Runtime 四之交换方法防止整个项目数组越界

数组取数据组越界的问题我想每个程序都遇到过无数次，我们这章不谈如何解决bug，不谈这样做
是否正确， 只谈如何通过runtime交换objectatidex方法导致crash的问题用于对runtime的
理解扩展(!!!注意，此方法并不会解决我们代码所产生的bug，仅仅会防止整个项目中数组的
越界crash问题，所以不要以为这样做了就可以不管bug了)

同上一章，我们需要建一个pch文件加到prefix headder中，在pch中我们包含这个category

众所周知try catch方法可以让编译器在遇到crash情况时抛出异常而不至于crash，那我们就通过
替换经常crash的objectatindex方法替换为try catch的lxzObjectAtIndex来实现

[demo传送门]https://github.com/joy-make/nsarrayBounds.git

category具体实现如下

#import "NSArray+LXZArray.h"
@implementation NSArray (LXZArray)

+(void)load{
[super load];
//定义方法a
  Method fromMethod = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(objectAtIndex:));
//定义方法b
  Method toMethod = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(lxzObjectAtIndex:));
//交换方法a和b
  method_exchangeImplementations(fromMethod, toMethod);
}

- (id)lxzObjectAtIndex:(NSInteger)index{
if (self.count-1

测试一下

@interface ViewController (){
    NSTimer *_timer;
}

@property (weak, nonatomic) IBOutlet UILabel *displayLabel;
@property (nonatomic,strong)NSArray *listArray;
@end

@implementation ViewController

static int repeats = 0;

- (void)viewDidLoad {
    
    [super viewDidLoad];
    
    __weak __typeof(&*self)weakSelf = self;
    _timer = [NSTimer scheduledTimerWithTimeInterval:0.4 repeats:10 block:^(NSTimer * _Nonnull timer) {
        weakSelf.displayLabel.text = repeats>=weakSelf.listArray.count-1?[NSString stringWithFormat:@"第%d个元素应该crash但没有crash",repeats++]:weakSelf.listArray[repeats++];
    }];
    _timer.fireDate = [NSDate distantFuture];

}

-(NSArray *)listArray{
    return _listArray =_listArray?:@[@"0",@"1",@"2"];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

- (IBAction)testListCrash:(id)sender {
    repeats = 0;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    _timer.fireDate = [NSDate distantPast];
    });
}