预防 app crash 之 unrecognized selector

曾无好事来相访，赖尔高文一起予。这篇文章主要讲述预防 app crash 之 unrecognized selector相关的知识，希望能为你提供帮助。
处理unrecognized selector异常原因假如封装一个方法，在其他模块调用该方法时，传入参数不匹配则crash。比如下面的方法：本应该传入的参数类型为NSMutableArray，如果传入的参数类型是NSArray，导致抛出 unrecognized selector异常

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- (void)doSomethingWithArray:(NSMutableArray *)arr{ [arr addObject:@"123"]; }

当然，通过 参数类型判断 也可以避免问题的发生：

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- (void)doSomethingWithArray:(NSMutableArray *)arr{ if ([arr isKindOfClass:[NSMutableArray class]]) { [arr addObject:@"123"]; }else{ CrashOnSimulator(@"??参数类型不对哦??"); } }

crash提醒：

1 2 3 void CrashOnSimulator(NSString *errorMsg) { if((TARGET_OS_SIMULATOR)){raise(SIGSTOP); } }

但是，有点地方可能忘记类型判断了怎么办，有全局拦截unrecognized selector 异常的方案吗？
分析如何全局拦截unrecognized selector 异常

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oc的消息发送机制咱们都熟悉了，通过superclass指针逐级向上查找该消息所对应的方法实现，如果遇到找不的方法，还有三次补救机制。我们可以通过上面三种方法中的一种，就可以避免unrecognized selector sent to instance
第一种方法：重写 NSObject 的forwardingTargetForSelector:

??filter unrecoginze seletor of intance only

思路

创建一个接收未知消息的类，暂且称之为Protector
创建一个NSObject 的分类，在分类中重写forwardingTargetForSelector: ，在这个方法中截获未实现的方法，转发给Protector。并为Protector 动态的添加未实现的方法，最后返回Protector 的实例对象。
在分类中新增一个安全的方法实现，来作为Protector 接收到的未知消息的实现

代码

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#import "NSObject+Protector.h" #import < objc/runtime.h> @implementation NSObject (Protector) #pragma clang diagnostic push #pragma clang diagnostic ignored "-Wobjc-protocol-method-implementation" - (id)forwardingTargetForSelector:(SEL)aSelector{ if ([self isCurrentClassInWhiteList]) { [[self class] warningDeveloper:aSelector]; Class protectorCls = NSClassFromString(@"ProtectorClassName"); if (!protectorCls){ protectorCls = objc_allocateClassPair([NSObject class], "ProtectorClassName", 0); objc_registerClassPair(protectorCls); } if (![self isExistSelector:aSelector inClass:protectorCls]){ class_addMethod(protectorCls, aSelector, [self safeImplementation:aSelector],[NSStringFromSelector(aSelector) UTF8String]); } Class Protector = [protectorCls class]; id instance = [[Protector alloc] init]; return instance; } else { return nil; } } #pragma clang diagnostic pop - (BOOL)isCurrentClassInWhiteList{ NSArray *classNameArray = @[@"NSNull",@"NSString",@"NSArray",@"NSDictionary",@"NSURL"]; for (NSString *className in classNameArray) { if ([self isKindOfClass:NSClassFromString(className)]) { return YES; } } return NO; } - (BOOL)isExistSelector: (SEL)aSelector inClass:(Class)currentClass{ BOOL isExist = NO; unsigned int methodCount = 0; Method *methods = class_copyMethodList(currentClass, & methodCount); for (int i = 0; i < methodCount; i++){ Method temp = methods[i]; SEL sel = method_getName(temp); NSString *methodName = NSStringFromSelector(sel); if ([methodName isEqualToString: NSStringFromSelector(aSelector)]){ isExist = YES; break; } } return isExist; } - (IMP)safeImplementation:(SEL)aSelector{ IMP imp = imp_implementationWithBlock(^(){ NSLog(@"PROTECTOR: %@ Done", NSStringFromSelector(aSelector)); }); return imp; } + (void)warningDeveloper:(SEL)aSelector{ #if DEBUG NSString *selectorStr = NSStringFromSelector(aSelector); NSLog(@"PROTECTOR: -[%@ %@]", [self class], selectorStr); NSLog(@"PROTECTOR: unrecognized selector \"%@\" sent to instance: %p", selectorStr, self); NSLog(@"PROTECTOR: call stack: %@", [NSThread callStackSymbols]); // @throw @"方法找不到"; #endif } @end

第二种方法：重写 NSObject 的methodSignatureForSelector(有些问题，下面有个最终版)

??filter unrecoginze seletor of class and intance
but 如果你使用了JSPatch、Aspects等对methodSignatureForSelector进行swizzle的第三方库，就别用这种方案了，有冲突，出现莫名的错误

思路

创建NSObject+Protector重写methodSignatureForSelector，判断current class是否在白名单？
- YES:返回一个空签名，啥也不做
- NO:返回正常的签名，走原来的逻辑

代码

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#import "NSObject+Protector.h" #import < objc/runtime.h> @implementation NSObject (Protector) #pragma clang diagnostic push #pragma clang diagnostic ignored "-Wobjc-protocol-method-implementation" - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{ return [[self class] __getMethodSignatureForSelector:aSelector]; } - (void)forwardInvocation:(NSInvocation *)anInvocation{ } #pragma clang diagnostic pop + (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{ return [self __getMethodSignatureForSelector:aSelector]; } + (void)forwardInvocation:(NSInvocation *)anInvocation{ } + (NSMethodSignature *)__getMethodSignatureForSelector:(SEL)aSelector{ if ([self isSubclassInWhiteListClass]) { [self warningDeveloper:aSelector]; return [NSMethodSignature signatureWithObjCTypes:"@"]; }else{ return [self instanceMethodSignatureForSelector:aSelector]; } } + (BOOL)isSubclassInWhiteListClass{ NSArray *classNameArray = @[@"NSNull",@"NSString",@"NSArray",@"NSDictionary",@"NSURL"]; for (NSString *className in classNameArray) { if ([self isSubclassOfClass:NSClassFromString(className)]) { return YES; } } return NO; } + (void)warningDeveloper:(SEL)aSelector{ #if DEBUG NSString *selectorStr = NSStringFromSelector(aSelector); NSLog(@"PROTECTOR: -[%@ %@]", [self class], selectorStr); NSLog(@"PROTECTOR: unrecognized selector \"%@\" sent to instance: %p", selectorStr, self); NSLog(@"PROTECTOR: call stack: %@", [NSThread callStackSymbols]); //@throw @"方法找不到"; #endif }

第三种方法：最终方案(解决methodSignatureForSelector的不足)第三方库对 methodSignatureForSelector进行了全局替换，而我们也在NSObject中进行了全局替换，冲突的点在于我们影响了第三库的自定义的Class。< br\>

如何避免呢？我们替换常用的的几个class就行了呗，是的，不过工作量有点大且重复，怎么办？用 define 来解决，如下：

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//NSArray+WBGProtector.m #import "NSArray+WBGProtector.h" #import < objc/runtime.h> #define WBG_PROTECT_CLASS_NAME(_classname_)\ @implementation _classname_ (WBGProtector)\ \ - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{\ return [[self class] __getMethodSignatureForSelector:aSelector type:@"instance"]; \ }\ \ - (void)forwardInvocation:(NSInvocation *)anInvocation{\ }\ \ + (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{\ return [self __getMethodSignatureForSelector:aSelector type:@"class"]; \ }\ \ + (void)forwardInvocation:(NSInvocation *)anInvocation{\ }\ \ + (NSMethodSignature *)__getMethodSignatureForSelector:(SEL)aSelector type:(id)type{\ NSString *errorMsg = [NSString stringWithFormat:@"PROTECTOR: -[%@ %@],unrecognized selector sent to %@",self,NSStringFromSelector(aSelector),type]; \ NSLog(@"%@",errorMsg); \ CrashOnSimulator(errorMsg); \ return [NSMethodSignature signatureWithObjCTypes:"@"]; \ }\ \ @end\ WBG_PROTECT_CLASS_NAME(NSArray) WBG_PROTECT_CLASS_NAME(NSDictionary) WBG_PROTECT_CLASS_NAME(NSString)

细节分析为什么需要白名单？
【预防 app crash 之 unrecognized selector】在app启动加载一些系统方法，总是莫名其名的报错甚至crash

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为什么去掉UIResponder?
isCurrentClassInWhiteList 中的classNameArray 本来是有 UIResponder的，但是后来测试发现UIWebView会出现异常！这里把UIResponder去掉了，毕竟过滤大部分的unrecognize selector主要的是NSArray和NSDictionary

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测试code

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// test Class method id clazz = [NSArray class]; [clazz viewDidLoad]; // test instance method NSMutableArray *arr = @{}; [arr addObject:@""]

遗留问题

需要判断自己项目中引入的第三方库没有通过category的方式去重写NSObject的方法methodSignatureForSelector /forwardInvocation 以及forwardingTargetForSelector
- 原因：一个category也不能可靠的覆盖另一个category中相同的类的相同的方法。例如UIViewController+A与UIViewController+B，都重写了viewDidLoad，我们就无法控制谁覆盖了谁。
如果第三方重写了，则在这里通过swizzling的方式替换具体的实现方法