预防 app crash 之 unrecognized selector

Posted 2020-09-24 小敏的博客

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：

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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

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？

在app启动加载一些系统方法，总是莫名其名的 报错 甚至crash

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

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