Android 如何从第二Activity调用首个Activity的Camera扫描功能

核心问题说明 #

你之前尝试直接new MainActivity()实例调用方法的方式不可行，Android Activity由系统统一管理生命周期，手动创建的实例不具备正常运行所需的上下文环境，无法调用相机等系统能力。

可行实现方案 #

通过startActivityForResult（或更高版本的Activity Result API）启动第一个Activity，携带扫描类型标记，第一个Activity完成扫描后根据标记将结果返回给第二个Activity，第二个Activity根据返回的扫描类型更新对应ImageView即可。

修改后第一个Activity（MainActivity）代码

public class MainActivity extends HasCameraOpenerActivity {
    // 全局参数键定义
    public static final String EXTRA_SCAN_TYPE = "scan_type";
    public static final int SCAN_TYPE_FIRST_LAUNCH = 0; // 首次启动扫描
    public static final int SCAN_TYPE_CASH_CARD = 1; // 扫描现金卡
    public static final int SCAN_TYPE_OTHER_CARD = 2; // 扫描另一张卡片
    public static final String EXTRA_SCANNED_URI = "scanned_uri";

    private Button button;
    private int currentScanType;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button = findViewById(R.id.scan_btn);

        // 接收启动参数判断扫描类型
        currentScanType = getIntent().getIntExtra(EXTRA_SCAN_TYPE, SCAN_TYPE_FIRST_LAUNCH);
        // 如果是从第二个Activity来的重扫请求，直接调起相机
        if (currentScanType != SCAN_TYPE_FIRST_LAUNCH) {
            pickCamera();
        }

        button.setOnClickListener(v -> pickCamera());
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if(resultCode == RESULT_OK && requestCode == 1){
            if (currentScanType == SCAN_TYPE_FIRST_LAUNCH) {
                // 首次启动扫描，跳转第二个Activity
                Intent intent = new Intent(MainActivity.this, MainActivity2.class);
                intent.putExtra(EXTRA_SCANNED_URI, image_uri.toString());
                startActivity(intent);
                finish();
            } else {
                // 重扫请求，把结果返回给第二个Activity
                Intent resultIntent = new Intent();
                resultIntent.putExtra(EXTRA_SCAN_TYPE, currentScanType);
                resultIntent.putExtra(EXTRA_SCANNED_URI, image_uri.toString());
                setResult(RESULT_OK, resultIntent);
                finish();
            }
        }
    }
}

// 相机基类无需修改
public abstract class HasCameraOpenerActivity extends Activity {
    protected Uri image_uri;
    public void pickCamera() {
        ContentValues values = new ContentValues();
        values.put(MediaStore.Images.Media.TITLE, "NewPic");
        values.put(MediaStore.Images.Media.DESCRIPTION, "Image to Text");
        image_uri = getContentResolver().insert(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, values);
        Intent cameraIntent = new Intent (MediaStore.ACTION_IMAGE_CAPTURE);
        cameraIntent.putExtra(MediaStore.EXTRA_OUTPUT, image_uri);
        startActivityForResult(cameraIntent, 1);
    }
}

修改后第二个Activity（MainActivity2）代码

public class MainActivity2 extends AppCompatActivity {
    // 请求码区分两个ImageView的重扫请求
    private static final int REQUEST_RESCAN_CASH_CARD = 1001;
    private static final int REQUEST_RESCAN_OTHER_CARD = 1002;
    private ImageView mPreviewCashCard;
    private ImageView mPreviewOtherCard;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        Button btnrescanCashCard = findViewById(R.id.rescanCashcard);
        Button btnrescanOtherCard = findViewById(R.id.rescanOtherCard);
        mPreviewCashCard = findViewById(R.id.imageView);
        mPreviewOtherCard = findViewById(R.id.imageView2);

        // 加载首次扫描的现金卡图片
        Bundle extras = getIntent().getExtras();
        if (extras != null) {
            String resultUri = extras.getString(MainActivity.EXTRA_SCANNED_URI);
            loadImageToView(resultUri, mPreviewCashCard);
        }

        // 现金卡重扫点击事件
        btnrescanCashCard.setOnClickListener(v -> {
            Intent rescanIntent = new Intent(MainActivity2.this, MainActivity.class);
            rescanIntent.putExtra(MainActivity.EXTRA_SCAN_TYPE, MainActivity.SCAN_TYPE_CASH_CARD);
            startActivityForResult(rescanIntent, REQUEST_RESCAN_CASH_CARD);
        });

        // 另一张卡片重扫点击事件
        btnrescanOtherCard.setOnClickListener(v -> {
            Intent rescanIntent = new Intent(MainActivity2.this, MainActivity.class);
            rescanIntent.putExtra(MainActivity.EXTRA_SCAN_TYPE, MainActivity.SCAN_TYPE_OTHER_CARD);
            startActivityForResult(rescanIntent, REQUEST_RESCAN_OTHER_CARD);
        });
    }

    // 接收重扫结果更新对应ImageView
    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (resultCode == RESULT_OK && data != null) {
            String scannedUri = data.getStringExtra(MainActivity.EXTRA_SCANNED_URI);
            int scanType = data.getIntExtra(MainActivity.EXTRA_SCAN_TYPE, -1);
            if (scanType == MainActivity.SCAN_TYPE_CASH_CARD) {
                loadImageToView(scannedUri, mPreviewCashCard);
            } else if (scanType == MainActivity.SCAN_TYPE_OTHER_CARD) {
                loadImageToView(scannedUri, mPreviewOtherCard);
            }
        }
    }

    // 公共图片加载方法
    private void loadImageToView(String uriStr, ImageView imageView) {
        if (uriStr == null || imageView == null) return;
        Uri myUri = Uri.parse(uriStr);
        try {
            Bitmap bitmap = MediaStore.Images.Media.getBitmap(this.getContentResolver(),myUri);
            imageView.setImageBitmap(bitmap);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

注意事项 #

• Android 6.0及以上版本需要提前动态申请相机、存储读写权限
• Android 7.0及以上版本传递文件Uri需要配置FileProvider，否则会触发Uri暴露异常
• 如果项目适配了AndroidX，建议使用Activity Result API替代已废弃的startActivityForResult方法，逻辑会更简洁

内容的提问来源于stack exchange，提问作者BootCamp