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their single child at 0,0 relative to their own origin, which
requires no computation or storage at all. some render objects
avoid computing the position of their children until the last
possible moment (for example, during the paint phase), to avoid
the computation entirely if they are not subsequently painted.
4 there exists one exception to this rule.
as discussed in the building widgets on demand
section, some widgets can be rebuilt as a result of a change in layout
constraints. if a widget marked itself dirty for unrelated reasons in
the same frame that it also is affected by a change in layout constraints,
it will be updated twice. this redundant build is limited to the
widget itself and does not impact its descendants.
5 a key is an opaque object optionally
associated with a widget whose equality operator is used to influence
the reconciliation algorithm.
6 for accessibility, and to give applications
a few extra milliseconds between when a widget is built and when it
appears on the screen, the viewport creates (but does not paint)
widgets for a few hundred pixels before and after the visible widgets.
7 this approach was first made popular by
facebook’s react library.
8 in practice, the t value is allowed
to extend past the 0.0-1.0 range, and does so for some curves. for
example, the “elastic” curves overshoot briefly in order to represent
a bouncing effect. the interpolation logic typically can extrapolate
past the start or end as appropriate. for some types, for example,
when interpolating colors, the t value is effectively clamped to
the 0.0-1.0 range.
<topic_end>
<topic_start>
understanding constraints
info note
if you are experiencing specific layout errors,
you might check out common flutter errors.
when someone learning flutter asks you why some widget
with width: 100 isn’t 100 pixels wide,
the default answer is to tell them to put that widget
inside of a center, right?
don’t do that.
if you do, they’ll come back again and again,
asking why some FittedBox isn’t working,
why that column is overflowing, or what
IntrinsicWidth is supposed to be doing.
instead, first tell them that flutter layout is very different
from HTML layout (which is probably where they’re coming from),
and then make them memorize the following rule:
flutter layout can’t really be understood without knowing
this rule, so flutter developers should learn it early on.
in more detail:
for example, if a composed widget contains a column
with some padding, and wants to lay out its two children
as follows:
the negotiation goes something like this:
widget: “hey parent, what are my constraints?”
parent: “you must be from 0 to 300 pixels wide,
and 0 to 85 tall.”
widget: “hmmm, since i want to have 5 pixels of padding,
then my children can have at most 290 pixels of width
and 75 pixels of height.”
widget: “hey first child, you must be from 0 to 290
pixels wide, and 0 to 75 tall.”
first child: “ok, then i wish to be 290 pixels wide,
and 20 pixels tall.”
widget: “hmmm, since i want to put my second child below the
first one, this leaves only 55 pixels of height for
my second child.”
widget: “hey second child, you must be from 0 to 290 wide,
and 0 to 55 tall.”
second child: “ok, i wish to be 140 pixels wide,
and 30 pixels tall.”
widget: “very well. my first child has position x: 5 and y: 5,
and my second child has x: 80 and y: 25.”
widget: “hey parent, i’ve decided that my size is going to be 300
pixels wide, and 60 pixels tall.”
<topic_end>
<topic_start>
limitations
flutter’s layout engine is designed to be a one-pass process.
this means that flutter lays out its widgets very efficiently,
but does result in a few limitations:
a widget can decide its own size only within the
constraints given to it by its parent.
this means a widget usually
can’t have any size it wants.
a widget can’t know and doesn’t decide its own position
in the screen, since it’s the widget’s parent who decides
the position of the widget.
since the parent’s size and position, in its turn,
also depends on its own parent, it’s impossible to
precisely define the size and position of any widget
without taking into consideration the tree as a whole.
if a child wants a different size from its parent and
the parent doesn’t have enough information to align it,
then the child’s size might be ignored.
be specific when defining alignment.
in flutter, widgets are rendered by their underlying
RenderBox objects. many boxes in flutter,
especially those that just take a single child,
pass their constraint on to their children.
generally, there are three kinds of boxes,