Objective-C通过runtime替换类方法

项目中用到了微信语音开放平台的SDK，可以实现语音读文字的功能，但是在iPhone X上一点就崩溃，查看日志，是崩在了这个方法[WXVoiceInformation getNetworkType]，查看log

Trapped uncaught exception 'NSUnknownKeyException', reason: '[ valueForUndefinedKey:]: this class is not key value coding-compliant for the key foregroundView.'

具体原因是iPhone X的状态栏UI层级发生了变化，多了一层，直接通过foregroundView这个key取的话就会崩溃（没想到微信语音的SDK用这样的方法）。
因为时间比较紧，再换其他第三方SDK来不及，所以就想着通过Method Swizzling替换这个方法。
首先想到的是通过Category在load方法里替换WXVoiceInformation的getNetworkType方法，但是WXVoiceInformation这个类并没有暴露出来，是SDK中的私有类，常规的通过Category进行黑魔法替换的路就要变一变了。
参考了网上的资料，具体的思路是新建一个类，命名为WXVoiceInformationHook，在AppDelegate中直接调用[WXVoiceInformationHook hook]进行替换，否则不起作用哦。直接上代码吧，也不多解释了，看注释吧
WXVoiceInformationHook.h如下

#import 

@interface WXVoiceInformationHook : NSObject

+ (void)hook;
- (NSString *)hook_getNetworkType;

@end

WXVoiceInformationHook.m如下

#import "WXVoiceInformationHook.h"

@implementation WXVoiceInformationHook

- (NSString *)hook_getNetworkType {
    return [WXVoiceInformationHook getNetWorkStates];
}

+ (void)hook  {
    Class aClass = objc_getClass("WXVoiceInformation");
    SEL sel = @selector(hook_getNetworkType);
    class_addMethod(aClass, sel, class_getMethodImplementation([self class], sel), "@");
    // 交换实现
    exchangeMethod(aClass, @selector(getNetworkType), sel);
}

void exchangeMethod(Class aClass, SEL oldSEL, SEL newSEL)
{
    //注意，因为getNetworkType是类方法所以要用class_getClassMethod
    Method oldMethod = class_getClassMethod(aClass, oldSEL);
    assert(oldMethod);
    //这里类方法和实例方法都可以用class_getInstanceMethod，求大神解释下
    Method newMethod = class_getInstanceMethod(aClass, newSEL);
    assert(newMethod);
    method_exchangeImplementations(oldMethod, newMethod);
}

+ (NSString *)getNetWorkStates{
    NSArray *children;
    UIApplication *app = [UIApplication sharedApplication];
    NSString *state = [[NSString alloc] init];
    //iPhone X
    if ([[app valueForKeyPath:@"_statusBar"] isKindOfClass:NSClassFromString(@"UIStatusBar_Modern")]) {
        children = [[[[app valueForKeyPath:@"_statusBar"] valueForKeyPath:@"_statusBar"] valueForKeyPath:@"foregroundView"] subviews];
        for (UIView *view in children) {
            for (id child in view.subviews) {
                //wifi
                if ([child isKindOfClass:NSClassFromString(@"_UIStatusBarWifiSignalView")]) {
                    state = @"wifi";
                }
                //2G 3G 4G
                if ([child isKindOfClass:NSClassFromString(@"_UIStatusBarStringView")]) {
                    if ([[child valueForKey:@"_originalText"] containsString:@"G"]) {
                        state = [child valueForKey:@"_originalText"];
                    }
                }
            }
        }
        if (state.length == 0) {
            state = @"无网络";
        }
    } else {
        children = [[[app valueForKeyPath:@"_statusBar"] valueForKeyPath:@"foregroundView"] subviews];
        for (id child in children) {
            if ([child isKindOfClass:NSClassFromString(@"UIStatusBarDataNetworkItemView")]) {
                //获取到状态栏
                switch ([[child valueForKeyPath:@"dataNetworkType"] intValue]) {
                    case 0:
                        state = @"无网络";
                        //无网模式
                        break;
                    case 1:
                        state = @"2G";
                        break;
                    case 2:
                        state = @"3G";
                        break;
                    case 3:
                        state = @"4G";
                        break;
                    case 5:
                        state = @"wifi";
                        break;
                    default:
                        break;
                }
            }
        }
    }
    return state;
}

@end