Question

我正在尝试集成一些代码，其中一条代码就是：

    mPreview = new CameraPreview(this, mCamera, previewCb, autoFocusCB);

当我在Activity中有它时，没有错误。当我把它放在我的片段中时，我收到此错误：预计：android.content.context 实际：这个（com.xxx.ScanFragment）

这是CameraPreview方法

                     public CameraPreview(Context context, Camera camera,
                     PreviewCallback previewCb,
                     AutoFocusCallback autoFocusCb)

正如我所说的，当它出现在活动文件中时没有错误，所以我认为问题是什么＆＃34; context＆＃34;在活动和碎片之间？

import android.content.Intent;
import android.content.pm.ActivityInfo;
import android.hardware.Camera;
import android.os.Bundle;
import android.os.Handler;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.FrameLayout;
import android.widget.TextView;

import com.parse.ParseUser;

import net.sourceforge.zbar.Config;
import net.sourceforge.zbar.Image;
import net.sourceforge.zbar.ImageScanner;
import net.sourceforge.zbar.Symbol;
import net.sourceforge.zbar.SymbolSet;

/**
 * Created by Jakub on 7/23/2014.
 */
public class Pay extends Fragment {
    private Button scanBtn;
    private TextView formatTxt, contentTxt;
    private Camera mCamera;
    private CameraPreview mPreview;
    private Handler autoFocusHandler;

    TextView scanText;
    Button scanButton;

    ImageScanner scanner;

    private boolean barcodeScanned = false;
    private boolean previewing = true;

    static {
        System.loadLibrary("iconv");
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_pay, container, false);

        return rootView;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);




        autoFocusHandler = new Handler();
        mCamera = getCameraInstance();

        /* Instance barcode scanner */
        scanner = new ImageScanner();
        scanner.setConfig(0, Config.X_DENSITY, 3);
        scanner.setConfig(0, Config.Y_DENSITY, 3);

        mPreview = new CameraPreview(getActivity(), mCamera, previewCb, autoFocusCB);
        FrameLayout preview = (FrameLayout)getActivity().findViewById(R.id.cameraPreview);
        Log.d(Payspot.TAG,"This is the string:" + preview.toString());
        preview.addView(mPreview);

        scanText = (TextView)getActivity().findViewById(R.id.scanText);

        scanButton = (Button)getActivity().findViewById(R.id.ScanButton);

        scanButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                if (barcodeScanned) {
                    barcodeScanned = false;
                    scanText.setText("Scanning...");
                    mCamera.setPreviewCallback(previewCb);
                    mCamera.startPreview();
                    previewing = true;
                    mCamera.autoFocus(autoFocusCB);
                }
            }
        });
    }




    public void onPause() {
        super.onPause();
        releaseCamera();
    }
    /** A safe way to get an instance of the Camera object. */
    public static Camera getCameraInstance(){
        Camera c = null;
        try {
            c = Camera.open();
        } catch (Exception e){
        }
        return c;
    }

    private void releaseCamera() {
        if (mCamera != null) {
            previewing = false;
            mCamera.setPreviewCallback(null);
            mCamera.release();
            mCamera = null;
        }
    }

    private Runnable doAutoFocus = new Runnable() {
        public void run() {
            if (previewing)
                mCamera.autoFocus(autoFocusCB);
        }
    };

    Camera.PreviewCallback previewCb = new Camera.PreviewCallback() {
        public void onPreviewFrame(byte[] data, Camera camera) {
            Camera.Parameters parameters = camera.getParameters();
            Camera.Size size = parameters.getPreviewSize();

            Image barcode = new Image(size.width, size.height, "Y800");
            barcode.setData(data);

            int result = scanner.scanImage(barcode);

            if (result != 0) {
                previewing = false;
                mCamera.setPreviewCallback(null);
                mCamera.stopPreview();

                SymbolSet syms = scanner.getResults();
                for (Symbol sym : syms) {
                    scanText.setText("barcode result " + sym.getData());
                    barcodeScanned = true;
                }
            }
        }
    };

    // Mimic continuous auto-focusing
    Camera.AutoFocusCallback autoFocusCB = new Camera.AutoFocusCallback() {
        public void onAutoFocus(boolean success, Camera camera) {
            autoFocusHandler.postDelayed(doAutoFocus, 1000);
        }
    };
}

Answer 1

在片段中，您必须调用getActivity()方法来获取上下文。因为片段必须与活动相关联并且具有活动上下文（活动扩展了上下文）。

Answer 2

Activity扩展了Context，但Fragments没有。这就是为什么你可以将this传递给上下文参数的原因。因此，错误消息将您的ScanFragment视为Context的参数，该参数不起作用。每个片段都与一个Activity的片段相关联。您可以使用getActivity()或getActivity().getApplicationContext()来接收在这些情况下使用的有效上下文。

您的新代码将是： mPreview = new CameraPreview(getActivity(), mCamera, previewCb, autoFocusCB);

活动和片段的上下文差异？

2 个答案: