Re: Scaffold API proposal

[Jeroen Zwartepoorte <jeroen xs4all nl>]

Hey Gustavo,

On Mon, 2004-01-05 at 09:57, Gustavo M. Giráldez wrote:
> Hi,
> 
> Happy New Year everyone!

Same to you and the rest of people on the list!

> After some days of thought and elaboration (and New Year's celebrations
> :-) I present you a first draft of the API proposal for Scaffold.  The
> proposal tries to address all the issues I enumerated in the thread
> "future of scaffold".
> 
> It's still a very rough draft, and there are lots of missing parts, but
> I want to get the ball rolling and gather some feedback before
> elaborating further.

Great.

> To design the interfaces I made a list of plugins I'd like to have in
> Scaffold and then tried to see the interactions between them.  My list
> goes something like:
> 
> - a project manager
> - a text document manager
> - a VCS plugin
> - text utilities
> - a debugger
> - glade plugin
> - devhelp plugin
> - a symbol browser
> - a personal information/mail plugin
> 
> I think the proposed interfaces cover a good amount of the interactions
> between those.  Surely I missed some interactions and some important
> plugins (in fact, right now I'm thinking a couple more).  Please feel
> free to append your wishlist :-)

This seems a pretty good list. There are a couple of additional plugins
i can think of:

- todo plugin
- unit testing plugin/manager
- planner plugin
- profiler

But only the todo plugin has any chance of appearing anytime soon :) The
rest is wishfull thinking at this point :)

> Note that while there are 9 interfaces outlined in the document, the
> actual number will be smaller.  This is because the shell interface
> involves the EventBus, the FileOpen service and the ShellUI, and those
> interfaces will not be implemented by any plugin.
> 
> Anyway, the document is attached.  Feedback of any kind is very welcome
> :-)

Will do. If i don't comment on a particular issue, it just means i agree
with it :)

> Cheers,
> Gustavo
> 
> 
> ______________________________________________________________________
> 			SCAFFOLD API PROPOSAL
> 
> 			     Version 0.1
> 
> 			    Jan 5th, 2004
> 
> 
> 
> 
> 1. BINARY INTERFACES
> ====================
> 
> [ NOTE: in all API declarations the first parameter which is a
> reference to the object itself is omitted for clarity. ]
> 
> ---
> The ValueContainer interface:
> 
> One of the main components of the shell is the ValueContainer.  The
> ValueContainer is an interface which can also be implemented by
> plugins which want to "take over" certain subpaths of the main
> hierarchy.
> 
> The shell's ValueContainer exposes a tree hierarchy where each of the
> nodes can hold a single GValue.  Paths in the tree are represented by
> strings, where each element of the path is separated by a special
> character or sequence of characters (still undecided, but the slash
> '/' will be used in the following examples).  The root of the
> hierarchy is not accesible, and thus the initial slash in a given path
> can be omitted.  Another way of looking at the model exposed by
> ValueContainer is as nested hashes.

You say the initial slash can be omitted, yet you use this throughout
this document. Contradiction? Seems a good idea to have an initial slash
imo.

> An implementation of the ValueContainer interface can support
> delegation.  This means, if a GValue which contains an object which in
> turn implements the ValueContainer interface is installed at a given
> path, all accesses to subpaths which have it as its parent will be
> delegated to the installed object.  Delegation should be transparent.
> 
> E.g. an object A which realizes the ValueContainer interface is
> installed at the shell's path '/Documents'.  An access to the path
> '/Documents/Untitled0' the shell's object hierarchy will be delegated
> to A (by using the ValueContainer interface) with a path of
> '/Untitled0'.
> 
> There's no such thing as a relative path as in a filesystem.  A path
> is an address, not a means of navigation.
>   
> 
>   /* sets a given value in the repository; returns TRUE if the value
>      could be set */
>   gboolean set_value (const gchar *path, GValue *value, GError **error);
> 
>   /* tries to retrieve a value for a given path, returning TRUE if the
>      value was set */
>   gboolean get_value (const gchar *path, GValue *value, GError **error);
> 
>   /* removes a value from the repository */
>   void unset_value (const gchar *path);

Might it not be better to call this clear_value?

>   /* gets a list of strings with the set immediate subpaths */
>   GSList *get_children (const gchar *parent);
> 
> 
> The repository offered by the shell's ValueContainer is the place
> where plugins should expose their services and go look for other
> plugins services.  A standard hierarchy should be defined, which will
> become part of the API, along with all the interfaces.
> 
>        [ UNDECIDED: what's the policy for replacing an existing value? ]

The current implementation emits a warning when a value is overwritten.
This is supposed to show the developer when he's overwriting something
he shouldn't. Don't know really whether this is a good solution.

> 
> ----
> The EventBus:
> 
> The EventBus interface provides a shell-wide notification mechanism.
> Events emitted in the bus are considered of interest of several
> plugins.  Notifications of lesser importance should be implemented
> using the standard GSignal mechanism.
> 
> Group of events are selectable through patterns by the use of
> wildcards (the g_pattern_spec_* API will be used for this).
> 
> While not enforced, it's expected that events will be namespaced.
> E.g.: "Shell::ready", "Shell::quit", "Build::start", "Build::stop",
> "Documents::change_active", etc..  The event names will be part of
> Scaffold's API too.
> 
> Event emission is synchronous.
> 
> 
>   /* registers a listener for a specific event pattern; use the returned
>      id to unregister the listener */
>   guint listen_event (const gchar *event_pattern, GClosure *closure);
> 
>   /* unregister a listener */
>   void unlisten_event (guint id);

Why not call these add_listener and remove_listener?

>   /* emits an event in the bus */
>   void emit_event (const gchar *event);
> 
> While presented as a separated interface, only the shell will
> implement the event bus.
> 
> 
> 
> ---
> The action UI merging interface UIActionTarget:
> 
> This interface will allow a plugin to merge action-type UI elements
> (menus, toolbars).  This API mirrors the GtkUIManager API.
> 
> 	[ DECISION: should we have each visual component implement this
> 	interface, or have a single implementation at the shell, and
> 	somehow namespace the UI elements (popups)?
> 
> 	I tend to prefer offering this interface and have each
> 	interested plugin implement it, since that allows greater
> 	flexibility.  For example, in the project manager we could
> 	have different popups depending on the row type and one place
> 	to add common actions to all types.  The individual types
> 	would be accessed as an interface in the shell's repository:
> 	'/Project/targets', '/Project/sources', '/Project/groups',
> 	while the common place through '/Project/all' (or similar).
> 
> 	If we decide to go the other way, we could eventually ditch
> 	this interface and have a shell method which would return the
> 	GtkUIManager. ]

I agree, the shell should only take care of the main window, toolbar,
statusbar and docking widget. The rest should be taken care of by the
respective plugins.

> 
>   /* install an action group */
>   void add_action_group (GtkActionGroup *group);
> 
>   /* remove an action group */
>   void remove_action_group (GtkActionGroup *group);
> 
>   /* merge UI elements to the component's UI; the returned id is used
>      for unmerging */
>   guint merge_ui (const gchar *ui);
> 
>   /* unmerge a previously merged UI */
>   void unmerge_ui (guint merge_id);
> 
> 
> The shell will have a <menu> element and a <toolbar> element at a
> minimum.  Plugins implementing the interface will have a <popup>
> element.
> 
> 	[ UNDECIDED: how to register and use several toolbars ]

Multiple toolbars should be an implementation detail. The shell should
allow a user to customize the way Actions(Groups) are displayed. It
should also allow a user to create a second toolbar etc. (kinda like how
epiphany handles toolbars).

> 
> 
> ---
> The FileOpen Service:
> 
> This service is offered by the shell in principle since it's way too
> common and will probably be used by almost all plugins.
> 
> The concept is as follows: a plugin which can open a certain type of
> document (selected by the mimetype) registers itself with the
> service.  When an open request is made, the shell checks to see if it
> has any registered provider and invokes the handler if so.  Otherwise,
> it opens the file using Gnome's standard machinery (gnome_url_show()
> or whatever).
> 
> Different plugins are expected to open different types of documents:
> the document manager will open text files, the glade plugin will open
> .glade files, the project manager will open .scaffold project files,
> etc..
> 
> 	[ NOTE: Having the shell implement this service also solves
> 	the problem of opening a file from the commandline; I don't
> 	like the shell exposing the full command line in the value
> 	repository ]
> 
> 
>   /* the open handler signature; if the opened document can be
>      manipulated the function should return a reference to the created
>      document, NULL otherwise */
>   typedef Document * (* FileHandler) (const gchar *uri, 
>                                       gboolean read_only,
>                                       GError **error);
> 
>   /* open a file and (if applicable) present it to the user; errors are
>      indicated in the error parameter, and NULL is a valid result */
>   Document *open_file (const gchar *uri, gboolean read_only, GError **error);
> 
>   /* register an open handler for a mime type */
>   void register_file_handler (const gchar *mime_type, FileHandler handler);
> 
>   /* deregister an open handler */
>   unregister_file_handler (FileHandler handler);

I'm not sure about this interface. I'd much rather see something like
the Command interface from JBuilder (see opentools.pdf) where plugins
can register command handlers. These would show up when you run scaffold
--help as well. The document manager would be the "default command
handler" and as such would get the command line passed as a parameter.

After startup, which plugins want to open a file? There's the
document-manager itself which adds several GUI elements for opening
files. Then there's the project-manager (when a user double-clicks on a
file in the project tree), but that should be handled by using the
document-manager interface, not the shell imo.

There is the matter of specifying a project file on the command line
(.scaffold), but that could be handled by the project-manager
registering some kind of mime-type handler with the document-manager.

I need to look at JBuilder's document manager before i can comment
further on this though. JBuilder has a pretty nice document manager
setup which allows for quite a lot of functionality & flexibility.

> 
> ---
> The Document interface:
> 
> The document interface represents a single file opened by the
> application.
> 
>   gchar *get_uri ();
> 
>   gchar *get_mime_type ();
> 
>   gboolean get_editable ();
> 
>   /* users of this method? */
>   save ();
> 

See above. Need to read up on JBuilder docs before commenting.

> 
> ---
> The TextDocument interface:
> 
> Implemented by textual documents, allows direct manipulation of the
> file contents.
> 
> Document regions are delimited by the point and the mark (ala emacs).
> Cut and copy operations affect the region.  Insert occurs at the point
> always.
> 
>   /* movement modifiers: a combination of one value from the following
>      mask values */
> 
>   #define RELATIVE_POS_SHIFT 0
>   #define STEP_SIZE_SHIFT    2
> 
>   /* relative position */
>   #define FROM_CURRENT 0 << RELATIVE_POS_SHIFT
>   #define FROM_START   1 << RELATIVE_POS_SHIFT
>   #define FROM_END     2 << RELATIVE_POS_SHIFT
> 
>   /* steps */
>   #define CHAR   0 << STEP_SIZE_SHIFT
>   #define WORD   1 << STEP_SIZE_SHIFT
>   #define LINE   2 << STEP_SIZE_SHIFT
>   #define COLUMN 3 << STEP_SIZE_SHIFT
> 
>   void set_mark (gboolean swap_with_point);
> 
>   gboolean move_point (gint modifier, gint offset);
> 
>   gchar *get_text (gboolean cut);
> 
>   void insert_text (const gchar *text);
> 
>   gint get_position (gint modifier);
> 

See above. Need to read up on JBuilder docs before commenting.

> 
> ---
> ShellUI:
> 
> The ShellUI interface deals with visual components in the shell's
> container (dock) and the preferences dialog.
> 
> 
>   /* adds a widget to the shell's dock; returns FALSE if there already
>      existed a widget with the same name */
>   gboolean add_dock_widget (GtkWidget   *widget,
>                             const gchar *name,
>                             const gchar *title,
>                             const gchar *pos_hint);

Don't forget a const gchar *stock_id parameter here :) Also, shouldn't
the pos_hint be an int (enum)?

>   /* adds a preference page to the preferences dialog; returns FALSE
>      if there already existed a page with the same name */
>   gboolean add_preferences_widget (GtkWidget *page,
> 				   const gchar *name,
> 				   const gchar *title,
> 				   const gchar *category);

I'm thinking a const gchar *path parameter instead of the name/title &
category implementation is better. The current way it does things to get
the treeview in the prefs dialog filled kinda sucks.

>   /* removes a widget from the shell */
>   void remove_widget (GtkWidget *widget);

Use the name parameter from the add call please. (or have a
remove_widget_by_name call).

>   /* makes sure the widget is visible to the user */
>   void present_widget (GtkWidget *widget);
> 
> 
> 	[ UNDECIDED: The preferences dialog should be unique among the
> 	application, but there might exist more than one shell at any
> 	given time.  How do we deal with the preference pages? ]

Each shell has a preferences dialog. When a new shell is created, new
plugin instances are created as well, which register pref pages. I don't
see a problem here. (synchronization between shells happens via gconf
notification)

> 	[ UNDECIDED: present_widget() should open the preferences
> 	dialog if the widget is a preference page? ]

Better to have a separate call for this, using the path parameter
probably. present_preference_page (const gchar *path);

> 
> 
> ---
> The data UI merging interface UIDataTarget:
> 
> Plugins which have visual elements which show information that can be
> extended by other plugins (e.g. the vcs plugin can add information to
> the project manager plugin) can implement this interface, which is
> roughly a stripped down GtkCellLayout.
> 
> GtkCellRenderers can be packed in groups.  Groups have an id and can
> have a title.  The main difference with the GtkCellLayout interface is
> the absence of a GtkTreeModel/GtkTreeIter pair to feed the set data
> function.  Instead a ValueContainer and a path into it is given.
> 
> 
>   /* set data function for the renderers */
>   typedef void (* CellDataFunc) (CellRenderer *cell, 
> 	 		         ValueContainer *container, 
> 			         const gchar *path, 
> 			         gpointer user_data);
> 
>   /* create a new group of renderers to pack cells into */
>   guint add_group (const gchar *group_title);
> 
>   /* pack a cell into the given group using the func to fill the data */
>   void pack_cell (guint cell_group,
>                   GtkCellRenderer *cell,
>                   gboolean expand,
>                   CellDataFunc func,
>                   gpointer user_data);
> 
>   /* destroy the cell group */
>   void remove_group (guint id);
> 
> 
> 	[ NOTE: I'm not really convinced with this interface, but I
> 	can't find another example (besides the vcs<->project
> 	interaction) which would use it. ]

I don't really know what to think of this. Looks all very strange to me
:) I think this can wait until we need to cross that bridge btw
(implement a vcs plugin).

> 
> 
> ---
> BuildTarget:
> 
> The interface presented by a buildable and (possibly) executable
> project target.
> 
> 	[ FIXME: this needs a lot of thought ]
> 
>   GList *list_sources ();
> 
>   add_source (const gchar *uri);
> 
>   remove_source (const gchar *uri);
> 
>   gboolean is_executable ();

Yeah, need more thought. Not a high priority imho though (for now). We
need to get the basics right first.

> 
> ---
> [ FIXME: Design the project interface ]
> [ FIXME: Design a reflection interface to discover runtime interfaces ]

This is an important one.

> [ FIXME: Other interfaces? ]
> 
> 
> 
> 2. STRING INTERFACE (repository hierarchy and event names)
> ==========================================================
> 
> Whatever the implementation of the above interfaces, there must be a
> standard way for plugins to look for specific well-known interfaces.
> As stated above, the value repository is the place for plugins to
> publish their services.  The missing part is how to name the services.
> 
> The following paths expressed from the shell's repository should lead
> to the specified services:
> 
> /Documents
> 	Text documents collection (each child of this node implements
> 	the TextDocument interface)
> 
> /Documents/Current
> 	The currently (or last) active text document
> 
> /Project
> 	The project interface and starting point to access all project
> 	elements such as targets and sources

How do we deal with project groups? Let's say we have a Scaffold project
group which contains gdl, gtksourceview, glimmer, gnome-build and the
scaffold project in it? Probably a bad example to start with, but you
get the idea.

So perhaps it should be /Projects and /Projects/Current.

> /Project/Targets
> 	Targets collection
> 
> /Session
> 	Session saved data.  This should present a ValueContainer
> 	interface.  Only serializable data should be allowed here.
> 
> 
> The following are standard event names:
> 
> Shell::Ready
> 	The shell has finished loading the plugins and it's ready to
> 	begin operations
> 
> Shell::Quit
> 	The user wants to exit the application
> 
> Session::Loaded
> 	The session plugin has finished loading the session data,
> 	which is now available at /Session
> 
> Session::Save
> 	The session plugin requests that all interested parties save
> 	session data to /Session
> 
> 
> 
> 3. STARTUP AND SHUTDOWN SEQUENCE
> ================================
> 
> Note: the session management is implemented by a plugin which also has
> the capability of opening session files (i.e. it registers an open
> handler).  The session data itself is sustained and accessed by a
> ValueContainer interface.

Should it really register an open handler? Do we want users to be able
to open a session file through the File->Open menuitem? IMHO it would be
better to have explicit Load Session and Save Session actions somewhere.

> The creation of a new shell involves the following operations and
> events:
> 
> - the shell is created
> - all the tools in the shell toolset are loaded in order
> - the shell opens all files given in the command line
> - the Shell::Ready event it emitted
>   - if there is a session plugin
>     - on emission of the Shell::Ready event, it loads the (default or
>       command line specified) session file and emits Session::Loaded
>     - all interested plugins can now load the session settings by
>       registering to listen to the Session::Loaded event
> 
> On shutdown (when the user quits the application or closes the shell
> window), the following sequence takes place:
> 
> - the event Shell::Quit is emitted
>   - all plugins can perform shutdown operations, including asking the
>     user to save unsaved data
>   - if there is a session plugin
>     - emits the Session::Save event
>       - all registered plugins save session data to /Session
>     - save the session file and perform shutdown operations
> - all tools are unloaded in reversed order
> - the shell is destroyed

I have one big issue with this: current implementation uses a shutdown
callback in ScaffoldTool which has a return value of gboolean. This
allows a plugin to return FALSE if it doesn't want the shell to quit (in
case a user answer's "Cancel" on the question of "Save documents?") How
is this handled in your scenario?

> 
> 	[ FIXME: what about loading a session file in the middle of
> 	another session? We need an event to signal all plugins to
> 	reset their contents. ]

Why not use Session::Loaded for this?

The proposal looks quite good overall. I'm anxious to see your
reflection proposal though because that's quite important if you want to
move away from having to include header files to use the various
interfaces.

On a side note: what do you think about using "sde_" as a prefix for
these methods? (sde stands for Scaffold Development Evironment) It's a
whole lot shorter than prefixing everything with "scaffold_". Same goes
for classes. Plugins would only use the "s" part of "sde". So
project-manager would use the prefix "spm" (Scaffold Project Manager).
Just a thought though.

Cheers,

Jeroen