IntentService in Xamarin PCL Solution - xamarin

Question

I am busy writing an application where the user needs to capture a lot of images and then they get packaged together with some text data and then they get uploaded to a local server. I want to implement the uploading on the Android platform through an Intent Service but I cannot find a good Xamarin Forms PCL example to show me how.
This is the method where I initialize the Intent to pass to the IntentService:
public async Task<bool> UploadAsync(Uri serviceAddress,
CaptureEntity capture,
List<ImageEntity> images)
{
try
{
Intent uploadIntent = new Intent();
uploadIntent.PutExtra("serviceAddress", serviceAddress.ToString());
uploadIntent.PutExtra("captureId", capture.WorkflowId.ToString());
StartService(uploadIntent);
return true;
}
catch (Exception exc)
{
App.logger.LogError(DateTime.Now, "Uploader", exc.ToString());
throw exc;
}
}
And this is the IntentService itself.
[Service]
public class ServiceIntent : IntentService
{
public ServiceIntent() : base("ServiceIntent")
{
}
//[return: GeneratedEnum]
public override StartCommandResult OnStartCommand(Intent intent, [GeneratedEnum] StartCommandFlags flags, int startId)
{
return base.OnStartCommand(intent, flags, startId);
}
public override void OnCreate()
{
base.OnCreate();
}
protected override void OnHandleIntent(Intent intent)
{
Uri serviceAddress = new Uri(intent.GetStringExtra("serviceAddress"));
Guid captureId = Guid.Parse(intent.GetStringExtra("captureId"));
CaptureEntity capture = new DatabaseConnection_Android().CreateConnection().Query<CaptureEntity>("SELECT * FROM [CaptureEntity]").Single(c => c.WorkflowId == captureId);
var images = new DatabaseConnection_Android().CreateConnection().Query<ImageEntity>("SELECT * FROM [ImageEntity]").Where(i => i.CaptureEntityId == capture.Id);
try
{
MultipartFormDataContent content = new MultipartFormDataContent();
StringContent strContent = new StringContent(
capture.XmlData,
Encoding.UTF8,
"text/xml");
IImageHandler handler = new ImageHandler_Droid();
HttpRequestMessage request = new HttpRequestMessage();
request.Headers.Add("workflow", capture.WorkflowId.ToString());
request.Method = HttpMethod.Post;
request.RequestUri = serviceAddress;
foreach (var image in images)
{
byte[] imageByte = handler.ReadAllBytes(image.ImagePath);
ByteArrayContent byteContent = new ByteArrayContent(imageByte);
byteContent.Headers.Add("Content-Type", "image/jpeg");
content.Add(byteContent, "file", image.ImageName);
}
content.Add(strContent, "text/xml");
request.Content = content;
using (HttpClient client = new HttpClient())
{
client.Timeout = TimeSpan.FromSeconds(180);
var response = client.SendAsync(
request,
HttpCompletionOption.ResponseContentRead).Result;
var readResponse = response.Content.ReadAsStringAsync().Result;
if (readResponse == "File uploaded.")
MessagingCenter.Send<CaptureEntity, string>(
capture,
"Completed",
"Success");
else if (readResponse.Contains("An error has occurred."))
MessagingCenter.Send<CaptureEntity, string>(
capture,
"Uploader",
String.Format(CultureInfo.InvariantCulture,
"Failed: {0}",
readResponse));
else
MessagingCenter.Send<CaptureEntity, string>(
capture,
"Uploader",
String.Format(CultureInfo.InvariantCulture,
"Failed: {0}",
readResponse));
}
}
catch (WebException webExc)
{
MessagingCenter.Send<string, string>("Uploader", "Failed",
String.Format(CultureInfo.InvariantCulture,
"{0} upload failed.\n{1}",
capture.DisplayName,
webExc.Message));
}
catch (TimeoutException timeExc)
{
MessagingCenter.Send<string, string>("Uploader", "Failed",
String.Format(CultureInfo.InvariantCulture,
"{0} upload failed.\n{1}",
capture.DisplayName,
timeExc.Message));
}
catch (Exception exc)
{
MessagingCenter.Send<string, string>("Uploader", "Failed",
String.Format(CultureInfo.InvariantCulture,
"{0} upload failed.\n{1}",
capture.DisplayName,
exc.Message));
}
}
}
Can anyone tell me what I am doing wrong as I am getting the following error when I want to start the service:
Java.Lang.NullPointerException: Attempt to invoke virtual method 'android.content.ComponentName android.content.Context.startService(android.content.Intent)' on a null object reference

Answer 1

In your Intent declaration you need to tell the service you want to call
Something like this:
var uploadIntent = new Intent(this, typeof(ServiceIntent));
Note: this represents the Context.
Update:
As mentioned in the comments your interface implementation cannot derive from Activity class. In order to have access to the Context to be able to call the StartService method and also create your Intent you can make it in two ways:
Using the Xamarin.Forms.Forms.Context:
public async Task<bool> UploadAsync(Uri serviceAddress,
CaptureEntity capture,
List<ImageEntity> images)
{
try
{
var context = Xamarin.Forms.Forms.Context;
var uploadIntent = new Intent(context, typeof(ServiceIntent));
uploadIntent.PutExtra("serviceAddress", serviceAddress.ToString());
uploadIntent.PutExtra("captureId", capture.WorkflowId.ToString());
context.StartService(uploadIntent);
return true;
}
catch (Exception exc)
{
App.logger.LogError(DateTime.Now, "Uploader", exc.ToString());
throw exc;
}
}
If you are using latest versions of Xamarin.Forms this global context was deprecated and they suggest to you local context instead. You can still use it though but in future updates of XF your app might break.
using CurrentActivity plugin:
public async Task<bool> UploadAsync(Uri serviceAddress,
CaptureEntity capture,
List<ImageEntity> images)
{
try
{
var context = CrossCurrentActivity.Current.Activity;
var uploadIntent = new Intent(context, typeof(ServiceIntent));
uploadIntent.PutExtra("serviceAddress", serviceAddress.ToString());
uploadIntent.PutExtra("captureId", capture.WorkflowId.ToString());
context.StartService(uploadIntent);
return true;
}
catch (Exception exc)
{
App.logger.LogError(DateTime.Now, "Uploader", exc.ToString());
throw exc;
}
}
This plugin can be installed from nugget and the setup is very straight forward. Basically it gives you access to the current activity and you can use it as your context to call the IntentService
Hope this helps.-

Answer 2

Here is the IntentService.
IntentService is a base class for Services that handle asynchronous requests (expressed as Intents) on demand. Clients send requests through startService(Intent) calls; the service is started as needed, handles each Intent in turn using a worker thread, and stops itself when it runs out of work.
In Android, we usually use IntentService to do asynchronous operator. As we all know, thread is also used to do asynchronous operator. The difference between IntentService and Thread is IntentService is Service which belongs to Android Component. So, the priority of IntentService is higher than Thread.
For example, there is a ActivityA which has a IntentService, and there is a ActivityB which has a Thread, both IntentService and Thread are working, and both ActivityA and ActivityB are al background Activity. Now, if your phone's system doesn't have extra resources, your ActivityB will be killed firstly.
About the Exception:
Java.Lang.NullPointerException: Attempt to invoke virtual method 'android.content.ComponentName android.content.Context.startService(android.content.Intent)' on a null object reference
That means you should use android.content.Context to call the StartService method. In Android, there are three kinds of Context. Application, Activity and Service. So you can call the StartService method in these three classes directly. If you are not in these three classes, you need pass the Context to your class, and then use the Context to call StartService.
I added Activity for this class' inheritance.
If you do this, your class will be a Activity, and you need to register it in your manifiest, add layout for your class, and it should have the lifecycle, and etc. It will not be what you want to get class. In Android, Activity is a Component, not normal class, so you can't inherit it unless you want your class to be a Activity.
Demo:
I have made a demo for you,