Container secrets: size allocation, part 6

Baselines

We are entering another of the more mysterious areas of GTK+ size allocation. Baselines move widgets from a simple box-with-width-and-height model to one where widgets can be aligned vertically in more interesting ways. The main place where this is matters is text. The readers eye is very sensitive to words moving up and down as you move along a line of text. Baselines are there to avoid that.

 

Since this is about aligning children vertically wrt. to each other, baselines are only relevant when the container is in horizontal orientation.

Measure above and below

Since children can now have a ‘forced’ alignment, simply t aking the maximum of the children’s heights is no longer sufficient. The alignment might cause children to ‘stick out’ at the top or the bottom, requiring a greater overall height. In order to handle this, we measure the parts ‘above the baseline’ and the parts ‘below the baseline’ separately, and maximize them separately.

for (i = 0; i < 3; i++) {
  gtk_widget_measure (child[i],
                      orientation,
                      sizes[i].minimum_size,
                      &child_min, &child_nat,
                      &child_min_baseline, &child_nat_baseline);

   below_min = MAX (below_min, child_min - child_min_baseline);
   above_min = MAX (above_min, child_min_baseline);
   below_nat = MAX (below_nat, child_nat - child_nat_baseline);
   above_nat = MAX (above_nat, child_nat_baseline);
}

total_min = above_min + below_min;
total_nat = above_nat + below_nat;

This code leaves out some details, such as dealing with children that don’t return a baseline.

Allocate on baseline

On the allocation side, there are two cases: either we are given a baseline that we have to align our children to, or we have to determine a baseline ourselves. In the latter case, we need to do essentially the same we already did for measuring: determine the below and above sizes separately, and use them to find our baseline:

for (i = 0; i < 3; i++) {
  if (gtk_widget_get_valign (child[i]) != GTK_ALIGN_BASELINE)
    continue;

  gtk_widget_measure (child[i],
                      GTK_ORIENTATION_VERTICAL,
                      child_size[i],
                      &child_min, &child_nat,
                      &child_min_baseline, &child_nat_baseline);

  below_min = MAX (below_min, child_min - child_min_baseline);
  below_nat = MAX (below_nat, child_nat - child_nat_baseline);
  above_min = MAX (above_min, child_min_baseline);
  above_nat = MAX (above_nat, child_nat_baseline);
}

When it comes to determining the baseline, we again have a choice to make. When there is more space available than the minimum, do we place the baseline as high as possible, or as low as possible, or somewhere in the middle? GtkBox has a ::baseline-position property to leave this choice to the user, and we do the same here.

switch (baseline_position) {
  case GTK_BASELINE_POSITION_TOP:
    baseline = above_min;
    break;
  case GTK_BASELINE_POSITION_CENTER:
    baseline = above_min + (height - (above_min + below_min)) / 2;
    break;
  case GTK_BASELINE_POSITION_BOTTOM:
    baseline = height - below_min;
    break;
}
Expanded, baseline position: center
Compressed, baseline position: top
Compressed, baseline position: center
Compressed, baseline position: bottom

Summary

This ends our journey through GTK+’s size allocation machinery. I hope you enjoyed it.

References

  1. Container secrets: size allocation
  2. Container secrets: size allocation, part 2
  3. Container secrets: size allocation, part 3
  4. Container secrets: size allocation, part 4
  5. Container secrets: size allocation, part 5
  6. The code with these changes

Container secrets: size allocation, part 5

Orientation

Many widgets in GTK+ can be oriented either horizontally or vertically. Anything from a separator to a toolbar is implementing the GtkOrientable interface to allow this to be changed at runtime, by setting the ::orientation property. So, obviously, GtkCenterBox should follow this pattern too.

I’m not explaining in detail how to add the interface and implement the property. The interesting part for us is how we are going to use the orientation property during size allocation.

Thankfully, much of our machinery is already written in terms of a single dimension, and can be applied to a height just as well as a width. What remains to be done is going through all the functions, and making sure that we take the orientation into account whenever we do something that depends on it. For example, we introduce a little helper to query the proper expand property.

static gboolean
get_expand (GtkWidget *widget,
            GtkOrientation orientation)
{
  if (orientation == GTK_ORIENTATION_HORIZONTAL)
    return gtk_widget_get_hexpand (widget);
  else
    return gtk_widget_get_vexpand (widget);
}

One thing to keep in mind is that some of the features we implement here only apply in horizontal orientation, such as right-to-left flipping, or baselines.

The measure() function changes to avoid hardcoding horizontal orientation:

if (orientation == self->orientation)
  gtk_center_box_measure_orientation (widget, orientation, for_size,
                                      minimum, natural,
                                      min_baseline, nat_baseline);
else
  gtk_center_box_measure_opposite (widget, orientation, for_size,
                                   minimum, natural,
                                   min_baseline, nat_baseline);

The size_allocate() function calls distribute() to distribute either the width or the height, depending on orientation:

if (self->orientation == GTK_ORIENTATION_HORIZONTAL) {
  size = width;
  for_size = height;
} else {
  size = height;
  for_size = width;
}
distribute (self, for_size, size, sizes);

After these straightforward, but tedious changes, we can orient a center box vertically:

References

  1. Container secrets: size allocation
  2. Container secrets: size allocation, part 2
  3. Container secrets: size allocation, part 3
  4. Container secrets: size allocation, part 4
  5. The code with these changes

Container secrets: size allocation, part 4

Height-for-width

This is where we enter the deeper parts of GTK+ size allocation. Height-for-width means that a widget does not have a single minimum size, but it might be able to accommodate a smaller width in return for getting a bigger height. Most widgets are not like this. The typical example for this behavior is a label that can wrap its text in multiple lines:

  

Height-for-width makes size allocation more expensive, so containers have to enable it explicitly, by setting a request mode. In general, containers should look at their children and use the request mode that is preferred by the majority of them. For simplicity, we just hardcode height-for-width here:

static GtkSizeRequestMode
gtk_center_box_get_request_mode (GtkWidget *widget)
{
  return GTK_SIZE_REQUEST_HEIGHT_FOR_WIDTH;
}

Measure both ways

The idiomatic way to write a measure() function that can handle height-for-width is to break it down into two cases: one where we are measuring along the orientation of the layout, and one where we are measuring in the opposite direction.

if (orientation == GTK_ORIENTATION_HORIZONTAL)
  measure_orientation (widget, for_size,
                       orientation,
                       minimum, natural,
                       minimum_baseline, natural_baseline);
else
  measure_opposite (widget, for_size,
                    orientation,
                    minimum, natural,
                    minimum_baseline, natural_baseline);

Measuring in the direction of the orientation is just like what our measure() function has done all along: we get a height, so we ask all children how much width they need for that height, and we sum up the answers.

Measuring in the opposite direction means to answer the question: given this width, how much height do you need ? We want to ask the children the same question, but what width should we give to each child ? We can’t just pass the full width to each child, since we don’t want them to overlap.

Distribute

To solve this, we need to distribute the available width among the children. This is just what our size_allocate() function is doing, so we need to break out the guts of size_allocate() out into a separate function.

Unsurprisingly, we will call the new function distribute().

static void
distribute (GtkCenterBox *self,
            int for_size,
            int size,
            GtkRequestedSize *sizes)
{
   /* Do whatever size_allocate() used to do
    * to determine sizes
    */

  sizes[0].minimum_size = start_size;
  sizes[1].minimum_size = center_size;
  sizes[2].minimum_size = end_size;
}

Now that we know how to get candidate widths for the children, we can complete the function to measure in the opposite direction. As before, we eventually return the maximum of the required heights for our children, since our layout is horizontal.

Note that orientation is GTK_ORIENTATION_VERTICAL in this case, so the min and nat values returned by the gtk_widget_measure() calls are heights.

distribute (self, -1, width, sizes);

gtk_widget_measure (start_widget,
                    orientation,
                    sizes[0].minimum_size,
                    &start_min, &start_nat,
                    &min_baseline, &nat_baseline);

gtk_widget_measure (center_widget,
                    orientation,
                    sizes[1].minimum_size,
                    &center_min, &center_nat,
                    &min_baseline, &nat_baseline);

gtk_widget_measure (end_widget,
                    orientation,
                    sizes[2].minimum_size,
                    &end_min, &end_nat,
                    &min_baseline, &nat_baseline);

*minimum = MAX (start_min, center_min, end_min);
*natural = MAX (start_nat, center_nat, end_nat);

Since we have now broken out the bulk of size_allocate() into the distribute() function, we can just call it from there and then do the remaining work that is necessary to assign positions to the children (since distribute already gave us the sizes).

Expanded
Slightly below natural size
Smaller
and smaller
and smaller

References

  1. Container secrets: size allocation
  2. Container secrets: size allocation, part 2
  3. Container secrets: size allocation, part 3
  4. The code with these changes
  5. Documentation of height-for-width geometry management

Container secrets: size allocation, part 3

 Expanding children

The next stop in our quest for featureful size allocation is the ::expand property. There are actually two of them,  ::hexpand and ::vexpand, and they have a somewhat interesting behavior of propagating upwards in the widget hierarchy. But that is not what we are going to discuss today, we simply want to give the children of our center box widget all the available space if they have the ::hexpand flag set.

Once again, the measure() implementation can stay as it is. And once again, we need to make a decision about who to expand first, if more than one child has an expand flag set. GtkBox tries to treat all its children the same, and evenly distributes the available extra space among all expanding children. We, on the other hand, prefer the center child, since it is the most important one.

But how do we go about this ? After some experimentation, I figured that if we already have to push the center child to the left or right because it does not fit, there is not point in making it even larger. Therefore we only respect the expand flag if that is not the case:

center_expand = gtk_widget_get_hexpand (center);

if (left_size > center_x)
  center_x = left_size;
else if (width - right_size < center_pos + center_size)
  center_x = width - center_width - right_size;
else if (center_expand) {
  center_width = width - 2 * MAX (left_size, right_size);
  center_x = (width / 2) - (center_width / 2);
}

After doing this, there may still be some space left that we can give to the outer children, if they are expanding:

if (left_expand)
  left_size = center_pos - left_pos;
if (right_expand)
  right_size = pos + width - (center_pos + center_width);
No expanding children
Center child expanding
End child expanding
Center and end children expanding

References

  1. Container secrets: size allocation
  2. Container secrets: size allocation, part 2
  3. The code with these changes

Container secrets: size allocation, part 2

Right-to-left languages

As the first thing in this series, we add back support for right-to-left languages.

It may be a little surprising if you are only used to writing software in English, but GTK+ has traditionally tried to ‘do the right thing’ automatically for languages that are written from right to left like Hebrew. Concretely, that means that we interpret the starting position in horizontal layouts to be on the right in these languages, i.e. we ‘flip’ horizontal arrangements. This can of course be overridden, by setting the ::text-direction property of the container. By default, the text direction gets determined from the locale.

This is of course very easy to do. The code I showed in the first post assumes left-to-right order of the children. We keep it that way and just reorder the children when necessary. Note that the measure() code doesn’t need any change, since it does not depend on the order at all. So, we just change size_allocate():

if (text_direction == rtl) {
  child[0] = last;
  child[1] = center;
  child[2] = first;
} else {
  child[0] = first;
  child[1] = center;
  child[2] = last;
}

One small gotcha that I haven’t mentioned yet: CSS assumes that :first-child is always the leftmost element, regardless of text direction. So, when we are moving child widgets from the left side to the right side in RTL context, we need to reorder the corresponding CSS nodes when the text direction changes.

if (direction == GTK_TEXT_DIR_LTR) {
  first = gtk_widget_get_css_node (start_widget);
  last = gtk_widget_get_css_node (end_widget);
} else {
  first = gtk_widget_get_css_node (end_widget);
  last = gtk_widget_get_css_node (start_widget);
}

parent = gtk_widget_get_css_node (box);
gtk_css_node_insert_after (parent, first, NULL);
gtk_css_node_insert_before (parent, last, NULL);

Natural size

Since this was easy, we’ll press on and make our size allocation respect natural size too.

In GTK+ every widget has not just a minimum size, which is the smallest size that it can usefully present itself in, but also a preferred or natural size. The measure() function returns both of these sizes.

For natural size, we can do slightly better than the code we showed last time. Back then, we simply calculated the natural size of the box by adding up the natural sizes of all children. But we want to center the middle child, so lets ask for enough room to give all children their natural size and put the middle child in the center:

*natural = child2_nat + 2 * MAX (child1_nat, child3_nat);

The size_allocate() function needs more work, and here we have some decisions to make. With 3 children vying for the available space, and the extra constraint imposed by centering, we could prefer to give more space to the outer children, or make the center child bigger. Since centering is the defining feature of this widget, I went with the latter choice.

So, how much space can we give to the center widget ? We don’t want to make it smaller than the minimum size, or bigger than the natural size, and we need to leave at least the minimum required space for the outer children.

center_size = CLAMP (width - (left_min + right_min),
                     center_min, center_nat);

Next, lets figure out how much space we can give to the outer children. We obviously can’t hand out more than we have left after giving the center child its part, and we have to split the remaining space evenly in order for centering to work (which is what avail in the code below is about). Again, want to respect the childs minimum and natural size.

avail = MIN ( (width - center_size) / 2,
              width - (center_size + right_min));
left_size = CLAMP (avail, left_min, left_nat);

And similarly for the right child. After determining the child sizes, all that is left is to assign the positions in the same way we’ve seen in part one: put the outer children at the very left and right, then center the middle child and push it to the right or left to avoid overlap.

Expanded
Natural size
Below natural size
Smaller
Minimum size

References

  1. Container secrets, size allocation
  2. The code with these changes

Container secrets: size allocation

I recently had an opportunity to reimplement size allocation for a container with all the things that GTK+ supports:

  • RTL support
  • Natural size
  • Align and expand
  • Height-for-width
  • Orientation support
  • Baselines

If you do a one-off container in an application, most of these may not matter, but in a general-purpose GTK+ widget, all of them are likely to become relevant sooner or later.

Since this is quite a lot of ground to cover, it will take a few posts to get through this. Lets get started!

The starting point

GtkCenterBox is a simple widget that can contain three child widgets – it is not a GtkContainer, at least not currently. In GTK+ 4, any widget can be a parent of other widgets. The layout that GtkCenterBox applies to its children is to center the middle child, as long as that is possible.  This is functionality that GtkBox provides in GTK+ 3, but the center child handling complicates an already complex container. Therefore we are moving it into a separate widget in GTK+ 4.

Expanded
Natural size
Below natural size
Minimum size

When I started looking at the GtkCenterBox size allocation code, it was very simple. The two methods to look at are measure() and size_allocate().

The measure implementation was just measuring the three children, adding up the minimum and natural sizes in horizontal direction, and taking their maximum in vertical direction. In rough pseudo-code:

if (orientation == GTK_ORIENTATION_HORIZONTAL) {
  *minimum = child1_min + child2_min + child3_min;
  *natural = child1_nat + child2_nat + child3_nat;
} else {
  *minimum = MAX(child1_min, child2_min, child3_min);
  *natural = MAX(child1_min, child2_min, child3_min);
}

The size_allocate implementation was putting the first child to the left, the last child to the right, and then placed the middle child in the center, eliminating overlaps by pushing it to the right or left, as needed.

child1_width = child1_min;
child2_width = child2_min;
child3_width = child3_min;
child1_x = 0;
child3_x = total_width - child3_width;
child2_x = total_width/2 - child2_width/2;
if (child2_x < child1_x + child1_width)
  child2_2 = child1_x + child1_width;
else if (child2_x + child2_width > child3_x)
  child2_x = child3_x - child2_width;

As you can see, this is pretty straightforward. Sadly, it does not have any of the features I listed above:

  • Children always get their minimum size
  • The ::expand property is not taken into account
  • The first child is always placed at the left, regardless of text direction
  • No vertical orientation
  • No height-for-width support
  • Baselines are ignored

Over the next few posts, I’ll try to show how to add these features back, hopefully clarifying some of the mysteries of how GTK+ size allocation works, along the way.

References

  1. First commit in the series
  2. Documentation for GtkWidget size allocation
  3. The code that we start from

Drag-and-Drop in lists, revisited

My previous post on Drag-and-Drop in lists made some compromises in order to present the simplest, fully functional implementation. One of these was that we could only draw the drop target highlight around entire rows, whereas the drop really inserts the dropped row between rows. Lets try to do better!

Changing the target

In the simplified version, we made every row a drop target. This time around, we will change this and have just the list itself accept drops.

gtk_drag_dest_set (list,
                   GTK_DEST_DEFAULT_MOTION|GTK_DEST_DEFAULT_DROP, 
                   entries, 1,
                    GDK_ACTION_MOVE);
g_signal_connect (list, "drag-data-received",
                  G_CALLBACK (drag_data_received), NULL);

If you compare this to what was done before, you may notice another change: Instead of GTK_DEST_DEFAULT_ALL, we now just request the default behavior for motion and drop. We no longer request the default behavior for highlighting, since we want to handle that ourselves.

In order to do so, we need to connect to the drag-motion and drag-leave signals. These are emitted on the drop target, i.e. the list:

 g_signal_connect (list, "drag-motion",
                   G_CALLBACK (drag_motion), NULL);
 g_signal_connect (list, "drag-leave",
                   G_CALLBACK (drag_leave), NULL);

Mind the gap

The basic idea for our improved drag highlighting is that we keep track of the two adjacent rows between which the drop will happen, and use the GTK+ CSS machinery to create a suitable highlight of the gap.

This requires a bit too much code to show in full here, but the idea is as follows: Find the row that is currently under the cursor with gtk_list_box_get_row_at_y(). Look at its allocation to to find out if the cursor is in the top or bottom half of the row. Depending on that, we pick either the following or the preceding row as the other member for our pair of rows.

row = gtk_list_box_get_row_at_y (list, y);
gtk_widget_get_allocation (row, &alloc);
if (y < alloc.y + alloc.height/2)
  {
    row_after = row;
    row_before = get_row_before (list, row);
  }
else
  {
    row_before = row;
    row_after = get_row_after (list, row);
  }

There are some corner cases which I am omitting here, e.g. when the hovered row is the first or last row, or when we are hovering over ’empty space’ in the list.

CSS Highlights

I said we would be using CSS for creating the highlight. The easiest way to do so is to just add style classes to the two rows we’ve found:

gtk_style_context_add_class (
                      gtk_widget_get_style_context (row_before),
                      "drag-hover-bottom");
gtk_style_context_add_class (
                      gtk_widget_get_style_context (row_after),
                      "drag-hover-top");

And then we will use some custom CSS to create our highlight. In case you are wondering: #4e9a06 is the drag highlight color used in the Adwaita theme.

.row.drag-hover-top {
  border-top: 1px solid #4e9a06; 
}
.row.drag-hover-bottom {
  border-bottom: 1px solid #4e9a06; 
}

This is again omitting the corner cases that I’ve mentioned before.

How did we get here ?

Dropping the row onto the place it came from does not achieve anything. It makes sense to mark the place where the drag started in some way to make this obvious. We can again use CSS for this, and add a style class to the dragged row in our drag_begin() method:

gtk_style_context_add_class (gtk_widget_get_style_context (row),
                             "drag-row");

In order to give the row a bit a highlight when we are hovering over it, we add an extra style class to it in our drag_motion() handler:

if (row == drag_row)
  {
    gtk_style_context_add_class (gtk_widget_get_style_context (row),
                                 "drag-hover");
    row_before = get_row_before (row);
    row_after = get_row_after (row);
  }
else …

And here is the CSS for these classes:

.row.drag-row {
  color: gray;
  background: alpha(gray,0.2);
 }
 .row.drag-row.drag-hover {
  border-top: 1px solid #4e9a06;
  border-bottom: 1px solid #4e9a06;
  color: #4e9a06;
}

Putting it all together

(Sorry about the broken cursors. We should really fix this in gnome-shell’s screen recorder.)

References

  1. The complete example, testlist3.c
  2. The GTK+ DND documentation

Container secrets

I recently spent some time tracking down a problem with GTK+ containers and drawing. Here is what I found out.

CSS drawing

In GTK+ 3.22, most, if not all, containers support the full CSS drawing model with multiple layers of backgrounds and borders. This is how, for example, GtkFrame draws its frame nowadays. But also containers that normally only arrange their children, such as GtkBox, can draw backgrounds and borders. The possibilities are endless!

For example, we can use a GtkBox to put a frame around a list box and a label, to make the label visually appear as part of the list. You can even make it colorful and fun, using some CSS like:

box.frame {
 border: 5px solid magenta;
}

Allocation and resizing

Traditionally, most containers in GTK+ are not doing any drawing of their own and just arrange their children, and thus there is no real need for them to do a full redraw when their size changes – it is enough to redraw the children. This is what gtk_container_set_reallocate_redraws() is about. And it defaults to FALSE in GTK+ 3, since we did not want to risk adding excessive redraws  whenever allocations change.

You can see where this is going: If I use the delete button to remove Butter and Salt from the list of ingredients, the allocation of the list, and thus of the box around it, will shrink, and we get a redraw problem.

The solution

If you plan to make plain layout containers draw backgrounds or borders, make sure to set reallocate-redraws to TRUE for the right widgets (in this case, the parent of the fun box).

gtk_container_reallocate_redraws (GTK_CONTAINER (parent), TRUE);

Note that gtk_container_reallocate_redraws() is deprecated in GTK+ 3.22, since we will get rid of it in GTK+ 4 and do the right thing automatically. But that shouldn’t stop you from using it to fix this issue.

Another (and maybe better) alternative is to use a container that is meant to draw a border, such as GtkFrame.

Logging and more

A while ago, GLib gained a new facility for ‘structured logging’. At the same time, it also gained support for writing logs to the systemd journal. Clearly, logging in GLib got a bit more complicated, and it can be a bit confusing.

This article is an attempt to clarify things.

Structured or not

The traditional GLib logging facilities are the g_message(), g_debug(), etc macros, which eventually call the g_log() function, which then uses the log handler that has been set with g_log_set_handler() to do the actual writing. You can put any information you like into your logs, but it has to all be formatted as a single string, the message.

g_debug ("You have %d eggs", 12 + 2);

g_log (G_LOG_DOMAIN,
       G_LOG_LEVEL_DEBUG,
       "You have %d eggs", 12 + 2);

With the new structured logging facilities, you call g_log_structured(), which then uses a log writer function to do the writing. So far, this is very similar to the older logging facility. The advantage of structured logs is that you can put multiple fields into the log without resorting to formatting it all into a string. Instead, you pass an array of log fields, which are key-value pairs.

g_log_structured (G_LOG_DOMAIN,
                  G_LOG_LEVEL_DEBUG,
                  "CODE_FILE", "mysource.c",
                  "CODE_LINE", 312,
                  "MESSSAGE_ID", "06d4df59e6c24647bfe69d2c27ef0b4e",
                  "MESSAGE", "You have %d eggs", 12 + 2);

CODE_FILE, CODE_LINE and MESSAGE_ID here are just examples for “standard” fields. You can also invent your own fields. Note that you can still use printf-style formatting for the MESSAGE field.

So GLib has two separate logging facilities now. To make matters a bit more interesting, we allow you to redirect the g_message(), g_debug(), etc wrapper macros to use g_log_structured() instead of g_log() under the covers. To do so, define the G_LOG_USE_STRUCTURED macro before including glib.h.

Why is this useful? For one thing, it saves you the trouble of replacing all your g_debug() and still lets you take some advantage of the structured logging – when used in this fashion, the traditional macros use separate fields for the log domain, the code file and line and some other fields, which can be helpful for filtering and searching in the resulting logs, in particular with the systemd journal.

Another advantage is that you can use a single backend, the log writer function, to control where both old and new logging calls end up.

Where have all my logs gone ?

Structured logging is frequently associated with the systemd journal. So, it is not surprising that people expect the g_log_structured() output to go the journal. And that is a really useful thing for services, or when you are launching an application e.g. from a desktop icon. But if you run it from the terminal, you probably expect to see its output right there.

To satisfy these competing demands, the GLib default log writer function tries to be smart. If it detects that stderr is redirected to the journald socket, then it writes its structured output to the journal. Otherwise, it formats a message and writes it to stderr.

Both GNOME Shell and DBus arrange for stderr to be redirected to the journal when they start apps or services. A way to explicitly redirect stderr to the journal is to run your application under systemd-cat:

systemd-cat my-app-that-logs

If you are sure that you want your logs to always go to the journal, you can tell GLib to use a log writer that does that:

g_log_set_writer_func (g_log_writer_journald, NULL, NULL)

Beyond the default

Even though the log writer functions that GLib provides by default should serve many needs, you might need to write your own. In that case, GLib has a number of useful functions that can help you, such as g_log_writer_format_fields(), g_log_writer_is_journald() or g_log_writer_supports_color().

Happy logging!

References

First steps with GSettings

We sometimes get questions about GSettings in #gtk+, and whether it is a good idea to use this API for ‘simple’ cases. The answer is a very clear Yes, in my opinion, and this article is trying to explain why.

Benefits

One of the nice things about GSettings is that it is a high-level API with backends for various native configuration systems. So, if you ever get around to porting your application to OS X or Windows, your application will automatically use the expected platform API to store its settings (the registry on Windows, and plists on OS X).

And even if your application will never be ported to those platforms, the dconf backend that is used on Linux has powerful features such as profiles and locks that let system administrators configure your application without you having to worry about it.

The documentation for GSettings unfortunately makes it appear more complicated than it is, since it doesn’t really try to hide the powerful features that are available to you (profiles, vendor overrides, translated defaults, complex types, bindings, etc).

So, here is a guide to first steps with GSettings, in a simple case.

Getting started

Lets get started with the simplest possible setting: a boolean.

The biggest hurdle to overcome is that GSettings insists on having a schema, which defines the datatype and default value for each key.

<schemalist>
  <schema path="/org/gnome/recipes/"       
         id="org.gnome.Recipes">
    <key type="b" name="use-metric">
      <default>true</default>
      <summary>Prefer metric units</summary>
      <description>
        This key determines whether values
        such as temperatures or weights will
        be displayed in metric units.
      </description>
    </key>
  </schema>
</schemalist>

Schemas need to be installed (the advantage of this is that tools such as dconf-editor can use the schema information). If you are using autotools, this is supported with macros. Just add

GLIB_GSETTINGS

to your configure.ac, and

gsettings_SCHEMAS = org.gnome.Recipes.gschema.xml
@GSETTINGS_RULES@

to Makefile.am. The setup with meson is similar.

Now that we’ve defined our key, we can obtain its value like this:

s = g_settings_new ("org.gnome.Recipes");
if (g_settings_get_boolean (s, "use-metric"))
  g_print ("Using metric units");

and we can set it like this:

s = g_settings_new ("org.gnome.Recipes");g_settings_set_boolean (s, "use-metric", TRUE);

Using GSettings for other basic types such as integers, floating point numbers, strings, etc. is very similar. Just use the appropriate getters and setters.

You may be wondering about the settings objects that we are creating here. It is fine to just create these whenever you need them. You can also create a global singleton if you prefer, but there is not real need to do so unless you want to monitor the setting for changes.

Next Steps: complex types

Beyond basic types, you have the full power of the GVariant type system available to store complex types. For example, if you need to store information about a circle in the plane, you could store it as a triple of type (ddd) storing the x, y coordinates of the center and the radius.

To handle settings with complex types in your code, use g_settings_get() and g_settings_set(), which return and accept the value in the form of a GVariant.

Next Steps: relocatable schemas

If your application uses accounts, you may want to look at relocatable schemas. A relocatable schema is what you need when you need multiple instances of the same configuration, stored separately. A typical example for this is accounts: your application allows to create more than one, and each of them has the same kind of configuration information associated with it.

GSettings handles this by omitting the path in the schema:

<schemalist>
  <schema id="org.gnome.Recipes.User">
    <key type="b" name="use-metric">
      <default>true</default>
    </key>
  </schema>
</schemalist>

Instead, we need to specify a path when we create the GSettings object:

s = g_settings_new_with_path ("org.gnome.Recipes.User",
                              "/org/gnome/Recipes/Account/mclasen");
if (g_settings_get_boolean (s, "use-metric"))
  g_print ("User mclasen is using metric units");

It is up to you to come up with a schema to map your accounts to unique paths.

Stumbling blocks

There are a few things to be aware of when using GSettings. One is that GLib needs to be able to find the compiled schema at runtime. This can be an issue when running your application out of the build directory without installing it. To handle this situation, you can set the GSETTINGS_SCHEMA_DIR environment variable to tell GLib where to find the compiled schema:

GSETTINGS_SCHEMA_DIR=build/data ./build/src/gnome-recipes

Another stumbling block is that GSettings reads the default values in the XML in the form of a serialized GVariant. This can be a bit surprising for the common case of a string, since it means that we need to put quotes around the string:

<default>'celsius'</default>

But these are minor issues, and easily avoided once you know about them.