从NSKeyedArchiver加载Singleton的状态 [英] Loading a Singleton's state from NSKeyedArchiver

问题描述

我有一个类，我已经做了一个单身汉，并能够使用NSKeyedArchiver保存它的状态，但我不能封装我的头围拉它的状态退出。

I have a class that I've made into a singleton and am able to save it's state using an NSKeyedArchiver, but I can't wrap my head around pulling it's state back out.

在加载的函数中有

Venue *venue = [Venue sharedVenue];
NSData *data = [[NSMutableData alloc] initWithContentsOfFile:[self dataFilePath]];
NSKeyedUnarchiver *unarchiver = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
venue = [unarchiver decodeObjectForKey:@"Venue"];
[unarchiver finishDecoding];

使用此代码，decodeObjectForKey会返回什么？它不能真正是Venue的另一个实例，它不会加载任何保存的值。

With this code, what does decodeObjectForKey return? It can't really be another instance of Venue and it's not loading in any of the saved values. Before I converted it to a singleton saving and loading was working fine.

推荐答案

这里是我认为这是错误的。你熟悉NSCoding，通常采用它通过使你的对象可编码通过encodeWithCoder：和initWithCoder：覆盖。什么会使这更简单是，你仍然可以使用NSCoding和NSCoders，而不重写这些方法。您可以对共享对象的状态进行编码，而不对共享对象本身进行编码。

Here's where I think this is going wrong. You're familiar with NSCoding and typically adopting it by making your object codable via encodeWithCoder: and initWithCoder: overrides. What will make this all simpler is that you can still use NSCoding, and NSCoders without overriding those methods. You can encode the state of the shared object without encoding the shared obejct itself. This prevents the awkward question of decoding the shared object.

这是一个我认为你可以做的例子：

Here's an example of what I think you could do:

@implementation MySharedObject
+ (id)sharedInstance {
    static id sharedInstance = nil;
    if (!sharedInstance) {
        sharedInstance = [[MyClass alloc] init];
    }
}

- (id)init {
    if ((self = [super init])) {
        NSData *data = /* probably from user defaults */
        if (data) { // Might not have any initial state.
            NSKeyedUnarchiver *coder = [[[NSKeyedUnarchiver alloc] initForReadingWithData:data] autorelease];
            myBoolean = [coder decodeBoolForKey:@"MyBoolean"];
            myObject = [[coder decodeObjectForKey:@"MyObject"] retain];
            [coder finishDecoding];
        }
    }
    return self;
}

- (void)saveState {
    NSMutableData *data = [NSMutableData data];
    NSKeyedArchiver *coder = [[[NSKeyedArchiver alloc] initForWritingWithMutableData:data] autorelease];
    [coder encodeBool:myBoolean forKey:@"MyBoolean"];
    [coder encodeObject:myObject forKey:@"MyObject"];
    [coder finishEncoding]
    // Save the data somewhere, probably user defaults...   
}
@end

这里我们有一个共享对象，它使用keyed归档来保持配置，但是我们不对共享对象本身进行编码。这避免了解码单例类的第二个实例的尴尬问题。

Here we have a shared object, and it uses keyed archive to persist configuration, but we don't encode the shared object itself. This avoid that awkward question of decoding a second instance of the singleton class.

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