21.網(wǎng)絡(luò)服務(wù)類型NSURLRequestNetworkServiceType
示例代碼：
typedef NS_ENUM(NSUInteger, NSURLRequestNetworkServiceType)
{ NSURLNetworkServiceTypeDefault = 0, // Standard internet traffic
NSURLNetworkServiceTypeVoIP = 1, // Voice over IP control traffic
NSURLNetworkServiceTypeVideo = 2, // Video traffic
NSURLNetworkServiceTypeBackground = 3, // Background traffic
NSURLNetworkServiceTypeVoice = 4 // Voice data};可以通過這個值來指定當(dāng)前的網(wǎng)絡(luò)類型，系統(tǒng)會跟據(jù)制定的網(wǎng)絡(luò)類型對很多方面進(jìn)行優(yōu)化，這個就設(shè)計到很細(xì)微的編程技巧了，可作為一個優(yōu)化的點備用。
