【异步加载专题】【原创分析】简单篇通过xml获取地址、本地文件缓存、异步加载

本文分析该内容:http://l5.yunpan.cn/lk/QkvuDuuCFWS89

首先我们看下作者的思路:

创建缓存目录 ->; 通过一个线程进行获取xml文件内容 ->; 对获取的xml文件进行解析获取所有的图片id、图片名称、图片地址 -> 在线程中通过Handler的Message创建listview的adapter -> 在adapter中实现异步加载图片资源.

//下一篇将分析:[异步加载专题][原创分析]中级篇本地缓存、ListView滑动停止加载、利用synchronized控制线程数机制、很好的文件结构
下面我们开始分析MainActivity:

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package com.example.synctask;
import java.io.File;
import java.util.List;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.widget.ListView;
public class MainActivity extends Activity {
protected static final int SUCCESS_GET_CONTACT = 0;
private ListView mListView;
private ImageAdapter mAdapter;
private File cache;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mListView = (ListView) findViewById(R.id.listView);
//创建缓存目录,系统一运行就得创建缓存目录的,
cache = new File(Environment.getExternalStorageDirectory(), "cache");
if(!cache.exists()){
cache.mkdirs();
}
//获取数据,主UI线程是不能做耗时操作的,所以启动子线程来做
new Thread(){
public void run() {
ContactService service = new ContactService();
List<contact> contacts = null;
try {
contacts = service.getContactAll();
} catch (Exception e) {
e.printStackTrace();
}
//子线程通过Message对象封装信息,并且用初始化好的,
//Handler对象的sendMessage()方法把数据发送到主线程中,从而达到更新UI主线程的目的
Message msg = new Message();
msg.what = SUCCESS_GET_CONTACT;
msg.obj = contacts;
mHandler.sendMessage(msg);
};
}.start();
}
private Handler mHandler = new Handler(){
public void handleMessage(android.os.Message msg) {
if(msg.what == SUCCESS_GET_CONTACT){
List<contact> contacts = (List<contact>) msg.obj;
mAdapter = new ImageAdapter(getApplicationContext(),contacts,cache);
mListView.setAdapter(mAdapter);
}
};
};
@Override
protected void onDestroy() {
super.onDestroy();
//清空缓存
File[] files = cache.listFiles();
for(File file :files){
file.delete();
}
cache.delete();
}
}

其中Contact用于存储从xml中读取到的对象类型:

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package com.example.synctask;
public class Contact {
int id;
String image;
String name;
public Contact() {
super();
}
public Contact(int id, String image, String name) {
this.id = id;
this.image = image;
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getImage() {
return image;
}
public void setImage(String image) {
this.image = image;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}

ContactService 用于获取网络资源:

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package com.example.synctask;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import org.xmlpull.v1.XmlPullParser;
import android.graphics.Bitmap;
import android.net.Uri;
import android.util.Xml;
public class ContactService {
/*
* 从服务器上获取数据
*/
public List<contact> getContactAll() throws Exception {
List<contact> contacts = null;
String Parth = "http://xxx.dreamtobe.cn:8080/xxx/list.xml";
URL url = new URL(Parth);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setConnectTimeout(30000);
conn.setReadTimeout(30000);
conn.setRequestMethod("GET");
if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {
InputStream is = conn.getInputStream();
// 这里获取数据直接放在XmlPullParser里面解析
contacts = xmlParser(is);
return contacts;
} else {
return null;
}
}
// 这里并没有下载图片下来,而是把图片的地址保存下来了
private List<contact> xmlParser(InputStream is) throws Exception {
List<contact> contacts = null;
Contact contact = null;
XmlPullParser parser = Xml.newPullParser();
parser.setInput(is, "UTF-8");
int eventType = parser.getEventType();
while ((eventType = parser.next()) != XmlPullParser.END_DOCUMENT) {
switch (eventType) {
case XmlPullParser.START_TAG:
if (parser.getName().equals("contacts")) {
contacts = new ArrayList<contact>();
} else if (parser.getName().equals("contact")) {
contact = new Contact();
contact.setId(Integer.valueOf(parser.getAttributeValue(0)));
} else if (parser.getName().equals("name")) {
contact.setName(parser.nextText());
} else if (parser.getName().equals("image")) {
contact.setImage(parser.getAttributeValue(0));
}
break;
case XmlPullParser.END_TAG:
if (parser.getName().equals("contact")) {
contacts.add(contact);
}
break;
}
}
return contacts;
}
/*
* 从网络上获取图片,如果图片在本地存在的话就直接拿,如果不存在再去服务器上下载图片
* 这里的path是图片的地址
*/
public Uri getImageURI(String path, File cache) throws Exception {
String name = MD5.getMD5(path) + path.substring(path.lastIndexOf("."));
File file = new File(cache, name);
// 如果图片存在本地缓存目录,则不去服务器下载
if (file.exists()) {
return Uri.fromFile(file);//Uri.fromFile(path)这个方法能得到文件的URI
} else {
// 从网络上获取图片
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setConnectTimeout(5000);
conn.setRequestMethod("GET");
conn.setDoInput(true);
if (conn.getResponseCode() == 200) {
InputStream is = conn.getInputStream();
FileOutputStream fos = new FileOutputStream(file);
byte[] buffer = new byte[1024];
int len = 0;
while ((len = is.read(buffer)) != -1) {
fos.write(buffer, 0, len);
}
is.close();
fos.close();
// 返回一个URI对象
return Uri.fromFile(file);
}
}
return null;
}
}

其中MainActivity中的线程:

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//获取数据,主UI线程是不能做耗时操作的,所以启动子线程来做
new Thread(){
public void run() {
ContactService service = new ContactService();
List<contact> contacts = null;
try {
contacts = service.getContactAll();
} catch (Exception e) {
e.printStackTrace();
}
//子线程通过Message对象封装信息,并且用初始化好的,
//Handler对象的sendMessage()方法把数据发送到主线程中,从而达到更新UI主线程的目的
Message msg = new Message();
msg.what = SUCCESS_GET_CONTACT;
msg.obj = contacts;
mHandler.sendMessage(msg);
};
}.start();

先通过

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contacts = service.getContactAll();

获得解析后的所有的Contact.
通过Hanlder的Message

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Message msg = new Message();
msg.what = SUCCESS_GET_CONTACT;
msg.obj = contacts;
mHandler.sendMessage(msg);

发送到

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private Handler mHandler = new Handler(){
public void handleMessage(android.os.Message msg) {
if(msg.what == SUCCESS_GET_CONTACT){
List<contact> contacts = (List<contact>) msg.obj;
mAdapter = new ImageAdapter(getApplicationContext(),contacts,cache);
mListView.setAdapter(mAdapter);
}
};
};

在handleMessage中创建ListView的Adapter.
在ImageAdapter中:

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@Override
public View getView(int position, View convertView, ViewGroup parent) {
// 1获取item,再得到控件
// 2 获取数据
// 3绑定数据到item
ViewHolder holder = null;
if (convertView == null) {
holder = new ViewHolder();
convertView = mInflater.inflate(R.layout.item, null);
holder.iv_header = (ImageView) convertView.findViewById(R.id.imageView);
holder.tv_name = (TextView) convertView.findViewById(R.id.textView);
convertView.setTag(holder);
}else {
holder = (ViewHolder)convertView.getTag();
}
Contact contact = contacts.get(position);
// 异步的加载图片 (线程池 + Handler ) ---> AsyncTask
asyncloadImage(holder.iv_header, contact.image);
holder.tv_name.setText(contact.name);
return convertView;
}

调用asyncloadImage顾名思义异步加载图片:

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private void asyncloadImage(ImageView iv_header, String path) {
ContactService service = new ContactService();
AsyncImageTask task = new AsyncImageTask(service, iv_header);
task.execute(path);
}

在其中创建联网下载图片的对象service传入以AsyncTask 为父类(暂且理解为可以更新UI的线程)的task对象.

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private final class AsyncImageTask extends AsyncTask<string, Integer, Uri> {
private ContactService service;
private ImageView iv_header;
public AsyncImageTask(ContactService service, ImageView iv_header) {
this.service = service;
this.iv_header = iv_header;
}
// 后台运行的子线程子线程
@Override
protected Uri doInBackground(String... params) {
try {
return service.getImageURI(params[0], cache);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
// 这个放在在ui线程中执行
@Override
protected void onPostExecute(Uri result) {
super.onPostExecute(result);
// 完成图片的绑定
if (iv_header != null && result != null) {
iv_header.setImageURI(result);
}
}
}

我们可以清晰的看到:
此类在:protected void onPostExecute(Uri result)是需要传入一个Uri类型的值得。
而protected Uri doInBackground(String… params)的返回值恰好是Uri(通过后面我们可以清晰的了解到,这个值就是传递给onPostExecute的)。
而这个线程的请求应该是通过:task.execute(path);
我们看下protected Uri doInBackground(String… params)

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// 后台运行的子线程子线程
@Override
protected Uri doInBackground(String... params) {
try {
return service.getImageURI(params[0], cache);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}

这个在后台运行的子线程子线程调通过getImageURI返回获取Uri.
我们看下:getImageURI:

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/*
* 从网络上获取图片,如果图片在本地存在的话就直接拿,如果不存在再去服务器上下载图片
* 这里的path是图片的地址
*/
public Uri getImageURI(String path, File cache) throws Exception {
String name = MD5.getMD5(path) + path.substring(path.lastIndexOf("."));
File file = new File(cache, name);
// 如果图片存在本地缓存目录,则不去服务器下载
if (file.exists()) {
return Uri.fromFile(file);//Uri.fromFile(path)这个方法能得到文件的URI
} else {
// 从网络上获取图片
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setConnectTimeout(5000); //5000毫秒 请求无连接直接返回Timeout
conn.setRequestMethod("GET"); //通过get请求
conn.setDoInput(true);
if (conn.getResponseCode() == 200) {
InputStream is = conn.getInputStream();
FileOutputStream fos = new FileOutputStream(file);
byte[] buffer = new byte[1024];
int len = 0;
while ((len = is.read(buffer)) != -1) {
fos.write(buffer, 0, len);
}
is.close();
fos.close();
// 返回一个URI对象
return Uri.fromFile(file);
}
}
return null;
}

这里已经备注的很清晰了,需要提到的是这里通过MD5来加密了获取到的图片的名字.
就是项目中涉及到的:

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package com.example.synctask;
import java.security.MessageDigest;
import java.util.Iterator;
public class MD5 {
public static String getMD5(String content) {
try {
MessageDigest digest = MessageDigest.getInstance("MD5");
digest.update(content.getBytes());
return getHashString(digest);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
public static String getHashString(MessageDigest digest) {
StringBuilder builder = new StringBuilder();
for (byte b : digest.digest()) {
builder.append(Integer.toHexString((b >> 4) & 0xf));
builder.append(Integer.toHexString(b & 0xf));
}
return builder.toString();
}
}

此时更新ui

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// 这个放在在ui线程中执行
@Override
protected void onPostExecute(Uri result) {
super.onPostExecute(result);
// 完成图片的绑定
if (iv_header != null && result != null) {
iv_header.setImageURI(result);
}
}

至此简单的异步加载就实现了。


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