第四章 Runtime应用：方法转换

swizzling应该只在+load中完成。
在 Objective-C 的运行时中，每个类有两个方法都会自动调用。
1、+load 是在一个类被初始装载时调用；
2、+initialize 是在应用第一次调用该类的类方法或实例方法前调用的。
两个方法都是可选的，并且只有在方法被实现的情况下才会被调用。

swizzling应该只在dispatch_once中完成，由于swizzling改变了全局的状态，所以我们需要确保每个预防措施在运行时都是可用的。
原子操作就是这样一个用于确保代码只会被执行一次的预防措施，就算是在不同的线程中也能确保代码只执行一次。
Grand Central Dispatch的dispatch_once满足了所需要的需求，并且应该被当做使用swizzling的初始化单例方法的标准。

代码示例
接口文件.h

#import <Foundation/Foundation.h>

@interface NSMutableArray (RunTime)

@end

实现文件.m

#import "NSMutableArray+RunTime.h"
#import <objc/runtime.h>

@implementation NSMutableArray (RunTime)

#pragma mark - 方法转换

+ (void)load
{
    // 方法1
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // 无效
//        Class class = [self class];
        // 有效
        Class class = NSClassFromString(@"__NSArrayM");

        SEL originalSelector = @selector(addObject:);
        SEL swizzledSelector = @selector(addObjectSafe:);

        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

        // judge the method named  swizzledMethod is already existed.
        BOOL didAddMethod = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
        // if swizzledMethod is already existed.
        if (didAddMethod)
        {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        }
        else
        {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });

    // 方法2
    // 无效（异常：Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'）
//        Class class = [self class];
    // 有效
//    Class class = NSClassFromString(@"__NSArrayM");
//    Method oldMethod = class_getInstanceMethod(class, @selector(addObject:));
//    Method newMethod = class_getInstanceMethod(class, @selector(addObjectSafe:));
//    method_exchangeImplementations(oldMethod, newMethod);
}

- (void)addObjectSafe:(id)anObject
{
    if (anObject)
    {
        [self addObjectSafe:anObject];
    }
}

@end

使用

#import "ViewController.h"
// 导入头文件
#import "NSMutableArray+RunTime.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    self.title = @"runtime";

    self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"click" style:UIBarButtonItemStyleDone target:self action:@selector(buttonClick)];

    // 方法转换
    NSMutableArray *array = [NSMutableArray arrayWithObjects:@"1", @"2", nil];
    NSLog(@"1 array: %@", array);
    [array addObject:@"devZhang"];
    NSLog(@"2 array: %@", array);
    NSString *item = nil;
    [array addObject:item]; // 执行该方法时不会闪退
    NSLog(@"3 array: %@", array);
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end