http://developer.android.com/reference/android/content/Context.html#WINDOW_SERVICE

当我在下面的代码段中使用它时,它会抛出一个错误,说它无法作为变量解析.

显示display =((WindowManager)getSystemService(WINDOW_SERVICE)).getDefaultDisplay();

这是完整的代码：

package com.commonsware.android.skeleton;
 
import android.app.Activity;
import android.content.Context;
import android.hardware.Camera;
import android.hardware.Camera.*;
import android.os.Bundle;
import android.view.Display;
import android.view.Surface;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.Window;
import android.view.WindowManager;
import android.widget.FrameLayout;
import java.io.IOException;
import java.util.List;
 
// ----------------------------------------------------------------------
 
public class SimpleBulbActivity extends Activity {
    private Preview mPreview;
    FrameLayout preview;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
 
        // Hide the window title.
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.main);
    }
 
    protected void onResume() {
        super.onResume();
        //Setup the FrameLayout with the Camera Preview Screen
        mPreview = new Preview(this);
        preview = (FrameLayout)findViewById(R.id.preview); 
        preview.addView(mPreview);
    }
 
}
 
// ----------------------------------------------------------------------
 
class Preview extends SurfaceView implements SurfaceHolder.Callback {
    SurfaceHolder mHolder;
    Camera mCamera;
 
    Preview(Context context) {
        super(context);
 
        // Install a SurfaceHolder.Callback so we get notified when the
        // underlying surface is created and destroyed.
        mHolder = getHolder();
        mHolder.addCallback(this);
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }
 
    public void surfaceCreated(SurfaceHolder holder) {
        // The Surface has been created,acquire the camera and tell it where
        // to draw.
        mCamera = Camera.open();
        try {
           mCamera.setPreviewDisplay(holder);
        } catch (IOException exception) {
            mCamera.release();
            mCamera = null;
            // TODO: add more exception handling logic here
        }
    }
 
    public void surfaceDestroyed(SurfaceHolder holder) {
        // Surface will be destroyed when we return,so stop the preview.
        // Because the CameraDevice object is not a shared resource,it's very
        // important to release it when the activity is paused.
        mCamera.stopPreview();
        mCamera.release();
        mCamera = null;
    }
 
 
    private Size getOptimalPreviewSize(List<Size> sizes,int w,int h) {
        final double ASPECT_TOLERANCE = 0.05;
        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 h) {
        // Now that the size is known,set up the camera parameters and begin
        // the preview.
        Camera.Parameters parameters = mCamera.getParameters();
 
        List<Size> sizes = parameters.getSupportedPreviewSizes();
        Size optimalSize = getOptimalPreviewSize(sizes,w,h);
        parameters.setPreviewSize(optimalSize.width,optimalSize.height);
 
        Display display = ((WindowManager)getSystemService(Context.WINDOW_SERVICE)).getDefaultDisplay();
 
        if(display.getRotation() == Surface.ROTATION_0)
        {
            parameters.setPreviewSize(h,w);                           
            mCamera.setDisplayOrientation(90);
        }
 
        if(display.getRotation() == Surface.ROTATION_90)
        {
            parameters.setPreviewSize(w,h);                           
        }
 
        if(display.getRotation() == Surface.ROTATION_180)
        {
            parameters.setPreviewSize(h,w);               
        }
 
        if(display.getRotation() == Surface.ROTATION_270)
        {
            parameters.setPreviewSize(w,h);
            mCamera.setDisplayOrientation(180);
        }
 
        mCamera.setParameters(parameters);
        mCamera.startPreview();
    }
 
}