轉載地址：http://www.eoeandroid.com/thread-158088-1-1.html

系統自帶的Gallery選中的item總是在元件的中間。但是有些時候我們需要把選中的元素放在左邊或者是Gallery一出來就要放在左邊。

修改Gallery靠左對齊的思路：
1、Gellary總是對center進行鎖定的，所以可以考慮修改它的center的位置，把center改成在left的位置就可以了。
Gallery中有個方法：

/** *@return The center of this Gallery. */ private int getCenterOfGallery() { return (getWidth() - mPaddingLeft -mPaddingRight) / 2 + mPaddingLeft; }

只可惜這個方法是private，所以沒有辦法重寫。那就把原始碼拷貝出來進行修改吧，但是當你拷貝處理的時候會發現很多的變數是沒有辦法用的，這是因為：

a.      

@ViewDebug.ExportedProperty  
This annotation can be used to markfields and methods to be dumped by the view server. Only non-void methods withno arguments can be annotated by this annotation.
 

這個很好理解，不作翻譯。

b.

 {@hide}
      Views which have been hidden or removedwhich need to be animated on their way out. This field should be made private,so it is hidden from the SDK.

這也很好理解，自己翻譯。
總結：Gallery的原始碼太過複雜所以這個方法只好放棄。
2、重寫Gallery。這個問題的關鍵是如何重寫，重寫什麼。

Gallery

package eoe.android.CustomGallery;

import android.R.attr;
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Camera;
import android.graphics.Matrix;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.animation.Transformation;
import android.widget.Gallery;

public class GalleryFlow extends Gallery {

        private Camera mCamera;
        private int mWidth;
        private int mPaddingLeft;
        private boolean flag;
        private static int firstChildWidth;
        private static int firstChildPaddingLeft;
        private int offsetX;

        public GalleryFlow(Context context) {
                super(context);
                mCamera = new Camera();
                this.setStaticTransformationsEnabled(true);
        }

        public GalleryFlow(Context context, AttributeSet attrs) {
                super(context, attrs);
                mCamera = new Camera();
                setAttributesValue(context, attrs);
                this.setStaticTransformationsEnabled(true);
        }

        public GalleryFlow(Context context, AttributeSet attrs, int defStyle) {
                super(context, attrs, defStyle);
                mCamera = new Camera();
                setAttributesValue(context, attrs);
                this.setStaticTransformationsEnabled(true);
        }

        private void setAttributesValue(Context context, AttributeSet attrs) {
                TypedArray typedArray = context.obtainStyledAttributes(attrs,
                                new int[] { attr.paddingLeft });
                mPaddingLeft = typedArray.getDimensionPixelSize(0, 0);
                typedArray.recycle();
        }

        protected boolean getChildStaticTransformation(View child, Transformation t) {
                t.clear();
                t.setTransformationType(Transformation.TYPE_MATRIX);
                mCamera.save();
                final Matrix imageMatrix = t.getMatrix();
                if (flag) {
                        firstChildWidth = getChildAt(0).getWidth();
                        firstChildPaddingLeft = getChildAt(0).getPaddingLeft();
                        flag = false;
                }
                offsetX = firstChildWidth / 2 + firstChildPaddingLeft + mPaddingLeft
                                - mWidth / 2;
                mCamera.translate(offsetX, 0f, 0f);
                mCamera.getMatrix(imageMatrix);
                mCamera.restore();
                return true;
        }

        
        @Override
        public boolean onTouchEvent(MotionEvent event) {
                event.offsetLocation(-offsetX, 0);
                return super.onTouchEvent(event);
        }

        protected void onSizeChanged(int w, int h, int oldw, int oldh) {
                if (!flag) {
                        mWidth = w * 2;
                        getLayoutParams().width = mWidth;
                        flag = true;
                }
                super.onSizeChanged(w, h, oldw, oldh);
        }
}