最近学习了一个视频公开课，讲到了利用HorizontalScrollView仿ViewPager设计的一个简单相册，其实主要用了ViewPager缓存的思想。此篇文章参考：Android自定义HorizontalScrollView打造超强Gallery效果（这篇文章与公开课的讲的大致一样）

这里简单说一下ViewPager的缓存机制

1.进入ViewPager时，加载当前页和后一页；

2.当滑动ViewPager至下一页时，加载后一页，此时第一页是不会销毁的，同时加载当前页的下一页。

其实就是默认加载3页，当前页，前一页和后一页。

而此HorizontalScrollView是默认加载两页的，这个要注意，不然调度代码会让人晕。

话不多说，上代码：

代码结构如下图：

一个View，一个Adapter，一个MainActivity，相信不用解释，大家也相当清楚了，典型的MVC模式~

package com.ssa.horizontalscrollview.myview; import java.util.HashMap; import java.util.Map; import com.ssa.horizontalscrollview.myUtils.DisplayUtil; import android.content.Context; import android.graphics.Color; import android.util.AttributeSet; import android.util.Log; import android.view.MotionEvent; import android.view.View; import android.view.View.OnClickListener; import android.widget.HorizontalScrollView; import android.widget.LinearLayout; public class GalleryHorizontalScrollView extends HorizontalScrollView implements OnClickListener { private LinearLayout mContainer;// MyHorizontalScrollView中的LinearLayout private int mChildWidth;// 子元素的宽度 private int mChildHeight;// 子元素的高度 private int mAllLastIndex;// 当前的最后一张的index private int mdisplayLastIndex;// 当前显示的最后一张的index private int mAllFirstIndex;// 当前的第一张index private GalleryHorizontalScrollViewAdapter mAdapter;// 数据适配器 private int mScreenWidth;// 屏幕的宽度 private int mCountOneScreen; private Map<View, Integer> mViewPos = new HashMap<View, Integer>(); private OnCurrentImageChangeListener mOnCurrentImageChangeListener; private OnClickImageChangeListener mOnClickImageChangeListener; public void setmOnCurrentImageChangeListener( OnCurrentImageChangeListener mListener) { this.mOnCurrentImageChangeListener = mListener; } public void setmOnClickImageListener(OnClickImageChangeListener mListener) { this.mOnClickImageChangeListener = mListener; } /** * 图片滚动时回调接口 */ public interface OnCurrentImageChangeListener { void onCurrentImgChanged(int position, View view); } /** * 点击图片时回调接口 */ public interface OnClickImageChangeListener { void onClickImageChangeListener(int position, View view); } public GalleryHorizontalScrollView(Context context, AttributeSet attrs) { super(context, attrs); // 获取屏幕宽度 mScreenWidth = getResources().getDisplayMetrics().widthPixels; } /** * 初始化数据，设置适配器 */ public void initData(GalleryHorizontalScrollViewAdapter mAdapter) { this.mAdapter = mAdapter; mContainer = (LinearLayout) getChildAt(0); final View view = mAdapter.getView(0, null, mContainer); mContainer.addView(view); if (mChildHeight == 0 && mChildWidth == 0) { /*int w = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED); int h = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED);*/ /** * 上面注释掉的是一位老师的写法，但我查了好多资料，用参数0和View.MeasureSpec.UNSPECIFIED是一种不太优美的做法； * 好的做法应该是 * 当View为match_parent时，无法测量出View的大小（任玉刚大神讲的，确实是这么一回事,这个具体的原因要结合源码分析，可以看一下任大神的博客） * 当View宽高为具体的数值时，比如100px： * int w =View.MeasureSpec.makeMeasureSpec(100, View.MeasureSpec.EXACTLY); * int h =View.MeasureSpec.makeMeasureSpec(100, View.MeasureSpec.EXACTLY); * view.measure(w, h); * 当View宽高为wrap_content时： * int w =View.MeasureSpec.makeMeasureSpec((1<<30)-1, View.MeasureSpec.AT_MOST); * int h =View.MeasureSpec.makeMeasureSpec((1<<30)-1, View.MeasureSpec.AT_MOST); * view.measure(w, h); * * 我的此View高度为固定的150dip,宽度为wrap_content */ int heightPx = DisplayUtil.dip2px(getContext(), 150); int w =View.MeasureSpec.makeMeasureSpec((1<<30)-1, View.MeasureSpec.AT_MOST); int h =View.MeasureSpec.makeMeasureSpec(heightPx, View.MeasureSpec.EXACTLY); view.measure(w, h); mChildHeight = view.getMeasuredHeight(); mChildWidth = view.getMeasuredWidth(); // 计算每次加载多少个item mdisplayLastIndex = mScreenWidth / mChildWidth; mCountOneScreen = mdisplayLastIndex + 1; initFirstScreenChildren(mdisplayLastIndex + 1); } } /** * 加载第一屏的元素 * * @param mDisplayCountOneScreen */ private void initFirstScreenChildren(int mDisplayCountOneScreen) { mContainer = (LinearLayout) getChildAt(0); mContainer.removeAllViews(); mViewPos.clear(); for (int i = 0; i < mDisplayCountOneScreen; i++) { View view = mAdapter.getView(i, null, mContainer); // 待完善的点击事件 view.setOnClickListener(this); mContainer.addView(view); mViewPos.put(view, i); mAllLastIndex = i; } // 初始化并刷新界面 if (null != mOnCurrentImageChangeListener) { notifyCurrentImgChanged(); } } private void notifyCurrentImgChanged() { // 先清除所有的背景颜色，点击时设置为蓝色 for (int i = 0; i < mContainer.getChildCount(); i++) { mContainer.getChildAt(i).setBackgroundColor(Color.WHITE); } mOnCurrentImageChangeListener.onCurrentImgChanged(mAllFirstIndex, mContainer.getChildAt(0)); } @Override public boolean onTouchEvent(MotionEvent ev) { /* * Log.e("X", getX()+""); Log.e("ChildX", * mContainer.getChildAt(0).getX()+""); Log.e("RawX",getLeft() +""); */ switch (ev.getAction()) { case MotionEvent.ACTION_MOVE: int scrollX = getScrollX(); Log.e("ScrollX", scrollX + ""); if (scrollX >= mChildWidth) { // 加载下一页，移除第一张 loadNextImg(); } if (scrollX == 0) { // 加载上一页，移除最后一张 loadPreImg(); } break; } return super.onTouchEvent(ev); } private void loadNextImg() {// 数组边界值计算 if (mAllLastIndex == mAdapter.getCount() - 1) { return; } // 移除第一张图片，且将水平滚动位置置0 scrollTo(0, 0); mViewPos.remove(mContainer.getChildAt(0)); mContainer.removeViewAt(0); // 获取下一张图片，并且设置onclick事件，且加入容器中 View view = mAdapter.getView(++mAllLastIndex, null, mContainer); view.setOnClickListener(this); mContainer.addView(view); mViewPos.put(view, mAllLastIndex); // 当前第一张图片小标 mAllFirstIndex++; // 如果设置了滚动监听则触发 if (mOnCurrentImageChangeListener != null) { notifyCurrentImgChanged(); } } private void loadPreImg() { if (mAllFirstIndex == 0) { return; } int index = mAllLastIndex - mCountOneScreen; if (index >= 0) { // 移除最后一张 int oldViewPos = mContainer.getChildCount() - 1; mViewPos.remove(mContainer.getChildAt(oldViewPos)); mContainer.removeViewAt(oldViewPos); // 将加入的View放在第一个位置 View view = mAdapter.getView(index, null, mContainer); mViewPos.put(view, index); mContainer.addView(view, 0); view.setOnClickListener(this); // 水平滚动位置向左移动View的宽度的像素 scrollTo(mChildWidth, 0); mAllLastIndex--; mAllFirstIndex--; if (null != mOnCurrentImageChangeListener) { notifyCurrentImgChanged(); } } } @Override public void onClick(View v) { if(null!=mOnClickImageChangeListener){ mOnClickImageChangeListener.onClickImageChangeListener(mViewPos.get(v), v); } } }

下面是Adapter的源码：

package com.ssa.horizontalscrollview.myview; import java.util.List; import com.ssa.horizontalscrollview.R; import android.content.Context; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.ImageView; import android.widget.TextView; public class GalleryHorizontalScrollViewAdapter { private LayoutInflater mInflater; private List<Integer> mDatas; public GalleryHorizontalScrollViewAdapter(Context context, List<Integer> mDatas) { mInflater = LayoutInflater.from(context); this.mDatas = mDatas; } public Object getItem(int position) { return mDatas.get(position); } public long getItemId(int position) { return position; } public int getCount() { return mDatas.size(); } public View getView(int position, View contentView, ViewGroup parent) { ViewHolder myHolder = null; if (null == contentView) { contentView = mInflater.inflate(R.layout.activity_gallery_item, parent, false); myHolder = new ViewHolder(contentView); contentView.setTag(myHolder); }else { myHolder = (ViewHolder)contentView.getTag(); } myHolder.ivImg.setImageResource(mDatas.get(position)); myHolder.tvText.setText("Img_"+position); return contentView; } private static class ViewHolder { ImageView ivImg; TextView tvText; public ViewHolder(View view) { ivImg = (ImageView)view.findViewById(R.id.iv_content); tvText =(TextView)view.findViewById(R.id.tv_index); } } }

下面是MainActivity的源码：

package com.ssa.horizontalscrollview; import java.util.ArrayList; import java.util.Arrays; import java.util.List; import android.app.Activity; import android.graphics.Color; import android.os.Bundle; import android.view.View; import android.widget.ImageView; import com.ssa.horizontalscrollview.myview.GalleryHorizontalScrollView; import com.ssa.horizontalscrollview.myview.GalleryHorizontalScrollView.OnClickImageChangeListener; import com.ssa.horizontalscrollview.myview.GalleryHorizontalScrollView.OnCurrentImageChangeListener; import com.ssa.horizontalscrollview.myview.GalleryHorizontalScrollViewAdapter; public class MainActivity extends Activity { private GalleryHorizontalScrollView mHorizontalScrollView; private GalleryHorizontalScrollViewAdapter mAdapter; private ImageView mImg; private List<Integer> mDatas = new ArrayList<Integer>(Arrays.asList( R.drawable.a, R.drawable.b, R.drawable.c, R.drawable.d, R.drawable.e,R.drawable.f,R.drawable.g)); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mImg = (ImageView)findViewById(R.id.iv_content); mHorizontalScrollView = (GalleryHorizontalScrollView)findViewById(R.id.mhsv_gallery_container); mAdapter = new GalleryHorizontalScrollViewAdapter(this, mDatas); mHorizontalScrollView.setmOnCurrentImageChangeListener(new OnCurrentImageChangeListener() { @Override public void onCurrentImgChanged(int position, View view) { mImg.setImageResource(mDatas.get(position)); view.setBackgroundColor(Color.parseColor("#6d9eeb")); } }); mHorizontalScrollView.setmOnClickImageListener(new OnClickImageChangeListener() { @Override public void onClickImageChangeListener(int position, View view) { mImg.setImageResource(mDatas.get(position)); } }); mHorizontalScrollView.initData(mAdapter); } }

至些，调试运行，读者会发现，整个相册会非常卡，

甚至有的图片还没有显示出来如img_4，看一下logcat，相信大家会发现原因：

信息已经提示的很清楚了，图片太大，

此时大家应该明白了，笔者故意选择了几张很大的图片加载，虽然没大到直接让应用崩掉，但是体验性已经变得非常差了，这是因为课堂上的老师讲课时用的图片都是几十K的小图片，加载当然不会有问题，所以要想使这个相册作为一个实用的相册，还要处理图片过大的问题，不然，依旧会造成OOM。

此时就用到这个工具类了：

package com.ssa.horizontalscrollview.myUtils; import android.content.res.Resources; import android.graphics.Bitmap; import android.graphics.BitmapFactory; public class BitmapUtil { public static Bitmap decodeSampledBitmapFromResources(Resources res, int resId, int reqWidth, int reqHeight) { final BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeResource(res, resId, options); options.inSampleSize = calculateInsampleSize(options, reqWidth, reqHeight); options.inJustDecodeBounds = false; return BitmapFactory.decodeResource(res, resId, options); } public static int calculateInsampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) { final int height = options.outHeight; final int width = options.outWidth; int inSampleSize = 1; if (height > reqHeight || width > reqWidth) { final int halfHeight = height / 2; final int halfWidth = width / 2; while ((halfHeight / inSampleSize) >= reqHeight && (halfWidth / inSampleSize) >= reqWidth) { inSampleSize *= 2; } } return inSampleSize; } }

添加了这个工具类，上面几个类的代码也要略微修改一下，具体怎么改，大家可以下载下面我上传的源码：

至于效果如下动图所示（生成的gif图有点卡，大家可以运行看效果）：

以上就是本文的全部内容，希望对大家学习Android软件编程有所帮助。