Big Nerd iOS Programming 第18章 Saving,Loading and Application States

18章 Saving,Loading and Application States
    Application Sandbox
    Constructing a file path
    NSKeyedArchiver and NSKeyedUnarchiver


    Application States and Transitions

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存档 
1.Archiving，永久存储数据。
Archiving:记录并存储属性
Unarchiving:从数据中重新重新创建对象。


实现NSCoding协议，实现两个方法 encodeWithCoder:, initWithCoder:


    - (void)encodeWithCoder:(NSCoder *)aCoder
{
   [aCoder encodeObject:self.itemName forKey:@"itemName"];
   [aCoder encodeObject:self.serialNumber forKey:@"serialNumber"];
   [aCoder encodeObject:self.dateCreated forKey:@"dateCreated"];
   [aCoder encodeObject:self.itemKey forKey:@"itemKey"];


   [aCoder encodeInt:self.valueInDollars forKey:@"valueInDollars"];
}


-(instancetype) initWithCoder:(NSCoder *)aDecoder
{
self = [super init];
if(self){
_itemName = [aDecoder decodeObjectForKey:@"itemName"];
}
return self;
}


2.应用沙箱， Application sandbox
程序的数据，缓存等。 不能访问其他应用的沙箱


！！！！！！！！！存储路径！！！！！！！！！！
-(NSString *)itemArchivePath
{
NSArray *documentDirectories = 
NSSearchPathForDirectoriesInDomain(NSDocumentDirectory,
NSUserDomainMask, YES);
NSString *documentDirectiry = [documentDirectories firstObject];
return [documentDirectories stringByAppendingPathComponent:@"items.archive"];


}


**************上面的到了存储的路径，以及药存储的模型数据。那什么时候出发存储已经什么时候加载数据呢？＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊＊


1.存储： 
使用NSKeyedArchiver when application exits.


eg. BNRItem
-(BOOL) saveChanges
{
NSString *path = [self itemArchivePath];
return [NSKeyedArchiver archiveRootObject:self.privateItems];
}


!archiveRootObject:toFile: 工作流程：
1.创建NSKeyedArchiver 对象
2.privateItems发送消息 encodeWithCoder: 并且把 NSKeyedArchiver的对象作为参数。
3.privateItems发送每个其中的字对象  encodeWithCoder,并且传入 NSKeyedArchiver
4.NSKeyedArchiver 写数据到系统文件夹中




当按下home键，applicationDidEnterBackground:会发送到applicationDelegate中，你可以在其中调用saveChanges
它会存在系统文件夹下，一个document文件夹中 item.archive


加载！！：
-(instancetype)initPrivate
{
self = [super init];
if(self){
NSString *path = [self itemArchivePath];
_privateItems = [NSKeyedUnarchiver unarchiveObjectWithFile:path];
if(!_privateItems){
_privateItems = [[NSMutableArray alloc] init];
}
}
return self;
}

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Application State and Transitions

Writing to the Filesystem with NSData

 NSNotificationCenter and Low-Memory Warnings

Model-View-Controller Store Design Pattern

    Cur:Application States Transitions
    Cur:Reading and Writing to the Filesystem
    Cur:the application bundle


Application State and Transitions
    当按下home键，应用进入后台，进入 background state，理解应用时如何在各个状态之间切换的：
        Not Running 
            -->Application Launches 
                    application:willFinishLaunchingWithOptions:
                    application:didFinishLaunchingWithOptions:
                    applicationDidBecomeActive:
        Active
            -->Home Button Pressed or incoming phone call
                    applicationWillResignActive:
        Inactive
            --->applicationDidEnterBackground:
        Background
            ---> After 5 seconds
        Suspended
            --->Application icon tapped
                    applicationWillEnterForeground:applicationDidBecomeActive:
            ---> system runs low on memory


当状态变化时，delegate会收到以上的一些信息，你可以在其中做些操作，比如保存数据等，


用NSData写入数据

保存图片
    -(NSString *)imagePathForKey:(NSString *)key
    {
        NSArray *documentDirectories = 
            NSSearchPathForDirectoriesInDomain(NSDocumentDirectory,NSUserDomainMask,YES);
        NSString *documentDirectory = [documentDirectories firstObject];
        return  [documentDirectory stringByAppendingPathComponent:key];

    }

// add a image to file system
    -(void)setImage:(UIImage *)image forKey:(NSString *)key
    {
        self.dictionary[key] = image;
        NSString *imagePath = [self imagePathForKey:key];
        NSData *data = UIImagegJPEGRepresentation(image,0.5);

        //write it to full path
        [data writeToFile:imagePath atomically:YES];
    }

// remove from file system
    -(void)deleteImageForKey:(NSString *)key
    {

        if(!key){
            return;
        }
        [self.dictionary removeObjectForKey:key];

        NSString *imagePath = [self imagePathForKey:key];
        [[NSFileManager defaultManager] removeItemAtPath: iamgePath error:nil];
    }

// load image from system
    -(UIImage *)imageForKey:(NSString *)key
    {
        // if posibble ,get it from dictionary
        UIImage *result = self.dictionary[key];
        if(!result){
            NSString *imagePath = [self imagePathForKey:key];
            result = [UIImage imageWithContentsOfFile:imagePath];

            // replace the cache
            if(result){
                self.dictionary[key] = result;
            }else{
                NSLog(@"Error, unable to fine %@", [self imagePathForKey:key]);
            }
        }
        return result;
    }

--------------------------------
通知中心，低内存警告⚠

    viewController收到    didReceiveMemoryWarning。
    你可以在其中清空掉图片缓存dictionary,待需要的时候再加载

你可以添加观察者，例如在BNRImageStore中：
    -(instancetype)init
    {
        self = [super init];
        if(self){
            _dictionary = [[NSMutableDictionary alloc] init];

            NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
            [nc addObserver:self selector:@selector(clearCache:) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
        }
        return self;
    }

delegate中的函数应该也是这样注册而来的

    -(void)clearCache:(NSNotification *)note
    {
        NSLog(@"flushing %d images out of the caches",[self.dictionary count]);
        [self.dictionary removeAllObjects];
    }

注意，此时如果没有显示或者没有被引用到的图片缓存将清除，如果当前正在显示，则等不显示到时候再🆑清除。 需要到时候自己再手动加载。

NSNotificationCenter.每个notification都有一个名字

可以在通知中带数据， 上面note 可以访问 userInfo。像cocos2d中的事件getUserData吧

Model-View-Controller-Store

Model表示数据的模型，而Store则正在的存储数据到本地，访问数据库等。
Model表示了比如一个carItem的数据模型：
    NSString *name;
    NSString *productYear;
    double price;

每个carItem作为store中dictionary的子项被存储到本地文件中

＊＊当你够建一个应用程序的时候，会自动创建一个 application bundle，其中包括了所有可执行待代码，图片，音频资源等等。所有程序在执行时用到的资源

你可以通过bundle来获取资源
NSBundle *applicationBundle = [NSBundle mainBundle];
NSString *path = [applicationBundle pathForResource:@"myImage" ofType:@"png"];

 这些都是只读的。不能在运行的时候修改或者添加资源，