需求：
讓程序能夠檢測網絡可用性，並在沒有網絡可用的時候能夠彈出對話框，並允許用戶點擊按鈕打開網絡設置

實踐：

1、使用reachabilityChanged來判斷網絡狀態
2、在Info.plist裡面加入
<key>SBUsesNetwork</key>  
<true/>  

其他功能
3、使用openurl跳轉到網絡設置頁面

[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"prefs:root=General&path=Network"]];  

補充：
另外，附上跳轉到設置其他頁面的鏈接：

（http://stackoverflow.com/questions/8246070/ios-launching-settings-restrictions-url-scheme）
prefs:root=General&path=About
prefs:root=General&path=ACCESSIBILITY
prefs:root=AIRPLANE_MODE
prefs:root=General&path=AUTOLOCK
prefs:root=General&path=USAGE/CELLULAR_USAGE
prefs:root=Brightness
prefs:root=General&path=Bluetooth
prefs:root=General&path=DATE_AND_TIME
prefs:root=FACETIME
prefs:root=General
prefs:root=General&path=Keyboard
prefs:root=CASTLE
prefs:root=CASTLE&path=STORAGE_AND_BACKUP
prefs:root=General&path=INTERNATIONAL
prefs:root=LOCATION_SERVICES
prefs:root=ACCOUNT_SETTINGS
prefs:root=MUSIC
prefs:root=MUSIC&path=EQ
prefs:root=MUSIC&path=VolumeLimit
prefs:root=General&path=Network
prefs:root=NIKE_PLUS_IPOD
prefs:root=NOTES
prefs:root=NOTIFICATIONS_ID
prefs:root=Phone
prefs:root=Photos
prefs:root=General&path=ManagedConfigurationList
prefs:root=General&path=Reset
prefs:root=Sounds&path=Ringtone
prefs:root=Safari
prefs:root=General&path=Assistant
prefs:root=Sounds
prefs:root=General&path=SOFTWARE_UPDATE_LINK
prefs:root=STORE
prefs:root=TWITTER
prefs:root=General&path=USAGE
prefs:root=VIDEO
prefs:root=General&path=Network/VPN
prefs:root=Wallpaper
prefs:root=WIFI
prefs:root=INTERNET_TETHERING