How to save the state of an activity?

Question

I have an activity A which shows images using Recyclerview and Firebase. When I click on an Image it opens in a new activity B. But when I back from Activity B to A, Activity A starts from the beginning. I want to resume activity A when I came back to it and starts from the image which was clicked.

I added this code but it is not working

intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

Manifests.xml

android:launchMode="singleInstance"

Activity A:

package com.cornicore.imagestatus;

import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;

import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

import java.io.ByteArrayOutputStream;

public class AttitideActivity extends AppCompatActivity {


    RecyclerView mRecyclerView;
    FirebaseDatabase mFirebaseDatabase;
    DatabaseReference mRef;

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

        ActionBar actionBar = getSupportActionBar();
        actionBar.setTitle("Status");

        mRecyclerView = findViewById(R.id.attitude_recyclerView);
        mRecyclerView.setHasFixedSize(true);


        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));


        mFirebaseDatabase = FirebaseDatabase.getInstance();
        mRef = mFirebaseDatabase.getReference("Attitude");

    }


    @Override
    protected void onStart() {
        super.onStart();

        FirebaseRecyclerAdapter<Model, ViewHolder> firebaseRecyclerAdapter =
                new FirebaseRecyclerAdapter<Model, ViewHolder>(
                        Model.class,
                        R.layout.row,
                        ViewHolder.class,
                        mRef
                ) {
                    @Override
                    protected void populateViewHolder(ViewHolder viewHolder, Model model, int position) {

                        viewHolder.setDetails(getApplicationContext(), model.getImage());

                    }

                    @Override
                    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {


                        ViewHolder viewHolder = super.onCreateViewHolder(parent, viewType);
                        viewHolder.setOnclickListener(new ViewHolder.ClickListener() {
                            @Override
                            public void onItemClick(View view, int position) {

                                ImageView mImageView = view.findViewById(R.id.rImageView);

                                Drawable mDrawable = mImageView.getDrawable();
                                Bitmap mBitmap = ((BitmapDrawable)mDrawable).getBitmap();

                                Intent intent = new Intent(view.getContext(), PostDetailsActivity.class);


                                ByteArrayOutputStream stream = new ByteArrayOutputStream();

                                mBitmap.compress(Bitmap.CompressFormat.PNG, 100,stream);
                                byte[] bytes = stream.toByteArray();
                                intent.putExtra("image", bytes);
                                intent.setFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);

                                startActivity(intent);


                            }

                            @Override
                            public void onItemLongClick(View view, int position) {



                            }
                        });



                        return viewHolder;
                    }
                };

        mRecyclerView.setAdapter(firebaseRecyclerAdapter);

    }
}

Activity B:

package com.cornicore.imagestatus;

import android.app.WallpaperManager;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.ColorDrawable;
import android.net.Uri;
import android.os.Build;
import android.os.Environment;
import android.support.annotation.NonNull;
import android.support.annotation.RequiresApi;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.widget.Button;
import android.widget.ImageView;
import android.Manifest;
import android.widget.Toast;

import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.text.SimpleDateFormat;
import java.util.Locale;
import java.util.concurrent.Executor;


public class PostDetailsActivity extends AppCompatActivity {

    Bitmap bitmap;

    ImageView mImageTv;

    Button mSaveBtn, mShareBtn, mWallBtn;

    private static final int WRITE_EXTERNAL_STORAGE_CODE = 1;

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_post_details);



        ActionBar actionBar = getSupportActionBar();
        actionBar.setTitle("Status Image");
        actionBar.setDisplayHomeAsUpEnabled(true);
        actionBar.setDisplayShowHomeEnabled(true);
        actionBar.setBackgroundDrawable(new ColorDrawable(0xf5000000));

        Window window = getWindow();
        window.setStatusBarColor(getResources().getColor(R.color.statusBlack));




        mImageTv = findViewById(R.id.full_imageView);

        mSaveBtn = findViewById(R.id.saveBtn);
        mShareBtn = findViewById(R.id.shareBtn);
        mWallBtn = findViewById(R.id.wallBtn);


        byte[] bytes = getIntent().getByteArrayExtra("image");
        Bitmap bmp = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);

        mImageTv.setImageBitmap(bmp);

        bitmap = ((BitmapDrawable)mImageTv.getDrawable()).getBitmap();

        mImageTv.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

            }
        });

        mSaveBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if (Build.VERSION.SDK_INT >=Build.VERSION_CODES.M){
                    if (checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_DENIED){
                        String[] permission = {Manifest.permission.WRITE_EXTERNAL_STORAGE};
                        requestPermissions(permission, WRITE_EXTERNAL_STORAGE_CODE);

                    }
                    else{

                        saveImage();
                    }
                }
                else {

                    saveImage();

                }

            }
        });

        mShareBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                shareImage();

            }
        });

        mWallBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                setImgWallpaper();

            }
        });


    }

    private void setImgWallpaper() {

        WallpaperManager myWallManager = WallpaperManager.getInstance(getApplicationContext());

        try{
            myWallManager.setBitmap(bitmap);
            Toast.makeText(this, "Wallpaper set ...", Toast.LENGTH_SHORT).show();

        }
        catch (Exception e){
            Toast.makeText(this, e.getMessage(), Toast.LENGTH_SHORT).show();

        }

    }

    private void shareImage() {

        try{
            File file = new File(getExternalCacheDir(), "sample.png");
            FileOutputStream fOut = new FileOutputStream(file);
            bitmap.compress(Bitmap.CompressFormat.PNG, 100, fOut);
            fOut.flush();
            fOut.close();
            file.setReadable(true, false);


            Intent intent = new Intent(Intent.ACTION_SEND);
            intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            intent.putExtra(Intent.EXTRA_TEXT, "");
            intent.putExtra(Intent.EXTRA_STREAM, Uri.fromFile(file));
            intent.setType("image/png");
            startActivity(Intent.createChooser(intent, "Share Via"));


        }
        catch (Exception e){

            Toast.makeText(this, e.getMessage(), Toast.LENGTH_SHORT).show();

        }

    }

    private void saveImage() {

        String timeStamp = new SimpleDateFormat("yyyymmdd_HHmmss", Locale.getDefault()).format(System.currentTimeMillis());

        File path = Environment.getExternalStorageDirectory();

        File dir = new File(path+"/Image_Status/");
        dir.mkdirs();

        String imageName = timeStamp + ".PNG";

        File file = new File(dir, imageName);

        OutputStream out;
        try{

            out = new FileOutputStream(file);
            bitmap.compress(Bitmap.CompressFormat.PNG, 100, out);
            out.flush();
            Toast.makeText(this, imageName+" saved to"+ dir, Toast.LENGTH_SHORT).show();

        }
        catch (Exception e){

            Toast.makeText(this, e.getMessage(), Toast.LENGTH_SHORT).show();


        }

    }


    @Override
    public boolean onSupportNavigateUp() {

        onBackPressed();
        return true;
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode){

            case WRITE_EXTERNAL_STORAGE_CODE: {
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
                    saveImage();
                }
                else {
                    Toast.makeText(this, "enable permission to save image", Toast.LENGTH_SHORT).show();
                }
            }

        }
    }


}

Answer 1

Remove this :

  }


    @Override
    protected void onStart() {
        super.onStart();

From A

Answer 2

问题的原因在于，当您返回A Activity时，将调用onStart（）方法，但是在创建活动时将调用一次onCreate（），只需从onStart（）中删除逻辑并将其添加到onCreate（）中即可。