# Migrating from other containers to GtkGrid

#GtkGrid is an attempt to write a comprehensive, legacy-free, box-layout container that is flexible enough to replace #GtkBox, #GtkTable and the like.

The layout model of GtkGrid is to arrange its children in rows and columns. This is done by assigning positions on a two-dimentions grid that stretches arbitrarily far in all directions. Children can span multiple rows or columns, too.

## GtkBox versus GtkGrid: packing

GtkBox works by arranging child widgets in a single line, either horizontally or vertically. It allows packing children from the beginning or end, using gtk_box_pack_start() and gtk_box_pack_end().

### A simple box

      box = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);

      gtk_box_pack_start (GTK_BOX (box), gtk_label_new ("One"), FALSE, FALSE, 0);
      gtk_box_pack_start (GTK_BOX (box), gtk_label_new ("Two"), FALSE, FALSE, 0);

This can be done with #GtkGrid as follows:

      grid = gtk_grid_new ();

      child1 = gtk_label_new ("One");
      gtk_grid_attach (GTK_GRID (grid), child1, 0, 0, 1, 1);
      child2 = gtk_label_new ("Two");
      gtk_grid_attach_next_to (GTK_GRID (grid), child2, child1, GTK_POS_RIGHT, 1, 1);

And similarly for gtk_box_pack_end(). In that case, you would use #GTK_POS_LEFT to place the grid children from left to right.

If you only need to pack children from the start, using gtk_container_add() is an even simpler alternative. GtkGrid places children added with gtk_container_add() in a single row or column according to its #GtkOrientable:orientation.

One difference to keep in mind is that the gtk_box_pack_start/pack_end functions allow you to place an arbitrary number of children from either end without ever 'colliding in the middle'. With GtkGrid, you have to leave enough space between the two ends, if you want to combine packing from both ends towards the middle. In practice, this should be easy to avoid; and GtkGrid simply ignores entirely empty rows or columns for layout and spacing.

On the other hand, GtkGrid is more flexible in that its grid extends indefinitively in both directions — there is no problem with using negative numbers for the grid positions. So, if you discover that you need to place a widget before your existing arrangement, you always can.

## GtkBox versus GtkGrid: sizing

When adding a child to a GtkBox, there are two hard-to-remember parameters (child properties, more exactly) named expand and fill that determine how the child size behaves in the main direction of the box. If expand is set, the box allows the position occupied by the child to grow when extra space is available. If fill is also set, the extra space is allocated to the child widget itself. Otherwise it is left 'free'. There is no control about the 'minor' direction; children are always given the full size in the minor direction.

GtkGrid does not have any custom child properties for controlling size allocation to children. Instead, it fully supports the newly introduced #GtkWidget:hexpand, #GtkWidget:vexpand, #GtkWidget:halign and #GtkWidget:valign properties.

The #GtkWidget:hexpand and #GtkWidget:vexpand properties operate in a similar way to the expand child properties of #GtkBox. As soon as a column contains a hexpanding child, GtkGrid allows the column to grow when extra space is available (similar for rows and vexpand). In contrast to GtkBox, all the extra space is always allocated to the child widget, there are no 'free' areas.

To replace the functionality of the fill child properties, you can set the #GtkWidget:halign and #GtkWidget:valign properties. An align value of #GTK_ALIGN_FILL has the same effect as setting fill to %TRUE, a value of #GTK_ALIGN_CENTER has the same effect as setting fill to %FALSE. The image below shows the effect of various combinations of halign and valign.

### Expansion and alignment

      box = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);

      gtk_box_pack_start (GTK_BOX (box), gtk_label_new ("One"), TRUE, FALSE, 0);
      gtk_box_pack_start (GTK_BOX (box), gtk_label_new ("Two"), TRUE, TRUE, 0);

This can be done with #GtkGrid as follows:

      grid = gtk_grid_new ();

      child1 = gtk_label_new ("One");
      gtk_widget_set_hexpand (child1, TRUE);
      gtk_widget_set_halign (child1, GTK_ALIGN_CENTER);
      gtk_grid_attach (GTK_GRID (grid), child1, 0, 0, 1, 1);
      child2 = gtk_label_new ("Two");
      gtk_widget_set_hexpand (child2, TRUE);
      gtk_widget_set_halign (child1, GTK_ALIGN_FILL);
      gtk_grid_attach_next_to (GTK_GRID (grid), child2, child1, GTK_POS_RIGHT, 1, 1);

One difference between the new GtkWidget expand properties and the GtkBox child property of the same name is that widget expandability is 'inherited' from children. What this means is that a container will become itself expanding as soon as it has an expanding child. This is typically what you want, it lets you e.g. mark the content pane of your application window as expanding, and all the intermediate containers between the content pane and the toplevel window will automatically do the right thing. This automatism can be overridden at any point by setting the expand flags on a container explicitly.

Another difference between GtkBox and GtkGrid with respect to expandability is when there are no expanding children at all. In this case, GtkBox will forcibly expand all children whereas GtkGrid will not. In practice, the effect of this is typically that a grid will 'stick to the corner' when the toplevel containing it is grown, instead of spreading out its children over the entire area. The problem can be fixed by setting some or all of the children to expand.

When you set the #GtkBox:homogeneous property on a GtkBox, it reserves the same space for all its children. GtkGrid does this in a very similar way, with #GtkGrid:row-homogeneous and #GtkGrid:column-homogeneous properties which control whether all rows have the same height and whether all columns have the same width.

## GtkBox versus GtkGrid: spacing

With GtkBox, you have to specify the #GtkBox:spacing when you construct it. This property specifies the space that separates the children from each other. Additionally, you can specify extra space to put around each child individually, using the #GtkBox:padding child property.

GtkGrid is very similar when it comes to spacing between the children, except that it has two separate properties, #GtkGrid:row-spacing and #GtkGrid:column-spacing, for the space to leave between rows and columns. Note that row-spacing is the space between rows, not inside a row. So, if you doing a horizontal layout, you need to set #GtkGrid:column-spacing.

GtkGrid doesn't have any custom child properties to specify per-child padding; instead you can use the #GtkWidget:margin property. You can also set different padding on each side with the #GtkWidget:margin-left, #GtkWidget:margin-right, #GtkWidget:margin-top and #GtkWidget:margin-bottom properties.

### Spacing in boxes

             box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 6);
             gtk_box_pack_start (GTK_BOX (box), child, FALSE, FALSE, 12);

This can be done with #GtkGrid as follows:

             grid = gtk_grid_new ();
             gtk_grid_set_row_spacing (GTK_GRID (grid), 6);
             g_object_set (child, "margin", 12, NULL);
             gtk_grid_attach (GTK_GRID (box), child, 0, 0, 1, 1);