آموزش LiveData در اندروید استودیو

LiveData چگونه کار می‌کند ؟

LiveData از observer pattern پیروی می‌کند. LiveData با تغییر وضعیت LifeCycle، اشیاء Observer را مطلع می‌کند. می‌توانید    کد خود را برای به روزرسانی UI در اشیاء Observer تلفیق کنید. به جای بروز کردن UI هر بار که داده‌های برنامه تغییر می‌کنند، Observer شما می‌تواند، هر زمان که تغییر ایجاد می‌کند، رابط کاربر را بروز کند.

No Memory Leak:

LiveData در اندروید در مقایسه با RxJava

LiveData در اندروید تا حدودی شبیه RxJava است، به جز اینکه LiveData از چرخه‌ی حیات آگاه یا lifecycle aware است.

LiveData چگونه در ViewModel اکتیویتی را بروز می‌کند؟

MutableLiveData

مثالی از LiveData در اندروید

implementation 'com.android.support:design:27.1.1'
implementation 'com.android.support:cardview-v7:27.1.1'
implementation 'android.arch.lifecycle:extensions:1.1.1

Android LiveData Code

کد طرح activity_main.xml در زیر آورده شده است:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="https://schemas.android.com/apk/res/android"
    xmlns:app="https://schemas.android.com/apk/res-auto"
    xmlns:tools="https://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">
        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />
    </android.support.design.widget.AppBarLayout>
    <RelativeLayout xmlns:android="https://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior">
        <android.support.v7.widget.RecyclerView
            android:id="@+id/recyclerView"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </RelativeLayout>
    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        app:srcCompat="@android:drawable/ic_input_add" />
</android.support.design.widget.CoordinatorLayout>

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView xmlns:android="https://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="8dp"
        android:gravity="center_vertical">
        <TextView
            android:id="@+id/tvUrl"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentStart="true"
            android:autoLink="web"
            android:padding="8dp"
            android:textColor="@android:color/black"
            android:textSize="20sp" />
        <TextView
            android:id="@+id/tvDate"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentLeft="true"
            android:layout_below="@+id/tvUrl" />
        <ImageButton
            android:id="@+id/btnDelete"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentEnd="true"
            android:layout_centerVertical="true"
            android:src="@android:drawable/ic_menu_delete" />
    </RelativeLayout>
</android.support.v7.widget.CardView>

کد کلاس DbSettings.java در زیر آورده شده است:

package com.sevenlearn.androidlivedata.db;
import android.provider.BaseColumns;
public class DbSettings {
    public static final String DB_NAME = "favourites.db";
    public static final int DB_VERSION = 1;
    public class DBEntry implements BaseColumns {
        public static final String TABLE = "fav";
        public static final String COL_FAV_URL = "url";
        public static final String COL_FAV_DATE = "date";
    }
}
The code for FavouritesDbHelper.java class is given below:
package com.journaldev.androidlivedata.db;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class FavouritesDBHelper extends SQLiteOpenHelper {
    public FavouritesDBHelper(Context context) {
        super(context, DbSettings.DB_NAME, null, DbSettings.DB_VERSION);
    }
    @Override
    public void onCreate(SQLiteDatabase db) {
        String createTable = "CREATE TABLE " + DbSettings.DBEntry.TABLE + " ( " +
                DbSettings.DBEntry._ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                DbSettings.DBEntry.COL_FAV_URL + " TEXT NOT NULL, " +
                DbSettings.DBEntry.COL_FAV_DATE + " INTEGER NOT NULL);";
        db.execSQL(createTable);
    }
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + DbSettings.DBEntry.TABLE);
        onCreate(db);
    }
}

package com.sevenlearn.androidlivedata;
public class Favourites {
    public long mId;
    public String mUrl;
    public long mDate;
    public Favourites(long id, String name, long date) {
        mId = id;
        mUrl = name;
        mDate = date;
    }
    public Favourites(Favourites favourites) {
        mId = favourites.mId;
        mUrl = favourites.mUrl;
        mDate = favourites.mDate;
    }
}

package com.sevenlearn.androidlivedata;
import android.app.Application;
import android.arch.lifecycle.AndroidViewModel;
import android.arch.lifecycle.MutableLiveData;
import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import com.journaldev.androidlivedata.db.DbSettings;
import com.journaldev.androidlivedata.db.FavouritesDBHelper;
import java.util.ArrayList;
import java.util.List;
public class FavouritesViewModel extends AndroidViewModel {
    private FavouritesDBHelper mFavHelper;
    private MutableLiveData<List<Favourites>> mFavs;
    FavouritesViewModel(Application application) {
        super(application);
        mFavHelper = new FavouritesDBHelper(application);
    }
    public MutableLiveData<List<Favourites>> getFavs() {
        if (mFavs == null) {
            mFavs = new MutableLiveData<>();
            loadFavs();
        }
        return mFavs;
    }
    private void loadFavs() {
        List<Favourites> newFavs = new ArrayList<>();
        SQLiteDatabase db = mFavHelper.getReadableDatabase();
        Cursor cursor = db.query(DbSettings.DBEntry.TABLE,
                new String[]{
                        DbSettings.DBEntry._ID,
                        DbSettings.DBEntry.COL_FAV_URL,
                        DbSettings.DBEntry.COL_FAV_DATE
                },
                null, null, null, null, null);
        while (cursor.moveToNext()) {
            int idxId = cursor.getColumnIndex(DbSettings.DBEntry._ID);
            int idxUrl = cursor.getColumnIndex(DbSettings.DBEntry.COL_FAV_URL);
            int idxDate = cursor.getColumnIndex(DbSettings.DBEntry.COL_FAV_DATE);
            newFavs.add(new Favourites(cursor.getLong(idxId), cursor.getString(idxUrl), cursor.getLong(idxDate)));
        }
        cursor.close();
        db.close();
        mFavs.setValue(newFavs);
    }
    public void addFav(String url, long date) {
        SQLiteDatabase db = mFavHelper.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(DbSettings.DBEntry.COL_FAV_URL, url);
        values.put(DbSettings.DBEntry.COL_FAV_DATE, date);
        long id = db.insertWithOnConflict(DbSettings.DBEntry.TABLE,
                null,
                values,
                SQLiteDatabase.CONFLICT_REPLACE);
        db.close();
        List<Favourites> favourites = mFavs.getValue();
        ArrayList<Favourites> clonedFavs;
        if (favourites == null) {
            clonedFavs = new ArrayList<>();
        } else {
            clonedFavs = new ArrayList<>(favourites.size());
            for (int i = 0; i < favourites.size(); i++) {
                clonedFavs.add(new Favourites(favourites.get(i)));
            }
        }
        Favourites fav = new Favourites(id, url, date);
        clonedFavs.add(fav);
        mFavs.setValue(clonedFavs);
    }
    public void removeFav(long id) {
        SQLiteDatabase db = mFavHelper.getWritableDatabase();
        db.delete(
                DbSettings.DBEntry.TABLE,
                DbSettings.DBEntry._ID + " = ?",
                new String[]{Long.toString(id)}
        );
        db.close();
        List<Favourites> favs = mFavs.getValue();
        ArrayList<Favourites> clonedFavs = new ArrayList<>(favs.size());
        for (int i = 0; i < favs.size(); i++) {
            clonedFavs.add(new Favourites(favs.get(i)));
        }
        int index = -1;
        for (int i = 0; i < clonedFavs.size(); i++) {
            Favourites favourites = clonedFavs.get(i);
            if (favourites.mId == id) {
                index = i;
            }
        }
        if (index != -1) {
            clonedFavs.remove(index);
        }
        mFavs.setValue(clonedFavs);
    }
}

package com.sevenlearn.androidlivedata;
import android.arch.lifecycle.Observer;
import android.arch.lifecycle.ViewModelProviders;
import android.content.DialogInterface;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.util.DiffUtil;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.TextView;
import java.util.Date;
import java.util.List;
public class MainActivity extends AppCompatActivity {
    private FavAdapter mFavAdapter;
    private FavouritesViewModel mFavViewModel;
    private List<Favourites> mFav;
    FloatingActionButton fab;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        fab = findViewById(R.id.fab);
        final RecyclerView recyclerView = findViewById(R.id.recyclerView);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        mFavViewModel = ViewModelProviders.of(this).get(FavouritesViewModel.class);
        final Observer<List<Favourites>> favsObserver = new Observer<List<Favourites>>() {
            @Override
            public void onChanged(@Nullable final List<Favourites> updatedList) {
                if (mFav == null) {
                    mFav = updatedList;
                    mFavAdapter = new FavAdapter();
                    recyclerView.setAdapter(mFavAdapter);
                } else {
                    DiffUtil.DiffResult result = DiffUtil.calculateDiff(new DiffUtil.Callback() {
                        @Override
                        public int getOldListSize() {
                            return mFav.size();
                        }
                        @Override
                        public int getNewListSize() {
                            return updatedList.size();
                        }
                        @Override
                        public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
                            return mFav.get(oldItemPosition).mId ==
                                    updatedList.get(newItemPosition).mId;
                        }
                        @Override
                        public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
                            Favourites oldFav = mFav.get(oldItemPosition);
                            Favourites newFav = updatedList.get(newItemPosition);
                            return oldFav.equals(newFav);
                        }
                    });
                    result.dispatchUpdatesTo(mFavAdapter);
                    mFav = updatedList;
                }
            }
        };
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                final EditText inUrl = new EditText(MainActivity.this);
                AlertDialog dialog = new AlertDialog.Builder(MainActivity.this)
                        .setTitle("New favourite")
                        .setMessage("Add a url link below")
                        .setView(inUrl)
                        .setPositiveButton("Add", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                                String url = String.valueOf(inUrl.getText());
                                long date = (new Date()).getTime();
                                mFavViewModel.addFav(url, date);
                            }
                        })
                        .setNegativeButton("Cancel", null)
                        .create();
                dialog.show();
            }
        });
        mFavViewModel.getFavs().observe(this, favsObserver);
    }
    public class FavAdapter extends RecyclerView.Adapter<FavAdapter.FavViewHolder> {
        @Override
        public FavViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item_row, parent, false);
            return new FavViewHolder(itemView);
        }
        @Override
        public void onBindViewHolder(FavViewHolder holder, int position) {
            Favourites favourites = mFav.get(position);
            holder.mTxtUrl.setText(favourites.mUrl);
            holder.mTxtDate.setText((new Date(favourites.mDate).toString()));
        }
        @Override
        public int getItemCount() {
            return mFav.size();
        }
        class FavViewHolder extends RecyclerView.ViewHolder {
            TextView mTxtUrl;
            TextView mTxtDate;
            FavViewHolder(View itemView) {
                super(itemView);
                mTxtUrl = itemView.findViewById(R.id.tvUrl);
                mTxtDate = itemView.findViewById(R.id.tvDate);
                ImageButton btnDelete = itemView.findViewById(R.id.btnDelete);
                btnDelete.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        int pos = getAdapterPosition();
                        Favourites favourites = mFav.get(pos);
                        mFavViewModel.removeFav(favourites.mId);
                    }
                });
            }
        }
    }
}

در کد بالا، ما کلاس Adapter را  برای RecyclerView در Activity تعریف کرده‌ایم.