Alpha不适用于这个简单的例子

Question

我的要求非常简单：我只想让相机预览前面有透明的抽屉。

最大的问题是alpha =＆＃34; 0＆＃34;似乎工作（没有图片），但一旦我设置了一些alpha值（即使＆＃34; 1＆＃34;），图片显示没有透明度！

布局：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <android.view.SurfaceView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/preview"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >
    </android.view.SurfaceView>

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent" 
        android:src="@drawable/test"
        android:alpha="5"/>
</RelativeLayout> 

代码：

/***
  Copyright (c) 2008-2012 CommonsWare, LLC
  (..)
*/


import android.app.Activity;
import android.hardware.Camera;
import android.os.Bundle;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.widget.Toast;

public class PreviewDemo extends Activity {
  private SurfaceView preview=null;
  private SurfaceHolder previewHolder=null;
  private Camera camera=null;
  private boolean inPreview=false;
  private boolean cameraConfigured=false;

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

    setContentView(R.layout.activity_main);

    preview=(SurfaceView)findViewById(R.id.preview);
    previewHolder=preview.getHolder();
    previewHolder.addCallback(surfaceCallback);
    previewHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
  }

  @Override
  public void onResume() {
    super.onResume();

    camera=Camera.open();
    startPreview();
  }

  @Override
  public void onPause() {
    if (inPreview) {
      camera.stopPreview();
    }

    camera.release();
    camera=null;
    inPreview=false;

    super.onPause();
  }

  private Camera.Size getBestPreviewSize(int width, int height,
                                         Camera.Parameters parameters) {
    Camera.Size result=null;

    for (Camera.Size size : parameters.getSupportedPreviewSizes()) {
      if (size.width<=width && size.height<=height) {
        if (result==null) {
          result=size;
        }
        else {
          int resultArea=result.width*result.height;
          int newArea=size.width*size.height;

          if (newArea>resultArea) {
            result=size;
          }
        }
      }
    }

    return(result);
  }

  private void initPreview(int width, int height) {
    if (camera!=null && previewHolder.getSurface()!=null) {
      try {
        camera.setPreviewDisplay(previewHolder);
      }
      catch (Throwable t) {
        Log.e("PreviewDemo-surfaceCallback",
              "Exception in setPreviewDisplay()", t);
        Toast
          .makeText(PreviewDemo.this, t.getMessage(), Toast.LENGTH_LONG)
          .show();
      }

      if (!cameraConfigured) {
        Camera.Parameters parameters=camera.getParameters();
        Camera.Size size=getBestPreviewSize(width, height,
                                            parameters);

        if (size!=null) {
          parameters.setPreviewSize(size.width, size.height);
          camera.setParameters(parameters);
          cameraConfigured=true;
        }
      }
    }
  }

  private void startPreview() {
    if (cameraConfigured && camera!=null) {
      camera.startPreview();
      inPreview=true;
    }
  }

  SurfaceHolder.Callback surfaceCallback=new SurfaceHolder.Callback() {
    public void surfaceCreated(SurfaceHolder holder) {
      // no-op -- wait until surfaceChanged()
    }

    public void surfaceChanged(SurfaceHolder holder,
                               int format, int width,
                               int height) {
      initPreview(width, height);
      startPreview();
    }

    public void surfaceDestroyed(SurfaceHolder holder) {
      // no-op
    }
  };
}

Answer 1

android:alpha是0-1的浮点值。如：

• 0是不可见的。
• 0.5是50％透明。
• 1完全不透明。

改为使用android:alpha="0.05"。