iOS如何彻底避免数组越界

我们先来看看有可能会出现的数组越界Crash的地方；

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- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {     WelfareItem *item = [_datasourceArray objectAtIndex:indexPath.row];//有可能会越界，你在下拉刷新时会用[_datasourceArray removeAllObjects]，这时你又点了某个cell就会Crash }   - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {     WelfareItem *item = _datasourceArray[indexPath.row];//有可能会越界，两个地方用了[tableView reloadData]；后一个有[_datasourceArray removeAllObjects]；前一个还没有执行完，就会Crash }

上面代码是有可能会越界的；出现Crash也不好复现，发出去的App总是能收到几条Crash；解决这个问题也很简单代码如下：

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- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {     WelfareItem *item = nil;     if (indexPath.row < [_datasourceArray count]) {//无论你武功有多高，有时也会忘记加         item = [_datasourceArray objectAtIndex:indexPath.row];     } }   - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {     WelfareItem *item = nil;     if (indexPath.row < [_datasourceArray count]) {         item = [_datasourceArray objectAtIndex:indexPath.row];     } }

问题又来了，无论你武功有多高，有时也会忘记加；所以我们要想一招制敌办法；我是想到了用Runtime把objectAtIndex方法替换一下；代码如下：

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/*!  @category  @abstract NSObject的Category  */ @interface NSObject (Util)   /*! @method swizzleMethod:withMethod:error: @abstract 对实例方法进行替换 @param oldSelector 想要替换的方法 @param newSelector 实际替换为的方法 @param error 替换过程中出现的错误，如果没有错误为nil */ + (BOOL)swizzleMethod:(SEL)originalSelector withMethod:(SEL)swizzledSelector error:(NSError **)error;   @end     #import "NSObject+Util.h" #import <objc runtime.h="">   @implementation NSObject (Util)   + (BOOL)swizzleMethod:(SEL)originalSelector withMethod:(SEL)swizzledSelector error:(NSError **)error {     Method originalMethod = class_getInstanceMethod(self, originalSelector);     if (!originalMethod) {         NSString *string = [NSString stringWithFormat:@" %@ 类没有找到 %@ 方法",NSStringFromClass([self class]),NSStringFromSelector(originalSelector)];         *error = [NSError errorWithDomain:@"NSCocoaErrorDomain" code:-1 userInfo:[NSDictionary dictionaryWithObject:string forKey:NSLocalizedDescriptionKey]];         return NO;     }           Method swizzledMethod = class_getInstanceMethod(self, swizzledSelector);     if (!swizzledMethod) {         NSString *string = [NSString stringWithFormat:@" %@ 类没有找到 %@ 方法",NSStringFromClass([self class]),NSStringFromSelector(swizzledSelector)];         *error = [NSError errorWithDomain:@"NSCocoaErrorDomain" code:-1 userInfo:[NSDictionary dictionaryWithObject:string forKey:NSLocalizedDescriptionKey]];         return NO;     }           if (class_addMethod(self, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod))) {         class_replaceMethod(self, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));     }     else {         method_exchangeImplementations(originalMethod, swizzledMethod);     }           return YES; }   @end     @implementation NSArray (ErrerManager)   + (void)load {     static dispatch_once_t onceToken;     dispatch_once(&onceToken, ^{         @autoreleasepool         {             [objc_getClass("__NSArrayI") swizzleMethod:@selector(objectAtIndex:) withMethod:@selector(swizzleObjectAtIndex:) error:nil];             [objc_getClass("__NSArrayM") swizzleMethod:@selector(objectAtIndex:) withMethod:@selector(swizzleObjectAtIndex:) error:nil];         };     }); }   - (id)swizzleObjectAtIndex:(NSUInteger)index {     if (index < self.count)     {         return [self swizzleObjectAtIndex:index];     }     NSLog(@"%@ 越界",self);     return nil;//越界返回为nil }   @end</objc>

有了上面代码我们用 [_datasourceArray objectAtIndex:indexPath.row] 就不会发生越界Crash了；越界
了会返回nil；看来是一个比较不错的解决方案；把app发出去吧，结果我们Crash比之前高了好几倍（越界的Crash没有了，出新的Crash了）；Crash如下

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1 tbreader 0x002b93e9 tbreader + 2098153 2 libsystem_platform.dylib 0x33a66873 _sigtramp + 34 3 libsystem_blocks.dylib 0x33941ae1 _Block_release + 216 4 libobjc.A.dylib 0x333c11a9 + 404 5 CoreFoundation 0x25ba23a9 _CFAutoreleasePoolPop + 16 6 UIKit 0x2912317f + 42 7 CoreFoundation 0x25c565cd + 20 8 CoreFoundation 0x25c53c8b + 278 9 CoreFoundation 0x25c54093 + 914 10 CoreFoundation 0x25ba2621 CFRunLoopRunSpecific + 476 11 CoreFoundation 0x25ba2433 CFRunLoopRunInMode + 106 12 GraphicsServices 0x2cf0a0a9 GSEventRunModal + 136 13 UIKit 0x2918c809 UIApplicationMain + 1440

都是这个Crash，出现在iOS7以上（含iOS7），关键还没有用户反馈有问题，Crash高了几倍没有一个用户反馈这种情况还是少见的，大家测试还复现不了；测试了一周终于复现了一样的Crash；是这样出现的，替换了objectAtIndex方法有输入的地方出来了软键盘按手机Home键就Crash了；此法不行只，只能另寻他策了。后来我们就给数组新增扩展方法代码如下

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@interface NSArray (SHYUtil)   /*!  @method objectAtIndexCheck:  @abstract 检查是否越界和NSNull如果是返回nil  @result 返回对象  */ - (id)objectAtIndexCheck:(NSUInteger)index;   @end     #import "NSArray+SHYUtil.h"   @implementation NSArray (SHYUtil)   - (id)objectAtIndexCheck:(NSUInteger)index {     if (index >= [self count]) {         return nil;     }           id value = [self objectAtIndex:index];     if (value == [NSNull null]) {         return nil;     }     return value; }   @end

把之前的代码 WelfareItem *item = [_datasourceArray objectAtIndex:indexPath.row] 改为 WelfareItem *item = [_datasourceArray objectAtIndexCheck:indexPath.row] 就可以了。这样就可以彻底解决数组越界 -[__NSArrayI objectAtIndex:]: index 100 beyond bounds [0 .. 1]' 错误了