译 ios 使用运行时规避数组等越界导致程序崩溃

作者: 瞌睡树懒 | 来源:发表于2019-01-15 13:18 被阅读7次

项目中经常遇到数组越界的情况，这是个很烦人的问题，并且面试中常遇到，就思考写了下如何避免这样的问题。
首先我们获取数组元素的方式分为:

    NSArray *array = @[@1,@2];
    NSLog(@"arrary: %@",[array objectAtIndex:2]);
    NSLog(@"arrary: %@",array[2]);
    
    NSMutableArray *mutArray = [NSMutableArray arrayWithArray:@[@1,@2]];
    NSLog(@"NSMutableArray: %@",[mutArray objectAtIndex:2]);
    NSLog(@"NSMutableArray: %@",mutArray[2]);

是的，通过objectAtIndex和[] 方式。

然后我们是不是第一个想法就是写分类然后重写？
嗯，我的确是试了一下，然后发现并没有用，系统这样提示我:

Category is implementing a method which will also be implemented by its primary class

WTF? 警告，说这里重写也没有用？那怎么办？
而且还有一个问题就是，objectAtIndex方法很明确了，那么[] 这是个什么鬼？这也能用方法来表示？是的。
有两种方式可以知道：
第一种直接看错误:

2017-12-29 19:25:36.510723+0800 OCTest[39152:4167702] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayI objectAtIndexedSubscript:]: index 3 beyond bounds [0 .. 1]'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007fff4150700b __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x00007fff680e5c76 objc_exception_throw + 48
    2   CoreFoundation                      0x00007fff41548514 _CFThrowFormattedException + 202
    3   CoreFoundation                      0x00007fff415b9201 -[__NSArrayI objectAtIndexedSubscript:] + 97
    4   OCTest                              0x0000000100000e55 main + 245
    5   libdyld.dylib                       0x00007fff68cd5115 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException

我们人为越界了一下，发现了错误所在
[__NSArrayI objectAtIndexedSubscript:]
是的，这里告诉我们就是它发生了错误.
第二种：

在main.m文件中写下如下代码:
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSArray *array = @[@1,@2];
        NSLog(@"arrary: %@",[array objectAtIndex:2]);
        NSLog(@"arrary: %@",array[3]);
    }
    return 0;
}

然后通过命令行:
clang -rewrite-objc main.m
得到
main.cpp
拉倒最后:
int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        NSArray *array = ((NSArray *(*)(Class, SEL, ObjectType  _Nonnull const * _Nonnull, NSUInteger))(void *)objc_msgSend)(objc_getClass("NSArray"), sel_registerName("arrayWithObjects:count:"), (const id *)__NSContainer_literal(2U, ((NSNumber *(*)(Class, SEL, int))(void *)objc_msgSend)(objc_getClass("NSNumber"), sel_registerName("numberWithInt:"), 1), ((NSNumber *(*)(Class, SEL, int))(void *)objc_msgSend)(objc_getClass("NSNumber"), sel_registerName("numberWithInt:"), 2)).arr, 2U);
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_2d_q947d5pn4z3dfyq0j4vqsq840000gn_T_main_9976d7_mi_0,((id (*)(id, SEL, NSUInteger))(void *)objc_msgSend)((id)array, sel_registerName("objectAtIndexedSubscript:"), (NSUInteger)1));
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_2d_q947d5pn4z3dfyq0j4vqsq840000gn_T_main_9976d7_mi_1,((id (*)(id, SEL, NSUInteger))(void *)objc_msgSend)((id)array, sel_registerName("objectAtIndex:"), (NSUInteger)1));
    }
    return 0;
}

通过代码我们也知道了[]调用的方式:
sel_registerName("objectAtIndexedSubscript:")
里面的objectAtIndexedSubscript方法就是当我们使用[]的时候底层调用的方法。
当然，它也无法重写。
知道了原因，我们却无法重写，真是一个悲伤的故事。于是我找啊找啊，就找到了一种可以替代方法的方法。不给上，哥就不上了? 那也太怂了是吧。
我找到的就是runtime中的替换方法。
官方解释网址:

https://developer.apple.com/documentation/objectivec/1418530-class_getinstancemethod

Method class_getInstanceMethod(Class cls, SEL name);
Returns a specified instance method for a given class.
为给定的类返回指定的实例方法。
我们先通过这个方法获取指定的实例方法
https://developer.apple.com/documentation/objectivec/1418769-method_exchangeimplementations

void method_exchangeImplementations(Method m1, Method m2);
Exchanges the implementations of two methods.
交换两种方法的实现。

然后我们再自己实现个方法，这方法里面我们做一下规避操作。比如用try catch把异常给捕捉起来，然后打上日志。就不用担心会崩溃，也不知道哪里发生了错误。当然具体的方法要根据业务场景自我实现。
下面的__NSArrayI 和__NSArrrayM分别代表不可变数组和可变数组的真实类型

NSLog(@"type of array:%@",[array class]);
NSLog(@"type of mutableArray:%@",[mArray class]);

结果如下:
2017-12-29 19:46:08.048887+0800 OCTest[39319:4202006] type of array:__NSArrayI
2017-12-29 19:46:08.048905+0800 OCTest[39319:4202006] type of mutableArray:__NSArrayM

通过runtime的method swizzling 交换方法的实现提前判断方法的参数是否符合要求

#import "NSArray+LXSRuntime.h"
#import <objc/runtime.h>
@implementation NSArray (LXSRuntime)
+(void)load
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // 替换不可变数组中的方法 objectAtIndex
        Method oldObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(objectAtIndex:));
        Method safeObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(safeObjectAtIndex:));
        method_exchangeImplementations(oldObjectAtIndex, safeObjectAtIndex);
        // 替换不可变数组中的方法 []调用的方法
        Method oldMutableObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(objectAtIndexedSubscript:));
        Method safeMutableObjectAtIndex =  class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(safeObjectAtIndexedSubscript:));
        method_exchangeImplementations(oldMutableObjectAtIndex, safeMutableObjectAtIndex);
        
        // 替换可变数组中的方法 objectAtIndex
        Method oldMObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(objectAtIndex:));
        Method safeMObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(safeMutableObjectAtIndex:));
        method_exchangeImplementations(oldMObjectAtIndex, safeMObjectAtIndex);
        // 替换可变数组中的方法  []调用的方法
        Method oldMMutableObjectAtIndex = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(objectAtIndexedSubscript:));
        Method safeMMutableObjectAtIndex =  class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(safeMutableObjectAtIndexedSubscript:));
        method_exchangeImplementations(oldMMutableObjectAtIndex, safeMMutableObjectAtIndex);
        
        Method sourceMethod2 = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(removeObjectAtIndex:));
        Method destMethod2 = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(safeRemoveObjectAtIndex:));
        
        method_exchangeImplementations(sourceMethod2, destMethod2);
        
        Method sourceMethod3 = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(insertObject:atIndex:));
        Method destMethod3 = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(safeInsertObject:atIndex:));
        
        method_exchangeImplementations(sourceMethod3, destMethod3);
        
        Method sourceMethod4 = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(replaceObjectAtIndex:withObject:));
        Method destMethod4 = class_getInstanceMethod(objc_getClass("__NSArrayM"), @selector(safeReplaceObjectAtIndex:withObject:));
        
        method_exchangeImplementations(sourceMethod4, destMethod4);
        
    });
}
#pragma mark - 数组的安全处理
- (id)safeObjectAtIndex:(NSUInteger)index{
    if (index > self.count - 1 || !self.count){
        @try {
            return [self safeObjectAtIndex:index];
        } @catch (NSException *exception) {
            NSLog(@"不可数组越界了");
            return nil;
        } @finally {
            
        }
    }
    else{
        return [self safeObjectAtIndex:index];
    }
}

- (id)safeObjectAtIndexedSubscript:(NSUInteger)index{
    if (index > self.count - 1 || !self.count){
        @try {
            return [self safeObjectAtIndexedSubscript:index];
        } @catch (NSException *exception) {
            NSLog(@"不可数组越界了");
            return nil;
        } @finally {
        }
    }
    else{
        return [self safeObjectAtIndexedSubscript:index];
    }
}
- (id)safeMutableObjectAtIndex:(NSUInteger)index{
    if (index > self.count - 1 || !self.count){
        @try {
            return [self safeMutableObjectAtIndex:index];
        } @catch (NSException *exception) {
            NSLog(@"可变数组越界了");
            return nil;
        } @finally {
            
        }
    }
    else{
        return [self safeMutableObjectAtIndex:index];
    }
}

- (id)safeMutableObjectAtIndexedSubscript:(NSUInteger)index{
    if (index > self.count - 1 || !self.count){
        @try {
            return [self safeMutableObjectAtIndexedSubscript:index];
        } @catch (NSException *exception) {
            NSLog(@"可变数组越界了");
            return nil;
        } @finally {
        }
    }
    else{
        return [self safeMutableObjectAtIndexedSubscript:index];
    }
}

-(void)safeRemoveObjectAtIndex:(NSInteger)index
{
    if (self.count <= index) {
        NSLog(@"Runtime Warning:index %li out of bound",index);
        return;
    }
    
    [self safeRemoveObjectAtIndex:index];
}
-(void)safeInsertObject:(id)object atIndex:(NSInteger)index
{
    if (!object) {
        NSLog(@"Runtime Warning:insert object can not be nil");
        return;
    }
    
    if (self.count < index) {
        NSLog(@"Runtime Warning:insert object at index %li out of bound",index);
        return;
    }
    
    [self safeInsertObject:object atIndex:index];
}
-(void)safeReplaceObjectAtIndex:(NSInteger)index withObject:(id)object
{
    if (index >= self.count) {
        NSLog(@"Runtime Warning:index %li out of bound",index);
        return;
    }
    
    if (!object) {
        NSLog(@"Runtime Warning:object can not be empty");
        return;
    }
    
    [self safeReplaceObjectAtIndex:index withObject:object];
}


@end

译 ios 使用运行时规避数组等越界导致程序崩溃

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