活动开始时的白色屏幕

Question

我不确定我是否正确理解这一点，但根据活动的生命周期，首先运行onCreate。因此，我试图通过调用onStart方法继续从那里运行活动。

不确定这是否是正确的事情。无论如何，我开始活动的那一刻（通过主要按钮）我看到的只是一个白色的屏幕。我有setContentView但似乎没有onCreate被运行。

我也有正确包含在清单中的活动，我很确定。

对所有这些仍然是新手，并且确实不确定该做什么，并希望对正确操作活动方法的方法提出一些建议。这个社区太棒了，过去帮了我很多，谢谢。

package example.myapplication;

import android.app.Activity;
import android.content.Context;
import android.hardware.Camera;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.widget.Button;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;

/** A basic Camera preview class */
public class CameraPreview extends Activity implements SurfaceHolder.Callback{

private android.hardware.Camera.PictureCallback mPicture;
private Camera mCamera;
private CameraPreview mPreview;
private SurfaceHolder mHolder;
private SurfaceHolder holder;

public void setContentView(int activity_camera) {}
public static final int MEDIA_TYPE_IMAGE = 1;
public static final int MEDIA_TYPE_VIDEO = 2;
private static final String TAG = "CameraPreview";

SurfaceView camera_preview;
Button button_capture;

public CameraPreview(){
    super();
}

@Override
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_camera);

    button_capture = (Button) findViewById(R.id.button_capture);
    camera_preview = (SurfaceView) findViewById(R.id.camera_preview);
    onStart();
}

public void onStart(Context context, Camera camera) {
    mCamera = camera;

    // Install a SurfaceHolder.Callback so we get notified when the
    // underlying surface is created and destroyed.
    mHolder = getHolder();
    mHolder.addCallback(this);
    // deprecated setting, but required on Android versions prior to 3.0
    mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
}

public void surfaceCreated(SurfaceHolder holder) {
    // The Surface has been created, now tell the camera where to draw the preview.
    try {
        mCamera.setDisplayOrientation(90);
        mCamera.setPreviewDisplay(holder);
        mCamera.startPreview();
    } catch (IOException e) {
        Log.d(TAG, "Error setting camera preview: " + e.getMessage());
    }
}

public void surfaceDestroyed(SurfaceHolder holder) {
    super.onDestroy();
    // empty. Take care of releasing the Camera preview in your activity.
}

public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
    // If your preview can change or rotate, take care of those events here.
    // Make sure to stop the preview before resizing or reformatting it.

    if (mHolder.getSurface() == null){
        // preview surface does not exist
        return;
    }

    // stop preview before making changes
    try {
        mCamera.stopPreview();
    } catch (Exception e){
        // ignore: tried to stop a non-existent preview
    }

    // set preview size and make any resize, rotate or
    // reformatting changes here

    // start preview with new settings
    try {
        mCamera.setPreviewDisplay(mHolder);
        mCamera.startPreview();

    } catch (Exception e){
        Log.d(TAG, "Error starting camera preview: " + e.getMessage());
    }
}

public void onPictureTaken(byte[] data, Camera camera) {
    File pictureFile = getOutputMediaFile(MEDIA_TYPE_IMAGE);
    if (pictureFile == null){
        Log.d(TAG, "Error creating media file, check storage permissions: ");
        return;
    }
    try {
        FileOutputStream fos = new FileOutputStream(pictureFile);
        fos.write(data);
        fos.close();

    } catch (FileNotFoundException e) {
        Log.d(TAG, "File not found: " + e.getMessage());

    } catch (IOException e) {
        Log.d(TAG, "Error accessing file: " + e.getMessage());
    }
}

/** Create a file Uri for saving an image or video */
private static Uri getOutputMediaFileUri(int type){
    return Uri.fromFile(getOutputMediaFile(type));
}

/** Create a File for saving an image or video */
private static File getOutputMediaFile(int type){
    // To be safe, you should check that the SDCard is mounted
    // using Environment.getExternalStorageState() before doing this.

    File mediaStorageDir = new File(Environment.getExternalStoragePublicDirectory(
            Environment.DIRECTORY_PICTURES), "MyCameraApp");
    // This location works best if you want the created images to be shared
    // between applications and persist after your app has been uninstalled.

    // Create the storage directory if it does not exist
    if (! mediaStorageDir.exists()){
        if (! mediaStorageDir.mkdirs()){
            Log.d("MyCameraApp", "failed to create directory");
            return null;
        }
    }

    // Create a media file name
    String timeStamp = new SimpleDateFormat("yyyyMMdd_HH:mm:ss").format(new Date());
    File mediaFile;
    if (type == MEDIA_TYPE_IMAGE){
        mediaFile = new File(mediaStorageDir.getPath() + File.separator +
                "IMG_"+ timeStamp + ".jpg");
    } else if(type == MEDIA_TYPE_VIDEO) {
        mediaFile = new File(mediaStorageDir.getPath() + File.separator +
                "VID_"+ timeStamp + ".mp4");
    } else {
        return null;
    }

    return mediaFile;
}

public void onClick() {
    // Add a listener to the Capture button
    Button captureButton = (Button) findViewById(R.id.button_capture);
    captureButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View arg0){
            // get an image from the camera
            mCamera.takePicture(null, null, mPicture);
        }
    });
}

public SurfaceHolder getHolder() {
    return holder;
}

public void setHolder(SurfaceHolder holder) {
    this.holder = holder;
}
}

Answer 1

不！生命周期方法（例如onCreate（），onStart（），onResume（）等）由android系统直接调用，不要明确自己打电话给他们请考虑活动生命周期here，以了解系统如何调用每个方法。

此外，查看代码时，不会在任何地方调用您自己的onStart()方法。看一下方法签名：

public void onStart(Context context, Camera camera) {

你只是无意中重载了android提供的默认onStart()方法。哪个不会导致问题，直到您在onCreate()中调用错误的方法，如下所示：

@Override
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_camera);

    button_capture = (Button) findViewById(R.id.button_capture);
    camera_preview = (SurfaceView) findViewById(R.id.camera_preview);
    onStart(); // <-- That is the activity's onStart, not yours!
}

表示整个块：

public void onStart(Context context, Camera camera) {
    mCamera = camera;

    // Install a SurfaceHolder.Callback so we get notified when the
    // underlying surface is created and destroyed.
    mHolder = getHolder();
    mHolder.addCallback(this);
    // deprecated setting, but required on Android versions prior to 3.0
    mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
}

永远不会被召唤。

要么将所有代码转储到onCreate()中，要么将方法命名为不同的方法，这样就不会让自己感到困惑。

Answer 2

您不需要调用onStart()方法。您只需要考虑onCreate（）方法。这就是你开始工作的地方。

此外，您正在调用setContentView(）方法错误。你应该这样称呼它：

setContentView(R.layout.activity_camera);

INSIDE您的onCreate()方法。