이미지뷰어

package com.naver.viewer01;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Environment;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import java.io.File;

public class MainActivity extends AppCompatActivity {
    Button btnPrev, btnNext;
    myPictureView myPicture;
    int curNum;
    File[] imageFiles;
    String imageFname;

    // 권한 식별 상수. 임의로 이름 지정.
    private static final int REQUEST_WRITE_STORAGE = 112;
    @Override

    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {

        super.onRequestPermissionsResult(requestCode, permissions, grantResults);

        switch (requestCode)
        {
            case REQUEST_WRITE_STORAGE: {
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED)
                {
                    //reload my activity with permission granted or use the features what required the permission
                } else
                {
                    Toast.makeText(this, "The app was not allowed to write to your storage. Hence, it cannot function properly. Please consider granting it this permission", Toast.LENGTH_LONG).show();
                }
            }
        }
    }

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

        //권한설정
        //SD card 쓰기 권한 허용을 사용자가 했는지 검사.
        boolean hasPermission = (ContextCompat.checkSelfPermission(this,
                Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED);
        if (!hasPermission) {
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                    REQUEST_WRITE_STORAGE);
        }

        btnPrev=(Button)findViewById(R.id.btnPrev);
        btnNext=(Button)findViewById(R.id.btnNext);
        myPicture=(myPictureView)findViewById(R.id.myPictureView1);

        imageFiles=new File(Environment.getExternalStorageDirectory().getAbsolutePath()+"/Pictures").listFiles();
        imageFname=imageFiles[0].toString();
        myPicture.imagePath=imageFname;

        btnPrev.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(curNum<=0){
                    Toast.makeText(getApplicationContext(),"첫번째 그림입니다.",Toast.LENGTH_LONG).show();
                }else{
                    curNum--;
                    imageFname=imageFiles[curNum].toString();
                    myPicture.imagePath=imageFname;
                    myPicture.invalidate();
                }
            }
        });
        btnNext.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(curNum>=imageFiles.length-1){
                    Toast.makeText(getApplicationContext(),"첫번째 그림입니다.",Toast.LENGTH_LONG).show();
                }else{
                    curNum++;
                    imageFname=imageFiles[curNum].toString();
                    myPicture.imagePath=imageFname;
                    myPicture.invalidate();
                }
            }
        });

    }
}