源码： https://github.com/lingochamp/okdownload
wiki： https://github.com/lingochamp/okdownload/wiki
整个代码风格和okhttp有点相似。
关于这个源码的分析，我结合之前写的一篇涉及“关于模块设计”主题的文章 学习方法总结 来分析。
（1）明确实现的功能
参考1：okhttp对外接口调用：
OkHttpClient client = new OkHttpClient();
Request request = new Request.Builder()
.url(url)
.build();
Call call = client.newCall(request);
call.enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
...
}
@Override
public void onResponse(Call call, Response response) throws IOException {
...
}
});
参考2: volley对外接口调用：
RequestQueue queue = Volley.newRequestQueue(this);
String url ="http://www.google.com";
StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
}
});
queue.add(stringRequest);
okdownload的wiki文档里，关于各种不同情况的下载，对外接口并不是统一的，我觉得OnDownload这个类应该是一个对外类。
// 在源码基础上略有修改
DownloadTask task = new DownloadTask.Builder(url, parentFile)
.setFilename(filename)
// the minimal interval millisecond for callback progress
.setMinIntervalMillisCallbackProcess(16)
// ignore the same task has already completed in the past.
.setPassIfAlreadyCompleted(false)
.build();
// 异步
OkDownload.with().enqueue(task, downloadlistener);
// 同步
OkDownload.with().execute(task, downloadlistener);
（2）分析实现这个功能需要哪些角色
OkDownload用到了Builder模式。涉及如下角色：
// 下载分发器
private DownloadDispatcher downloadDispatcher;
// 回调分发器
private CallbackDispatcher callbackDispatcher;
// 下载存储类
private DownloadStore downloadStore;
// 下载连接
private DownloadConnection.Factory connectionFactory;
// 文件处理
private ProcessFileStrategy processFileStrategy;
// 下载策略
private DownloadStrategy downloadStrategy;
// 下载输出流
private DownloadOutputStream.Factory outputStreamFactory;
下载分发器 DownloadDispatcher
private final List<DownloadCall> readyAsyncCalls;
private final List<DownloadCall> runningAsyncCalls;
private final List<DownloadCall> runningSyncCalls;
private final List<DownloadCall> finishingCalls;
private volatile ExecutorService executorService;
private int maxParallelRunningCount = 5;
public void enqueue(DownloadTask[] tasks);
public void enqueue(DownloadTask task);
public void execute(DownloadTask task);
public boolean cancel(IdentifiedTask task);
public void cancel(IdentifiedTask[] tasks);
public boolean cancel(int id);
public void cancelAll();
回调分发器 CallbackDispatcher
private final DownloadListener transmit;
private final Handler uiHandler;
public DownloadListener dispatch();
public class DefaultTransmitListener implements interface DownloadListener {
void taskStart(DownloadTask task);
void connectTrialStart(DownloadTask task, Map<String, List<String>> requestHeaderFields);
void connectTrialEnd(DownloadTask task, int responseCode, Map<String, List<String>> responseHeaderFields);
void downloadFromBeginning(DownloadTask task, BreakpointInfo info, ResumeFailedCause cause);
void downloadFromBreakpoint(DownloadTask task, BreakpointInfo info);
void connectStart(DownloadTask task, int blockIndex, Map<String, List<String>> requestHeaderFields);
void connectEnd(DownloadTask task, int blockIndex, int responseCode, Map<String, List<String>> responseHeaderFields);
void fetchStart(DownloadTask task, int blockIndex, long contentLength);
void fetchProgress(DownloadTask task, int blockIndex, long increaseBytes);
void fetchEnd(DownloadTask task, int blockIndex, long contentLength);
void taskEnd(DownloadTask task, EndCause cause, Exception realCause);
}
下载存储类 DownloadStore
默认实现类是BreakpointStoreOnSQLite
protected final BreakpointSQLiteHelper helper;
protected final BreakpointStoreOnCache onCache;
BreakpointInfo get(int id);
BreakpointInfo createAndInsert(DownloadTask task);
int findOrCreateId(DownloadTask task);
boolean update(BreakpointInfo breakpointInfo);
void remove(int id);
下载连接 DownloadConnection，使用简单工厂模式
void addHeader(String name, String value);
boolean setRequestMethod(@NonNull String method);
Connected execute();
void release();
interface Factory {
DownloadConnection create(String url) throws IOException;
}
文件处理 ProcessFileStrategy
public MultiPointOutputStream createProcessStream(@NonNull DownloadTask task,
@NonNull BreakpointInfo info,
@NonNull DownloadStore store) {
return new MultiPointOutputStream(task, info, store);
}
public void completeProcessStream(@NonNull MultiPointOutputStream processOutputStream,
@NonNull DownloadTask task) {
}
public void discardProcess(@NonNull DownloadTask task);
而MultiPointOutputStream是支持分段处理的输出流，FileChannel支持seek
public void write(int blockIndex, byte[] bytes, int length);
public void cancel();
public void done(int blockIndex);
public void cancelAsync();
下载策略 DownloadStrategy
// 1 connection: [0, 1MB)
private static final long ONE_CONNECTION_UPPER_LIMIT = 1024 * 1024; // 1MiB
// 2 connection: [1MB, 5MB)
private static final long TWO_CONNECTION_UPPER_LIMIT = 5 * 1024 * 1024; // 5MiB
// 3 connection: [5MB, 50MB)
private static final long THREE_CONNECTION_UPPER_LIMIT = 50 * 1024 * 1024; // 50MiB
// 4 connection: [50MB, 100MB)
private static final long FOUR_CONNECTION_UPPER_LIMIT = 100 * 1024 * 1024; // 100MiB
public int determineBlockCount(@NonNull DownloadTask task, long totalLength);
public boolean isUseMultiBlock(final boolean isAcceptRange);
下载输出流 DownloadOutputStream，使用简单工厂模式
void write(byte[] b, int off, int len);
void close();
void flushAndSync();
void seek(long offset);
void setLength(long newLength);
interface Factory {
DownloadOutputStream create(Context context, File file, int flushBufferSize);
DownloadOutputStream create(Context context, Uri uri, int flushBufferSize);
boolean supportSeek();
}
默认实现类DownloadUriOutputStream，成员变量
private final FileChannel channel;
private final ParcelFileDescriptor pdf;
private final BufferedOutputStream out;
private final FileOutputStream fos;
（3）理清各角色之间的关系
（4）细化各角色的具体工作
DownloadConnection：使用的okhttp
DownloadStore：内存存储和db
支持分段下载，分段规则由DownloadStrategy决定
public class BreakpointInfo {
private final int id;
private final String url;
private String etag;
private File targetFile;
private final List<BlockInfo> blockInfoList;
}
public class BlockInfo {
@IntRange(from = 0)
private final long startOffset;
@IntRange(from = 0)
private final long contentLength;
private final AtomicLong currentOffset;
}