| A Form is a Screen that contains
an arbitrary mixture of items: images,
read-only text fields, editable text fields, editable date fields, gauges,
choice groups, and custom items. In general, any subclass of the
Item Item class may be contained within a form.
The implementation handles layout, traversal, and scrolling.
The entire contents of the Form scrolls together.
Item Management
The items contained within a Form may be edited
using append, delete,
insert, and set methods. Items within a
Form are referred to by their
indexes, which are consecutive integers in the range from zero to
size()-1,
with zero referring to the first item and size()-1
to the last item.
An item may be placed within at most one
Form. If the application
attempts to place an item into a Form, and the
item is already owned by this
or another Form, an
IllegalStateException is thrown.
The application must
remove the item from its currently containing Form
before inserting it into
the new Form.
If the Form is visible on the display when
changes to its contents are
requested by the application, updates to the display take place as soon
as it is feasible for the implementation to do so.
Applications need not take any special action to refresh a
Form's display
after its contents have been modified.
Layout
Layout policy in Form is organized around
rows. Rows are typically
related to the width of the screen, respective of margins, scroll bars, and
such. All rows in a particular Form will have the
same width. Rows do not
vary in width based on the Items contained within
the Form, although they
may all change width in certain circumstances, such as when a scroll bar
needs to be added or removed. Forms generally do not scroll
horizontally.
Forms grow vertically and scroll vertically as
necessary. The height
of a Form varies depending upon the number of rows
and the height of
each row. The height of each row is determined by the items that are
positioned on that row. Rows need not all have the same height.
Implementations may also vary row heights to provide proper padding or
vertical alignment of Item labels.
An implementation may choose to lay out Items in a
left-to-right or right-to-left direction depending upon the language
conventions in use. The same choice of layout direction must apply to all
rows within a particular Form.
Prior to the start of the layout algorithm, the
Form is considered to
have one empty row at the top. The layout algorithm considers each Item
in turn, starting at Item zero and proceeding in
order through each Item
until the last Item in the Form
has been processed.
If the layout direction (as described above) is left-to-right, the
beginning of the row is the left edge of the Form. If the
layout direction is right-to-left, the beginning of the row is the right
edge of the Form. Items are laid out at the
beginning of each row, proceeding across each row in the chosen layout
direction, packing as many Items onto each row as will fit,
unless a condition occurs that causes the packing of a row to be terminated
early.
A new row is then added, and
Items are packed onto it
as described above. Items are packed onto rows,
and new rows are added
below existing rows as necessary until all Items
have been processed by
the layout algorithm.
The layout algorithm has a concept of a current alignment.
It can have the value LAYOUT_LEFT,
LAYOUT_CENTER, or LAYOUT_RIGHT.
The value of the current alignment at the start of the layout algorithm
depends upon the layout direction in effect for this Form. If
the layout direction is left-to-right, the initial alignment value must be
LAYOUT_LEFT. If the layout direction is right-to-left, the
initial alignment value must be LAYOUT_RIGHT.
The current alignment changes when the layout
algorithm encounters an Item that has one of the layout
directives LAYOUT_LEFT, LAYOUT_CENTER, or
LAYOUT_RIGHT. If none of these directives is present on an
Item, the current layout directive does not change. This
rule has the effect of grouping the contents of the
Form into sequences of consecutive Items
sharing an alignment value. The alignment value of each Item
is maintained internally to the Form and does not affect the
Items' layout value as reported by the
Item.getLayout Item.getLayout method.
The layout algorithm generally attempts to place an item on the same
row as the previous item, unless certain conditions occur that cause a
"row break." When there is a row break, the current item
will be placed
at the beginning of a new row instead of being placed after
the previous item, even if there is room.
A row break occurs before an item if any of the following
conditions occurs:
- the previous item has a row break after it;
- it has the
LAYOUT_NEWLINE_BEFORE directive; or
- it is a
StringItem whose contents starts with
"\n";
- it is a
ChoiceGroup, DateField,
Gauge, or a TextField, and the
LAYOUT_2 directive is not set; or
- this
Item has a LAYOUT_LEFT,
LAYOUT_CENTER, or LAYOUT_RIGHT directive
that differs from the Form's current alignment.
A row break occurs after an item if any of the following
conditions occurs:
- it is a
StringItem whose contents ends with
"\n"; or
- it has the
LAYOUT_NEWLINE_AFTER directive; or
- it is a
ChoiceGroup, DateField,
Gauge, or a TextField, and the
LAYOUT_2 directive is not set.
The presence of the LAYOUT_NEWLINE_BEFORE or
LAYOUT_NEWLINE_AFTER directive does not cause
an additional row break if there is one already present. For example,
if a LAYOUT_NEWLINE_BEFORE directive appears on a
StringItem whose contents starts with "\n",
there is only a single row break. A similar rule applies with a
trailing "\n" and LAYOUT_NEWLINE_AFTER.
Also, there is only a single row
break if an item has the LAYOUT_NEWLINE_AFTER directive
and the next item has the LAYOUT_NEWLINE_BEFORE directive.
However, the presence of consecutive "\n" characters,
either within a single StringItem or in adjacent
StringItems, will cause as many row breaks as there are
"\n" characters. This will cause empty rows to be present.
The height of an empty row is determined by the prevailing font height of
the StringItem within which the "\n" that ends the
row occurs.
Implementations may provide additional conditions under which a row
break occurs. For example, an implementation's layout policy may lay out
labels specially, implicitly causing a break before every
Item that has a
label. Or, as another example, a particular implementation's user
interface style may dictate that a DateField item always appears on a row
by itself. In this case, this implementation may cause row breaks to occur
both before and after every DateField item.
Given two items with adjacent Form indexes, if
none of the specified
or implementation-specific conditions for a row break between them
occurs, and if space permits, these items should be placed on the same
row.
When packing Items onto a row, the width of the
item is compared with
the remaining space on the row. For this purpose, the width used is the
Item's preferred width, unless the
Item has the LAYOUT_SHRINK
directive,
in which case the Item's minimum width is used. If
the Item is too wide
to fit in the space remaining on the row, the row is considered to be
full, a new row is added beneath this one, and the
Item is laid out on
this new row.
Once the contents of a row have been determined, the space available on
the row is distributed by expanding items and by adding space between
items. If any items on this row have the
LAYOUT_SHRINK directive (that is,
they are shrinkable), space is first distributed to these items. Space is
distributed to each of these items proportionally to the difference between
the each Item's preferred size and its minimum
size. At this stage, no
shrinkable item is expanded beyond its preferred width.
For example, consider a row that has 30 pixels
of space available and
that has two shrinkable items A and
B. Item A's preferred size is
15 and
its minimum size is 10. Item B's
preferred size is 30 and its minimum
size is 20. The difference between
A's preferred and minimum size is
5,
and B's difference is 10. The
30 pixels are distributed to these items
proportionally to these differences. Therefore, 10
pixels are
distributed to item A and 20
pixels to item B.
If after expanding all the shrinkable items to their preferred widths,
there is still space left on the row, this remaining space is distributed
equally among the Items that have the
LAYOUT_EXPAND directive (the
stretchable Items). The presence of any
stretchable items on a row will
cause the Items on this row to occupy the full
width of the row.
If there are no stretchable items on this row, and there is still space
available on this row, the Items are packed as tightly as
possible and are placed on the row according to the alignment value shared
by the Items on this row. (Since changing the current
alignment causes a row break, all Items on the same row must
share the same alignment value.) If the alignment value is
LAYOUT_LEFT, the Items are positioned at the left
end of the row and the remaining space is placed at the right end of the
row. If the alignment value is LAYOUT_RIGHT, the
Items are positioned at the right end of the row and the
remaining space is placed at the left end of the row. If the alignment
value is LAYOUT_CENTER, the Items are positioned
in the middle of the row such that the remaining space on the row is
divided evenly between the left and right ends of the row.
Given the set of items on a particular row, the heights of these
Items are inspected. For each Item, the height
that is used is the preferred height, unless the Item has the
LAYOUT_VSHRINK directive, in which case the
Item's minimum height is used.
The height of the tallest
Item determines the
height of the row. Items that have the
LAYOUT_VSHRINK directive are expanded to their preferred
height or to the height of the row, whichever is smaller.
Items that are still shorter than the
row height and that
have the LAYOUT_VEXPAND directive will expand to
the height of the row.
The LAYOUT_VEXPAND directive on an item will never
increase the height
of a row.
Remaining Items shorter than the row height
will be positioned
vertically within the row using the LAYOUT_TOP,
LAYOUT_BOTTOM, and
LAYOUT_VCENTER directives. If no vertical layout directive is
specified, the item must be aligned along the bottom of the row.
StringItems are treated specially in the above
algorithm. If the
contents of a StringItem (its string value,
exclusive of its label) contain
a newline character ("\n"), the string should be split at
that point and
the remainder laid out starting on the next row.
If one or both dimensions of the preferred size of
a StringItem have been locked, the StringItem
is wrapped to fit that width and height and is treated as a
rectangle whose minimum and preferred width and height are the width and
height of this rectangle. In this case, the
LAYOUT_SHRINK, LAYOUT_EXPAND,
and LAYOUT_VEXPAND directives are ignored.
If both dimensions of the preferred size of a StringItem
are unlocked, the text from the StringItem may be wrapped
across multiple rows. At the point in the layout algorithm where the width
of the Item is compared to the remaining space on the row, as
much text is taken from the beginning of the StringItem as
will fit onto the current row. The contents of this row are then
positioned according to the current alignment value. The remainder of the
text in the StringItem is line-wrapped to the full width of as
many new rows as are necessary to accommodate the text. Each full row is
positioned according to the current alignment value. The last line of the
text might leave space available on its row. If there is no row break
following this StringItem, subsequent Items are
packed into the remaining space and the contents of the row are positioned
according to the current alignment value. This rule has the effect of
displaying the contents of a StringItem as a paragraph of text
set flush-left, flush-right, or centered, depending upon whether the
current alignment value is LAYOUT_LEFT,
LAYOUT_RIGHT, or LAYOUT_CENTER, respectively.
The preferred width and height of a StringItem wrapped across
multiple rows, as reported by the
Item.getPreferredWidth Item.getPreferredWidth and
Item.getPreferredHeight Item.getPreferredHeight methods, describe the width and height of the bounding rectangle of the
wrapped text.
ImageItems are also treated specially by the above
algorithm. The foregoing rules concerning the horizontal alignment value
and the LAYOUT_LEFT, LAYOUT_RIGHT, and
LAYOUT_CENTER directives, apply to ImageItems
only when the LAYOUT_2 directive is also present on that item.
If the LAYOUT_2 directive is not present on an
ImageItem, the behavior of the LAYOUT_LEFT,
LAYOUT_RIGHT, and LAYOUT_CENTER directives is
implementation-specific.
A Form's layout is recomputed automatically as
necessary. This may
occur because of a change in an Item's size caused
by a change in its
contents or because of a request by the application to change the Item's
preferred size. It may also occur if an Item's
layout directives are
changed by the application. The application does not need to perform
any specific action to cause the Form's layout to
be updated.
For all cases where text is wrapped,
line breaks must occur at each newline character
('\n' = Unicode 'U+000A').
If space does not permit
the full text to be displayed it is truncated at line breaks.
If there are no suitable line breaks, it is recommended that
implementations break text at word boundaries.
If there are no word boundaries, it is recommended that
implementations break text at character boundaries.
Labels that contain line breaks may be truncated at the line
break and cause the rest of the label not to be shown.
User Interaction
When a Form is present on the display the user
can interact
with it and its Items indefinitely (for instance,
traversing from Item
to Item
and possibly
scrolling). These traversing and scrolling operations do not cause
application-visible events. The system notifies
the application when the user modifies the state of an interactive
Item
contained within the Form. This notification is
accomplished by calling the
ItemStateListener.itemStateChanged itemStateChanged() method of the listener declared to the Form with the
Form.setItemStateListener setItemStateListener() method.
As with other Displayable objects, a
Form can declare
Command commands and declare a command listener with the
Displayable.setCommandListener setCommandListener() method.
CommandListener CommandListener objects are distinct from
ItemStateListener ItemStateListener objects, and they are declared
and invoked separately.
Notes for Application Developers
- Although this class allows creation of arbitrary combination of
components
the application developers should keep the small screen size in mind.
Form is designed to contain a small number of
closely related
UI elements.
- If the number of items does not fit on the screen, the
implementation may choose to make it scrollable or to fold some components
so that a separate screen appears when the element is edited.
See Also: Item
since:
MIDP 1.0 |
