How to receive response from server in Xamarin.Forms - xamarin

Question

I have created an HTTP Client that sends data to my server. This data will query my server that will return a JSON object. How can I receive the JSON Object response from my server and insert it into my database?
The code below will send a ContactID to my server and my server will return a JSON Object How can I get the JSON Object from my server? What will I add to my code? I have added
var data = await response.Content.ReadAsStringAsync();
but I don't know how to proceed.
try
{
var db = DependencyService.Get<ISQLiteDB>();
var conn = db.GetConnection();
var sql = "SELECT * FROM tblUser WHERE ContactID = '" + contact + "'";
var getUser = conn.QueryAsync<UserTable>(sql);
var resultCount = getUser.Result.Count;
//Check if the user has been sync
if (resultCount < 1)
{
try
{
syncStatus.Text = "Syncing user to server...";
var link = Constants.requestUrl + "Host=" + host + "&Database=" + database + "&Contact=" + contact + "&Request=8qApc8";
string contentType = "application/json";
JObject json = new JObject
{
{ "ContactID", contact }
};
HttpClient client = new HttpClient();
var response = await client.PostAsync(link, new StringContent(json.ToString(), Encoding.UTF8, contentType));
var data = await response.Content.ReadAsStringAsync();
if (response.IsSuccessStatusCode)
{
var content = await response.Content.ReadAsStringAsync();
var userresult = JsonConvert.DeserializeObject<IList<UserData>>(content);
var count = userresult.Count;
for (int i = 0; i < count; i++)
{
try
{
syncStatus.Text = "Syncing user to server...";
var item = userresult[i];
var contactID = item.ContactID;
var userID = item.UserID;
var userPassword = item.UserPassword;
var userType = item.UserType;
var userStatus = item.UserStatus;
var lastSync = item.LastSync;
var serverUpdate = item.ServerUpdate;
var mobileUpdate = item.MobileUpdate;
var user = new UserTable
{
ContactID = Convert.ToInt32(contactID),
UserID = userID,
UserPassword = userPassword,
UserType = userType,
UserStatus = userStatus,
LastSync = lastSync,
ServerUpdate = serverUpdate,
MobileUpdate = mobileUpdate
};
await conn.InsertAsync(user);
}
catch (Exception ex)
{
Console.Write("Syncing user error " + ex.Message);
}
}
}
}
catch (Exception ex)
{
Console.Write("Syncing User Error " + ex.Message);
}
}
My PHP code will query my database with the ContactID received from Xamarin HTTP Client.
$json_str = file_get_contents('php://input');
$json_obj = json_decode($json_str);
$ContactID = $json_obj->ContactID;
$sql = "SELECT * FROM tblUser WHERE ContactID = '$ContactID'";
$result = mysqli_query($conn, $sql);
$count = mysqli_num_rows($result);
if($count > 0){
while ($row = #mysqli_fetch_array($result)) {
$decr = CryptRC4(FromHexDump($row['UserPassword']), $key);
$ar[] = array(
'ContactID' => $row['ContactID'],
'UserID' => $row['UserID'],
'UserPassword' => $decr,
'UserType' => $row['UserType'],
'UserStatus' => $row['UserStatus'],
'LastSync' => $sync,
'ServerUpdate' => $row['ServerUpdate'],
'MobileUpdate' => $row['MobileUpdate']
);
print json_encode($ar);
//Update LastSync DateTime
$sql = "UPDATE tblUser SET LastSync = '$sync' WHERE ContactID = '$ContactID'";
mysqli_query($conn, $sql);
}
}

Answer 1

Last statement in your example above gives list of json objects in string format.
var data = await response.Content.ReadAsStringAsync();
You need to convert that back to list of objects. To let your project know about definition of the object, create a plain class with public properties (Something like below)
public class UserLog
{
public int ContactId { get; set; }
public string Log { get; set; }
public DateTime LogDate { get; set; }
}
Add Newtonsoft.Json (by James Newton-King) Nuget package to your project so that you can work with json.
To convert content of the variable 'data' into list of UserLog objects, write code like
var list = NewtonsoftUtil<IList<UserLog>>.DeserializeObject(data);
(Add using Newtonsoft.Json; at top of the file)
Please let me know if this helps.

Answer 2

The answers above missing one important point -> efficiency.
There is no need to allocate a string in memory, especially if your JSON is big. Streams can do much better then strings:
// Read the response as stream
var stream = await response.Content.ReadAsStreamAsync();
// Use the next method for deserialization
T DeserializeJsonFromStream<T>(Stream stream)
{
if (stream == null || stream.CanRead == false)
return default(T);
using (var sr = new StreamReader(stream))
using (var jtr = new JsonTextReader(sr))
{
var js = new JsonSerializer();
return js.Deserialize<T>(jtr);
}
}
P.S.: Code example is bases on Json.NET.
P.S.S.: There are many good articles on this topic, I could recommend to get familiar with the next one.

Answer 3

Assuming that you have done everything correctly. In other words, You're be able to sent your contactID and get back a json.
Let's say your json structure is something like:
{"firstname" : "Doe",
"lastname" : "foo"
"age" : "27"}
One possible way to retrieve the data is as below:
using Newtonsoft.Json;
//after PostAsync()
if (response.IsSuccessStatusCode)
{
var content = await response.Content.ReadAsStringAsync();
JObject jContent = (JObject)JsonConvert.DeserializeObject(content);
string firstName = (string)jContent.GetValue("firstname")
string lastName = (string)jContent.GetValue("lastname");
int age = (int)jContent.GetValue("age");
}
Newtonsoft is available on Nuget. you need to install it if you have not done so.
Improved solution
What if your json has many key/values pairs like below:
{ key1 : value1,
key2 : value2,
key3 : value3,
...
key10 : value10}
Then it is not a good idea by doing :
string foo1 = (string)jContent.GetValue("key1");
string foo2 = (string)jContent.GetValue("key2");
//...
string foo10 = (string)jContent.GetValue("key10");
To handle this case, you can create a class:
public class Foo
{
public string Foo1 {get;set;}
public string Foo2 {get;set;}
//...
public string Foo2 {get;set;}
}
Then, you can do as simple as below:
if (response.IsSuccessStatusCode)
{
var content = await response.Content.ReadAsStringAsync();
Foo foo = JsonConvert.DeserializeObject<Foo>(content);
}
The improved solution referecnced from www.newtonsoft.com. Go there and check out other ways of using the library.