如何使用 AsyncTask 更新 TextView

Question

Background Story:

This app listens to my gmail account for latest mail and retrieves it. It then displays the content of the mail on the app.

Problem:

I'm new to Android Programming. Now before I get bashed for not doing my research, let me tell you guys that I did. I know I am not allowed to update TextView in doInBackground() method. And I know that I have to do it onPostExecute instead. My question is how do I do that? How do I make use of onPostExecute to update my label in the context of my application. Thank you so much!

EDIT:

All I want is to display the latest content of my mail onto the TextView.

public class MainActivity extends AppCompatActivity {
    //Alarm Sound Global Declaration
    private TextView lblDegrees;
    @RequiresApi(api = Build.VERSION_CODES.O)
    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Permit all, Remove strict mode
        StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
        StrictMode.setThreadPolicy(policy);

        //Declaration of Widgets
        lblDegrees = (TextView) findViewById(R.id.lblDegrees);
        new receiveMail().execute();
}

public class receiveMail extends AsyncTask {
        protected Object doInBackground(Object[] objects) {
            Store store = null;
            Properties props = new Properties();
            props.setProperty("mail.store.protocol", "imaps");

            try {
                Session session = Session.getInstance(props, null);
                store = session.getStore();
                store.connect("imap.gmail.com", 993, "*****@gmail.com", "****"); //Commented this on purpose
                final Folder inbox = store.getFolder("INBOX");
                inbox.open(Folder.READ_WRITE);
                int msgCount;
                do {//go through all the unread emails in the inbox at least once
                    msgCount = inbox.getMessageCount();//set how many messages are in the inbox when the array is created
                    // Fetch unseen messages from inbox folder
                    Message[] messages = inbox.search(
                            new FlagTerm(new Flags(Flags.Flag.SEEN), false));
                    for (javax.mail.Message msg : messages) {
                        //process emails hereSystem.out.println("---------------------------------------------------");
                        System.out.println("Unread Messages : " + inbox.getUnreadMessageCount());
                        System.out.println("Send Date : "  + msg.getSentDate());
                        System.out.println("Subject : " + msg.getSubject());
                        System.out.println("Content : " + msg.getContent().toString());
                        //Substring the celsius here later
                    }
                    //if a new message came in while reading the messages start the loop over and get all unread messages
                } while (inbox.getMessageCount() != msgCount);
                //add listener
                inbox.addMessageCountListener(new MessageCountAdapter() {
                    @RequiresApi(api = Build.VERSION_CODES.O)
                    @Override
                    public void messagesAdded(MessageCountEvent ev) {
                        //Show notification here
                        show_Notification();
                        alarm_On();
                        Message[] messages = ev.getMessages();
                        for (Message msg : messages) {
                            //process emails here
                            try {

                                System.out.println("Unread Messages : " + inbox.getUnreadMessageCount());
                            } catch (MessagingException e) {
                                e.printStackTrace();
                            }
                            try {
                                System.out.println("Send Date : "  + msg.getSentDate());
                            } catch (MessagingException e) {
                                e.printStackTrace();
                            }
                            try {
                                System.out.println("Subject : " + msg.getSubject());
                            } catch (MessagingException e) {
                                e.printStackTrace();
                            }
                            try {
                                System.out.println("Content : " + msg.getContent().toString());
                            } catch (IOException e) {
                                e.printStackTrace();
                            } catch (MessagingException e) {
                                e.printStackTrace();
                            }
                        }
                    }
                });
                // wait for new messages
                while (inbox.isOpen()) {
                    //every 25 minutes poke the server with a inbox.getMessageCount() to keep the connection active/open
                    ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
                    final Runnable pokeInbox = new Runnable() {
                        @Override
                        public void run() {
                            try {
                                System.out.println(inbox.getMessageCount());
                            } catch (MessagingException ex) {
                                //nothing doin'
                            }
                        }
                    };
                    scheduler.schedule(pokeInbox, 20, TimeUnit.MINUTES);
                    ((IMAPFolder) inbox).idle();
                }
            } catch (FolderClosedException e) {
                e.printStackTrace();
                System.out.println("error connection was dropped");
                if (store != null) {
                    try {
                        store.close();
                    } catch (MessagingException ex) {
                        ex.printStackTrace();
                    }
                }
//            loadUnreadEmails();//restarts listening for email if the connection times out
            } catch (IOException e) {
                e.printStackTrace();
            } catch (NoSuchProviderException e) {
                e.printStackTrace();
            } catch (MessagingException e) {
                e.printStackTrace();
            } finally {
                if (store != null) {
                    try {
                        store.close();
                    } catch (MessagingException e) {
                        e.printStackTrace();
                    }
                }
            }

            return store;
        }


        protected void onPostExecute(String result) {
            // execution of result of Long time consuming operation
            lblDegrees.setText(result);

        }

        protected void onPreExecute() {

        }

        protected void onProgressUpdate(String text) {


        }
    }

Answer 1

Don't run on the UI thread, because this defeats the whole idea of asynchronous execution (and this might only work while it's an inner class)... this creates a dependency where there should be none.

But write an interface eg. with a method onCompleted() , and let the Activity implement it - then the Activity which implements that interface can be passed as listener into the constructor of AsyncTask - and be triggered .onPostExecute() . The same interface can also be used for non-standard progress updates and alike, when adding further callback methods.

---

This would be one simple interface :

public interface SomeListener {
    void onCompleted(String result);
}

This would be the declaration of the class:

public class MainActivity extends AppCompatActivity implements SomeListener ...

And the constructor of the AsyncTask :

private SomeListener listener = null;

public void receiveMail(SomeListener listener) {
    this.listener = listener;
}

To be triggered onPostExecute with:

if(this.listener != null) {
    this.listener.onCompleted("some result");
}

And in the callback, one can then update the TextView :

@override
public void onCompleted(String result) {
    this.lblDegrees.setText(result);
}