IOS 降低线上版本Crash率

IOS 防止Crash 组件WTSafeGuard

背景

由于Object-C本身的不安全性，导致很容易产生Crash。在这些Crash，很多我们可以利用自定义手段，进行避免。这样可以降低线上版本的Crash率，提升用户
体验。WTSafeGuard 避免APP Crash 组件，目前能做到的还很有限。

UIKit Called on Non-Main Thread

UIKit不是线程安全的，执行UIKit操作如果不在主线程很可能造成程序Crash。所以我们对Hook，UIView 的setNeedsLayout，layoutIfNeeded，layoutSubviews，setNeedsUpdateConstraints方法。如果执行以上函数没有在主队列，通过强行将执行代码，在主队列执行。

- (void)wt_safe_setNeedsLayout
{
    if(![NSThread isMainThread]){
        dispatch_async(dispatch_get_main_queue(), ^{
            NSAssert(false, @"wt_safe_setNeedsLayout failed");
            [self wt_safe_setNeedsLayout];
        });
    }else{
        [self wt_safe_setNeedsLayout];
    }
}

- (void)wt_safe_layoutIfNeeded
{
    if(![NSThread isMainThread]){
        dispatch_async(dispatch_get_main_queue(), ^{
            NSAssert(false, @"wt_safe_layoutIfNeeded failed");
            [self wt_safe_layoutIfNeeded];
        });
    }else{
        [self wt_safe_layoutIfNeeded];
    }
}

- (void)wt_safe_layoutSubviews
{
    if(![NSThread isMainThread]){
        dispatch_async(dispatch_get_main_queue(), ^{
            NSAssert(false, @"wt_safe_layoutSubviews failed");
            [self wt_safe_layoutSubviews];
        });
    }else{
        [self wt_safe_layoutSubviews];
    }
}

- (void)wt_safe_setNeedsUpdateConstraints
{
    if(![NSThread isMainThread]){
        dispatch_async(dispatch_get_main_queue(), ^{
            NSAssert(false, @"wt_safe_setNeedsUpdateConstraints failed");
            [self wt_safe_setNeedsUpdateConstraints];
        });
    }else{
        [self wt_safe_setNeedsUpdateConstraints];
    }
}

避免 Foundation 类Carsh

NSString

+ (instancetype)stringWithUTF8String:(const char *)bytes
- (instancetype)initWithString:(NSString *)aString
- (instancetype)initWithUTF8String:(const char *)nullTerminatedCString
- (instancetype)initWithFormat:(NSString *)format locale:(id)locale arguments:(va_list)argList
- (NSString *)stringByAppendingString:(NSString *)aString
- (unichar)characterAtIndex:(NSUInteger)index
- (void)getCharacters:(unichar *)buffer range:(NSRange)range
- (NSRange)rangeOfCharacterFromSet:(NSCharacterSet  *)searchSet 
                            options:(NSStringCompareOptions)mask
                              range:(NSRange)searchRange
                                    
- (NSRange)rangeOfString:(NSString *)searchString
                options:(NSStringCompareOptions)mask
                  range:(NSRange)searchRange
                 locale:(NSLocale *)locale
- (NSString *)substringFromIndex:(NSUInteger)from
- (NSString *)substringWithRange:(NSRange)range
- (NSString *)substringToIndex:(NSUInteger)to
- (void)getLineStart:(NSUInteger *)startPtr   
                 end:(NSUInteger *)lineEndPtr                                         
                 contentsEnd:(NSUInteger *)contentsEndPtr
                   forRange:(NSRange)range

NSAttributedString

hook 方法：对传入参数range 进行check，如果range有问题，直接返回nil

- (NSAttributedString *)attributedSubstringFromRange:(NSRange)range;

NSFileManager

- (nullable NSDirectoryEnumerator<NSURL *> *)enumeratorAtURL:(NSURL *)url includingPropertiesForKeys:(nullable NSArray<NSURLResourceKey> *)keys options:(NSDirectoryEnumerationOptions)mask errorHandler:(nullable BOOL (^)(NSURL *url, NSError *error))handler

NSIndexPath

- (void)getIndexes:(NSUInteger *)indexes range:(NSRange)positionRang

NSJSONSerialization

+ (NSData *)dataWithJSONObject:(id)obj options:(NSJSONWritingOptions)opt error:(NSError **)error

NSDictionary

hook 方法：

+ (id)sharedKeySetForKeys:(NSArray<KeyType <NSCopying>> *)keys
- (instancetype)initWithObjects:(const ObjectType _Nonnull [_Nullable])objects forKeys:(const KeyType <NSCopying> _Nonnull [_Nullable])keys
- (instancetype)initWithObjects:(const ObjectType _Nonnull [_Nullable])objects forKeys:(const KeyType <NSCopying> _Nonnull [_Nullable])keys count:(NSUInteger)cnt

NSMutableDictionary

hook 方法：

+ (NSMutableDictionary<KeyType, ObjectType> *)dictionaryWithSharedKeySet:(id)keyset
- (void)setObject:(ObjectType)anObject forKey:(KeyType <NSCopying>)aKey;
- (void)removeObjectForKey:(KeyType)aKey;

NSSet

- (instancetype)WT_initWithObjects:(const id [])objects count:(NSUInteger)cnt
- (void)addObject:(id)object;
- (void)makeObjectsPerformSelector:(SEL)aSelector
- (void)makeObjectsPerformSelector:(SEL)aSelector
                        withObject:(id)argument

NSMutableSet

- (void)addObject:(id)anObject

NSMutableString

- (void)setString:(NSString *)aString
- (void)appendString:(NSString *)aString
- (void)deleteCharactersInRange:(NSRange)range
- (void)insertString:(NSString *)aString atIndex:(NSUInteger)loc
- (void)replaceCharactersInRange:(NSRange)range withString:(NSString *)aString
-  (NSUInteger)replaceOccurrencesOfString:(NSString *)target
                                     withString:(NSString *)replacement
                                        options:(NSStringCompareOptions)options
                                          range:(NSRange)searchRange

NSURL

+ (NSURL *)fileURLWithPath:(NSString *)path
+ (NSURL *)fileURLWithPath:(NSString *)path isDirectory:(BOOL)isDir
+ (NSURL *)fileURLWithPathComponents:(NSArray<NSString *> *)components
+ (NSURL *)fileURLWithPath:(NSString *)path
                      isDirectory:(BOOL)isDir
                    relativeToURL:(NSURL *)baseURL
- (instancetype)initWithString:(NSString *)URLString relativeToURL:(NSURL *)baseURL
- (instancetype)initFileURLWithPath:(NSString *)path
- (instancetype)initFileURLWithPath:(NSString *)path
                             relativeToURL:(NSURL *)baseURL
- (instancetype)initFileURLWithPath:(NSString *)path
                               isDirectory:(BOOL)isDir
                             relativeToURL:(NSURL *)baseURL

2. KVO
3. 容器越界（NSArray， NSDictionary,...）
4. unrecognized selector crash (这个很多时候是由于class使用错误导致)
5. NSTimer 导致crash

---

KVO Crash

项目中KVO crash 占比很高， 主要原因为，添加删除不对称导致。
解决方法为，添加Map进行缓存。
不过这个方案，目前还有缺陷。

unrecognized selector crash

这个就比较简单了，直接上代码：

    [NSObject jr_swizzleMethod:@selector(forwardingTargetForSelector:) withMethod:@selector(WT_safeForwardingTargetForSelector:) error:&error];
    
    - (id)WT_safeForwardingTargetForSelector:(SEL)aSelector
{
    NSMethodSignature *signature = [self methodSignatureForSelector:aSelector];
    if ([self respondsToSelector:aSelector] || signature) {
        return [self WT_safeForwardingTargetForSelector:aSelector];
    }
    
    return [WTSafeGuard createFakeForwardTargetObject:self selector:aSelector];
}