check Network ConnectivityManager , connect to web service and return JSON HttpUURLConnection OkHttp Retrofit

1 check network connection.java

Sending and Managing Network Requests: 
http://guides.codepath.com/android/Sending-and-Managing-Network-Requests

Core java: 
. HttpUURLConnection: core java và android, low level, ta tự parse kết quả và tự làm request 
Library: 
. OkHttp(library của Android): http://square.github.io/okhttp/
. http://loopj.com/android-async-http/ (library )
. handle the entire process of sending and parsing network requests for us in a more robust and easy-to-use way.
. Retrofit
http://guides.codepath.com/android/Consuming-APIs-with-Retrofit
which uses OkHttp and makes it easier to make more RESTful API calls. 

=> There can be a bit of a learning curve when using these libraries, so your best bet when first learning is to use Android Async Http Client. With OkHttp you also have to deal with the complexity of whether your callbacks need to be run on the main thread to update the UI, 
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#Cachematters for networking
https://www.youtube.com/watch?v=7lxVqqWwTb0&list=PLWz5rJ2EKKc9CBxr3BVjPTPoDPLdPIFCE&index=11

TH khi 1 thông tin trên mạng cần lấy thường xuyên -> cache sẽ tăng performance, nếu ta tự viết cách cache thì phải 
chỉ 1 file cache lưu trên đĩa size bao nhiêu. Cache áp dúng mọi TH httprequest (tin hay image).
https://developer.android.com/samples/DisplayingBitmaps/src/com.example.android.displayingbitmaps/util/DiskLruCache.html?utm_campaign=android_series_#cachematters_for_networking_101315&utm_source=anddev&utm_medium=yt-annt

hay dơn giản hơn là sd thư viện có tích hợp networking + cache:
http://square.github.io/okhttp/
https://developer.android.com/training/volley/index.html?utm_campaign=android_series_#cachematters_for_networking_101315&utm_source=anddev&utm_medium=yt-annt
http://square.github.io/picasso/
http://stackoverflow.com/search?q=android+image+load+http

* Using the Network Traffic tool
https://developer.android.com/studio/profile/ddms.html#network?utm_campaign=android_series_#cachematters_for_networking_101315&utm_source=anddev&utm_medium=yt-annt
thấy khoảng thời gian, size của data transfer.
 
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Optimizing Network Request Frequencies 
https://www.youtube.com/watch?v=nDHeuEM30ks&list=PLWz5rJ2EKKc9CBxr3BVjPTPoDPLdPIFCE&index=12
chtr này nói về sync less: vd trong fb khi update new feed:
  . khi pull to refresh: cần sync now.
  . khi dag đợi server trả về or đang push data: ko cần sync.

Ta ko cần sync để hỏi server xem có tin mới ko, sẽ tối ưu hơn khi server có tin mới sẽ send cho ta ->
lúc đó mới cần sycn 
Xem thêm về google cloud message: https://developers.google.com/cloud-messaging/

Khi cần sync liên tục: cach tối ưu, sycn ko có tin mới thì thời gian sync tiếp theo x2, cứ thế.

Hay tìm hiểu xem hành động của con người(họ dang di độ, đi xe) đẻ xem liệu họ có cần sync ko 
https://plus.google.com/+MagnusHyttsten/posts
vd nếu phone đang nằm chừng 8 tiếng mà đột nhien wake up và move thì ta biết là hãy bắt đầu sync đi.

hay nếu điện thoại đang cắm sạc (-> sync ko sao) ... -> các pattern này thì nhờ google api detect
GcmNetworkManager
https://developers.google.com/android/reference/com/google/android/gms/gcm/GcmNetworkManager

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KIEM TRA XEM CÓ ĐƯỜNG CONNECTION ĐẾN MỤC TIÊU KHÔNG TRƯỚC KHI TẠO KẾT NỐI TỚI MỤC TIÊU
 <!--TODO 1 thêm quyền truy cập internet-->
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
What's the difference between ACCESS_NETWORK_STATE and INTERNET?
http://stackoverflow.com/questions/19642032/whats-the-difference-between-access-network-state-and-internet
https://developer.android.com/reference/android/Manifest.permission.html#ACCESS_NETWORK_STATE

ACCESS_NETWORK_STATE: Allows applications to access information about networks
INTERNET: Allows applications to open network sockets.
In short, the INTERNET permission lets you use the internet, whereas ACCESS_NETWORK_STATE will just give you information about the network, such as whether you are connected to a network at all.
You can use either one without needing the other. If you don't check that you have a valid network connection before trying to use the Internet, your HTTP requests will simply fail.

-> You dont have to check if there is a connection or not, if there isnt the request will fail or whatever. If there is no internet the HTTP request will fail and you will jet some sort of exception that you will have to handle

Chú ý: Note that having an active network interface doesn't guarantee that a particular networked service is available or that the internet is actually connected. Network issues, server downtime, low signal, captive portals, content filters and the like can all prevent your app from reaching a server. For instance you can't tell for sure if your app can reach Twitter until you receive a valid response from the Twitter service.

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Determining and Monitoring the Connectivity Status
https://developer.android.com/training/monitoring-device-state/connectivity-monitoring.html
Check connect khi, check trước khi quyết đinh có sync hay ko, nếu sync phải kết nối tới internet và kết nối tới server đợi nó rất tốn time.
 Determine if You Have an Internet Connection: There's no need to schedule an update based on an Internet resource if you aren't connected to the Internet.
 Determine the Type of your Internet Connection: By querying the type of the active network, as shown below, you can alter your refresh rate based on the bandwidth available. Mobile data costs tend to be significantly higher than Wi-Fi, so in most cases, your app's update rate should be lower when on mobile connections. Similarly, downloads of significant size should be suspended until you have a Wi-Fi connection.
Monitor for Changes in Connectivity: Having disabled your updates, it's important that you listen for changes in connectivity in order to resume them once an Internet connection has been established.
 	Broadcast <action android:name="android.net.conn.CONNECTIVITY_CHANGE"/>
 	Changes to a device's connectivity can be very frequent—this broadcast is triggered every time you move between mobile data and Wi-Fi. As a result, it's good practice to monitor this broadcast only when you've previously suspended updates or downloads in order to resume them. 

 => check for Internet connectivity before beginning an update and, should there be none, suspend further updates until connectivity is restored.
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ConnectivityManager
https://developer.android.com/reference/android/net/ConnectivityManager.html
Class that answers queries about the state of network connectivity. It also notifies applications when network connectivity changes. Get an instance of this class by calling Context.getSystemService(Context.CONNECTIVITY_SERVICE).

The primary responsibilities of this class are to:

Monitor network connections (Wi-Fi, GPRS, UMTS, etc.)
Send broadcast intents when network connectivity changes
Attempt to "fail over" to another network when connectivity to a network is lost
Provide an API that allows applications to query the coarse-grained or fine-grained state of the available networks
Provide an API that allows applications to request and select networks for their data traffic
    
    public class Connectivity {

    /**
     * Get the network info
     *
     * @param context
     * @return
     */
    public static NetworkInfo getNetworkInfo(Context context) {
        ConnectivityManager cm = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        return cm.getActiveNetworkInfo();
    }

    /**
     * Check if there is any connectivity
     *
     * @param context
     * @return
     */
    public static boolean isConnected(Context context) {
        NetworkInfo info = Connectivity.getNetworkInfo(context);
        return (info != null && info.isConnected());
    }

    /**
     * Check if there is any connectivity to a Wifi network
     *
     * @param context
     * @return
     */
    public static boolean isConnectedWifi(Context context) {
        NetworkInfo info = Connectivity.getNetworkInfo(context);
        return (info != null && info.isConnected() && info.getType() == ConnectivityManager.TYPE_WIFI);
    }

    /**
     * Check if there is any connectivity to a mobile network
     *
     * @param context
     * @return
     */
    public static boolean isConnectedMobile(Context context) {
        NetworkInfo info = Connectivity.getNetworkInfo(context);
        return (info != null && info.isConnected() && info.getType() == ConnectivityManager.TYPE_MOBILE);
    }

    /**
     * Check if there is fast connectivity
     *
     * @param context
     * @return
     */
    public static boolean isConnectedFast(Context context) {
        NetworkInfo info = Connectivity.getNetworkInfo(context);
        return (info != null && info.isConnected() && Connectivity.isConnectionFast(info.getType(), info.getSubtype()));
    }

    /**
     * Check if the connection is fast
     *
     * @param type
     * @param subType
     * @return
     */
    public static boolean isConnectionFast(int type, int subType) {
        if (type == ConnectivityManager.TYPE_WIFI) {
            return true;
        } else if (type == ConnectivityManager.TYPE_MOBILE) {
            switch (subType) {
                case TelephonyManager.NETWORK_TYPE_1xRTT:
                    return false; // ~ 50-100 kbps
                case TelephonyManager.NETWORK_TYPE_CDMA:
                    return false; // ~ 14-64 kbps
                case TelephonyManager.NETWORK_TYPE_EDGE:
                    return false; // ~ 50-100 kbps
                case TelephonyManager.NETWORK_TYPE_EVDO_0:
                    return true; // ~ 400-1000 kbps
                case TelephonyManager.NETWORK_TYPE_EVDO_A:
                    return true; // ~ 600-1400 kbps
                case TelephonyManager.NETWORK_TYPE_GPRS:
                    return false; // ~ 100 kbps
                case TelephonyManager.NETWORK_TYPE_HSDPA:
                    return true; // ~ 2-14 Mbps
                case TelephonyManager.NETWORK_TYPE_HSPA:
                    return true; // ~ 700-1700 kbps
                case TelephonyManager.NETWORK_TYPE_HSUPA:
                    return true; // ~ 1-23 Mbps
                case TelephonyManager.NETWORK_TYPE_UMTS:
                    return true; // ~ 400-7000 kbps
            /*
			 * Above API level 7, make sure to set android:targetSdkVersion
			 * to appropriate level to use these
			 */
                case TelephonyManager.NETWORK_TYPE_EHRPD: // API level 11
                    return true; // ~ 1-2 Mbps
                case TelephonyManager.NETWORK_TYPE_EVDO_B: // API level 9
                    return true; // ~ 5 Mbps
                case TelephonyManager.NETWORK_TYPE_HSPAP: // API level 13
                    return true; // ~ 10-20 Mbps
                case TelephonyManager.NETWORK_TYPE_IDEN: // API level 8
                    return false; // ~25 kbps
                case TelephonyManager.NETWORK_TYPE_LTE: // API level 11
                    return true; // ~ 10+ Mbps
                // Unknown
                case TelephonyManager.NETWORK_TYPE_UNKNOWN:
                default:
                    return false;
            }
        } else {
            return false;
        }
    }

}

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Cach khac1:   // TODO: 5/20/2016 4 check internet connection by pinging to google
private Boolean isOnline() {
        Runtime runtime = Runtime.getRuntime();
        try {
            Process ipProcess = runtime.exec("/system/bin/ping -c 1 8.8.8.8");
            int     exitValue = ipProcess.waitFor();
            return (exitValue == 0);
        } catch (IOException e)          { e.printStackTrace(); }
        catch (InterruptedException e) { e.printStackTrace(); }
        return false;
    }
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2 send http request in a asynctask.java

TẠO KẾT NỐI ĐẾN MỤC TIÊU SAU KHI ĐÃ BIẾT CHÍNH XÁC LÀ ĐÃ KẾT NỐI ĐƯỢC
https://gist.github.com/PhongHuynh93/42be08bd07bd06926723ca1f3b0b4484