Руководство Android AsyncTaskLoader

package org.o7planning.asynctaskloaderexample;

import android.content.Context;
import android.os.SystemClock;

import androidx.loader.content.AsyncTaskLoader;

import java.util.ArrayList;
import java.util.List;

public class UserAccountTaskLoader extends AsyncTaskLoader<List<UserAccount>> {

    private String param1;
    private String param2;

    public UserAccountTaskLoader(Context context, String param1, String param2) {
        super(context);
        this.param1 = param1;
        this.param2 = param2;
    }

    @Override
    public List<UserAccount> loadInBackground() {
        // Do something, for example:
        // - Download data from URL and parse it info Java object.
        // - Query data from Database into Java object.

        List<UserAccount> list = new ArrayList<UserAccount>();
        list.add(new UserAccount("tom", "tom@example.com", "Tom"));
        list.add(new UserAccount("jerry", "jerry@example.com", "Jerry"));
        list.add(new UserAccount("donald", "donald@example.com", "Donald"));

        SystemClock.sleep(2000); // 2 Seconds.

        return list;
    }
}

package org.o7planning.asynctaskloaderexample;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.loader.app.LoaderManager;
import androidx.loader.content.Loader;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;

import java.util.List;

public class MainActivity extends AppCompatActivity //
        implements LoaderManager.LoaderCallbacks<List<UserAccount>>,
                   Loader.OnLoadCanceledListener<List<UserAccount>> {

    private static final String LOG_TAG = "AndroidExample";
    private static final int LOADER_ID_USERACCOUNT = 10000;

    private Button buttonLoad;
    private Button buttonCancel;
    private ProgressBar progressBar;
    private TextView textView;

    private static final String KEY_PARAM1 = "SomeKey1";
    private static final String KEY_PARAM2 = "SomeKey2";

    private LoaderManager loaderManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        this.buttonLoad = (Button) this.findViewById(R.id.button_load);
        this.buttonCancel = (Button) this.findViewById(R.id.button_cancel);
        this.progressBar = (ProgressBar) this.findViewById(R.id.progressBar);
        this.textView = (TextView) this.findViewById(R.id.textView);

        // Hide ProgressBar.
        this.progressBar.setVisibility(View.GONE);
        this.buttonCancel.setEnabled(false);

        this.buttonLoad.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                clickButtonLoad();
            }
        });

        this.buttonCancel.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                clickButtonCancel();
            }
        });

        this.loaderManager = LoaderManager.getInstance(this);
    }

    // User click on "Load Data" button.
    private void clickButtonLoad() {
        this.textView.setText("");
        Log.i(LOG_TAG, "loadUserAccount");
        LoaderManager.LoaderCallbacks<List<UserAccount>> loaderCallbacks = this;

        // Arguments:
        Bundle args = new Bundle();
        args.putString(KEY_PARAM1, "Some value1");
        args.putString(KEY_PARAM2, "Some value2");

        // You can pass a null args to a Loader
        Loader<List<UserAccount>> loader =  this.loaderManager.initLoader(LOADER_ID_USERACCOUNT, args, loaderCallbacks);
        try {
            loader.registerOnLoadCanceledListener(this); // Loader.OnLoadCanceledListener
        } catch(IllegalStateException e) {
            // There is already a listener registered
        }
        loader.forceLoad(); // Start Loading..
    }

    // User click on "Cancel" button.
    private void clickButtonCancel() {
        Log.i(LOG_TAG, "cancelLoadUserAccount");
        Loader<List<UserAccount>> loader = this.loaderManager.getLoader(LOADER_ID_USERACCOUNT);
        if(loader != null)  {
            boolean cancelled = loader.cancelLoad();
        }
    }

    // Implements method of LoaderManager.LoaderCallbacks
    @NonNull
    @Override
    public Loader<List<UserAccount>> onCreateLoader(int id, @Nullable Bundle args) {
        Log.i(LOG_TAG, "onCreateLoader");

        this.progressBar.setVisibility(View.VISIBLE); // To show

        if(id == LOADER_ID_USERACCOUNT) {
            this.buttonLoad.setEnabled(false);
            this.buttonCancel.setEnabled(true);
            // Parameters:
            String param1 = (String) args.get(KEY_PARAM1);
            String param2 = (String) args.get(KEY_PARAM2);
            // Return a Loader.
            return new UserAccountTaskLoader(MainActivity.this, param1, param2);
        }
        throw new RuntimeException("TODO..");
    }

    // Implements method of LoaderManager.LoaderCallbacks
    @Override
    public void onLoadFinished(@NonNull Loader<List<UserAccount>> loader, List<UserAccount> data) {
        Log.i(LOG_TAG, "onLoadFinished");

        if(loader.getId() == LOADER_ID_USERACCOUNT) {
            // Destroy a Loader by ID.
            this.loaderManager.destroyLoader(loader.getId());

            StringBuilder sb = new StringBuilder();

            for(UserAccount userAccount: data)  {
                sb.append("Username:").append(userAccount.getUserName()).append("\t") //
                        .append("Email:").append(userAccount.getEmail()).append("\n");
            }
            this.textView.setText(sb.toString());
            // Hide ProgressBar.
            this.progressBar.setVisibility(View.GONE);
            this.buttonLoad.setEnabled(true);
            this.buttonCancel.setEnabled(false);
        }
    }

    // Implements method of LoaderManager.LoaderCallbacks
    @Override
    public void onLoaderReset(@NonNull Loader<List<UserAccount>> loader) {
        Log.i(LOG_TAG, "onLoaderReset");

        this.textView.setText("");
    }

    // Implements method of Loader.OnLoadCanceledListener
    @Override
    public void onLoadCanceled(@NonNull Loader<List<UserAccount>> loader) {
        Log.i(LOG_TAG, "onLoadCanceled");

        if(loader.getId() == LOADER_ID_USERACCOUNT) {
            // Destroy a Loader by ID.
            this.loaderManager.destroyLoader(loader.getId());

            this.progressBar.setVisibility(View.GONE); // To hide
            this.buttonLoad.setEnabled(true);
            this.buttonCancel.setEnabled(false);
        }
    }
}

package org.o7planning.asynctaskloaderexample;

public class UserAccount {

    private String userName;
    private String email;
    private String fullName;

    public UserAccount(String userName, String email, String fullName) {
        this.userName = userName;
        this.email = email;
        this.fullName = fullName;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getFullName() {
        return fullName;
    }

    public void setFullName(String fullName) {
        this.fullName = fullName;
    }

}