iOS 数组越界 Crash加工经验

我们先来看看有可能会出现的数组越界Crash的地方。

- (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]。前一个还没有运行完，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；解决问题也非常easy代码例如以下：

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    WelfareItem *item = nil;
    if (indexPath.row < [_datasourceArray count]) {//不管你武功有多高，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，不管你武功有多高。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。代码例如以下：

/*!
 @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，假设没有错误为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

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。看来是一个比較不错的解决方式。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例如以下

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。出如今iOS7以上（含iOS7），关键还没实用户反馈有问题，Crash高了几倍没有一个用户反馈这样的情况还是少见的，大家測试还复现不了；測试了一周最终复现了一样的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；此法不行仅仅，仅仅能另寻他策了。

后来我们就给数组新增扩展方法代码例如以下

@interface NSArray (SHYUtil)

/*!
 @method objectAtIndexCheck:
 @abstract 检查是否越界和NSNull假设是返回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] 在上面。因此，有可能解决数组边界 -[__NSArrayI objectAtIndex:]: index 100 beyond bounds [0 .. 1]' 错误