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Minor edits for readability to section 4

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Paul Schulz 2022-03-04 15:20:33 +10:30
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src/sec4.src.md
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@ -4,10 +4,10 @@
### GtkLabel ### GtkLabel
We made a window and show it on the screen in the previous section. In the previous section we made a window and displayed it on the screen.
Now we go on to the next topic, widgets in the window. Now we go on to the next topic, where we add widgets to this window.
The simplest widget is GtkLabel. The simplest widget is GtkLabel.
It is a widget with a string in it. It is a widget with text in it.
@@@include @@@include
misc/lb1.c misc/lb1.c
@ -32,7 +32,7 @@ cd misc; diff pr4.c lb1.c
This tells us: This tells us:
- The definition of a variable `lab` is added. - The definition of a new variable `lab` is added.
- The title of the window is changed. - The title of the window is changed.
- A label is created and connected to the window as a child. - A label is created and connected to the window as a child.
@ -44,20 +44,20 @@ But these two relationships are totally different.
Don't be confused. Don't be confused.
In the program `lb1.c`, `lab` is a child widget of `win`. In the program `lb1.c`, `lab` is a child widget of `win`.
Child widgets are always located in their parent widget on the screen. Child widgets are always located in their parent widget on the screen.
See the window appeared on the screen. See how the window has appeared on the screen.
The window includes the label. The application window includes the label.
The window `win` doesn't have any parents. The window `win` doesn't have any parents.
We call such a window top-level window. We call such a window top-level window.
One application can have two or more top-level windows. An application can have more than one top-level window.
### GtkButton ### GtkButton
Next widget is GtkButton. The next widget to introduce is GtkButton.
It has a label or icon on it. It displays a button on the screen with a label or icon on it.
In this subsection, we will make a button with a label. In this subsection, we will make a button with a label.
When a button is clicked on, it emits a "clicked" signal. When the button is clicked, it emits a "clicked" signal.
The following program shows how to catch the signal and do something. The following program shows how to catch the signal to then do something.
@@@include @@@include
misc/lb2.c misc/lb2.c
@ -70,16 +70,16 @@ Finally, connects a "clicked" signal of the button to a handler (function) `clic
So, if `btn` is clicked, the function `click_cb` is invoked. So, if `btn` is clicked, the function `click_cb` is invoked.
The suffix "cb" means "call back". The suffix "cb" means "call back".
Name the program `lb2.c` and save it. Name the program `lb2.c` and save it.
Now compile and run it. Now compile and run it.
![Screenshot of the label](../image/screenshot_lb2.png){width=11.205cm height=6.945cm} ![Screenshot of the label](../image/screenshot_lb2.png){width=11.205cm height=6.945cm}
A window with the button appears. A window with the button appears.
Click the button (it is a large button, you can click everywhere in the window), then a string "Clicked." appears on the terminal. Click the button (it is a large button, you can click everywhere in the window), then a string "Clicked." appears on the terminal.
It shows the handler was invoked by clicking the button. It shows the handler was invoked by clicking the button.
It's fairly good for us to make sure that the clicked signal was caught and the handler was invoked. It's good that we make sure that the clicked signal was caught and the handler was invoked by using `g_print`.
However, using g_print is out of harmony with Gtk which is a GUI library. However, using g_print is out of harmony with Gtk which is a GUI library.
So, we will change the handler. So, we will change the handler.
The following code is `lb3.c`. The following code is `lb3.c`.
@ -94,17 +94,17 @@ And the difference between `lb2.c` and `lb3.c` is as follows.
cd misc; diff lb2.c lb3.c cd misc; diff lb2.c lb3.c
@@@ @@@
The change is: The changes are:
- The function `g_print` in `lb2.c` was deleted and two lines above are inserted instead. - The function `g_print` in `lb2.c` was deleted and the two lines above are inserted instead.
- The label of `btn` is changed from "Click me" to "Quit". - The label of `btn` is changed from "Click me" to "Quit".
- The fourth argument of `g_signal_connect` is changed from `NULL` to `win`. - The fourth argument of `g_signal_connect` is changed from `NULL` to `win`.
Most important is the fourth argument of `g_signal_connect`. The most important change is the fourth argument of `g_signal_connect`.
It is described as "data to pass to handler" in the definition of `g_signal_connect` in [GObject API Reference](https://docs.gtk.org/gobject/func.signal_connect.html). This argument is described as "data to pass to handler" in the definition of `g_signal_connect` in [GObject API Reference](https://docs.gtk.org/gobject/func.signal_connect.html).
Therefore, `win` which is a pointer to GtkApplicationWindow is passed to the handler as a second parameter `user_data`. Therefore, `win` which is a pointer to GtkApplicationWindow is passed to the handler as a second parameter `user_data`.
Then, the handler cast it to a pointer to GtkWindow and call `gtk_window_destroy` to destroy the top-level window. The handler then casts it to a pointer to GtkWindow and calls `gtk_window_destroy` to destroy the top-level window.
Then, the application quits. The application then quits.
### GtkBox ### GtkBox
@ -148,4 +148,3 @@ Then, these two buttons are appended to the box.
The handler corresponds to `btn1` toggles its label. The handler corresponds to `btn1` toggles its label.
The handler corresponds to `btn2` destroys the top-level window and the application quits. The handler corresponds to `btn2` destroys the top-level window and the application quits.