常常會看到這樣的崩潰日誌-[__NSArrayM objectAtIndex:]: index4 beyond bounds [0 ..1]，陣列越界。

舉個列子：

    int index = 3;
    NSArray *array = @[@"z", @"x", @"c"];
    [array objectAtIndex:index];

這段程式碼執行會崩潰，陣列越界，解決辦法有很多，比如我們可以嘗試增加一個條件判斷：

    int index = 3;
    NSArray *array = @[@"z", @"x", @"c"];
    if (index < array.count) {
        [array objectAtIndex:index];
    }
 

或者使用try...catch捕獲異常：

    int index = 3;
    NSArray *array = @[@"z", @"x", @"c"];
    @try {
        //可能丟擲異常的程式碼
        [array objectAtIndex:index];
    }
    @catch (NSException *exception) {
        //處理異常
        NSLog(@"exception: %@", exception.reason);
    }
    @finally {
        //finally程式碼塊是可選的，如果寫了，不管有沒有異常，block內的程式碼都會被執行
        NSLog(@"finally");
    }
 

上面方法已經可以避免crash，為了避免冗餘的程式碼，寫一個NSArray的分類，利用runtime替換NSArray的物件方法objectAtIndex：，在這裡進行判斷，捕獲異常：

#import <Foundation/Foundation.h>

@interface NSArray (Crash)

@end

/*** ---------------分割線--------------- ***/ 

#import "NSArray+Crash.h"
#import <objc/runtime.h>

@implementation NSArray (Crash)

+ (void)load
{
    [super load];
    
    //替換不可變陣列方法
    Method oldObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(objectAtIndex:));
    Method newObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(objectAtSafeIndex:));
    method_exchangeImplementations(oldObjectAtIndex, newObjectAtIndex);
    
    //替換可變陣列方法
    Method oldMutableObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(objectAtIndex:));
    Method newMutableObjectAtIndex =  class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(mutableObjectAtSafeIndex:));
    method_exchangeImplementations(oldMutableObjectAtIndex, newMutableObjectAtIndex);
}

- (id)objectAtSafeIndex:(NSUInteger)index
{
    if (index > self.count - 1 || !self.count) {
        @try {
            return [self objectAtSafeIndex:index];
        }
        @catch (NSException *exception) {
            NSLog(@"exception: %@", exception.reason);
            return nil;
        }
        
    }else {
        return [self objectAtSafeIndex:index];
    }
}

- (id)mutableObjectAtSafeIndex:(NSUInteger)index
{
    if (index > self.count - 1 || !self.count) {
        @try {
            return [self mutableObjectAtSafeIndex:index];
        }
        @catch (NSException *exception) {
            NSLog(@"exception: %@", exception.reason);
            return nil;
        }
        
    }else {
        return [self mutableObjectAtSafeIndex:index];
    }
}

@end