Update/refresh field within a certain time interval

Sam Mettenmeyer

7y3.2kC#

Hello altogether,

In my Xamarin.Android-App, when opening a special page (activity), there is a table displayed on create. A ListAdapter defines the columns within that table. I have a function (Get_Status) which gets me an external int. The int is amongst other data displayed in each row in that table, but differs in it's value (depending on the received data). But this function is only called once per row (on create). How can I call this function multible time during runtime within a certain time interval? I want that if the received int changes, the display of the int in the table changes as well without calling the whole activity again, so just automatically update/refresh its display field every, lets say, 2 seconds.

Can anyone just give me a hint how to do this?

Please see my current code, maybe it's easier to help me.

Activity:

protected override void OnCreate(Bundle bundle)
{
//...
var lv = FindViewById (Resource.Id.ListView);
lv.Adapter = new ListAdapter(this, Resource.Layout.List, list.CurrentList, Constants.ServerIP);
}

class ListAdapter : ArrayAdapter

public ListAdapter(Context Context, int ListId, List list, string serverip) : base(Context, ListId, Geraete)
{
this.List = list;
}

public override View GetView(int position, View convertView, ViewGroup parent)
{
View v = convertView;
if (v == null)
{
LayoutInflater inflater = (LayoutInflater) Context.GetSystemService(Context.LayoutInflaterService);
v = inflater.Inflate(Resource.Layout.List, parent, false);
}
if (Get_Status(List[position]) == 1)
{
v.FindViewById(Resource.Id.ImageView).SetImageResource(Resource.Drawable.green);
}
if (Get_Status(List[position]) != 1)
{
v.FindViewById(Resource.Id.ImageView).SetImageResource(Resource.Drawable.red);
}
return v;
}

I tried several technics myself, unfortunately without success. I will share my efforts as well, maybe you see instantly where the failure was.

1.) using System.Threading

var thread = new Thread(() => MethodToRun(garaet));
thread.Start();

void MethodToRun(Geraet geraet)
{
while (true)
{
Thread.Sleep(2000);
var a = new Activity();
a.RunOnUiThread(() => {
int status = Get_Status(geraet);
if (status == 1)
{
v.FindViewById(Resource.Id.ImageView).SetImageResource(Resource.Drawable.green);
}
if (status != 1)
{
v.FindViewById(Resource.Id.ImageView).SetImageResource(Resource.Drawable.red);
}
});
}
}

Received error message (MethodToRun Line 6): Java.Lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

2.) With a handler:

handler = new Handler(Looper.MainLooper);
actions = new Runnable(() =>
{
int status = Get_Geraetestatus(Geraetelist[position]);
handler.Post(() =>
{
ImageView display = v.FindViewById(Resource.Id.ImageView);
led.SetImageResource(status == 1 ? Resource.Drawable.green: Resource.Drawable.red);
PostAction();
});
});

private void PostAction()
{
handler.PostDelayed(() => {
new Thread(actions).Start();
}, 2000);
}

Received error message: none (but it doesn't work either, the display is missing completely, debug mode showed that he skips completely everything from line 3 [actions = new Runnable] and jumps directly to return v;)

3.) ThreadWithState

ThreadWithState tws = new ThreadWithState(Serverip, Geraetelist[position]);
Thread thread = new Thread(new ThreadStart(tws.ThreadProc));
thread.Start();

public class ThreadWithState

{
public void ThreadProc()
{
while (true)
{
Thread.Sleep(1000);
Activity a = new Activity();
a.RunOnUiThread(() => { geraetestatus = Get_Geraetestatus(geraet); });
}
}
}

Received error message (Thread Parameter Line 2): Could not convert "System.Threading.ThreadStart" to "Java.Lang.IRunnable" (Although in my instruction it's nearly the same code: https://docs.microsoft.com/de-de/dotnet/standard/threading/creating-threads-and-passing-data-at-start-time )

Hoping that anyone can help me. Thanks in advance for answers and best regards

21 Replies

Know the answer? Post it — somebody with the same question will find it here.

Amit Gupta7y ago
Accepted answer
Your whole code will look like this:

// adapter class
Activity parentActivity;
        public override  View GetView(int position, View convertView, ViewGroup parent)
        {
            View v = convertView;

            if (v == null)
            {
                LayoutInflater inflater = (LayoutInflater)Context.GetSystemService(Context.LayoutInflaterService);
                v = inflater.Inflate(Resource.Layout.List, parent, false);
            }

            v.FindViewById(Resource.Id.MachineNumberTextView).Text = List[position].Key.ToString();
            v.FindViewById(Resource.Id.MachineNameTextView).Text = List[position].Desc;
            Task.Run(new Action(async() =>
            {
                int x = await Get_State("");
                // Threading doesnot allow to play with UI, so we need to run on UI thread and this is a part of activity class, so we need to pass the activity object
                parentActivity?.RunOnUiThread(() =>
                {
                    TextView tv = v.FindViewById(Resource.Id.MachineStateTextView);
                    tv.Text = (state == 1 ? "yes" : "no");
                });
            }));


            return base.GetView(position, convertView, parent);
        }
//on create function on main activity, change it according to your requirement.
protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            SetContentView(Resource.Layout.Unlock);
            // (...)
            lv = FindViewById(Resource.Id.ListView);
            var adapter= new ListAdapter(this, Resource.Layout.List, MyListClass.CurrentList, serverip);
            adapter.parentActivity = this;
            lv.Adapter = adapter;
            startTimer();

            // notify dataset changed event will now be called as
            adapter.NotifyDataSetChanged();
            // (...)
        }
Do let me know, if it solves your problem.

Good Luck
+1
Amit Gupta7y ago
It totally depends upon your server response, generally it response within 50~100 ms. Moreover, I suggest you to post a seperate question for your new query because it already reached very long trailing posts
0
Sam Mettenmeyer7y ago
My friend, can you just briefly tell me:

What is the time interval the Get_State()-method is executed? So how often are the machines asked about their state? Do you know what I mean?^^
0
Sam Mettenmeyer7y ago
Sorry for my mistakes I got the point and corrected everything. Thank you again so much really kind of you. Now basically everything works.

I am right now testing everything, so sorry for me not writing earlier. I am going to write you again tomorrow to report about the tests.

One thing I already found out: If I remove the power cable out of one of these physical machines in behind, the state isn't refreshed. But as said, more details tomorrow.

Again so much thanks for everything. If you want, you can send me your bank account details to [email protected] , i will tip you!
+1
Amit Gupta7y ago
1. Here is the mistake you had done, please read my code carefully

UnlockActivity().RunOnUiThread(() =>

it should be only

parentActivity?.RunOnUiThread(() =>

// ? (question mark is used to check the variable is null or not, it is not a method, so you cannot use ( )

2. You have to define parentActivity here like this
Activity parentActivity;
public override View GetView(int position, View convertView, ViewGroup parent)
{
Hope you got the point
+1
Sam Mettenmeyer7y ago
Hello Amit and thank you so much for your code.

You write "we need to pass the activity object." The activity where the table is displayed, is called "UnlockActivity". So I changed your code to:
Task.Run(new Action(async () =>
{
    int state = await Get_Geraetestatus(Geraetelist[position].Ip);
    // Threading doesnot allow to play with UI, so we need to run on UI thread and this is a part of activity class, so we need to pass the activity object
    UnlockActivity().RunOnUiThread(() =>
    {
        TextView tv = v.FindViewById(Resource.Id.MachineStateTextView);
        tv.Text = (state == 1 ? "yes" : "no");
    });
}));
Hope this was correct.
Unfortunately, when being on the so called "UnlockActivity", I cannot do neither
adapter.UnlockActivity = this;
(Error Message: "UnlockActivity" is a "method group". An assignment is therefore not possible.)
nor
adapter.parentActivity = this;
(Error Message: "ListAdapter" does not contain a definition for "parentActivity", or there wasn't found an availiable parentActivity-extension-method, which accepts its first argument of type "ListAdapter" (maybe a using-directive or assembly reference is missing)).
Am I doing this right? Or what is the correct statement to write here?
And in addition: do we still need the startTimer();-method? Or can I remove it as we do "NotifyDataSetChanged" at a different point now?

Best regards
+1
Sam Mettenmeyer7y ago
Ye, but only putting async keyword makes the error message read "Return-Type of an async-method must be "void", "Task" or "Task":
public override async View GetView(int position, View convertView, ViewGroup parent)
{ /*...*/}
+1
Amit Gupta7y ago
As mentioned earlier, you only have to put async keyword and not the Task. It will be as usual
+1
Sam Mettenmeyer7y ago
Mmmh, unfortunately, when making the calling function async, following error message appears:
"ListAdapter.GetView(int, View, ViewGroup)": The return-type has to be "View" to fit the overridden member "ArrayAdapter.GetView(int, View, ViewGroup)".

Please see the following code:
class ListAdapter : ArrayAdapter
{
    List List;
    string ip;

    public ListAdapter(Context Context, int ListId, List List, string serverip) : base(Context, ListId, List)
    {
        this.Geraetelist = Geraete;
        this.ip = serverip;
    }

    public override async Task GetView(int position, View convertView, ViewGroup parent)//here I did the change, before, it was public override View GetView(...)
    {
        View v = convertView;

        if (v == null)
        {
            LayoutInflater inflater = (LayoutInflater)Context.GetSystemService(Context.LayoutInflaterService);
            v = inflater.Inflate(Resource.Layout.List, parent, false);
        }

        v.FindViewById(Resource.Id.MachineNumberTextView).Text = List[position].Key.ToString();
        v.FindViewById(Resource.Id.MachineNameTextView).Text = List[position].Desc;

        int state = await Get_State(List[position].IpAddress);
        TextView tv = v.FindViewById(Resource.Id.MachineStateTextView);
        tv.Text = (state == 1 ? "yes" : "no");

        return v;
    }
Problem seem to be that the function is part of an adapter, which means a class (ArrayAdapter) is extended and through this a special member within that class is overridden.
+1
Amit Gupta7y ago
That's how async works, make your calling function async and put await when calling the function int state = await Get_State(List[position].IpAddress)
+1
Sam Mettenmeyer7y ago
Alright your first version works, at least it is syntactical correct. Thank you very much for your code.
But now the Get_State()-function returns Task. I have the following statement in my ListAdapter : ArrayAdapter:
int state = Get_State(List[position].IpAddress);
TextView tv = v.FindViewById(Resource.Id.MachineStateTextView);
tv.Text = (state == 1 ? "yes" : "no");
Unfortunately he underlines Get_State(List[position].IpAddress); saying: "The type "System.Threading.Tasks.Task" cannot be converted implicitly into "int"".

So what do I have to do now with this returned Task?^^
+1
Amit Gupta7y ago
I am not with my laptop, It might be possible that function have async enabled
private async Task<int> Get_State(string IpAddressOfMachine)
{
    var request = HttpWebRequest.Create(string.Format(Constants.Path, Serverip, Constants.WebservicePort, IpAddressOfMachine));
    request.ContentType = "application/json";
    request.Method = "GET";

    using (HttpWebResponse response = await  request.GetResponseAsync() as HttpWebResponse)
    {
        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
        {
            var content = reader.ReadToEnd();

            var result = System.Text.RegularExpressions.Regex.Replace(content, @"[^0-9]", "");

            if (result == string.Empty)
            {
                return -2;
            }
            else
            {
                return Int32.Parse(result);
            }
        }
    }
}
If not then make whole method under async programming like below
private async Task<int> Get_State(string IpAddressOfMachine)
{
    int val=-2;
    await Task.Run(new Action(()=>
{
    var request = HttpWebRequest.Create(string.Format(Constants.Path, Serverip, Constants.WebservicePort, IpAddressOfMachine));
    request.ContentType = "application/json";
    request.Method = "GET";

    using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
    {
        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
        {
            var content = reader.ReadToEnd();

            var result = System.Text.RegularExpressions.Regex.Replace(content, @"[^0-9]", "");

    val = result==""? -2: Convert.ToInt32(result)
         }
    }
}));
    return val;
}
Do correct some minor error code.

Good Luck !!
+1
Sam Mettenmeyer7y ago
Hello Amit and thank you really much for your helping efforts.

Please excuse, as I am pretty new in programming, that I have troubles implementing this.

Let me show you the `Get_State()`-Function:
private int Get_State(string IpAddressOfMachine)
{
    var request = HttpWebRequest.Create(string.Format(Constants.Path, Serverip, Constants.WebservicePort, IpAddressOfMachine/*becomes something like "http://192.168.1.1:5555/api/get_state?ip=192.168.1.2"*/));
    request.ContentType = "application/json";
    request.Method = "GET";

    using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
    {
        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
        {
            var content = reader.ReadToEnd();

            var result = System.Text.RegularExpressions.Regex.Replace(content, @"[^0-9]", "");

            if (result == string.Empty)
            {
                return -2;
            }
            else
            {
                return Int32.Parse(result);
            }
        }
    }
}
Refering to your second step: Where do I have to add async / await-statements?^^
Would it maybe be anyhow possible that you assist me to do that all? :(

Thanks again for everything and best regards
+1
Amit Gupta7y ago
Okay! Got your point, you can optimize your code. 1. Initialize the adapter seperately and then pass to the listview's adapter. 2. Use async to load the data 3. If you can get each system's status individually then you can use index of the adapter to notifyDataSetChanged(index), this can reduce the some load behaviour. 4. You can run NotifyDataSetChanged(..) in async function instead of running on main thread
+1
Sam Mettenmeyer7y ago
Ahm, okay let me briefly describe the functionality, so it's maybe easier for you to get what I am trying to do.

The list is not that huge. Maybe maximum 40 or 50 entries. On create, the app fills this list from a database. Each entry is one physical machine in behind. So the table which is displayed upon activity just shows the machines and their state.

Please see the following example:

Machine Number (Key) Description / Name Online?
123456789 "machine number 1" no
987654321 "machine number 2" yes
123123123 "machine number 3" no

The Get_State-Function always asks the machine whether it's reachable or not. If yes, the machine sends 1, if not the function runs into a timeout. Additionally, a machine can be reachable, but not useable. In this case, the machine sends something different than 1.

So most likely, at least I think so, if machine not reachable, waiting on the timeout makes the app that slow.

As I tried to describe I am simply trying to refresh the state-display of those machines, so if they are online (actually useable) or not, hope this was now understandable somehow^^
+1
Amit Gupta7y ago
If the list you are adding is huge then you can use recycler view, that is much more optimized than listview.

Yes, of course you can use async function but I am unaware about your app functionality.
+2
Sam Mettenmeyer7y ago
Okay, thanks so much for your code, everything works, but now the app unfortunately is extreeeeeeeeemly slowly. Can we make that whole refreshment stuff async anyhow? :X
0
Amit Gupta7y ago
Try like below
((ListAdapter)lv.Adapter).NotifyDataSetChanged();
0
Sam Mettenmeyer7y ago
Hello Amit and thank you for your answer.

Do you talk about something like this?
public class MyActivity : Activity
{
    ListView lv;

    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.Unlock);
        // (...)
        lv = FindViewById(Resource.Id.ListView);
        lv.Adapter = new ListAdapter(this, Resource.Layout.List, MyListClass.CurrentList, serverip);
        startTimer();
        // (...)
    }

    public void startTimer()
        {
                System.Timers.Timer Timer = new System.Timers.Timer();
                Timer.Start();
                Timer.Interval = 1000;
                Timer.Enabled = true;
                Timer.Elapsed += (object sender, System.Timers.ElapsedEventArgs e) =>
                {
                    RunOnUiThread(() =>
                    {
                            lv.Adapter.NotifyDataSetChanged();
                    });
                };
                Timer.Start();
        }
Unfortunately, he underlines NotifyDataSetChanged(); saying "IListAdapter" does not contain a definition for "NotifyDataSetChanged();" and an there was no available NotifyDataSetChanged-Extention-Method found, which accepts first argument of type "IListAdapter" (maybe a using-directive or assembly-reference is missing). Can you maybe tell me if ListView is the correct class for defining the adapter in the beginning? Or is something different needed? Because in the beginning, I cannot write var, I have to specify the type of variable. Or do you talk about a completely different adapter?^^
0
Amit Gupta7y ago
If you want to update tge listview when any data changes then you can call adapter.NotifyDataSetChanged()
0
Amit Gupta7y ago
You can simply extract the listAdapter to a seperate method and call it whenever you want, you can use Threading timer or stopwatch timer to run at specific interval.
0

Machine Number (Key)	Description / Name	Online?
123456789	"machine number 1"	no
987654321	"machine number 2"	yes
123123123	"machine number 3"	no