Android中图片的三级缓存策略
在开发过程中，经常会碰到进行请求大量的网络图片的例子，如果处理的不好，很容易造成oom。对于避免oom的方法，无非就是进行图片的压缩，及时的回收不用的图片。这些看似简单但是处理起来其实涉及的知识特别广泛。在这里主要讲解图片的缓存，通过缓存也是个很好的避免oom的途径。最近常用的到自然是LruCache了，它里面有一个LindedHashMap链式表，而且这个表是按最近最少使用算法排序的，最近使用的往往拍的靠前。最少使用的往往处于队尾，当需要回收利用的时候，最后面的那个元素是会被清除掉的。LruCache主要实现了内存缓存，这里还会讲解DiskLruCache，这是GitHub开源库提供的使用文件缓存的一种基于最近最少使用的硬盘缓存类。同时顺带讲解图片的简单压缩方法。那么，接下来就需要了解LruCache,DiskLruCache,BitmapFactory。
一、LruCache
①、LruCache的构造函数解析，首先看源码：
public LruCache(int maxSize) {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
this.maxSize = maxSize;
this.map = new LinkedHashMap(0, 0.75f, true);
}
1、maxSize:当我们使用构造函数的时候，如果没有重写sizeOf方法的话，这个值代表在此cache里面能够缓存的实体个数（Bitmap,int,String......）的最大值。如果重写了sizeOf方法的话，这个值代表所有缓存实体所占用的字节大小的总和。此值必须大于0.
2、map:这是LruCache里面用于存放实体对象的链式表，前面说了，Lrucache是基于最近最少使用算法来决定淘汰哪个实体的，这个算法其实不是LruCache实现的，而是LinkedHashMap实现的。在这里，map=new LinkedHashMap中的第三个参数，如果传值true的话，那么链式表里面的对象会根据最近最少使用算法来排序，最近最少使用的对象，就越往后。如果传值false的话，就是按插入值得顺序在链式表中排序。
②、重新设置Lrucache对象缓存大小的最大值：
public void resize(int maxSize) {}此方法用于重新这是缓存大小，当新的maxSize比原来的值要大的时候，只是将新的值赋值给了原来的值，并没有做其他的事情。而当新的值比原来的值小的时候，那么就会把已经缓存的对象从队列里移除，直到缓存的大小小于或者等于新的maxSize的值。
③、根据key来创建一个值：
protected V create(K key) {}④、根据key来获取一个值：
public final V get(K key) {}根据key从缓存中获取相应的值，如果缓存中存在该值就返回。如果不存在，就会调用create方法去创建一个值，注意，如果你没有重写这个方法的话，默认返回的都是null值。如果create了一个新的值（不能为null）且不与原来cache里面的实体产生冲突的话，就会把该值压进cache里面，并返回。如果cache既不存在该值也不能通过create创建一个新值，那么此方法返回null。
⑤、在缓存中添加一个键值对：
public final V put(K key, V value) {}将一个键值对添加进缓存，注意key和value均不能为null，此方法如果成功将键值对添加进缓存，那么就会返回null。如果返回的值不为null，说明缓存里面已经存在了该key对应的值，不能再进行添加。
⑥、将某个对象从缓存区中移除：
public final V remove(K key) {}⑦、计算某个实体占用的内存空间：
protected int sizeOf(K key, V value) {}此方法很重要此方法特别重要。这个方法默认的实现是每次有一个实体添加至缓存，就＋1，导致后续计算缓存是否足够容纳实体的时候，是通过判断实体个数来计算，而不是根据实体占用的空间来比较。所以一般来说，这个方法都需要重写，此方法应该返回当前key对应的实体占用的空间大小。
⑧、清除缓存空间占用的所有对象：
public final void evictAll(){}此方法会依次将所有存储的对象移除。
⑨、返回缓存中的所有对象：
public synchronized final Map snapshot(){} 会返回按照最近最少使用排序的LinkedHashMap对象，此对象保存了所有的缓存对象。例子在后面给出，现在先总的介绍要用到的知识。
二、DiskLruCache
DiskLruCache并不是android提供的api，而是一个开源库的代码，这个类受到了Google的强烈推荐，所以，这里将介绍它，它其实是一个使用文件系统的采用最近最少用算法的缓存类。DiskLruCache是一个拥有一定空间的文件系统方式的缓存对象，其中的每一个缓存实体都包含有一个字符串类型的key，和固定数量的文件（一个key可以对应多个文件，这些文件都保存着缓存数据），这些文件是按顺序排列的，通常来说是一些文件或者输入流的字节形式的。每一个文件得长度都必须介于0-Integer.MAXVALUE之间。
DiskLruCache是利用文件系统的文件夹来存储数据的，缓存对象会经常对此文件夹进行删除文件获取覆盖文件的操作，因此该文件夹必须是该DiskLruCache专用的。当我们进行DiskLruCache对象创建的时候，应该指定一个缓存大小给它，当缓存数据的大小超过限制的大小的时候，它会在后台将一些缓存实体删除掉，直到缓存的大小达到限制的值。同时要注意的一点时，这个限制值不是绝对的，比如DiskLruCache进行文件删除的时候，缓存的容量可能会暂时的超越限制的值。
当想要更新或者创建一个缓存实体的话，应该调用DiskLruCache的edit方法获取一个Editor对象，此对象如果是null表示当前的值不可编辑。此方法必须和Editor.commit或者Editor.abort对应。另外就是，使用这个类进行缓存的时候，应该捕获一些常见的I\O操作异常，因为这个类在进行写文件的时候如果发现错误，只会修改失败，并不会导致操作上的失败。后续提到的缓存对象指的是DiskLruCache，缓存实体指的是保存在缓存对象里面的具体对象。
①、DiskLruCache对象的获取：
public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)参数解析：
directory:表示存储缓存数据的文件。
appVersion:当前应用的appVersion，主要作用在于书写缓存日志的时候，用来标识缓存数据的版本号。
valueCount:每个缓存实体能够对应的文件的个数。key->多个缓存数据
maxSoze:DiskLruCache最大能够缓存的数据容量大小。
②、获取缓存实体，为了获取缓存实体，先要了解一下Snopshot，这是一个DiskLruCache的内部类，它代表了一个缓存实体的值。关于这个类，有两个重要的常用方法：如下：
public InputStream getInputStream(int index) public String getString(int index)前面提到过一个key是可以对应多个缓存文件的，这里的index就是指该key所对应的第index文件，可以通过第一种方法获得该文件的输入流读取内容。如果该文件保存的是字符串的内容，可以通过第二种方法直接获取到字符串值。另外，读取完文件的内容后，需要调用close方法将给key对应的缓存文件关闭。那么如何获取某个key对应的缓存实体的Snopshot对象内，如下：
public synchronized Snapshot get(String key)③、修改，添加缓存内容。同样的，我们需要先了解Editor这个类，这也是DiskLruCache提供的内部类。这个类代表了某个可编辑的缓存实体。这个类也提供了两个方法进行读取缓存数据的内容，如下：
public InputStream newInputStream(int index)以及
public String getString(int index) 和前面一样，第一个方法获取的是文件的输入流，第二个方法获取的是文件的内容转换为字符串之后的值。上述两个方法用于读取文件内容。如果是要进行编辑内容的话，需要调用如下方法：
public OutputStream newOutputStream(int index)用于获取某个key对应下标的文件输出流，后续我们需要把缓存的内容通过它写入缓存。调用这个之后，必须调用commit或者abort方法，来确认是否修改或者覆盖缓存内容。
同样的，要想获取一个缓存实体的可编辑对象，需要通过如下方法：
public Editor edit(String key)此方法会返回key对应的可编辑对象，注意，这里即使之前没有key相关的缓存对象，通过此方法就会在文件系统里新建一个key相关的缓存对象，因此也会返回一个可编辑对象。如果返回的是null，说明这个key对应的可编辑对象正在被调用，当前进程无法调用。
④、删除某个缓存实体。
public synchronized boolean remove(String key) ⑤、关闭缓存对象：
public synchronized void close() ⑥、删除所有的缓存数据：
public void delete()
上述两个类可用于存储任何类型的数据，在这里主要以缓存图片为例子给大家讲解，所以接下来还需要了解一下BitmapFactory这个类。
三、BitmapFactory
public static Bitmap decodeStream(InputStream is, Rect outPadding, Options opts)参数解析：
/*
* Copyright (C) 2011 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.cw.cache;
import java.io.BufferedInputStream;
import java.io.BufferedWriter;
import java.io.Closeable;
import java.io.EOFException;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.Reader;
import java.io.StringWriter;
import java.io.Writer;
import java.lang.reflect.Array;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
/**
******************************************************************************
* Taken from the JB source code, can be found in:
* libcore/luni/src/main/java/libcore/io/DiskLruCache.java
* or direct link:
* https://android.googlesource.com/platform/libcore/+/android-4.1.1_r1/luni/src/main/java/libcore/io/DiskLruCache.java
******************************************************************************
*
* A cache that uses a bounded amount of space on a filesystem. Each cache
* entry has a string key and a fixed number of values. Values are byte
* sequences, accessible as streams or files. Each value must be between {@code
* 0} and {@code Integer.MAX_VALUE} bytes in length.
*
* The cache stores its data in a directory on the filesystem. This
* directory must be exclusive to the cache; the cache may delete or overwrite
* files from its directory. It is an error for multiple processes to use the
* same cache directory at the same time.
*
*
This cache limits the number of bytes that it will store on the
* filesystem. When the number of stored bytes exceeds the limit, the cache will
* remove entries in the background until the limit is satisfied. The limit is
* not strict: the cache may temporarily exceed it while waiting for files to be
* deleted. The limit does not include filesystem overhead or the cache
* journal so space-sensitive applications should set a conservative limit.
*
*
Clients call {@link #edit} to create or update the values of an entry. An
* entry may have only one editor at one time; if a value is not available to be
* edited then {@link #edit} will return null.
*
* - When an entry is being created it is necessary to
* supply a full set of values; the empty value should be used as a
* placeholder if necessary.
*
- When an entry is being edited, it is not necessary
* to supply data for every value; values default to their previous
* value.
*
* Every {@link #edit} call must be matched by a call to {@link Editor#commit}
* or {@link Editor#abort}. Committing is atomic: a read observes the full set
* of values as they were before or after the commit, but never a mix of values.
*
* Clients call {@link #get} to read a snapshot of an entry. The read will
* observe the value at the time that {@link #get} was called. Updates and
* removals after the call do not impact ongoing reads.
*
*
This class is tolerant of some I/O errors. If files are missing from the
* filesystem, the corresponding entries will be dropped from the cache. If
* an error occurs while writing a cache value, the edit will fail silently.
* Callers should handle other problems by catching {@code IOException} and
* responding appropriately.
*/
public final class DiskLruCache implements Closeable {
static final String JOURNAL_FILE = "journal";
static final String JOURNAL_FILE_TMP = "journal.tmp";
static final String MAGIC = "libcore.io.DiskLruCache";
static final String VERSION_1 = "1";
static final long ANY_SEQUENCE_NUMBER = -1;
private static final String CLEAN = "CLEAN";
private static final String DIRTY = "DIRTY";
private static final String REMOVE = "REMOVE";
private static final String READ = "READ";
private static final Charset UTF_8 = Charset.forName("UTF-8");
private static final int IO_BUFFER_SIZE = 8 * 1024;
/*
* This cache uses a journal file named "journal". A typical journal file
* looks like this:
* libcore.io.DiskLruCache
* 1
* 100
* 2
*
* CLEAN 3400330d1dfc7f3f7f4b8d4d803dfcf6 832 21054
* DIRTY 335c4c6028171cfddfbaae1a9c313c52
* CLEAN 335c4c6028171cfddfbaae1a9c313c52 3934 2342
* REMOVE 335c4c6028171cfddfbaae1a9c313c52
* DIRTY 1ab96a171faeeee38496d8b330771a7a
* CLEAN 1ab96a171faeeee38496d8b330771a7a 1600 234
* READ 335c4c6028171cfddfbaae1a9c313c52
* READ 3400330d1dfc7f3f7f4b8d4d803dfcf6
*
* The first five lines of the journal form its header. They are the
* constant string "libcore.io.DiskLruCache", the disk cache's version,
* the application's version, the value count, and a blank line.
*
* Each of the subsequent lines in the file is a record of the state of a
* cache entry. Each line contains space-separated values: a state, a key,
* and optional state-specific values.
* o DIRTY lines track that an entry is actively being created or updated.
* Every successful DIRTY action should be followed by a CLEAN or REMOVE
* action. DIRTY lines without a matching CLEAN or REMOVE indicate that
* temporary files may need to be deleted.
* o CLEAN lines track a cache entry that has been successfully published
* and may be read. A publish line is followed by the lengths of each of
* its values.
* o READ lines track accesses for LRU.
* o REMOVE lines track entries that have been deleted.
*
* The journal file is appended to as cache operations occur. The journal may
* occasionally be compacted by dropping redundant lines. A temporary file named
* "journal.tmp" will be used during compaction; that file should be deleted if
* it exists when the cache is opened.
*/
private final File directory;
private final File journalFile;
private final File journalFileTmp;
private final int appVersion;
private final long maxSize;
private final int valueCount;
private long size = 0;
private Writer journalWriter;
private final LinkedHashMap lruEntries
= new LinkedHashMap(0, 0.75f, true);
private int redundantOpCount;
/**
* To differentiate between old and current snapshots, each entry is given
* a sequence number each time an edit is committed. A snapshot is stale if
* its sequence number is not equal to its entry's sequence number.
*/
private long nextSequenceNumber = 0;
/* From java.util.Arrays */
@SuppressWarnings("unchecked")
private static T[] copyOfRange(T[] original, int start, int end) {
final int originalLength = original.length; // For exception priority compatibility.
if (start > end) {
throw new IllegalArgumentException();
}
if (start < 0 || start > originalLength) {
throw new ArrayIndexOutOfBoundsException();
}
final int resultLength = end - start;
final int copyLength = Math.min(resultLength, originalLength - start);
final T[] result = (T[]) Array
.newInstance(original.getClass().getComponentType(), resultLength);
System.arraycopy(original, start, result, 0, copyLength);
return result;
}
/**
* Returns the remainder of 'reader' as a string, closing it when done.
*/
public static String readFully(Reader reader) throws IOException {
try {
StringWriter writer = new StringWriter();
char[] buffer = new char[1024];
int count;
while ((count = reader.read(buffer)) != -1) {
writer.write(buffer, 0, count);
}
return writer.toString();
} finally {
reader.close();
}
}
/**
* Returns the ASCII characters up to but not including the next "\r\n", or
* "\n".
*
* @throws EOFException if the stream is exhausted before the next newline
* character.
*/
public static String readAsciiLine(InputStream in) throws IOException {
// TODO: support UTF-8 here instead
StringBuilder result = new StringBuilder(80);
while (true) {
int c = in.read();
if (c == -1) {
throw new EOFException();
} else if (c == '\n') {
break;
}
result.append((char) c);
}
int length = result.length();
if (length > 0 && result.charAt(length - 1) == '\r') {
result.setLength(length - 1);
}
return result.toString();
}
/**
* Closes 'closeable', ignoring any checked exceptions. Does nothing if 'closeable' is null.
*/
public static void closeQuietly(Closeable closeable) {
if (closeable != null) {
try {
closeable.close();
} catch (RuntimeException rethrown) {
throw rethrown;
} catch (Exception ignored) {
}
}
}
/**
* Recursively delete everything in {@code dir}.
*/
// TODO: this should specify paths as Strings rather than as Files
public static void deleteContents(File dir) throws IOException {
File[] files = dir.listFiles();
if (files == null) {
throw new IllegalArgumentException("not a directory: " + dir);
}
for (File file : files) {
if (file.isDirectory()) {
deleteContents(file);
}
if (!file.delete()) {
throw new IOException("failed to delete file: " + file);
}
}
}
/** This cache uses a single background thread to evict entries. */
private final ExecutorService executorService = new ThreadPoolExecutor(0, 1,
60L, TimeUnit.SECONDS, new LinkedBlockingQueue());
private final Callable cleanupCallable = new Callable() {
@Override public Void call() throws Exception {
synchronized (DiskLruCache.this) {
if (journalWriter == null) {
return null; // closed
}
trimToSize();
if (journalRebuildRequired()) {
rebuildJournal();
redundantOpCount = 0;
}
}
return null;
}
};
private DiskLruCache(File directory, int appVersion, int valueCount, long maxSize) {
this.directory = directory;
this.appVersion = appVersion;
this.journalFile = new File(directory, JOURNAL_FILE);
this.journalFileTmp = new File(directory, JOURNAL_FILE_TMP);
this.valueCount = valueCount;
this.maxSize = maxSize;
}
/**
* Opens the cache in {@code directory}, creating a cache if none exists
* there.
*
* @param directory a writable directory
* @param appVersion
* @param valueCount the number of values per cache entry. Must be positive.
* @param maxSize the maximum number of bytes this cache should use to store
* @throws IOException if reading or writing the cache directory fails
*/
public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)
throws IOException {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
if (valueCount <= 0) {
throw new IllegalArgumentException("valueCount <= 0");
}
// prefer to pick up where we left off
DiskLruCache cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
if (cache.journalFile.exists()) {
try {
cache.readJournal();
cache.processJournal();
cache.journalWriter = new BufferedWriter(new FileWriter(cache.journalFile, true),
IO_BUFFER_SIZE);
return cache;
} catch (IOException journalIsCorrupt) {
// System.logW("DiskLruCache " + directory + " is corrupt: "
// + journalIsCorrupt.getMessage() + ", removing");
cache.delete();
}
}
// create a new empty cache
directory.mkdirs();
cache = new DiskLruCache(directory, appVersion, valueCount, maxSize);
cache.rebuildJournal();
return cache;
}
private void readJournal() throws IOException {
InputStream in = new BufferedInputStream(new FileInputStream(journalFile), IO_BUFFER_SIZE);
try {
String magic = readAsciiLine(in);
String version = readAsciiLine(in);
String appVersionString = readAsciiLine(in);
String valueCountString = readAsciiLine(in);
String blank = readAsciiLine(in);
if (!MAGIC.equals(magic)
|| !VERSION_1.equals(version)
|| !Integer.toString(appVersion).equals(appVersionString)
|| !Integer.toString(valueCount).equals(valueCountString)
|| !"".equals(blank)) {
throw new IOException("unexpected journal header: ["
+ magic + ", " + version + ", " + valueCountString + ", " + blank + "]");
}
while (true) {
try {
readJournalLine(readAsciiLine(in));
} catch (EOFException endOfJournal) {
break;
}
}
} finally {
closeQuietly(in);
}
}
private void readJournalLine(String line) throws IOException {
String[] parts = line.split(" ");
if (parts.length < 2) {
throw new IOException("unexpected journal line: " + line);
}
String key = parts[1];
if (parts[0].equals(REMOVE) && parts.length == 2) {
lruEntries.remove(key);
return;
}
Entry entry = lruEntries.get(key);
if (entry == null) {
entry = new Entry(key);
lruEntries.put(key, entry);
}
if (parts[0].equals(CLEAN) && parts.length == 2 + valueCount) {
entry.readable = true;
entry.currentEditor = null;
entry.setLengths(copyOfRange(parts, 2, parts.length));
} else if (parts[0].equals(DIRTY) && parts.length == 2) {
entry.currentEditor = new Editor(entry);
} else if (parts[0].equals(READ) && parts.length == 2) {
// this work was already done by calling lruEntries.get()
} else {
throw new IOException("unexpected journal line: " + line);
}
}
/**
* Computes the initial size and collects garbage as a part of opening the
* cache. Dirty entries are assumed to be inconsistent and will be deleted.
*/
private void processJournal() throws IOException {
deleteIfExists(journalFileTmp);
for (Iterator i = lruEntries.values().iterator(); i.hasNext(); ) {
Entry entry = i.next();
if (entry.currentEditor == null) {
for (int t = 0; t < valueCount; t++) {
size += entry.lengths[t];
}
} else {
entry.currentEditor = null;
for (int t = 0; t < valueCount; t++) {
deleteIfExists(entry.getCleanFile(t));
deleteIfExists(entry.getDirtyFile(t));
}
i.remove();
}
}
}
/**
* Creates a new journal that omits redundant information. This replaces the
* current journal if it exists.
*/
private synchronized void rebuildJournal() throws IOException {
if (journalWriter != null) {
journalWriter.close();
}
Writer writer = new BufferedWriter(new FileWriter(journalFileTmp), IO_BUFFER_SIZE);
writer.write(MAGIC);
writer.write("\n");
writer.write(VERSION_1);
writer.write("\n");
writer.write(Integer.toString(appVersion));
writer.write("\n");
writer.write(Integer.toString(valueCount));
writer.write("\n");
writer.write("\n");
for (Entry entry : lruEntries.values()) {
if (entry.currentEditor != null) {
writer.write(DIRTY + ' ' + entry.key + '\n');
} else {
writer.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
}
}
writer.close();
journalFileTmp.renameTo(journalFile);
journalWriter = new BufferedWriter(new FileWriter(journalFile, true), IO_BUFFER_SIZE);
}
private static void deleteIfExists(File file) throws IOException {
// try {
// Libcore.os.remove(file.getPath());
// } catch (ErrnoException errnoException) {
// if (errnoException.errno != OsConstants.ENOENT) {
// throw errnoException.rethrowAsIOException();
// }
// }
if (file.exists() && !file.delete()) {
throw new IOException();
}
}
/**
* Returns a snapshot of the entry named {@code key}, or null if it doesn't
* exist is not currently readable. If a value is returned, it is moved to
* the head of the LRU queue.
*/
public synchronized Snapshot get(String key) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (entry == null) {
return null;
}
if (!entry.readable) {
return null;
}
/*
* Open all streams eagerly to guarantee that we see a single published
* snapshot. If we opened streams lazily then the streams could come
* from different edits.
*/
InputStream[] ins = new InputStream[valueCount];
try {
for (int i = 0; i < valueCount; i++) {
ins[i] = new FileInputStream(entry.getCleanFile(i));
}
} catch (FileNotFoundException e) {
// a file must have been deleted manually!
return null;
}
redundantOpCount++;
journalWriter.append(READ + ' ' + key + '\n');
if (journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}
return new Snapshot(key, entry.sequenceNumber, ins);
}
/**
* Returns an editor for the entry named {@code key}, or null if another
* edit is in progress.
*/
public Editor edit(String key) throws IOException {
return edit(key, ANY_SEQUENCE_NUMBER);
}
private synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (expectedSequenceNumber != ANY_SEQUENCE_NUMBER
&& (entry == null || entry.sequenceNumber != expectedSequenceNumber)) {
return null; // snapshot is stale
}
if (entry == null) {
entry = new Entry(key);
lruEntries.put(key, entry);
} else if (entry.currentEditor != null) {
return null; // another edit is in progress
}
Editor editor = new Editor(entry);
entry.currentEditor = editor;
// flush the journal before creating files to prevent file leaks
journalWriter.write(DIRTY + ' ' + key + '\n');
journalWriter.flush();
return editor;
}
/**
* Returns the directory where this cache stores its data.
*/
public File getDirectory() {
return directory;
}
/**
* Returns the maximum number of bytes that this cache should use to store
* its data.
*/
public long maxSize() {
return maxSize;
}
/**
* Returns the number of bytes currently being used to store the values in
* this cache. This may be greater than the max size if a background
* deletion is pending.
*/
public synchronized long size() {
return size;
}
private synchronized void completeEdit(Editor editor, boolean success) throws IOException {
Entry entry = editor.entry;
if (entry.currentEditor != editor) {
throw new IllegalStateException();
}
// if this edit is creating the entry for the first time, every index must have a value
if (success && !entry.readable) {
for (int i = 0; i < valueCount; i++) {
if (!entry.getDirtyFile(i).exists()) {
editor.abort();
throw new IllegalStateException("edit didn't create file " + i);
}
}
}
for (int i = 0; i < valueCount; i++) {
File dirty = entry.getDirtyFile(i);
if (success) {
if (dirty.exists()) {
File clean = entry.getCleanFile(i);
dirty.renameTo(clean);
long oldLength = entry.lengths[i];
long newLength = clean.length();
entry.lengths[i] = newLength;
size = size - oldLength + newLength;
}
} else {
deleteIfExists(dirty);
}
}
redundantOpCount++;
entry.currentEditor = null;
if (entry.readable | success) {
entry.readable = true;
journalWriter.write(CLEAN + ' ' + entry.key + entry.getLengths() + '\n');
if (success) {
entry.sequenceNumber = nextSequenceNumber++;
}
} else {
lruEntries.remove(entry.key);
journalWriter.write(REMOVE + ' ' + entry.key + '\n');
}
if (size > maxSize || journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}
}
/**
* We only rebuild the journal when it will halve the size of the journal
* and eliminate at least 2000 ops.
*/
private boolean journalRebuildRequired() {
final int REDUNDANT_OP_COMPACT_THRESHOLD = 2000;
return redundantOpCount >= REDUNDANT_OP_COMPACT_THRESHOLD
&& redundantOpCount >= lruEntries.size();
}
/**
* Drops the entry for {@code key} if it exists and can be removed. Entries
* actively being edited cannot be removed.
*
* @return true if an entry was removed.
*/
public synchronized boolean remove(String key) throws IOException {
checkNotClosed();
validateKey(key);
Entry entry = lruEntries.get(key);
if (entry == null || entry.currentEditor != null) {
return false;
}
for (int i = 0; i < valueCount; i++) {
File file = entry.getCleanFile(i);
if (!file.delete()) {
throw new IOException("failed to delete " + file);
}
size -= entry.lengths[i];
entry.lengths[i] = 0;
}
redundantOpCount++;
journalWriter.append(REMOVE + ' ' + key + '\n');
lruEntries.remove(key);
if (journalRebuildRequired()) {
executorService.submit(cleanupCallable);
}
return true;
}
/**
* Returns true if this cache has been closed.
*/
public boolean isClosed() {
return journalWriter == null;
}
private void checkNotClosed() {
if (journalWriter == null) {
throw new IllegalStateException("cache is closed");
}
}
/**
* Force buffered operations to the filesystem.
*/
public synchronized void flush() throws IOException {
checkNotClosed();
trimToSize();
journalWriter.flush();
}
/**
* Closes this cache. Stored values will remain on the filesystem.
*/
public synchronized void close() throws IOException {
if (journalWriter == null) {
return; // already closed
}
for (Entry entry : new ArrayList(lruEntries.values())) {
if (entry.currentEditor != null) {
entry.currentEditor.abort();
}
}
trimToSize();
journalWriter.close();
journalWriter = null;
}
private void trimToSize() throws IOException {
while (size > maxSize) {
// Map.Entry toEvict = lruEntries.eldest();
final Map.Entry toEvict = lruEntries.entrySet().iterator().next();
remove(toEvict.getKey());
}
}
/**
* Closes the cache and deletes all of its stored values. This will delete
* all files in the cache directory including files that weren't created by
* the cache.
*/
public void delete() throws IOException {
close();
deleteContents(directory);
}
private void validateKey(String key) {
if (key.contains(" ") || key.contains("\n") || key.contains("\r")) {
throw new IllegalArgumentException(
"keys must not contain spaces or newlines: \"" + key + "\"");
}
}
private static String inputStreamToString(InputStream in) throws IOException {
return readFully(new InputStreamReader(in, UTF_8));
}
/**
* A snapshot of the values for an entry.
*/
public final class Snapshot implements Closeable {
private final String key;
private final long sequenceNumber;
private final InputStream[] ins;
private Snapshot(String key, long sequenceNumber, InputStream[] ins) {
this.key = key;
this.sequenceNumber = sequenceNumber;
this.ins = ins;
}
/**
* Returns an editor for this snapshot's entry, or null if either the
* entry has changed since this snapshot was created or if another edit
* is in progress.
*/
public Editor edit() throws IOException {
return DiskLruCache.this.edit(key, sequenceNumber);
}
/**
* Returns the unbuffered stream with the value for {@code index}.
*/
public InputStream getInputStream(int index) {
return ins[index];
}
/**
* Returns the string value for {@code index}.
*/
public String getString(int index) throws IOException {
return inputStreamToString(getInputStream(index));
}
@Override public void close() {
for (InputStream in : ins) {
closeQuietly(in);
}
}
}
/**
* Edits the values for an entry.
*/
public final class Editor {
private final Entry entry;
private boolean hasErrors;
private Editor(Entry entry) {
this.entry = entry;
}
/**
* Returns an unbuffered input stream to read the last committed value,
* or null if no value has been committed.
*/
public InputStream newInputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
if (!entry.readable) {
return null;
}
return new FileInputStream(entry.getCleanFile(index));
}
}
/**
* Returns the last committed value as a string, or null if no value
* has been committed.
*/
public String getString(int index) throws IOException {
InputStream in = newInputStream(index);
return in != null ? inputStreamToString(in) : null;
}
/**
* Returns a new unbuffered output stream to write the value at
* {@code index}. If the underlying output stream encounters errors
* when writing to the filesystem, this edit will be aborted when
* {@link #commit} is called. The returned output stream does not throw
* IOExceptions.
*/
public OutputStream newOutputStream(int index) throws IOException {
synchronized (DiskLruCache.this) {
if (entry.currentEditor != this) {
throw new IllegalStateException();
}
return new FaultHidingOutputStream(new FileOutputStream(entry.getDirtyFile(index)));
}
}
/**
* Sets the value at {@code index} to {@code value}.
*/
public void set(int index, String value) throws IOException {
Writer writer = null;
try {
writer = new OutputStreamWriter(newOutputStream(index), UTF_8);
writer.write(value);
} finally {
closeQuietly(writer);
}
}
/**
* Commits this edit so it is visible to readers. This releases the
* edit lock so another edit may be started on the same key.
*/
public void commit() throws IOException {
if (hasErrors) {
completeEdit(this, false);
remove(entry.key); // the previous entry is stale
} else {
completeEdit(this, true);
}
}
/**
* Aborts this edit. This releases the edit lock so another edit may be
* started on the same key.
*/
public void abort() throws IOException {
completeEdit(this, false);
}
private class FaultHidingOutputStream extends FilterOutputStream {
private FaultHidingOutputStream(OutputStream out) {
super(out);
}
@Override public void write(int oneByte) {
try {
out.write(oneByte);
} catch (IOException e) {
hasErrors = true;
}
}
@Override public void write(byte[] buffer, int offset, int length) {
try {
out.write(buffer, offset, length);
} catch (IOException e) {
hasErrors = true;
}
}
@Override public void close() {
try {
out.close();
} catch (IOException e) {
hasErrors = true;
}
}
@Override public void flush() {
try {
out.flush();
} catch (IOException e) {
hasErrors = true;
}
}
}
}
private final class Entry {
private final String key;
/** Lengths of this entry's files. */
private final long[] lengths;
/** True if this entry has ever been published */
private boolean readable;
/** The ongoing edit or null if this entry is not being edited. */
private Editor currentEditor;
/** The sequence number of the most recently committed edit to this entry. */
private long sequenceNumber;
private Entry(String key) {
this.key = key;
this.lengths = new long[valueCount];
}
public String getLengths() throws IOException {
StringBuilder result = new StringBuilder();
for (long size : lengths) {
result.append(' ').append(size);
}
return result.toString();
}
/**
* Set lengths using decimal numbers like "10123".
*/
private void setLengths(String[] strings) throws IOException {
if (strings.length != valueCount) {
throw invalidLengths(strings);
}
try {
for (int i = 0; i < strings.length; i++) {
lengths[i] = Long.parseLong(strings[i]);
}
} catch (NumberFormatException e) {
throw invalidLengths(strings);
}
}
private IOException invalidLengths(String[] strings) throws IOException {
throw new IOException("unexpected journal line: " + Arrays.toString(strings));
}
public File getCleanFile(int i) {
return new File(directory, key + "." + i);
}
public File getDirtyFile(int i) {
return new File(directory, key + "." + i + ".tmp");
}
}
}
package com.cw.cache;
import android.graphics.Bitmap;
import android.support.v4.util.LruCache;//为了兼容3.1之前的版本，请使用支持包中的LruCache
import android.util.Log;
/**
* Created by Myy on 2016/7/26.
* 内存缓存
*/
public class MemoryCache {
private static LruCache cache = null;
private MemoryCache() {
}
private static class MemoryCacheHolder {
private static MemoryCache cache = new MemoryCache();
}
public static MemoryCache getInstance() {
if (cache == null) {
initCache();
}
return MemoryCacheHolder.cache;
}
private static void initCache() {
int maxMemory = (int) Runtime.getRuntime().maxMemory();
Log.i("最大内存", maxMemory + "");
cache = new LruCache(maxMemory / 4) {
@Override
protected int sizeOf(String key, Bitmap value) {
Log.i("图片内存", value.getByteCount() + "");
return value.getByteCount();
}
};
}
public Bitmap get(String url) {
if (url == null || url.length() == 0)
throw new NullPointerException("url不可为空");
String key = StringUtils.urlToKey(url);
if (cache == null)
throw new RuntimeException("cache初始化失败");
return cache.get(key);
}
/**
* 缓存一个Bitmap，返回null表示该url对应的值已存在，添加失败。
*
* @param url
* @param bitmap
* @return
*/
public Bitmap put(String url, Bitmap bitmap) {
if (url == null || url.length() == 0 || bitmap == null)
throw new NullPointerException("url或者图像不可为空");
String key = StringUtils.urlToKey(url);
if (cache == null)
throw new RuntimeException("cache初始化失败");
return cache.put(key, bitmap);
}
/**
* 清除内存中的所有缓存数据
*/
public void clearCache() {
if (cache == null)
throw new RuntimeException("cache初始化失败");
cache.evictAll();
}
}
package com.cw.cache;
import android.content.Context;
import android.content.pm.PackageManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Environment;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileDescriptor;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
/**
* Created by Myy on 2016/7/26.
* 文件缓存
*/
public class DiskCache {
private static DiskLruCache cache = null;
private static Context c = null;
private DiskCache() {
}
private static class DiskCacheHolder {
private static DiskCache diskCache = new DiskCache();
}
public static DiskCache getInstance(Context context) {
if (cache == null) {
c = context.getApplicationContext();
initCache();
}
return DiskCacheHolder.diskCache;
}
private static void initCache() {
File fileDirectory = getFileDirectory();//保存缓存文件的目录
int appVersion = getAppVersion();
int valueCount = 1;//这里设置每个key只对应一个缓存实体
int maxSize = 100 * 1024 * 1024;//100MB的缓存空间
try {
//如果缓存目录存在缓存文件则直接使用，否则会在缓存目录新建缓存相关的文件
cache = DiskLruCache.open(fileDirectory, appVersion, valueCount, maxSize);
} catch (IOException e) {
e.printStackTrace();
}
}
private static int getAppVersion() {
try {
return c.getPackageManager().getPackageInfo(c.getPackageName(), 0).versionCode;
} catch (PackageManager.NameNotFoundException e) {
e.printStackTrace();
}
return 1;
}
private static File getFileDirectory() {
String path = null;
if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
path = Environment.getExternalStorageDirectory().getPath() + File.separator + "bitmap";
} else {
path = Environment.getDataDirectory().getPath() + File.separator + "bitmap";
}
File file = new File(path);
if (!file.exists())
file.mkdir();
return file;
}
/**
* 清除缓存数据
*/
public void clearCache() {
if (cache == null)
throw new RuntimeException("初始化失败");
try {
cache.delete();
} catch (IOException e) {
e.printStackTrace();
}
}
/**
* 缓存图片
*
* @param url
* @param bitmap
*/
public void put(String url, Bitmap bitmap) {
if (url == null || url.length() == 0 || bitmap == null)
throw new NullPointerException("url或者图像不可为空");
String key = StringUtils.urlToKey(url);
if (cache == null)
throw new RuntimeException("cache初始化失败");
DiskLruCache.Editor editor = null;
OutputStream os = null;
try {
editor = cache.edit(key);//注意后面调用的abort或者commit方法
if (editor != null) {
os = editor.newOutputStream(0);
if (!bitmap.compress(Bitmap.CompressFormat.PNG, 100, os)) {
throw new RuntimeException("图片压缩失败");
}
}
} catch (Exception e) {
e.printStackTrace();
try {
os.close();
editor.abort();
} catch (IOException e1) {
e1.printStackTrace();
}
} finally {
try {
os.close();
editor.commit();
} catch (IOException e) {
e.printStackTrace();
}
}
}
/**
* 获取缓存图片
*
* @param url
* @return
*/
public Bitmap get(String url) {
if (url == null || url.length() == 0)
throw new NullPointerException("url不可为空");
String key = StringUtils.urlToKey(url);
if (cache == null)
throw new RuntimeException("cache初始化失败");
DiskLruCache.Snapshot snapshot = null;
InputStream is = null;
Bitmap bitmap = null;
try {
snapshot = cache.get(key);//返回null表示该值不存在或当前正处于不可读状态。
if (snapshot != null) {
is = snapshot.getInputStream(0);
bitmap = BitmapFactory.decodeStream(is);
} else
return null;
} catch (IOException e) {
e.printStackTrace();
try {
snapshot.close();//这里不需要手动关闭is，因为此方法会将刚刚获取的文件流全部关闭
} catch (Exception e1) {
e1.printStackTrace();
} finally {
snapshot.close();
}
}
return bitmap;
}
}
package com.cw.cache;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
/**
* Created by Myy on 2016/7/26.
*/
public class StringUtils {
public static String urlToKey(String url) {
StringBuilder sb = new StringBuilder();
try {
MessageDigest digest = MessageDigest.getInstance("MD5");
digest.update(url.getBytes());
byte[] bytes = digest.digest();
for (int i = 0; i < bytes.length; i++)
sb.append(Integer.toHexString(0xff & bytes[i]));
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
}
return sb.toString().length() == 0 ? null : sb.toString();
}
public static String[] urlS = new String[]
{
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fimage53.360doc.com%2FDownloadImg%2F2012%2F07%2F2317%2F25701259_6.jpg",
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fimg.pconline.com.cn%2Fimages%2Fupload%2Fupc%2Ftx%2Fwallpaper%2F1308%2F16%2Fc2%2F24549817_1376646910888.jpg",
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fwww.deskcar.com%2Fdesktop%2Ffengjing%2F2013312114415%2F3.jpg",
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fimage.tianjimedia.com%2FuploadImages%2F2012%2F011%2FR5J8A0HYL5YV.jpg",
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fwww.bz55.com%2Fuploads%2Fallimg%2F111017%2F13264160c-25.jpg",
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fwww.bz55.com%2Fuploads1%2Fallimg%2F120130%2F1_120130225951_1.jpg",
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fwww.bz55.com%2Fuploads%2Fallimg%2F130618%2F1-13061PU440.jpg",
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fimg.pconline.com.cn%2Fimages%2Fupload%2Fupc%2Ftx%2Fsoftbbs%2F1008%2F26%2Fc0%2F4984165_1282800005719_1024x1024soft.jpg",
"http://image.baidu.com/search/down?tn=download&ipn=dwnl&word=download&ie=utf8&fr=result&url=http%3A%2F%2Fimg.pconline.com.cn%2Fimages%2Fupload%2Fupc%2Ftx%2Fwallpaper%2F1307%2F10%2Fc3%2F23153824_1373426670894.jpg",
"http://img2.imgtn.bdimg.com/it/u=331221080,82593678&fm=206&gp=0.jpg",
"http://img4.imgtn.bdimg.com/it/u=562407178,1662987234&fm=206&gp=0.jpg",
"http://img3.imgtn.bdimg.com/it/u=2324814778,3433509063&fm=206&gp=0.jpg",
"http://img4.imgtn.bdimg.com/it/u=3570507366,2497738850&fm=206&gp=0.jpg",
"http://img4.imgtn.bdimg.com/it/u=846199408,2794756692&fm=206&gp=0.jpg",
"http://img5.imgtn.bdimg.com/it/u=221456928,362190599&fm=206&gp=0.jpg",
"http://img4.imgtn.bdimg.com/it/u=1969253456,2193232238&fm=206&gp=0.jpg",
"http://img3.imgtn.bdimg.com/it/u=2695781595,4041188434&fm=206&gp=0.jpg",
"http://img0.imgtn.bdimg.com/it/u=3662196526,1418421672&fm=206&gp=0.jpg",
"http://img3.imgtn.bdimg.com/it/u=664997347,4191517248&fm=206&gp=0.jpg",
"http://img5.imgtn.bdimg.com/it/u=3641843242,2739246521&fm=206&gp=0.jpg",
"http://img3.imgtn.bdimg.com/it/u=2686058747,1067524060&fm=206&gp=0.jpg",
"http://img2.imgtn.bdimg.com/it/u=84383536,2556612772&fm=206&gp=0.jpg",
"http://img3.imgtn.bdimg.com/it/u=224917259,3388622236&fm=206&gp=0.jpg"
};
}
package com.cw.cache;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Rect;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.widget.ImageView;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
* Created by Myy on 2016/7/26.
*/
public class ImageLoader {
private static DiskCache diskCache;
private static MemoryCache memoryCache;
private static ExecutorService pool = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
private static Handler handler = null;
private ImageLoader() {
}
private static class ImageLoaderHolder {
private static ImageLoader imageLoader = new ImageLoader();
}
public static ImageLoader getInstance(Context context) {
if (diskCache == null || memoryCache == null) {
diskCache = DiskCache.getInstance(context);
memoryCache = MemoryCache.getInstance();
handler = new Handler(Looper.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
ViewHolder holder = (ViewHolder) msg.obj;
Bitmap bitmap = holder.bitmap;
ImageView imageView = holder.imageView;
if (imageView.getTag().equals(holder.url))
imageView.setImageBitmap(bitmap);
}
};
}
return ImageLoaderHolder.imageLoader;
}
/**
* 读取图片
*
* @param url
* @param imageView
*/
public void getBitmap(String url, ImageView imageView) {
Bitmap bitmap = null;
imageView.setTag(url);
if (diskCache == null || memoryCache == null) {
throw new RuntimeException("初始化失败");
} else {
//先从内存中读取，然后文件读取，最后网络读取
bitmap = memoryCache.get(url);
if (bitmap == null) {
bitmap = diskCache.get(url);
if (bitmap == null) {
getBitmapFromNet(url, imageView);
} else {
displayBitmap(imageView, bitmap, url);
}
} else {
displayBitmap(imageView, bitmap, url);
}
}
}
/**
* 从网络获取图片
*
* @param urls
*/
private synchronized void getBitmapFromNet(final String urls, final ImageView imageView) {
pool.execute(new Runnable() {
@Override
public void run() {
try {
URL url = new URL(urls);
HttpURLConnection con = (HttpURLConnection) url.openConnection();
InputStream is = con.getInputStream();
Bitmap bitmap = BitmapFactory.decodeStream(is);
if (bitmap != null) {
memoryCache.put(urls, bitmap);
diskCache.put(urls, bitmap);
displayBitmap(imageView, bitmap, urls);
}
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* 显示图片
*
* @param imageView
* @param bitmap
*/
private void displayBitmap(ImageView imageView, Bitmap bitmap, String url) {
ViewHolder holder = new ViewHolder();
holder.bitmap = bitmap;
holder.imageView = imageView;
holder.url = url;
Message msg = Message.obtain(handler, 0);
msg.obj = holder;
msg.sendToTarget();
}
private class ViewHolder {
Bitmap bitmap;
ImageView imageView;
String url;
}
}
然后是图片适配器，ImageAdapter：
package com.cw.cache;
import android.content.Context;
import android.os.Parcelable;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
/**
* Created by Myy on 2016/7/26.
*/
public abstract class ImageAdapter extends ArrayAdapter {
private LayoutInflater inflater = null;
private int resource = 0;
private Context context;
public ImageAdapter(Context context, int resource) {
super(context, resource, 0, StringUtils.urlS);
inflater = LayoutInflater.from(context);
this.resource = resource;
this.context = context;
}
/**
* 用于判断是否处于空闲状态
*
* @return
*/
public abstract boolean getIdle();
@Override
public View getView(int position, View convertView, ViewGroup parent) {
Log.i("sss", getIdle() + "");
if (convertView == null) {
convertView = inflater.inflate(resource, null);
ImageView imageView = (ImageView) convertView.findViewById(R.id.imageView);
ViewHolder viewHolder = new ViewHolder();
viewHolder.imageView = imageView;
convertView.setTag(viewHolder);
}
View view = convertView;
ImageView imageView = ((ViewHolder) view.getTag()).imageView;
if (getIdle()) {
ImageLoader.getInstance(context).getBitmap((String) getItem(position), imageView);
}
return view;
}
private class ViewHolder {
ImageView imageView;
}
}
package com.cw.cache;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.AbsListView;
import android.widget.ListView;
public class MainActivity extends AppCompatActivity implements AbsListView.OnScrollListener {
private boolean isIdle = true;
private ListView listView;
private ImageAdapter adapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listView = (ListView) findViewById(R.id.listView);
listView.setOnScrollListener(this);
adapter = new ImageAdapter(this, R.layout.image_layout) {
@Override
public boolean getIdle() {
return isIdle;
}
};
listView.setAdapter(adapter);
}
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
Log.i("xxx",scrollState+"");
if (scrollState == AbsListView.OnScrollListener.SCROLL_STATE_IDLE) {
isIdle = true;
adapter.notifyDataSetChanged();//注意，因为当处于闲置状态是，getView方法不会被调用，此时需要手动刷新listView
}
else
isIdle = false;
}
@Override
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
}
}
当我们运行过一遍之后，关掉网络，再次打开（注意彻底清除任务）如果还能显示图片，说明我们的缓存目的达到了。
---------文章写自：HyHarden---------
--------博客地址：http://blog.csdn.net/qq_25722767-----------