Android:ViewPager详细解释（异步网络负载图片，有图片缓存，）并与导航点

android 应用。准则欢迎页面。 和图像旋转木马特征， 或者没有很多其他的内容显示在一个页面。以被划分成多个页面，在这一刻viewpager这是非常容易使用。

首先看下效果：

以下是一个样例。带异步网络载入图片，并带导航小圆点

package com.example.viewpagertest;

import java.io.IOException;
import java.lang.ref.SoftReference;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.params.CoreConnectionPNames;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v4.view.ViewPager.OnPageChangeListener;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;

import com.example.viewpagertest.MainActivity.AsyncImageLoader.ImageCallback;

/***
 *
 * 说明：ViewPager ，带小圆点导航。适配器採用PagerAdapter，基本能够满足需求
 * 也能够採用FragmentPagerAdapter。有人说，Fragment能够更好的适应平板和手机。
 * 而且能够更好的代码重用。详细这些优点大家试一下就知道了。
 *
 * @author andylaw
 *
 * 很多其它内容请查看博客：http://blog.csdn.net/lyc66666666666
 *
 */

@SuppressLint("NewApi")
public class MainActivity extends Activity {

	private ViewPager view_pager;

	private LayoutInflater inflater;

	// 图片的地址，这里能够从server获取
	String[] urls = new String[]{

			"http://a.hiphotos.baidu.com/image/pic/item/3bf33a87e950352ad6465dad5143fbf2b2118b6b.jpg",
			"http://a.hiphotos.baidu.com/image/pic/item/c8177f3e6709c93d002077529d3df8dcd0005440.jpg",
			"http://f.hiphotos.baidu.com/image/pic/item/7aec54e736d12f2ecc3d90f84dc2d56285356869.jpg",
			"http://e.hiphotos.baidu.com/image/pic/item/9c16fdfaaf51f3de308a87fc96eef01f3a297969.jpg",
			"http://d.hiphotos.baidu.com/image/pic/item/f31fbe096b63f624b88f7e8e8544ebf81b4ca369.jpg",
			"http://h.hiphotos.baidu.com/image/pic/item/11385343fbf2b2117c2dc3c3c88065380cd78e38.jpg",
			"http://c.hiphotos.baidu.com/image/pic/item/3801213fb80e7bec5ed8456c2d2eb9389b506b38.jpg"

	};

	private ImageView image;
	private View item ;
	private MyAdapter adapter ;
	private ImageView[] indicator_imgs = new ImageView[7];//存放引到图片数组

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		view_pager = (ViewPager) findViewById(R.id.view_pager);
		List<View> list = new ArrayList<View>();
		inflater = LayoutInflater.from(this);

		/**
		 * 创建多个item （每一条viewPager都是一个item）
		 * 从server获取完数据（如文章标题、url地址） 后，再设置适配器
		 */
		for (int i = 0; i < 7; i++) {
			item = inflater.inflate(R.layout.item, null);
			((TextView) item.findViewById(R.id.text_view)).setText("第 " + i+ " 个 viewPager");
			list.add(item);
		}

		//创建适配器。 把组装完的组件传递进去
		adapter = new MyAdapter(list);
		view_pager.setAdapter(adapter);

		//绑定动作监听器：如翻页的动画
		view_pager.setOnPageChangeListener(new MyListener());

		initIndicator();
	}

	/**
	 * 初始化引导图标
	 * 动态创建多个小圆点，然后组装到线性布局里
	 */
	private void initIndicator(){

		ImageView imgView;
		View v = findViewById(R.id.indicator);// 线性水平布局，负责动态调整导航图标

		for (int i = 0; i < 7; i++) {
			imgView = new ImageView(this);
			LinearLayout.LayoutParams params_linear = new LinearLayout.LayoutParams(10,10);
			params_linear.setMargins(7, 10, 7, 10);
			imgView.setLayoutParams(params_linear);
			indicator_imgs[i] = imgView;

			if (i == 0) { // 初始化第一个为选中状态

				indicator_imgs[i].setBackgroundResource(R.drawable.indicator_focused);
			} else {
				indicator_imgs[i].setBackgroundResource(R.drawable.indicator);
			}
			((ViewGroup)v).addView(indicator_imgs[i]);
		}

	}

	/**
	 * 适配器。负责装配 、销毁  数据  和  组件 。
	 */
	private class MyAdapter extends PagerAdapter {

		private List<View> mList;

		private AsyncImageLoader asyncImageLoader;

		public MyAdapter(List<View> list) {
			mList = list;
			asyncImageLoader = new AsyncImageLoader();
		}

		/**
		 * Return the number of views available.
		 */
		@Override
		public int getCount() {
			// TODO Auto-generated method stub
			return mList.size();
		}

		/**
		 * Remove a page for the given position.
		 * 滑动过后就销毁 ，销毁当前页的前一个的前一个的页！

* instantiateItem(View container, int position)
		 * This method was deprecated in API level . Use instantiateItem(ViewGroup, int)
		 */
		@Override
		public void destroyItem(ViewGroup container, int position, Object object) {
			// TODO Auto-generated method stub
			container.removeView(mList.get(position));

		}

		@Override
		public boolean isViewFromObject(View arg0, Object arg1) {
			// TODO Auto-generated method stub
			return arg0==arg1;
		}

		/**
		 * Create the page for the given position.
		 */
		@Override
		public Object instantiateItem(final ViewGroup container, final int position) {

			Drawable cachedImage = asyncImageLoader.loadDrawable(
					urls[position], new ImageCallback() {

						public void imageLoaded(Drawable imageDrawable,
								String imageUrl) {

							View view = mList.get(position);
							image = ((ImageView) view.findViewById(R.id.image));
							image.setBackground(imageDrawable);
							container.removeView(mList.get(position));
							container.addView(mList.get(position));
							// adapter.notifyDataSetChanged();

						}
					});

			View view = mList.get(position);
			image = ((ImageView) view.findViewById(R.id.image));
			image.setBackground(cachedImage);

			container.removeView(mList.get(position));
			container.addView(mList.get(position));
			// adapter.notifyDataSetChanged();

			return mList.get(position);

		}

	}

	/**
	 * 动作监听器，可异步载入图片
	 *
	 */
	private class MyListener implements OnPageChangeListener{

		@Override
		public void onPageScrollStateChanged(int state) {
			// TODO Auto-generated method stub
			if (state == 0) {
				//new MyAdapter(null).notifyDataSetChanged();
			}
		}

		@Override
		public void onPageScrolled(int arg0, float arg1, int arg2) {
			// TODO Auto-generated method stub

		}

		@Override
		public void onPageSelected(int position) {

			// 改变全部导航的背景图片为：未选中
			for (int i = 0; i < indicator_imgs.length; i++) {

				indicator_imgs[i].setBackgroundResource(R.drawable.indicator);

			}

			// 改变当前背景图片为：选中
			indicator_imgs[position].setBackgroundResource(R.drawable.indicator_focused);
		}

	}

	/**
	 * 异步载入图片
	 */
	static class AsyncImageLoader {

		// 软引用。使用内存做暂时缓存 （程序退出。或内存不够则清除软引用）
		private HashMap<String, SoftReference<Drawable>> imageCache;

		public AsyncImageLoader() {
			imageCache = new HashMap<String, SoftReference<Drawable>>();
		}

		/**
		 * 定义回调接口
		 */
		public interface ImageCallback {
			public void imageLoaded(Drawable imageDrawable, String imageUrl);
		}

		/**
		 * 创建子线程载入图片
		 * 子线程载入完图片交给handler处理（子线程不能更新ui。而handler处在主线程，能够更新ui）
		 * handler又交给imageCallback。imageCallback需要自己来实现，在这里能够对回调參数进行处理
		 *
		 * @param imageUrl ：需要载入的图片url
		 * @param imageCallback：
		 * @return
		 */
		public Drawable loadDrawable(final String imageUrl,
				final ImageCallback imageCallback) {

			//假设缓存中存在图片  ，则首先使用缓存
			if (imageCache.containsKey(imageUrl)) {
				SoftReference<Drawable> softReference = imageCache.get(imageUrl);
				Drawable drawable = softReference.get();
				if (drawable != null) {
					imageCallback.imageLoaded(drawable, imageUrl);//运行回调
					return drawable;
				}
			}

			/**
			 * 在主线程里运行回调，更新视图
			 */
			final Handler handler = new Handler() {
				public void handleMessage(Message message) {
					imageCallback.imageLoaded((Drawable) message.obj, imageUrl);
				}
			};

			/**
			 * 创建子线程訪问网络并载入图片 ，把结果交给handler处理
			 */
			new Thread() {
				@Override
				public void run() {
					Drawable drawable = loadImageFromUrl(imageUrl);
					// 下载完的图片放到缓存里
					imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
					Message message = handler.obtainMessage(0, drawable);
					handler.sendMessage(message);
				}
			}.start();

			return null;
		}

		/**
		 * 下载图片  （注意HttpClient 和httpUrlConnection的差别）
		 */
		public Drawable loadImageFromUrl(String url) {

			try {
				HttpClient client = new DefaultHttpClient();
				client.getParams().setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT, 1000*15);
				HttpGet get = new HttpGet(url);
				HttpResponse response;

				response = client.execute(get);
				if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
					HttpEntity entity = response.getEntity();

					Drawable d = Drawable.createFromStream(entity.getContent(),
							"src");

					return d;
				} else {
					return null;
				}
			} catch (ClientProtocolException e) {
				e.printStackTrace();
			} catch (IOException e) {
				e.printStackTrace();
			}

			return null;
		}

		//清除缓存
		public void clearCache() {

			if (this.imageCache.size() > 0) {

				this.imageCache.clear();
			}

		}

	}

}

以下是layout主文件：

<span style="font-size:14px;"><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin" >

    <RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent" >

        <android.support.v4.view.ViewPager
            android:id="@+id/view_pager"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" >
        </android.support.v4.view.ViewPager>

       <LinearLayout
            android:id="@+id/indicator"
            android:layout_width="fill_parent"
            android:layout_height="100px"
            android:layout_marginTop="500px"
            android:gravity="center_horizontal"
            android:orientation="horizontal" >
        </LinearLayout>

    </RelativeLayout>

</LinearLayout>
</span>

以下是每一条item文件：

<span style="font-size:14px;"><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:orientation="vertical" >

   <TextView
       android:id="@+id/text_view"
       android:layout_width="fill_parent"
       android:layout_height="wrap_content"/>

   <ImageView
       android:id="@+id/image"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        />

</LinearLayout></span>

代码地址

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