/Users/SpongeBob/Library/Developer/CoreSimulator/Devices/5A46B8A4-F547-4B6A-B5B8-F76A6E13998F/data/Containers/Data/Application/02DF6B84-32DF-4082-A7AE-EF4D9A110E42/tmp/

    @IBOutlet weak var imgView: UIImageView!

    var str = "https://www.baidu.com/img/bdlogo.png"

    // 在Swift中，只需要一行搞定“懒加载”
    // 参考： http://swiftist.org/topics/129
    lazy var imgCache = NSMutableDictionary()

//    lazy var cache : NSMutableDictionary = {
//        let dic = NSMutableDictionary()
//
//        return dic
//    }()

    // MARK: lazy load array   ???
//    lazy var name: String = "Spongebob"
//    var arr: NSArray {
//        get {
//            return NSArray()
//        }
//    }

    /// 将网络图片缓存到本地沙盒
    func downloadPicToCache() {

        println("网络下载图片")

        var urlPic = NSURL(string: str)

        var data = NSData(contentsOfURL: urlPic!)

        var image = UIImage(data: data!)
        self.imgView.image = image

        // 写入沙盒
        if data != nil {
            println((str as NSString).appendCacheDir())

            data?.writeToFile((str as NSString).appendCacheDir(), atomically: true);
        }
    }

    /// 加载网络图片
    func loadImg() {
        var img = UIImage(contentsOfFile: (str as NSString).appendCacheDir())

        if self.imgCache[str] != nil {

            println("从内存中加载图片")

            self.imgView.image = self.imgCache[str] as? UIImage
        } else {

            if img != nil {
                println("从沙盒（磁盘）加载图片")

                println((str as NSString).appendCacheDir())

                // 将图片添加到图片缓冲池中
                self.imgCache.setObject(img!, forKey: str)

                self.imgView.image = img
            } else {
                downloadPicToCache()
            }

        }
    }

/*
///  拼接文档目录
- (NSString *)appendDocumentDir;

///  拼接缓存目录
- (NSString *)appendCacheDir;

///  拼接临时目录
- (NSString *)appendTmpDir;
*/

- (NSString *)appendDocumentDir {

    NSString *dir = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];

    // stringByAppendingPathComponent拼接时，会自动加上反斜杠"/"
    // self为字符串本身，lastPathComponent获取文件路径最后的字符串！
    return [dir stringByAppendingPathComponent:self.lastPathComponent];
}

- (NSString *)appendCacheDir {
    NSString *dir = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];

    return [dir stringByAppendingPathComponent:self.lastPathComponent];
}

- (NSString *)appendTmpDir {
    return [NSTemporaryDirectory() stringByAppendingPathComponent:self.lastPathComponent];
}