Question

我正在尝试使用NSKeyedArchiver在iOS应用程序上保存一些持久数据以写入文件，我想稍后使用NSKeyedUnarchiver检索此数据。我创建了一个非常基本的应用程序来测试一些代码，但没有成功。以下是我正在使用的方法：

- (void)viewDidLoad
{
    [super viewDidLoad];

    Note *myNote = [self loadNote];

    myNote.author = @"MY NAME";

    [self saveNote:myNote]; // Trying to save this note containing author's name

    myNote = [self loadNote]; // Trying to retrieve the note saved to the file

    NSLog(@"%@", myNote.author); // Always logs (null) after loading data
}

-(NSString *)filePath
{
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,  NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *filePath = [documentsDirectory stringByAppendingPathComponent: @"myFile"];
    return filePath;
}

-(void)saveNote:(Note*)note
{
    bool success = [NSKeyedArchiver archiveRootObject:note toFile:[self filePath]];
    NSLog(@"%i", success); // This line logs 1 (success)
}

-(Note *)loadNote
{
    return [NSKeyedUnarchiver unarchiveObjectWithFile:[self filePath]];
}

我用来测试此代码的类如下：

Note.h

#import <Foundation/Foundation.h>

@interface Note : NSObject <NSCoding>

@property NSString *title;
@property NSString *author;
@property bool published;

@end

Note.m

#import "Note.h"

@implementation Note

-(id)initWithCoder:(NSCoder *)aDecoder
{
    if (self = [super init])
    {
        self.title = [aDecoder decodeObjectForKey:@"title"];
        self.author = [aDecoder decodeObjectForKey:@"author"];
        self.published = [aDecoder decodeBoolForKey:@"published"];
    }
    return self;
}

-(void)encodeWithCoder:(NSCoder *)aCoder
{
    [aCoder encodeObject:self.title forKey:@"title"];
    [aCoder encodeObject:self.author forKey:@"author"];
    [aCoder encodeBool:self.published forKey:@"published"];
}

@end

我已经看过使用NSUserDefaults（https://blog.soff.es/archiving-objective-c-objects-with-nscoding）的类似示例，但我想将此数据保存到文件中，因为据我所知NSUserDefaults主要用于存储用户首选项，而不是一般数据。我错过了什么吗？提前谢谢。

Answer 1

考虑一下您的应用首次运行时会发生什么，并且您调用loadNote:方法并且尚未保存任何内容。

该行：

Note *myNote = [self loadNote];

将导致myNote为nil，因为没有加载任何内容。现在想想如何在代码的其余部分中进行级联。

您需要处理没有保存数据的初始情况。

Note *myNote = [self loadNote];
if (!myNote) {
    myNote = [[Note alloc] init];
    // Setup the initial note as needed
    myNote.title = ...
}

无法使用NSKeyedArchiver / NSKeyedUnarchiver在文件上设置/检索数据

1 个答案: