macOS 12.0.1 (Monterey) XIB 加载失败；抛出异常 "This coder is expecting the replaced object ... to be returned from NSClassSwapper"

Question

我有包含自定义对象的 XIB，其中之一实际上是一个 class 集群，其 -init 方法总是 returns 相同的单例对象。

基本上：

- (instancetype)init
{
    self = [super init];
    if (HelpLinkHelperSingleton==nil)
        {
        // This is the first instance of DDHelpLink: make it the immortal singleton
        HelpLinkHelperSingleton = self;
        }
    else
        {
        // Not the first DDHelpLink object to be created: discard this instance
        //  and return a reference to the shared singleton
        self = HelpLinkHelperSingleton;
        }
    return self;
}

从 macOS 12.0.1 开始，加载 XIB 会抛出此异常：

This coder is expecting the replaced object 0x600002a4f680 to be returned from NSClassSwapper.initWithCoder instead of <DDHelpLink: 0x600002a487a0>

我尝试实施 <NSSecureCoding> 并做同样的事情，但这也不起作用。

还有办法在 NIB 中使用 class 集群吗？

Answer 1

我通过使用 XIB 中的代理对象将消息转发给单例解决了这个问题。

@interface HelpLinkHelperProxy : NSObject
@end

@implementation HelpLinkHelperProxy
{
    HelpLinkHelper* _singleton;
}

- (void) forwardInvocation:(NSInvocation*)invocation
{
    if (_singleton == nil)
    {
        _singleton = [HelpLinkHelper new];
    }

    if ([_singleton respondsToSelector:[invocation selector]])
    {
        [invocation invokeWithTarget:_singleton];
    }
    else
    {
        [super forwardInvocation:invocation];
    }
}

@end

如果我们从 NSProxy 而不是 NSObject 继承子类，解决方案将如下所示：

@interface HelpLinkHelperProxy : NSProxy
@end

@implementation HelpLinkHelperProxy
{
    HelpLinkHelper* _singleton;
}

- (instancetype) init
{
    _singleton = [HelpLinkHelper new];
    return self;
}

- (NSMethodSignature*) methodSignatureForSelector:(SEL)sel
{
    return [_singleton methodSignatureForSelector:sel];
}

- (void) forwardInvocation:(NSInvocation*)invocation
{
    if ([_singleton respondsToSelector:[invocation selector]])
    {
        [invocation invokeWithTarget:_singleton];
    }
    else
    {
        [super forwardInvocation:invocation];
    }
}

+ (BOOL) respondsToSelector:(SEL)aSelector
{
    return [HelpLinkHelper respondsToSelector:aSelector];
}

@end