java - 安卓 Java : Modification of instance variable inside button click handler

Question

在 Button OnClick() 处理程序中分配/修改类的实例变量有一种奇怪的行为。

如下所示，实例变量“CurrentFile”在 imgBtn.setOnClickListener() 处理程序中进行修改。但是当在 onActivityResult() 方法中访问该变量时，该变量包含值“null”。

根据我的理解，“GetPicActivity” Activity 在Camera Activity 返回之前不会被销毁。因此，实例变量“CurrentFile”不应为空。

如果我缺少一些基础知识，请帮忙。

import java.io.File;
import java.io.IOException;

import com.favoritepics.R;

import android.app.Activity;
import android.content.Intent;
import android.database.Cursor;
import android.graphics.drawable.ColorDrawable;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.provider.MediaStore;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.Window;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.Toast;

public class GetPicActivity extends Activity {

protected static final int ID_REQ_IMAGE_CAPTURE = 1;
protected static final int ID_REQ_PICK_PHOTO = 0;
protected File currentFile = null;  /// Goal is "to modify this variable inside Button onClick handlers"

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);

    getWindow().setBackgroundDrawable(
            new ColorDrawable(android.graphics.Color.DKGRAY));

    setContentView(R.layout.activity_get_pic);

    // set handler image gallery
    ImageButton imgBtn = (ImageButton) findViewById(R.id.imgGallery);
    imgBtn.setOnClickListener(new ImageButton.OnClickListener() {

        @Override
        public void onClick(View v) {
            // create intent to take picture on camera
            Intent pickPhoto = new Intent(
                    Intent.ACTION_PICK,
                    android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);

            startActivityForResult(pickPhoto, ID_REQ_PICK_PHOTO);
        }
    });

    // set handler for camera image
    imgBtn = (ImageButton) findViewById(R.id.imgCamera);
    imgBtn.setOnClickListener(new ImageButton.OnClickListener() {

        @Override
        public void onClick(View v) {
            Intent takePicture = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
            File tempDir = Environment.getExternalStorageDirectory();
            File tempFile = null;

            try {

                currentFile = File
                        .createTempFile("_jpeg_", ".jpg", tempDir);

            } catch (IOException exception) {
                Toast.makeText(
                        GetPicActivity.this,
                        "Problem occured during creation of temp file! "
                                + exception.getMessage(),
                        Toast.LENGTH_SHORT).show();
            }

            if (currentFile != null) {
                takePicture.putExtra(MediaStore.EXTRA_OUTPUT,
                        Uri.fromFile(currentFile));
                startActivityForResult(takePicture, ID_REQ_IMAGE_CAPTURE);

            }
        }
    });

    // set handler for cancel button
    Button btnCancel = (Button) findViewById(R.id.btnCancel);

    btnCancel.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            finish();
        }
    });
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    switch (requestCode) {
        case ID_REQ_IMAGE_CAPTURE:
           if (resultCode == RESULT_OK) {
              Intent newIntent = new Intent();
              newIntent.putExtra("TYPE", requestCode);
              newIntent.putExtra("PATH", currentFile.getAbsolutePath());
              setResult(RESULT_OK, newIntent);
                finish();
           }

           break;

        default:
           break;
    }
  }
}

点击此处查看Logcat messages

我打算做的步骤:

1, Create a temporary file in External storage for the destination of Camera picture

2, Start Camera to take a picture by passing the URI of temporary file

3, OnActivityResult() from camera, return the (temporary) File path to previous activity for further process

但我看到的是，

一旦相机启动并拍摄照片，就会再次创建 GetPicActivity。因此，实例变量“CurrentFile”将为空并且在 OnActivityResult() 中，我无法获取临时文件的文件路径。

为什么启动Camera后GetPicActivity就被销毁了？

Answer 1

启动相机后，相机将以“横向”屏幕方向显示。

甚至现有 Activity 堆栈的方向似乎也更改为“横向”。

所以，当Camera的结果返回到GetPicActivity方法OnActivityResult()时，GetPicActivity被再次创建。但现在，实例变量“CurrentFile”为空。这会给出一个 NullPointerException。