUpInside),                                 @"UIControlEventTouchUpOutside": @(UIControlEventTouchUpOutside),                                 @"UIControlEventTouchCancel": @(UIControlEventTouchCancel),                                 @"UIControlEventValueChanged": @(UIControlEventValueChanged),                                 @"UIControlEventEditingDidBegin": @(UIControlEventEditingDidBegin),                                 @"UIControlEventEditingChanged": @(UIControlEventEditingChanged),                                 @"UIControlEventEditingDidEnd": @(UIControlEventEditingDidEnd),                                 @"UIControlEventEditingDidEndOnExit": @(UIControlEventEditingDidEndOnExit),                                 @"UIControlEventAllTouchEvents": @(UIControlEventAllTouchEvents),                                 @"UIControlEventAllEditingEvents": @(UIControlEventAllEditingEvents),                                 @"UIControlEventApplicationReserved": @(UIControlEventApplicationReserved),                                 @"UIControlEventSystemReserved": @(UIControlEventSystemReserved),                                 @"UIControlEventAllEvents": @(UIControlEventAllEvents)                                 }  retain];

+(UIControlEvents)eventWithName:(NSString *)name{    return [[XY_DicControlStringEvent objectForKey:name] integerValue];}

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.

当一些条件语句难以让人看清他的目的时,

- (void)showName:(NSString *)name{    if (name != nil)    {        if (name.length > 0)        {            NSLog(@"showName");        }        else        {            NSLog(@"name.length is zero");        }    }    else    {        NSLog(@"name is nil");    }}

我们可以用卫语句来使得主体逻辑更加清晰

- (void)showName:(NSString *)name{    if (name == nil)    {        NSLog(@"name is nil");        return;    }    if (name.length == 0)    {        NSLog(@"name.length is zero");        return;    }    NSLog(@"showName");}

我们还可以用三目来优化代码,当然了这个只是说可以,实际不建议用.
如果真的的要用,请按照这种写法:

    BOOL b = YES;    // 普通写法    if (b)    {        printf("11");    }    else    {        printf("22");    }    // 三目写法    (b) ?    ({        printf("11");    })    :    ({        printf("22");    });