RealmCursor

[cmelchior]

Fixes #438

Missing
ContentProviders exposes a cursor, which works fine if used within a app. However it breaks if shared with another app:

• Accessing an ContentProvider in another app is done using IPC (Blocked by Add multi process support #1091)
• The ContentProvider is started on App1 main thread
• App2 is accessing ContentProvider using a Binder thread, meaning that opening the Realm in onCreate causes Thread exceptions.

We can ship RealmCursor with the explicit caveat that it doesn't work with ContentProviders across apps, but it is not ideal.

Description
This adds a Cursor implementation for Realm data. The main challenge is mapping the very limited types in SQLite to the more expressive types in Realm.

The current approach is to respect the Cursor interface as much as possible as that is what will be exposed in ContentProviders and other framework classes.

This has the following implications:

// Simple types
Boolean:  becomes a Cursor.INTEGER and will return 0 or 1 when calling Cursor.getInt()
Date: becomes a Cursor.INTEGER. getLong() return ms. since epoch. getInt() and getShort() will throw exception.

// Links/LinkLists: No immediately good solution is available. Suggestion is adding support  for the following syntax. Disallowing accessing RealmLists might also be an option as it doesn't map well to a table layout anyway?
int calculatedIndex = cursor.getIndex("foo.columnString");
int calculatedIndex = cursor.getIndex("fooList[0].columnString");

I have a single new method setIdColumn(String columnName) that creates a virtual "_id" field used by many framework classes. This should be acceptable as it is defined when the Cursor is created and the developer know it is a ReamCursor. Adding other nice methods like getDate() should be avoided as client code should only interact with the cursor using the Interface.

Note that RealmCursor just reuses the tableview backing the RealmResults. This means that any changes to RealmResults will be reflected in the RealmCursor. I am not sure this behaviour is developer friendly but AFAIK we cannot lock the tableview until we start implementing Async queries.

@emanuelez @kneth @bmunkholm

[kneth]

We use this is almost all our test modules. Why don't we move to a helper class?

[nhachicha]

It's safe to call contentObservable.unregisterAll() at this point.

[cmelchior]

Good catch. It has been added.

[cmelchior]

Reverting to WIP status. There are some details about ContentObservers and NotificationUri's that needs to be discussed as it is probably also part of #989

[zaki50]

This branch can be removed.

Constant values are always inlined(http://docs.oracle.com/javase/specs/jls/se7/html/jls-13.html#jls-13.1-110-C).
This means there will be no reference to Cursor.FIELD* in class file.

[yuriy-larin]

Is Cursor example still not ready?

[cmelchior]

No, there is work on it here though: realm/realm-android-adapters#53
Anything in particular you are missing?

[yuriy-larin]

I'm trying to switch from SQLite to Realm while I'm on early stage of project.
I have code with :
implements LoaderCallbacks
SimpleCursorAdapter
static class MyCursorLoader extends CursorLoader
trying to rewrite it for Realm.
As I understand, I have to use something like:
RealmResults results = realm.where(User.class).findAll();
in Fragment class instead. Is that correct?

[yuriy-larin]

What is also not quite clear for me from documentation...
How to fetch limited number of instances, something equal to 'LIMIT 10' functionality?

[cmelchior]

realm.where(User.class).findAllAsync() is better as that will run the query on a background thread. No need for loaders in that case.

We don't support LIMIT yet, but since RealmResults are lazy-loaded, you normally don't need it anyway. You can use results.subList() instead though if you want to restrict the result.

[yuriy-larin]

Thank you for suggestion.

Today I also stumbled upon the limitation trying to extend class from external library.
#761
Inheritance / Polymorphism
I had to make copy of code instead.

[cmelchior]

Closing this PR as it is now being tracked realm/realm-android-adapters#6 instead