Gtk4-tutorial/src/sec10.src.md

# Instance and class

This section and the following four sections are explanations about the next version of the text file editor (tfe).
It is tfe5.
It has many changes from the prior version.
All the sources are listed in [Section 15](sec15.src.md).
They are located in two directories, [src/tfe5](tfe5) and [src/tfetextview](tfetextview).

## Encapsulation

We've divided C source file into two parts.
But it is not enough in terms of encapsulation.

- `tfe.c` includes everything other than TfeTextView.
It should be divided at least into two parts, `tfeapplication.c` and `tfenotebook.c`.
- Header files also need to be organized.

However, first of all, I'd like to focus on the object TfeTextView.
It is a child object of GtkTextView and has a new member `file` in it.
The important thing is to manage the Gfile object pointed by `file`.

- What is necessary to GFile when generating (or initializing) TfeTextView?
- What is necessary to GFile when destructing TfeTextView?
- TfeTextView should read/write a file by itself or not?
- How it communicates with objects outside?

You need to know at least class, instance and signals before thinking about them.
I will explain them in this section and the next section.
After that I will explain:

- Organizing functions.
- How to use GtkFileChooserDialog

## GObject and its children

GObject and its children are objects, which have both class and instance.
First, think about instance of objects.
Instance is structured memories and described as C language structure.
The following is a structure of TfeTextView.

~~~C
/* This typedef statement is automatically generated by the macro G_DECLARE_FINAL_TYPE */
typedef struct _TfeTextView TfeTextView;

struct _TfeTextView {
  GtkTextView parent;
  GFile *file;
};
~~~

The members of the structure are:

- `parent` is the instance structure of GtkTextView which is the parent object of TfeTextView.
- `file` is a pointer to GFile. It can be NULL if no file corresponds to the TfeTextView object.

Notice the program above is the declaration of the structure, not the definition.
So, no memories are allocated at this moment.
They are to be allocated when `tfe_text_view_new` function is invoked.

You can find the declaration of the ancestors of TfeTextView in the source files of GTK and GLib.
The following is extracts from the source files (not exactly the same).

~~~C
typedef struct _GObject GObject;
typedef struct _GObject GInitiallyUnowned;
struct  _GObject
{
  GTypeInstance  g_type_instance;
  volatile guint ref_count;
  GData         *qdata;
};

typedef struct _GtkWidget GtkWidget;
struct _GtkWidget
{
  GInitiallyUnowned parent_instance;
  GtkWidgetPrivate *priv;
};

typedef struct _GtkTextView GtkTextView;
struct _GtkTextView
{
  GtkWidget parent_instance;
  GtkTextViewPrivate *priv;
};
~~~

In each structure, its parent instance is declared at the top of members.
So, every ancestors is included in the child instance.
This is very important.
It guarantees a child widget to derive all the features from ancestors.
The structure of `TfeTextView` is like the following diagram.

![The structure of the instance TfeTextView](../image/TfeTextView.png){width=14.39cm height=2.16cm}


## Generate TfeTextView instance

The function `tfe_text_view_new` generates a new TfeTextView instance.

@@@include
tfetextview/tfetextview.c tfe_text_view_new
@@@

When this function is run, the following procedure is gone through.

1. Initialize GObject part in TfeTextView instance.
2. Initialize GtkWidget part in TfeTextView instance.
3. Initialize GtkTextView part in TfeTextView instance.
4. Initialize TfeTextView part in TfeTextView instance.

Step one through three is done automatically.
Step four is done by the function `tfe_text_view_init`.

> In the same way, `gtk_text_view_init`, `gtk_widget_init` and `g_object_init` is the initialization functions of GtkTextView, GtkWidget and GObject respectively.
> You can find them in the GTK or GLib source files.

@@@include
tfetextview/tfetextview.c tfe_text_view_init
@@@

This function just initializes `tv->file` to be `NULL`.

## Functions and Classes

In Gtk, all objects derived from GObject have class and instance.
Instance is memories which has a structure defined by C structure declaration as I mentioned in the previous two subsections.
Each object can have more than one instance.
Those instances have the same structure.
Instance, which is structured memories, only keeps status of the instance.
Therefore, it is insufficient to define its behavior.
We need at least two things.
One is functions and the other is class.

You've already seen many functions.
For example, `tfe_text_view_new` is a function to generate TfeTextView instance.
These functions are similar to object methods in object oriented languages such as Java or Ruby.
Functions are public, which means that they are expected to be used by other objects.

Class comprises mainly pointers to functions.
Those functions are used by the object itself or its descendant objects.
For example, GObject class is declared in `gobject.h` in GLib source files.

@@@include
class_gobject.c
@@@

I'd like to explain some of the members.
There's a pointer to the function `dispose` in line 22.

~~~C
void (*dispose) (GObject *object);
~~~

The declaration is a bit complicated.
The asterisk before the identifier `dispose` means pointer.
So, the pointer `dispose` points to a function which has one parameter, which points a GObject structure, and returns no value.
In the same way, line 23 says `finalize` is a pointer to the function which has one parameter, which points a GObject structure, and returns no value.

~~~C
void (*finalize) (GObject *object);
~~~

Look at the declaration of `_GObjectClass` so that you would find that most of the members are pointers to functions.

- 10: A function pointed by `constructor` is called when the instance is generated. It completes the initialization of the instance.
- 22: A function pointed by `dispose` is called when the instance destructs itself.
Destruction process is divided into two phases.
The first one is called disposing.
In this phase, the instance releases all the references to other instances.
The second phase is finalizing.
- 23: A function pointed by `finalize` finishes the destruction process.
- The other pointers point to functions which are called while the instance lives.

## TfeTextView class 

TfeTextView class is a structure and it includes all its ancestors' class in it.
Let's look at all the classes from GObject, which is the top level object, to TfeTextView object, which is the lowest.

    GObject -- GInitiallyUnowned -- GtkWidget -- GtkTextView -- TfeTextView

The following is extracts from the source files (not exactly the same).

@@@include
classes.c
@@@

- 105-107: This three lines are generated by the macro G\_DECLARE\_FINAL\_TYPE.
So, they are not written in either `tfe_text_view.h` or `tfe_text_view.c`.
- 2, 73, 106: Each derived class puts its parent class at the first member of its structure.
It is the same as instance structures.
- Class members in ancestors are open to the descendant class.
So, they can be changed in `tfe_text_view_class_init` function.
For example, the `dispose` pointer in GObjectClass will be overridden later in `tfe_text_view_class_init`.
(Override is an object oriented programming terminology.
Override is rewriting ancestors' class methods in the descendant class.)
- Some class methods are often overridden.
`set_property`, `get_property`, `dispose`, `finalize` and `constructed` are such methods.

TfeTextViewClass includes its ancestors' class in it.
It is illustrated in the following diagram.

![The structure of TfeTextView Class](../image/TfeTextViewClass.png){width=16.02cm height=8.34cm}

## Destruction of TfeTextView

Every Object derived from GObject has a reference count.
If an object A refers an object B, then A keeps a pointer to B in A and at the same time increases the reference count of B by one with the function `g_object_ref (B)`.
If A doesn't need B any longer, then A discards the pointer to B (usually it is done by assigning NULL to the pointer) and decreases the reference count of B by one with the function `g_object_unref (B)`.

If two objects A and B refer to C, then the reference count of C is two.
If A no longer needs C, A discards the pointer to C and decreases the reference count in C by one.
Now the reference count of C is one.
In the same way, if B no longer needs C, B discards the pointer to C and decreases the reference count in C by one.
At this moment, no object refers C and the reference count of C is zero.
This means C is no longer useful.
Then C destructs itself and finally the memories allocated to C is freed.

![Reference count of B](../image/refcount.png){width=15.855cm height=2.475cm}

The idea above is based on an assumption that an object referred by nothing has reference count of zero.
When the reference count drops to zero, the object starts its destruction process.
The destruction process is split into two phases: disposing and finalizing.
In the disposing process, the object invokes the function pointed by `dispose` in its class to release all references to other objects.
In the finalizing process, it invokes the function pointed by `finalize` in its class to complete the destruction process.
These functions are also called handlers or methods.
For example, dispose handler or dispose method.

In the destruction process of TfeTextView, the reference count of widgets related to TfeTextView is automatically decreased.
But GFile pointed by `tv->file` needs to decrease its reference count by one.
You must write the code in the dispose handler `tfe_text_view_dispose`.

@@@include
tfetextview/tfetextview.c tfe_text_view_dispose 
@@@

- 5,6: If `tv->file` points a GFile, decrease its reference count.
`g_clear_object` decreases the reference count and assigns NULL to `tv->file`.
In dispose handlers, we usually use `g_clear_object` rather than `g_object_unref`.
- 8: invokes parent's dispose handler. (This will be explained later.)

In the disposing process, the object uses the pointer in its class to call the handler.
Therefore, `tfe_text_view_dispose` needs to be registered in the class when the TfeTextView class is initialized.
The function `tfe_text_view_class_init` is the class initialization function and it is declared in the replacement produced by `G_DEFINE_TYPE` macro.

~~~C
static void
tfe_text_view_class_init (TfeTextViewClass *class) {
  GObjectClass *object_class = G_OBJECT_CLASS (class);

  object_class->dispose = tfe_text_view_dispose;

}
~~~

Each ancestors' class has been generated before TfeTextViewClass is generated.
Therefore, there are four classes and each class has a pointer to each dispose handler.
Look at the following diagram.
There are four classes -- GObjectClass (GInitiallyUnownedClass), GtkWidgetClass, GtkTextViewClass and TfeTextViewClass.
Each class has its own dispose handler -- `dh1`, `dh2`, `dh3` and `tfe_text_view_dispose`.

![dispose handlers](../image/dispose_handler.png){width=14.925cm height=4.455cm}

Now, look at the `tfe_text_view_dispose` program above.
It first releases the reference to GFile object pointed by `tv->file`.
Then it invokes its parent's dispose handler in line 8.

~~~C
G_OBJECT_CLASS (tfe_text_view_parent_class)->dispose (gobject);
~~~

`tfe_text_view_parent_class`,which is made by `G_DEFINE_TYPE` macro, is a pointer that points the parent object class.
Therefore, `G_OBJECT_CLASS (tfe_text_view_parent_class)->dispose` points the handler `dh3` in the diagram above.
And `gobject` is a pointer to TfeTextView instance which is casted as a GObject instance.
`dh3` releases all the references to objects in the GtkTextView part (it is actually the private area pointed by `prev`) in TfeTextView instance.
After that, `dh3` calls `dh2`, and `dh2` calls `dh1`.
Finally all the references are released.