Android 是否按顺序执行任务？

Question

我的项目中 Android Studio 出现了一个非常有趣的行为。对不同活动的调用没有按顺序执行！

我正在构建一个项目，其中的调用是这样的：

button.setOnClickListener()
{
    (1) call to cusom camera acivity
    (2) call to activity showing preview of latest captured image         
    (3) call to a doCrop() function
}

但是执行的时候，实际的流程是：

    (1) call to the doCrop() function
    (2) call to activity showing preview of image captured
    (3) call to cusom camera acivity

自定义相机 activity 处理所有必要的 SurfaceHolder 和 SurfaceView 操作。

这种效果是因为 SurfaceView 布局创建和销毁需要更多时间并且 android 首先切换到更简单的任务吗？

即便如此，它也应该跳到 preview activity 而不是 doCrop() 调用。

这里发生了什么？请指点!

谢谢！

编辑：

命名为：

MainActivity - 主要 activity

Preview - 创建相机实例

CameraPreview - 处理 SurfaceView 等

ImagePreview - 显示指定图片

Main activity代码：

photo.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {

                File temp = new File(//path to store image);
                imageUri=Uri.fromFile(temp);

                 Intent intent = new Intent(MainActivity.this,Preview.class);

                startActivity(intent);

               // Image Preview Activity
                Intent intent1=new Intent(MainActivity.this,ImagePreview.class);
                startActivity(intent1);

                //Crop function
                doCrop();

            }

        });

preview activity代码：

public class Preview extends Activity {
    private static final String TAG = "CamTestActivity";
    CameraPreview preview;
    Button buttonClick;
    Camera camera;
    Activity act;
    Context ctx;
    Uri uri;


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ctx = this;
        act = this;
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().addFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN);

        setContentView(R.layout.activity_preview);

        preview = new CameraPreview(this, (SurfaceView)findViewById(R.id.surfaceView));
        preview.setLayoutParams(new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
        ((FrameLayout) findViewById(R.id.layout)).addView(preview);
        preview.setKeepScreenOn(true);

        buttonClick = (Button) findViewById(R.id.btnCapture);

        buttonClick.setOnClickListener(new OnClickListener()
        {
            public void onClick(View v) {
                camera.takePicture(shutterCallback, rawCallback, pngCallback);
            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        int numCams = Camera.getNumberOfCameras();
        if(numCams > 0){
            try{
                camera = Camera.open(0);
                camera.startPreview();
                preview.setCamera(camera);
            } catch (RuntimeException ex){
                Toast.makeText(ctx, getString(R.string.camera_not_found), Toast.LENGTH_LONG).show();
            }
        }
    }

    @Override
    protected void onPause() {
        if(camera != null) {
            camera.stopPreview();
            preview.setCamera(null);
            camera.release();
            camera = null;
        }
        super.onPause();
    }

    private void resetCam() {
        camera.startPreview();
        preview.setCamera(camera);
    }

    private void refreshGallery(File file) {
        Intent mediaScanIntent = new Intent( Intent.ACTION_MEDIA_SCANNER_SCAN_FILE);
        mediaScanIntent.setData(Uri.fromFile(file));
        sendBroadcast(mediaScanIntent);
    }

    ShutterCallback shutterCallback = new ShutterCallback() {
        public void onShutter() {

        }
    };

    PictureCallback rawCallback = new PictureCallback() {
        public void onPictureTaken(byte[] data, Camera camera) {

        }
    };

    PictureCallback pngCallback = new PictureCallback() {
        public void onPictureTaken(byte[] data, Camera camera) {
            new SaveImageTask().execute(data);
            resetCam();
            Log.d(TAG, "onPictureTaken - png");
        }
    };

    private class SaveImageTask extends AsyncTask<byte[], Void, Void> {
        @Override
        protected Void doInBackground(byte[]... data) {
              // save the image
            }
            catch (Exception e) { e.printStackTrace();}

            return null;
        }
    }
}

CameraPreview代码：

class CameraPreview extends ViewGroup implements SurfaceHolder.Callback {
    private final String TAG = "Preview";

    SurfaceView mSurfaceView;
    SurfaceHolder mHolder;
    Size mPreviewSize;
    List<Size> mSupportedPreviewSizes;
    Camera mCamera;

    CameraPreview(Context context, SurfaceView sv) {
        super(context);

        mSurfaceView = sv;

        mHolder = mSurfaceView.getHolder();
        mHolder.addCallback(this);
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }

    public void setCamera(Camera camera) {
        mCamera = camera;
        if (mCamera != null) {
            mSupportedPreviewSizes = mCamera.getParameters().getSupportedPreviewSizes();
            requestLayout();

            // get Camera parameters
            Camera.Parameters params = mCamera.getParameters();

            List<String> focusModes = params.getSupportedFocusModes();
            if (focusModes.contains(Camera.Parameters.FOCUS_MODE_AUTO)) {
                // set the focus mode
                params.setFocusMode(Camera.Parameters.FOCUS_MODE_AUTO);

                // set Camera parameters
                mCamera.setParameters(params);
            }
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        final int width = resolveSize(getSuggestedMinimumWidth(), widthMeasureSpec);
        final int height = resolveSize(getSuggestedMinimumHeight(), heightMeasureSpec);
        setMeasuredDimension(width, height);

        if (mSupportedPreviewSizes != null) {
            mPreviewSize = getOptimalPreviewSize(mSupportedPreviewSizes, width, height);
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        if (changed && getChildCount() > 0) {
            final View child = getChildAt(0);

            final int width = r - l;
            final int height = b - t;

            int previewWidth = width;
            int previewHeight = height;
            if (mPreviewSize != null) {
                previewWidth = mPreviewSize.width;
                previewHeight = mPreviewSize.height;
            }

            // Center the child SurfaceView within the parent.
            if (width * previewHeight > height * previewWidth) {
                final int scaledChildWidth = previewWidth * height / previewHeight;
                child.layout((width - scaledChildWidth) / 2, 0,
                        (width + scaledChildWidth) / 2, height);
            } else {
                final int scaledChildHeight = previewHeight * width / previewWidth;
                child.layout(0, (height - scaledChildHeight) / 2,
                        width, (height + scaledChildHeight) / 2);
            }
        }
    }

    public void surfaceCreated(SurfaceHolder holder) {
        // The Surface has been created, acquire the camera and tell it where
        // to draw.
        try {
            if (mCamera != null) {
                mCamera.setPreviewDisplay(holder);
            }
        } catch (IOException exception) {
            Log.e(TAG, "IOException caused by setPreviewDisplay()", exception);
        }
    }

    public void surfaceDestroyed(SurfaceHolder holder) {
        // Surface will be destroyed when we return, so stop the preview.
        if (mCamera != null) {
            mCamera.stopPreview();
        }
    }


    private Size getOptimalPreviewSize(List<Size> sizes, int w, int h) {
        final double ASPECT_TOLERANCE = 0.1;
        double targetRatio = (double) w / h;
        if (sizes == null) return null;
        Size optimalSize = null;
        double minDiff = Double.MAX_VALUE;

        int targetHeight = h;

        // Try to find an size match aspect ratio and size
        for (Size size : sizes) {
            double ratio = (double) size.width / size.height;
            if (Math.abs(ratio - targetRatio) > ASPECT_TOLERANCE) continue;
            if (Math.abs(size.height - targetHeight) < minDiff) {
                optimalSize = size;
                minDiff = Math.abs(size.height - targetHeight);
            }
        }

        // Cannot find the one match the aspect ratio, ignore the requirement
        if (optimalSize == null) {
            minDiff = Double.MAX_VALUE;
            for (Size size : sizes) {
                if (Math.abs(size.height - targetHeight) < minDiff) {
                    optimalSize = size;
                    minDiff = Math.abs(size.height - targetHeight);
                }
            }
        }
        return optimalSize;
    }

    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        if(mCamera != null) {
            Camera.Parameters parameters = mCamera.getParameters();
            parameters.setPreviewSize(mPreviewSize.width, mPreviewSize.height);
            requestLayout();

            mCamera.setParameters(parameters);
            mCamera.startPreview();
        }
    }

}

Answer 1

我认为这更像是一个设计问题。您真的应该一次打开多个 activity 吗？

当 activity 启动时，它被放在历史堆栈的顶部，这可能给人一种它正在以相反方式发生的错觉。

但在看到代码之前我无法进一步评论。

更新：是的，设计问题。

这是它应该的工作方式：

MainActivity 中的 Onclick 操作应仅使用 startActivityForResult() 打开预览 activity。
用户拍照，触发setResult()并关闭相机activity。
结果返回到 MainActivity，触发 onActivityResult()
onActivityResult 中的代码启动 ImagePreview activity。
ImagePreview 中的 onCreate 函数触发 doCrop()；

我建议您仔细阅读此处的 Android 文档： http://developer.android.com/guide/components/activities.html

Android 不如其他平台灵活（或更有帮助），因此您必须很好地理解 API.

Answer 2

Activity 启动代码是异步的，如果同时启动多个活动，如下所示：

startActivity(intent1);
startActivity(intent2);
startActivity(intent3);

不保证它们会按顺序显示给用户，即 "launch order"。

这是预期的行为，您通常不应该这样做，因为这表明您的应用程序存在设计缺陷

Answer 3

所以您要做的是启动 3 个活动 simultaneously.These，所有活动都需要在 onActivityResult() 中处理。但是一个用户不能同时访问 3 个活动。所以修改你的流程。应该是这样的

启动 CameraActivity
在你调用 activity 的 onActivityResult() 中，你调用 do crop 并做你想做的事。

您需要重新审视您的工作流程，因为：

您没有启动 3 个活动。
您正在根据用户交互触发事件，因此不应从您的代码中控制函数的顺序。

Answer 4

您的任务执行顺序错误，因为它们被异步调用。要按照您指定的顺序执行它们，请调用 synchronous，可以使用 startActivityForResult(intent, requestCode);

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