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Objective-C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.

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");
    });

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