本章节翻译自《Beginning-Android-4-Application-Development》,如有翻译不当的地方,敬请指出。
原书购买地址http://www.amazon.com/Beginning-Android-4-Application-Development/dp/1118199545/Projections
managedQuery()的第二个参数(CursorLoader类的第三个参数)控制查询返回列的数量。之前的代码,传入的是null,就是把所有的列都取到。
Cursor c; if (android.os.Build.VERSION.SDK_INT < 11) { c = managedQuery(allContacts, null, null, null, null); } else { CursorLoader cursorLoader = new CursorLoader(this, allContacts, null, null, null, null); c = cursorLoader.loadInBackground(); }可以创建一个数组,指定要查询哪些列。
Uri allContacts = ContactsContract.Contacts.CONTENT_URI; String[] projection = new String[] { ContactsContract.Contacts._ID, ContactsContract.Contacts.DISPLAY_NAME, ContactsContract.Contacts.HAS_PHONE_NUMBER }; Cursor c; if (android.os.Build.VERSION.SDK_INT < 11) { c = managedQuery(allContacts, projection, null, null, null); } else { CursorLoader cursorLoader = new CursorLoader(this, allContacts, projection, null, null, null); c = cursorLoader.loadInBackground(); }
这样,就指定的需要查询的列:名字和是否有手机号。
Filtering
managedQuery()方法的第3和第4个参数(CursorLoader类的第4和第5个参数)。这两个参数指定了SQL语句中的WHERE条件,进行条件查询。
举个例子,下面的代码返回名字以"三"为结尾的联系人。
Uri allContacts = ContactsContract.Contacts.CONTENT_URI; String[] projection = new String[] { ContactsContract.Contacts._ID, ContactsContract.Contacts.DISPLAY_NAME, ContactsContract.Contacts.HAS_PHONE_NUMBER }; Cursor c; if (android.os.Build.VERSION.SDK_INT < 11) { c = managedQuery(allContacts, projection, ContactsContract.Contacts.DISPLAY_NAME + " LIKE ?", new String[] { "%三" }, null); } else { CursorLoader cursorLoader = new CursorLoader(this, allContacts, projection, ContactsContract.Contacts.DISPLAY_NAME + " LIKE ?", new String[] { "%三" }, null); c = cursorLoader.loadInBackground(); }经查询,会返回“张三”。
Sorting
managedQuery()和CursorLoader的最后一个参数。这个参数指定了SQL语句的ORDER BY条件,进行结果排序。
下面的代码对联系人进行名字的升序排列。
Uri allContacts = ContactsContract.Contacts.CONTENT_URI; String[] projection = new String[] { ContactsContract.Contacts._ID, ContactsContract.Contacts.DISPLAY_NAME, ContactsContract.Contacts.HAS_PHONE_NUMBER }; Cursor c; if (android.os.Build.VERSION.SDK_INT < 11) { c = managedQuery(allContacts, projection, ContactsContract.Contacts.DISPLAY_NAME + " LIKE ?", new String[] { "%三" }, ContactsContract.Contacts.DISPLAY_NAME + " ASC"); } else { CursorLoader cursorLoader = new CursorLoader(this, allContacts, projection, ContactsContract.Contacts.DISPLAY_NAME + " LIKE ?", new String[] { "%三" }, ContactsContract.Contacts.DISPLAY_NAME + " ASC"); c = cursorLoader.loadInBackground(); }