安卓 :以编程方式初始化具有参数的XML对象

Question

我有一个名为CoverFlowClass的类。我曾经这样做是为了创建一个类，

CoverFlowClass coverFlow = new CoverFlowClass(this)
coverFlow.setAdapter(new ImageAdapter(this));

但是现在我想使用 XML，以便我可以进行布局。我创建了一个这样的XML代码：

<id.co.ajsmsig.display.CoverFlowClass
    android:id="@+id/coverflow"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
/>

但是当我像这样从代码创建时：

CoverFlowClass coverFlow=  (CoverFlowClass) findViewById(R.id.coverflow);
coverFlow.setAdapter(new ImageAdapter(this));

我的应用强制关闭。我发现的 LogCat 中的错误日志是：

04-18 07：46：34.804： E/AndroidRuntime（1847）： at id.co.ajsmsig.display.CoverFlowMain.onCreate（CoverFlowMain.java：76）.

第 76 行是：

coverFlow.setAdapter(new ImageAdapter(this));

我认为是因为该类在其构造函数中使用了一个参数（= new CoverFlowClass（this）），而 findViewById 没有给出任何参数。我仍然不知道，如何做到这一点？任何想法都是值得赞赏的。

谢谢

P.s. 下面是 CoverFlow 类源代码：

---

package id.co.ajsmsig.display;

import android.content.Context;
import android.graphics.Camera;
import android.graphics.Matrix;
import android.util.AttributeSet;
import android.view.View;
import android.view.animation.Transformation;
import android.widget.Gallery;
import android.widget.ImageView;
public class CoverFlowClass extends Gallery {
    /**
     * Graphics Camera used for transforming the matrix of ImageViews
     */
    private Camera mCamera = new Camera();
    /**
     * The maximum angle the Child ImageView will be rotated by
     */    
    private int mMaxRotationAngle = 60;
    /**
     * The maximum zoom on the centre Child
     */
    private int mMaxZoom = -120;
    /**
     * The Centre of the Coverflow 
     */   
    private int mCoveflowCenter;
    public CoverFlowClass(Context context) {
  super(context);
  this.setStaticTransformationsEnabled(true);
 }
    public CoverFlowClass(Context context, AttributeSet attrs) {
  super(context, attrs);
        this.setStaticTransformationsEnabled(true);
 }
    public CoverFlowClass(Context context, AttributeSet attrs, int defStyle) {
   super(context, attrs, defStyle);
   this.setStaticTransformationsEnabled(true);   
  }
    /**
     * Get the max rotational angle of the image
  * @return the mMaxRotationAngle
  */
    public int getMaxRotationAngle() {
  return mMaxRotationAngle;
 }
 /**
    * Set the max rotational angle of each image
  * @param maxRotationAngle the mMaxRotationAngle to set
  */
    public void setMaxRotationAngle(int maxRotationAngle) {
  mMaxRotationAngle = maxRotationAngle;
 }
 /**
    * Get the Max zoom of the centre image
  * @return the mMaxZoom
  */
    public int getMaxZoom() {
  return mMaxZoom;
 }
 /**
    * Set the max zoom of the centre image
  * @param maxZoom the mMaxZoom to set
  */
    public void setMaxZoom(int maxZoom) {
        mMaxZoom = maxZoom;
    }
 /**
     * Get the Centre of the Coverflow
     * @return The centre of this Coverflow.
     */
    private int getCenterOfCoverflow() {
        return (getWidth() - getPaddingLeft() - getPaddingRight()) / 2 + getPaddingLeft();
    }
    /**
     * Get the Centre of the View
     * @return The centre of the given view.
     */
    private static int getCenterOfView(View view) {
        return view.getLeft() + view.getWidth() / 2;
    }  
    /**
     * {@inheritDoc}
  *
  * @see #setStaticTransformationsEnabled(boolean) 
  */ 
    protected boolean getChildStaticTransformation(View child, Transformation t) {
  final int childCenter = getCenterOfView(child);
  final int childWidth = child.getWidth() ;
  int rotationAngle = 0;
  t.clear();
  t.setTransformationType(Transformation.TYPE_MATRIX);
        if (childCenter == mCoveflowCenter) {
            transformImageBitmap((ImageView) child, t, 0);
        } else {      
            rotationAngle = (int) (((float) (mCoveflowCenter - childCenter)/ childWidth) *  mMaxRotationAngle);
            if (Math.abs(rotationAngle) > mMaxRotationAngle) {
             rotationAngle = (rotationAngle < 0) ? -mMaxRotationAngle : mMaxRotationAngle;   
            }
            transformImageBitmap((ImageView) child, t, rotationAngle);         
        }    
  return true;
 }
 /**
    * This is called during layout when the size of this view has changed. If
  * you were just added to the view hierarchy, you're called with the old
  * values of 0.
  *
  * @param w Current width of this view.
  * @param h Current height of this view.
  * @param oldw Old width of this view.
  * @param oldh Old height of this view.
     */
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        mCoveflowCenter = getCenterOfCoverflow();
        super.onSizeChanged(w, h, oldw, oldh);
     }
    /**
    * Transform the Image Bitmap by the Angle passed 
    * 
    * @param imageView ImageView the ImageView whose bitmap we want to rotate
    * @param t transformation 
    * @param rotationAngle the Angle by which to rotate the Bitmap
    */
    private void transformImageBitmap(View child, Transformation t, int rotationAngle) {            
      mCamera.save();
      final Matrix imageMatrix = t.getMatrix();;
      final int imageHeight = child.getLayoutParams().height;;
      final int imageWidth = child.getLayoutParams().width;
      final int rotation = Math.abs(rotationAngle);
      mCamera.translate(0.0f, 0.0f, 100.0f);
      //As the angle of the view gets less, zoom in     
      if ( rotation < mMaxRotationAngle ) {
       float zoomAmount = (float) (mMaxZoom +  (rotation * 1.5));
       mCamera.translate(0.0f, 0.0f, zoomAmount);          
      } 
      mCamera.rotateY(rotationAngle);
      mCamera.getMatrix(imageMatrix);               
      imageMatrix.preTranslate(-(imageWidth/2), -(imageHeight/2)); 
      imageMatrix.postTranslate((imageWidth/2), (imageHeight/2));
      mCamera.restore();
 }
}

Answer 1

抱歉，此响应有点晚，但也许它会帮助遇到类似问题的人。它还认为这听起来像一个 NullPointerException，所以如果你打算在 XML 布局中使用 Coverflow，请确保在调用任何 findViewByID 之前设置ContentView（R.layout.main）。您可以打印 coverFlow 的值以确保它不再为空。

Answer 2

听起来

你有一个NullPointerException：findViewById调用实际上并没有找到视图，因为它从未被实例化。您可能需要为CoverFlowClass实现一个构造函数，该构造函数采用"属性集"。也许是这样的：

public CoverFlowClass(Context context, AttributeSet attrs) 
{
    super(context, attrs);
        // do whatever other initialization is necessary
}