Question

所以当我的应用程序终止时，我正在尝试将一个自定义对象的NSMutableArray（一个代表Course Planner应用程序的大学课程的“课程”）写入文件，然后将该数组从文件中读取到相关的ViewController中将在应用程序启动时使用数据。

以下是相关代码：

CoursesAppDelegate.m：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    // Override point for customization after application launch.
    coursesViewController = [[SampleHomeScreen alloc] initWithNibName:@"SampleHomeScreen" bundle:nil];

    NSString *filePath = [self dataFilePath];
    if ([[NSFileManager defaultManager] fileExistsAtPath:filePath]) {
        [coursesViewController setCourses:[NSKeyedUnarchiver unarchiveObjectWithFile: filePath]];
    }

    UIApplication *app = [UIApplication sharedApplication];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationWillTerminate:)name:UIApplicationWillTerminateNotification object:app];

    [window addSubview:coursesViewController.view];
    [window makeKeyAndVisible];

    return YES;
}

- (NSString *)dataFilePath {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *path = [documentsDirectory stringByAppendingPathComponent:@"data.plist"];
    NSLog(@"%@", path);
    return path;
}

/**
 applicationWillTerminate: saves changes in the application's managed object context before the application terminates.
 */
- (void)applicationWillTerminate:(UIApplication *)application {
    NSLog(@"%@", [coursesViewController courses]);
    [NSKeyedArchiver archiveRootObject:[coursesViewController courses] toFile:[self dataFilePath]];
}

Course.h：

@interface Course : NSObject <NSCoding> {
    NSString *name; //e.g. ECS 189H
    double grade, totalWeight; //course grade in %
    NSMutableArray *list;
}

@property (nonatomic, retain) NSString *name;
@property (nonatomic) double grade, totalWeight;
@property (nonatomic, retain) NSMutableArray *list;

-(Course *)initWithName:(NSString *)courseName;

@end

Course.m：

@implementation Course

@synthesize name, grade, totalWeight, list;

-(Course *)initWithName:(NSString *)courseName {
    name = [courseName retain];
    grade = -1.0;
    totalWeight = 0.0;
    list = [[NSMutableArray alloc] init];
    [super init];
    return self;
}

-(Course *)initWithCoder:(NSCoder *)aDecoder {
    self.name = [aDecoder decodeObjectForKey:@"name"];
    self.grade = [aDecoder decodeDoubleForKey:@"grade"];
    self.totalWeight = [aDecoder decodeDoubleForKey:@"totalWeight"];
    self.list = [aDecoder decodeObjectForKey:@"list"];
    [super init];
    return self;
}

- (void) encodeWithCoder: (NSCoder *)coder
{
    [coder encodeObject:name forKey:@"name"];   
    [coder encodeDouble:grade forKey:@"grade"];
    [coder encodeDouble:totalWeight forKey:@"totalWeight"];
    [coder encodeObject:list forKey:@"list"];
}

-(void)dealloc {
    [name release];
    [list release];
    [super dealloc];
}

@end

[coursesViewController courses]是保存课程对象的NSMutableArray。我知道它拥有有效的数据。

问题是， 1：仅当我从xcode运行它时，应用程序才会保存到data.plist（即在xcode中单击“编译并运行”）。 2：它从plist加载数据，但所有保存的是课程名称和grade和totalWeight的默认值（分别为-1和0）。所以他们真的得到了保存，好像首先在他们身上调用了initWithName。

这是我第一次真正深入研究一个相当高级的iOS应用程序，因此我是一个新手，我可能遗漏了一些重要的信息。如果是这种情况，请告诉我，我会更新问题。

谢谢！ -HT

P.S。如果它是相关的，我将info.plist中的doNotRunInBackground设置为true。

Answer 1

您正在尝试在对象初始化之前设置值。然后初始化将重置您的值。

-(Course *)initWithName:(NSString *)courseName {
    name = [courseName retain];              // <- Accessing ivar before self is initialized
    grade = -1.0;                            // <- Accessing ivar before self is initialized
    totalWeight = 0.0;                       // <- Accessing ivar before self is initialized
    list = [[NSMutableArray alloc] init];    // <- Accessing ivar before self is initialized
    [super init];                            // initialization resets your values !!!!
    return self;
}

此外，你忽略了super的init返回值，98％的情况下可以正常工作，但我建议总是使用正确的初始化方案：

- (id)init {
    if (self = [super init]) {
        // It's save to access ivars here
    }
    return self
}

在Cocoa中，init方法可以返回一个不同的对象，然后是已分配的对象。所以你必须将自己分配给super的init。

所以，你的init应该是这样的：

-(Course *)initWithName:(NSString *)courseName {
    if (self = [super init]) {
        name = [courseName retain];
        grade = -1.0;
        totalWeight = 0.0;
        list = [[NSMutableArray alloc] init];
    }
    return self;
}

同样适用于initWithCoder:(NSCoder *)coder。

为什么我的<nscoding>对象只有一个属性正确写入文件？</nscoding>

1 个答案: