本文共 19000 字，大约阅读时间需要 63 分钟。

在阅读本文之前，你需要了解Handler作为Android中的线程间通信机制究竟是如何运作的，可以参考

源码

/** * A helper class to help make handling asynchronous {@link ContentResolver} * queries easier. */public abstract class AsyncQueryHandler extends Handler {
       private static final String TAG = "AsyncQuery";    private static final boolean localLOGV = false;    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
   
     mResolver;    private static Looper sLooper = null;    private Handler mWorkerThreadHandler;    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;    }    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);                        // Calling getCount() causes the cursor window to be filled,                        // which will make the first access on the main thread a lot faster.                        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;            }            // passing the original token value back to the caller            // on top of the event values in arg1.            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();        }    }    public AsyncQueryHandler(ContentResolver cr) {        super();        mResolver = new WeakReference
    
     (cr);        synchronized (AsyncQueryHandler.class) {            if (sLooper == null) {                HandlerThread thread = new HandlerThread("AsyncQueryWorker");                thread.start();                sLooper = thread.getLooper();            }        }        mWorkerThreadHandler = createHandler(sLooper);    }    protected Handler createHandler(Looper looper) {        return new WorkerHandler(looper);    }    /**     * This method begins an asynchronous query. When the query is done     * {@link #onQueryComplete} is called.     *     * @param token A token passed into {@link #onQueryComplete} to identify     *  the query.     * @param cookie An object that gets passed into {@link #onQueryComplete}     * @param uri The URI, using the content:// scheme, for the content to     *         retrieve.     * @param projection A list of which columns to return. Passing null will     *         return all columns, which is discouraged to prevent reading data     *         from storage that isn't going to be used.     * @param selection A filter declaring which rows to return, formatted as an     *         SQL WHERE clause (excluding the WHERE itself). Passing null will     *         return all rows for the given URI.     * @param selectionArgs You may include ?s in selection, which will be     *         replaced by the values from selectionArgs, in the order that they     *         appear in the selection. The values will be bound as Strings.     * @param orderBy How to order the rows, formatted as an SQL ORDER BY     *         clause (excluding the ORDER BY itself). Passing null will use the     *         default sort order, which may be unordered.     */    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;        args.uri = uri;        args.projection = projection;        args.selection = selection;        args.selectionArgs = selectionArgs;        args.orderBy = orderBy;        args.cookie = cookie;        msg.obj = args;        mWorkerThreadHandler.sendMessage(msg);    }    /**     * Attempts to cancel operation that has not already started. Note that     * there is no guarantee that the operation will be canceled. They still may     * result in a call to on[Query/Insert/Update/Delete]Complete after this     * call has completed.     *     * @param token The token representing the operation to be canceled.     *  If multiple operations have the same token they will all be canceled.     */    public final void cancelOperation(int token) {        mWorkerThreadHandler.removeMessages(token);    }    /**     * This method begins an asynchronous insert. When the insert operation is     * done {@link #onInsertComplete} is called.     *     * @param token A token passed into {@link #onInsertComplete} to identify     *  the insert operation.     * @param cookie An object that gets passed into {@link #onInsertComplete}     * @param uri the Uri passed to the insert operation.     * @param initialValues the ContentValues parameter passed to the insert operation.     */    public final void startInsert(int token, Object cookie, Uri uri,            ContentValues initialValues) {        // Use the token as what so cancelOperations works properly        Message msg = mWorkerThreadHandler.obtainMessage(token);        msg.arg1 = EVENT_ARG_INSERT;        WorkerArgs args = new WorkerArgs();        args.handler = this;        args.uri = uri;        args.cookie = cookie;        args.values = initialValues;        msg.obj = args;        mWorkerThreadHandler.sendMessage(msg);    }    /**     * This method begins an asynchronous update. When the update operation is     * done {@link #onUpdateComplete} is called.     *     * @param token A token passed into {@link #onUpdateComplete} to identify     *  the update operation.     * @param cookie An object that gets passed into {@link #onUpdateComplete}     * @param uri the Uri passed to the update operation.     * @param values the ContentValues parameter passed to the update operation.     */    public final void startUpdate(int token, Object cookie, Uri uri,            ContentValues values, String selection, String[] selectionArgs) {        // Use the token as what so cancelOperations works properly        Message msg = mWorkerThreadHandler.obtainMessage(token);        msg.arg1 = EVENT_ARG_UPDATE;        WorkerArgs args = new WorkerArgs();        args.handler = this;        args.uri = uri;        args.cookie = cookie;        args.values = values;        args.selection = selection;        args.selectionArgs = selectionArgs;        msg.obj = args;        mWorkerThreadHandler.sendMessage(msg);    }    /**     * This method begins an asynchronous delete. When the delete operation is     * done {@link #onDeleteComplete} is called.     *     * @param token A token passed into {@link #onDeleteComplete} to identify     *  the delete operation.     * @param cookie An object that gets passed into {@link #onDeleteComplete}     * @param uri the Uri passed to the delete operation.     * @param selection the where clause.     */    public final void startDelete(int token, Object cookie, Uri uri,            String selection, String[] selectionArgs) {        // Use the token as what so cancelOperations works properly        Message msg = mWorkerThreadHandler.obtainMessage(token);        msg.arg1 = EVENT_ARG_DELETE;        WorkerArgs args = new WorkerArgs();        args.handler = this;        args.uri = uri;        args.cookie = cookie;        args.selection = selection;        args.selectionArgs = selectionArgs;        msg.obj = args;        mWorkerThreadHandler.sendMessage(msg);    }    /**     * Called when an asynchronous query is completed.     *     * @param token the token to identify the query, passed in from     *            {@link #startQuery}.     * @param cookie the cookie object passed in from {@link #startQuery}.     * @param cursor The cursor holding the results from the query.     */    protected void onQueryComplete(int token, Object cookie, Cursor cursor) {        // Empty    }    /**     * Called when an asynchronous insert is completed.     *     * @param token the token to identify the query, passed in from     *        {@link #startInsert}.     * @param cookie the cookie object that's passed in from     *        {@link #startInsert}.     * @param uri the uri returned from the insert operation.     */    protected void onInsertComplete(int token, Object cookie, Uri uri) {        // Empty    }    /**     * Called when an asynchronous update is completed.     *     * @param token the token to identify the query, passed in from     *        {@link #startUpdate}.     * @param cookie the cookie object that's passed in from     *        {@link #startUpdate}.     * @param result the result returned from the update operation     */    protected void onUpdateComplete(int token, Object cookie, int result) {        // Empty    }    /**     * Called when an asynchronous delete is completed.     *     * @param token the token to identify the query, passed in from     *        {@link #startDelete}.     * @param cookie the cookie object that's passed in from     *        {@link #startDelete}.     * @param result the result returned from the delete operation     */    protected void onDeleteComplete(int token, Object cookie, int result) {        // Empty    }    @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;        }    }}
    
   

背景

/** * A helper class to help make handling asynchronous {
   @link ContentResolver} * queries easier. */

这是07年的代码，从最开头的类注释来看，写这个helper类的目的是用于简化ContentResolver异步查询。出于历史因素，你可以在某些Android应用——比如Dialer——源码中看到它的身影。不过就实际而言，后来出现的Loader在可代替的情况下比它更好用。

虽然如此，作为官方范例，在Handler的使用上还是有着不少学习之处。

解读

先看构造方法：

public AsyncQueryHandler(ContentResolver cr) {        super();        mResolver = new WeakReference
   
    (cr);        synchronized (AsyncQueryHandler.class) {            if (sLooper == null) {                HandlerThread thread = new HandlerThread("AsyncQueryWorker");                thread.start();                sLooper = thread.getLooper();            }        }        mWorkerThreadHandler = createHandler(sLooper);    }
   

它做了这样几件事：

1. super()调用父亲构造方法，令此AsyncQueryHandler工作在主线程中（参见Handler源码，其中有mLooper = Looper.myLooper();）。 2. 将ContentResolver作为弱引用持有 3. 在类同步语句块中，如果由static修饰的sLooper为null，创建一个工作线程thread，然后得到sLooper的值。 参考资料： 4. 通过sLooper，创建在工作线程中工作的mWorkerThreadHandler 。因为是同一个sLooper，所以不同AsyncQueryHandler实例的mWorkerThreadHandler 工作在同一个工作线程中。

HandlerThread专门设计用于与Handler协同工作，从设计角度而言，这样关系紧密的两个类，要么用A持有B的方式扩展使用，要么就反过来。这里是由Handler持有HandlerThread，与HandlerThread持有Handler的方式相比各有优缺点。

这里封装的具体操作无非是増删改查，我们看一种就好：

/**     * This method begins an asynchronous query. When the query is done     * {@link #onQueryComplete} is called.     *     * @param token A token passed into {@link #onQueryComplete} to identify     *  the query.     * @param cookie An object that gets passed into {@link #onQueryComplete}     * @param uri The URI, using the content:// scheme, for the content to     *         retrieve.     * @param projection A list of which columns to return. Passing null will     *         return all columns, which is discouraged to prevent reading data     *         from storage that isn't going to be used.     * @param selection A filter declaring which rows to return, formatted as an     *         SQL WHERE clause (excluding the WHERE itself). Passing null will     *         return all rows for the given URI.     * @param selectionArgs You may include ?s in selection, which will be     *         replaced by the values from selectionArgs, in the order that they     *         appear in the selection. The values will be bound as Strings.     * @param orderBy How to order the rows, formatted as an SQL ORDER BY     *         clause (excluding the ORDER BY itself). Passing null will use the     *         default sort order, which may be unordered.     */    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;        args.uri = uri;        args.projection = projection;        args.selection = selection;        args.selectionArgs = selectionArgs;        args.orderBy = orderBy;        args.cookie = cookie;        msg.obj = args;        mWorkerThreadHandler.sendMessage(msg);    }

注释已经十分详细，做的事情也十分简单，用obtainMessage()从循环池中拿出一个Message,放上表示动作的标志位msg.arg1 = EVENT_ARG_QUERY;，然后构造一个专门用于放数据的静态内部类WorkerArgs ，传入的参数全部放入其中，再作为Object传给Messagemsg.obj = args;，最后是发出MessagemWorkerThreadHandler.sendMessage(msg);。

再看下WorkerThreadHandler是如何工作的：

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);                        // Calling getCount() causes the cursor window to be filled,                        // which will make the first access on the main thread a lot faster.                        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;            }            // passing the original token value back to the caller            // on top of the event values in arg1.            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();        }    }

根据传来的msg.arg1;判定是什么事件，处理后通过args.handler即AsyncQueryHandler本身构造回复用的Message，传入数据的同时reply.obj = args;继续传入标志位reply.arg1 = msg.arg1;，然后回复即可。

注意这里告诉了我们一个使用Cursor时提速的技巧：

// Calling getCount() causes the cursor window to be filled,                        // which will make the first access on the main thread a lot faster.                        if (cursor != null) {                            cursor.getCount();                        }

最后就是主线程中的处理了：

@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;        }    }

通过标志位判定，来进行増删改查后的回调处理。

本篇至此结束，下次见。

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