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【安卓笔记】AsyncQueryHandler源码剖析
AsyncQueryHandler类用于提供异步数据库操作（增删改查）功能。本文视图从源码角度分析这个类。
其内部封装了Looper和Handler，这个消息循环工作在子线程。另提供4个常量标识当前操作类型。ContentResolver由构造器注入。
工作线程的handler，即mWorkThreadHandler的handleMessage方法负责操作数据库：
AsyncQueryHandler在构造的时候，会创建一个HandlerThread，start之，然后获取其Looper，并利用其Looper构建子线程消息循环，并与mWorkThreadHandler绑定：
WorkerHandler:
1.AsyncQueryHandler使用方式：
AsyncQueryHandler handler = new AsyncQueryHandler(this.getContentResolver())
{
@Override
protected void onQueryComplete(int token, Object cookie,Cursor cursor)
{
}
@Override
protected void onUpdateComplete(int token, Object cookie, int result)
{
}
@Override
protected void onInsertComplete(int token, Object cookie, Uri uri)
{
}
@Override
protected void onDeleteComplete(int token, Object cookie, int result)
{
}
};
handler.startQuery(token, cookie, uri, projection, selection, selectionArgs, orderBy);
handler.startDelete(token, cookie, uri, selection, selectionArgs);
handler.startInsert(token, cookie, uri, initialValues);
handler.startUpdate(token, cookie, uri, values, selection, selectionArgs);2.源码分析
分析源码前，从整体上把握这个类：每个AsyncQueryHandler对象都会开启一个后台线程，在线程中执行与ContentProvider组件的数据交互，进行增删改查。调用时，可以通过AsyncQueryHandler.startXXX系列方法将请求打包发送到后台线程，当相关处理完成后，会将结果异步回传给主线程并调用AsyncQueryHandler.onXXXComplete方法通知调用者。调用者每次请求时，需要传入一个整型值token作为这次请求的标识，当该请求完成后进行回调时，会将token传回，帮助调用者确定这是哪一次请求。
AsyncQueryHandler是个抽象类，不能直接被实例化，这点需要注意：
public abstract class AsyncQueryHandler extends Handler
其内部封装了Looper和Handler，这个消息循环工作在子线程。另提供4个常量标识当前操作类型。ContentResolver由构造器注入。
private static final int EVENT_ARG_QUERY = 1;
private static final int EVENT_ARG_INSERT = 2;
private static final int EVENT_ARG_UPDATE = 3;
private static final int EVENT_ARG_DELETE = 4;
/* package */ final WeakReference<ContentResolver> mResolver;
private static Looper sLooper = null;
private Handler mWorkerThreadHandler;工作线程的handler，即mWorkThreadHandler的handleMessage方法负责操作数据库：
protected class WorkerHandler extends Handler {
public WorkerHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
final ContentResolver resolver = mResolver.get();
if (resolver == null) return;
WorkerArgs args = (WorkerArgs) msg.obj;
int token = msg.what;
int event = msg.arg1;
switch (event) {
case EVENT_ARG_QUERY:
Cursor cursor;
try {
cursor = resolver.query(args.uri, args.projection,
args.selection, args.selectionArgs,
args.orderBy);
if (cursor != null) {
cursor.getCount();
}
} catch (Exception e) {
Log.w(TAG, "Exception thrown during handling EVENT_ARG_QUERY", e);
cursor = null;
}
args.result = cursor;
break;
case EVENT_ARG_INSERT:
args.result = resolver.insert(args.uri, args.values);
break;
case EVENT_ARG_UPDATE:
args.result = resolver.update(args.uri, args.values, args.selection,
args.selectionArgs);
break;
case EVENT_ARG_DELETE:
args.result = resolver.delete(args.uri, args.selection, args.selectionArgs);
break;
}
//将结果返回给AsyncQueryHandler的handleMessage处理
Message reply = args.handler.obtainMessage(token);
reply.obj = args;
reply.arg1 = msg.arg1;
if (localLOGV) {
Log.d(TAG, "WorkerHandler.handleMsg: msg.arg1=" + msg.arg1
+ ", reply.what=" + reply.what);
}
reply.sendToTarget();
}
}AsyncQueryHandler在构造的时候，会创建一个HandlerThread，start之，然后获取其Looper，并利用其Looper构建子线程消息循环，并与mWorkThreadHandler绑定：
public AsyncQueryHandler(ContentResolver cr) {
super();
mResolver = new WeakReference<ContentResolver>(cr);
synchronized (AsyncQueryHandler.class) {
if (sLooper == null) {
HandlerThread thread = new HandlerThread("AsyncQueryWorker");
thread.start();//启动子线程,启动子线程消息循环
sLooper = thread.getLooper();
}
}
mWorkerThreadHandler = createHandler(sLooper);
}
protected Handler createHandler(Looper looper) {
return new WorkerHandler(looper);
}这个类提供了WorkerArgs封装数据库操作的请求参数： protected static final class WorkerArgs {
public Uri uri;
public Handler handler;
public String[] projection;
public String selection;
public String[] selectionArgs;
public String orderBy;
public Object result;
public Object cookie;
public ContentValues values;
}当外界调用startXXX方法时，其实会构造WorkerArgs对象，然后将这个对象以消息的形式发送给mWorkThreadHandler
，处理完成后将结果发送给AsyncQueryHandler本身：
startXXX:
public void startQuery(int token, Object cookie, Uri uri,
String[] projection, String selection, String[] selectionArgs,
String orderBy) {
// Use the token as what so cancelOperations works properly
Message msg = mWorkerThreadHandler.obtainMessage(token);
msg.arg1 = EVENT_ARG_QUERY;
WorkerArgs args = new WorkerArgs();
args.handler = this;//注意这行制定了Handler为本身，即AsyncQueryHandler
args.uri = uri;
args.projection = projection;
args.selection = selection;
args.selectionArgs = selectionArgs;
args.orderBy = orderBy;
args.cookie = cookie;
msg.obj = args;
mWorkerThreadHandler.sendMessage(msg);//交给子线程处理
}WorkerHandler:
Message reply = args.handler.obtainMessage(token);
reply.obj = args;
reply.arg1 = msg.arg1;
...
reply.sendToTarget();//在startXXX方法中已经制定了target为AsyncQuertHandler本身最终，操作结果交由AsyncQueryHandler类的handleMessage处理： @Override
public void handleMessage(Message msg) {
WorkerArgs args = (WorkerArgs) msg.obj;
if (localLOGV) {
Log.d(TAG, "AsyncQueryHandler.handleMessage: msg.what=" + msg.what
+ ", msg.arg1=" + msg.arg1);
}
int token = msg.what;
int event = msg.arg1;
// pass token back to caller on each callback.
switch (event) {
case EVENT_ARG_QUERY:
onQueryComplete(token, args.cookie, (Cursor) args.result);
break;
case EVENT_ARG_INSERT:
onInsertComplete(token, args.cookie, (Uri) args.result);
break;
case EVENT_ARG_UPDATE:
onUpdateComplete(token, args.cookie, (Integer) args.result);
break;
case EVENT_ARG_DELETE:
onDeleteComplete(token, args.cookie, (Integer) args.result);
break;
}
}而AsyncQueryHandler类的handleMessage方法回调了onXXXcomplete系列方法。我们重写onXXXcomplete方法即可拿到数据库操作结果。
这个设计太赞了！！！！
【安卓笔记】AsyncQueryHandler源码剖析
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