这个问题在这里已经有了答案：

firebase必须上传两次图像才能显示（1个答案）
两天前关门了。
我正在为我的学校项目创建一个应用程序，我的编码没有那么先进，所以我提前为我的错误编码道歉。所以我尝试获取下载uri，这样一旦用户用图片完成表单，我就可以将详细信息上传到实时数据库，以便在我的应用程序的其他部分中使用。

//I use this code below, I got it from the Firebase documents//

     ref.getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
                @Override
                public void onSuccess(Uri uri) {
                    // picUri is a globally declared string and I use it to set my database values,
                    picUri=(uri.toString());

                }
            }).addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception exception) {
                    // Handle any errors
                }
            });

我相信这与firebase方法是异步的有关？所以uri.tostring（）不会被解析为全局字符串变量？因为我一直在玩，我意识到当我使用.setvalue（object）将我的对象写入实时数据库时，uri确实会被捕获，但是那样会太乱，然后我就无法捕获其他值。我到处读到我需要写一封回信，有很多指南，我真的把自己搞糊涂了。
有人能告诉我如何理解回调和实现回调来获得这个值的指南或youtube教程吗？或者是我做错了什么，请建议，谢谢。
照片和其他参数都被成功记录了下来，只是照片的uri我无法捕捉(我暂时用picuri替换了categoryinput，以便更轻松地运行测试）
我是一个新用户，所以我不能上传图片，但数据库和布局的图片是：https://i.ibb.co/ybdszwb/databasestack.png
https://i.ibb.co/svg2zxl/stackoverflow.png
我将输入我的全部代码只是为了获取信息

package com.example.giventake;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.graphics.Bitmap;
import android.net.Uri;
import android.os.Bundle;
import android.provider.MediaStore;
import android.view.View;
import android.widget.*;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.storage.FirebaseStorage;
import com.google.firebase.storage.OnProgressListener;
import com.google.firebase.storage.StorageReference;
import com.google.firebase.storage.UploadTask;

import java.io.IOException;

public class ListPage extends AppCompatActivity {

    /*Declare all variables I'll be using*/
    private EditText title, category, description;
    private ImageView image, image2, image3;
    private FirebaseAuth mAuth;
    private FirebaseStorage storage;
    private StorageReference storageReference;
    private FirebaseDatabase rootNode;
    private DatabaseReference referenceItems;
    private Uri filePath;
    private Uri filePath2;
    private Uri filePath3;
    private final int PICK_IMAGE_REQUEST = 1;
    private final int PICK_IMAGE2_REQUEST = 2;
    private final int PICK_IMAGE3_REQUEST = 3;
    String picUri;
    /*Declare all variables I'll be using*/

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

        /*Initialise all instances of variables*/
        title = (EditText) findViewById(R.id.editTextItemTitle);
        category = (EditText) findViewById(R.id.editTextItemCategory);
        description = (EditText) findViewById(R.id.editTextItemDescription);
        image = (ImageView) findViewById(R.id.imgViewPic);
        image2 = (ImageView) findViewById(R.id.imgViewPic2);
        image3 = (ImageView) findViewById(R.id.imgViewPic3);
        image.setImageResource(R.drawable.ic_baseline_list_24);
        image2.setImageResource(R.drawable.ic_baseline_list_24);
        image3.setImageResource(R.drawable.ic_baseline_list_24);
        storage = FirebaseStorage.getInstance();
        storageReference = storage.getReference();
        mAuth = FirebaseAuth.getInstance();
        rootNode = FirebaseDatabase.getInstance();
        referenceItems = rootNode.getReference("Items");

        /*Initialise all instances of variables*/

    }

    /*This is to upload images from your phones direction*/
    public void onFirstPicClick(View view) {
        Intent i = new Intent();
        i.setType("image/*");
        i.setAction(Intent.ACTION_GET_CONTENT);
        startActivityForResult(Intent.createChooser(i, "Select Picture"), PICK_IMAGE_REQUEST);
    }

    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
            if (requestCode == PICK_IMAGE_REQUEST && resultCode == RESULT_OK
                    && data != null && data.getData() != null) {
                filePath = data.getData();
                try {
                    Bitmap bitmap = MediaStore.Images.Media.getBitmap(getContentResolver(), filePath);
                    image.setImageBitmap(bitmap);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

    /*This is to upload images from your phones direction*/

    public void onSubmitClicked(View view) {
        //Fetching data
        String titleInput = title.getText().toString().trim();
        //String categoryInput = category.getText().toString().trim();
        String descriptionInput = description.getText().toString().trim();
        String itemId = referenceItems.child(mAuth.getCurrentUser().getUid()).push().getKey();
        referenceItems = referenceItems.child(mAuth.getCurrentUser().getUid()).child(itemId);

        if (filePath != null) {
            StorageReference ref = storageReference.child("Images").child(mAuth.getCurrentUser().getUid()).child(itemId).child("Pic1");
            ref.putFile(filePath)
                    .addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
                        @Override
                        public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                            // Toast.makeText(ListPage.this, "Uploaded", Toast.LENGTH_SHORT).show();
                        }
                    })
                    .addOnFailureListener(new OnFailureListener() {
                        @Override
                        public void onFailure(@NonNull Exception e) {
                            Toast.makeText(ListPage.this, "Picture upload failed" + e.getMessage(), Toast.LENGTH_SHORT).show();
                        }
                    })
                    .addOnProgressListener(new OnProgressListener<UploadTask.TaskSnapshot>() {
                        @Override
                        public void onProgress(UploadTask.TaskSnapshot taskSnapshot) {
                            double progress = (100.0 * taskSnapshot.getBytesTransferred() / taskSnapshot
                                    .getTotalByteCount());
                        }
                    });

            ref.getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
                @Override
                public void onSuccess(Uri uri) {
                    // Got the download URL for 'users/me/profile.png'
                    picUri=(uri.toString());

                }
            }).addOnFailureListener(new OnFailureListener() {
                @Override
                public void onFailure(@NonNull Exception exception) {
                    // Handle any errors
                }
            });
        }

        //ItemsHelperClass item = new ItemsHelperClass(titleInput, categoryInput, descriptionInput, itemId);
        ItemsHelperClass item = new ItemsHelperClass(titleInput,picUri,descriptionInput,itemId);

        referenceItems.setValue(item).addOnCompleteListener(new OnCompleteListener<Void>() {
                                                                @Override
                                                                public void onComplete(@NonNull Task<Void> task) {
                                                                    if (task.isSuccessful()) {
                                                                        Intent i = new Intent(ListPage.this, MainActivity.class);
                                                                        startActivity(i);
                                                                        finish();
                                                                        // getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new HomeFragment()).commit();
                                                                    } else {
                                                                        //If it fails to add
                                                                        Toast.makeText(ListPage.this, "Something went wrong, please try again", Toast.LENGTH_LONG).show();
                                                                    }
                                                                }
                                                            }
        );
    }

}