Android应用中是怎么调用系统相册中的照片的

 <Button
  android:id="@+id/button2"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_marginTop="5dp"
  android:layout_marginEnd="52dp"
  android:layout_marginRight="52dp"
  android:text="choose"
  app:layout_constraintEnd_toEndOf="parent"
  app:layout_constraintTop_toTopOf="parent" />

 <ImageView
  android:id="@+id/imageView"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_marginTop="29dp"
  app:layout_constraintEnd_toEndOf="parent"
  app:layout_constraintStart_toStartOf="parent"
  app:layout_constraintTop_toBottomOf="@+id/button"
  app:srcCompat="@mipmap/ic_launcher_round" />

 private Uri imageUri;
 private ImageView imageView;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  imageView = findViewById(R.id.imageView);
  Button button1 = findViewById(R.id.button2);
  button1.setOnClickListener(new View.OnClickListener() {
   @Override
   public void onClick(View v) {
   //动态申请权限
    if (ContextCompat.checkSelfPermission(MainActivity.this,Manifest.permission
      .WRITE_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED){
     ActivityCompat.requestPermissions(MainActivity.this,new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},1);
    }else{
    //执行启动相册的方法
     openAlbum();
    }
   }
  });
  }
//获取权限的结果
@Override
 public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
  if (requestCode == 1){
   if (grantResults.length>0&&grantResults[0] == PackageManager.PERMISSION_GRANTED) openAlbum();
   else Toast.makeText(MainActivity.this,"你拒绝了",Toast.LENGTH_SHORT).show();
  }
 }

//启动相册的方法
private void openAlbum(){
  Intent intent = new Intent("android.intent.action.GET_CONTENT");
  intent.setType("image/*");
  startActivityForResult(intent,2);
 }

@Override
 protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
 if (requestCode == 2){
 //判断安卓版本
    if (resultCode == RESULT_OK&&data!=null){
    if (Build.VERSION.SDK_INT>=19)
    handImage(data);
    else handImageLow(data);
   }
  }
 }

//安卓版本大于4.4的处理方法
@RequiresApi(api = Build.VERSION_CODES.KITKAT)
 private void handImage(Intent data){
  String path =null;
  Uri uri = data.getData();
  //根据不同的uri进行不同的解析
  if (DocumentsContract.isDocumentUri(this,uri)){
   String docId = DocumentsContract.getDocumentId(uri);
   if ("com.android.providers.media.documents".equals(uri.getAuthority())){
    String id = docId.split(":")[1];
    String selection = MediaStore.Images.Media._ID+"="+id;
    path = getImagePath(MediaStore.Images.Media.EXTERNAL_CONTENT_URI,selection);
   }else if("com.android.providers.downloads.documents".equals(uri.getAuthority())){
    Uri contentUri = ContentUris.withAppendedId(Uri.parse("content://downloads/public_downloads"),Long.valueOf(docId));
    path = getImagePath(contentUri,null);
   }
  }else if ("content".equalsIgnoreCase(uri.getScheme())){
   path = getImagePath(uri,null);
  }else if ("file".equalsIgnoreCase(uri.getScheme())){
   path = uri.getPath();
  }
  //展示图片
  displayImage(path);
 }


//安卓小于4.4的处理方法
private void handImageLow(Intent data){
  Uri uri = data.getData();
  String path = getImagePath(uri,null);
  displayImage(path);
 }

//content类型的uri获取图片路径的方法
private String getImagePath(Uri uri,String selection) {
  String path = null;
  Cursor cursor = getContentResolver().query(uri,null,selection,null,null);
  if (cursor!=null){
   if (cursor.moveToFirst()){
    path = cursor.getString(cursor.getColumnIndex(MediaStore.Images.Media.DATA));
   }
   cursor.close();
  }
  return path;
 }

//根据路径展示图片的方法
private void displayImage(String imagePath){
  if (imagePath != null){
   Bitmap bitmap = BitmapFactory.decodeFile(imagePath);
   imageView.setImageBitmap(bitmap);
  }else{
   Toast.makeText(this,"fail to set image",Toast.LENGTH_SHORT).show();
  }
 }