Widgets in this package:
Why are these widgets an indispensable tool?
When your desired layout feels too complex for Columns and Rows,
AlignPositioned is a real
lifesaver. Flutter is very composable, which is good, but sometimes it's unnecessarily complex to
translate some layout requirement into a composition of simpler widgets.
AlignPositioned aligns, positions, sizes, rotates and transforms its child in relation to both
the container and the child itself. In other words, it lets you easily and directly define where
and how a widget should appear in relation to another.
For example, you can tell it to position the top-left of its child at 15 pixels to the left of the
top-left corner of the container, plus move it two thirds of the child's height to the bottom plus
10 pixels, and then rotate 15 degrees.
Do you even know how to start doing this by composing basic Flutter widgets? Maybe, but
AlignPositioned it's much easier, and it takes a single widget.
AlignPositioned is specially helpful for explicit animations
(those that use a
controller), since you can just calculate the final position, size and rotation
you want for each frame. Without it, you may find yourself having to animate a composition of
AnimChain widgets are helpful for
implicit animations, which are very easy to create. If you change their parameters they animate
automatically, interpolating between the old and new parameter values.
How it works
as a dependency
pubspec.yaml file, then import it:
child to the
AlignPositioned or the
AnimatedAlignPositioned, and then one or more of
the following parameters:
AlignPositioned( child: child, alignment: ..., dx: ..., dy: ..., moveByChildWidth: ..., moveByChildHeight: ..., moveByContainerWidth: ..., moveByContainerHeight: ..., moveVerticallyByChildWidth: ..., moveHorizontallyByChildHeight: ..., moveVerticallyByContainerWidth: ..., moveHorizontallyByContainerHeight: ..., childWidth: ..., childHeight: ..., minChildWidth: ..., minChildHeight: ..., maxChildWidth: ..., maxChildHeight: ..., childWidthRatio: ..., childHeightRatio: ..., minChildWidthRatio: ..., minChildHeightRatio: ..., maxChildWidthRatio: ..., maxChildHeightRatio: ..., rotateDegrees: ..., matrix4Transform: ..., wins: ..., touch: ..., );
Let's study each parameter in detail:
Align and Position parameters
alignment parameter works as expected. For example,
Alignment.bottomRight represents the bottom right of the container, and
represents the center of the container. The distance from -1.0 to +1.0 is the distance from one side
of the rectangle to the other side of the rectangle.
alignment works just like the alignment for the
aligning the child inside the container.
Touch.outside, then the alignment happens outside the container; and if
Touch.middle, the center of the child will be aligned to the container edge.
As another example, if
Alignment(1.0, 0.0) makes the child's right
side touch the right side of the container (it touches the container from the inside).
Alignment(1.0, 0.0) makes the child's left side touch the
right side of the container (it touches the container from the outside).
Alignment(1.0, 0.0) makes the child's center touch the
right side of the container (it touches the container from the middle).
dy can be positive or negative, and move the child horizontally and
vertically, in pixels.
moveByChildHeight can be positive or negative, and move the
child horizontally and vertically, but the unit here is not pixels, but child widths and heights.
moveByContainerHeight can be positive or negative, and move
the child horizontally and vertically, but the unit here is not pixels, but container widths and
moveHorizontallyByContainerHeight allow you to move in some
direction according to the size (width or height) of the orthogonal direction. For example,
moveByChildWidth: 0.2 would move horizontally by 20% of the child's width,
moveVerticallyByChildWidth would move vertically by 20% of the child's width.
Align and Position Examples
The below image shows the center of the child positioned 15 pixels to the right of the top-left corner of the container:
AlignPositioned( child: child, alignment: Alignment.topLeft, touch: Touch.inside, dx: 15.0, // Move 4 pixels to the right. moveByChildWidth: -0.5, // Move half child width to the left. moveByChildHeight: -0.5); // Move half child height to the top.
Then, to move the child one container width to the right, and one container height to the bottom:
AlignPositioned( child: child, alignment: Alignment.topLeft, touch: Touch.inside, dx: 15.0, // Move 4 pixels to the right. moveByChildWidth: -0.5, // Move half child width to the left. moveByChildHeight: -0.5, // Move half child height to the top. moveByContainerWidth: 1.0, // Move one container width to the right. moveByContainerHeight: 1.0); // Move one container height to the bottom.
Please try the example that showcases the effects seen below:
Optionally, you can also define the child size:
childWidthis the child width, in pixels.
childHeightis the child height, in pixels.
minChildWidthis the minimum width, in pixels. It has precedence over
minChildHeightis the minimum height, in pixels. It has precedence over
maxChildWidthis the maximum width, in pixels. It has precedence over
maxChildHeightis the maximum height, in pixels. It has precedence over
childWidthRatiois the child width, as a fraction of the container width. If between 0.0 and 1.0, the child will be smaller than its container. If more than 1.0, the child will be larger than its container. If you define both
childWidththey will be added.
childHeightRatiois the child height, as a fraction of the container height. If between 0.0 and 1.0, the child will be smaller than its container. If more than 1.0, the child will be larger than its container. If you define both
childHeightthey will be added.
minChildWidthRatiois the minimum child width, as a fraction of the container width. It has precedence over
childWidth. If both
minChildWidthRatioare defined, both will be applied (the minimum will be the larger one).
minChildHeightRatio. is the minimum child height, as a fraction of the container height. It has precedence over
childHeight. If both
minChildHeightRatioare defined, both will be applied (the minimum will be the larger one).
maxChildWidthRatiois the maximum child width, as a fraction of the container width. It has precedence over
childWidth. If both
maxChildWidthRatioare defined, both will be applied (the maximum will be the smaller one).
maxChildHeightRatiois the maximum child height, as a fraction of the container height. It has precedence over
childHeight. If both
maxChildHeightRatioare defined, both will be applied (the maximum will be the smaller one).
winsdecides what happens if the minimum size is larger than the maximum size. If
Wins.min, the default, the minimum size will be used. If
Wins.max, the maximum size will be used.
Rotate and Transform
Optionally, you can also define rotation and transformation:
rotateDegreesis the rotation, in degrees (1 turn is 360 degrees). The position of the axis of the rotation (the "origin") depends on the
alignmentparameter and the parent. So, for example,
Alignment.centermeans the axis of rotation is at the center of the parent.
matrix4Transformlets you apply any transformation to the child. This uses Matrix4Transform instead of Matrix4, since it's easier to use. However, you can still use Matrix4 directly with the constructor
One widget relative to another
AlignPositioned.relative() factory if you have a main widget, and you want to
position/size/rotate/translate another widget relative to the main one, but the second is not a
child of the first.
Example, to center the main container widget, and then put a relative child widget vertically below it (in the Y-axis):
Center( child: AlignPositioned.relative( container: widgetA(), child: widgetB(), moveByContainerHeight: 0.5, moveByChildHeight: 0.5));
invert parameter controls which widget overlaps the other. If
container widget is below the
child widget in the Z-axis (will be painted before).
invert is true, the
container widget to be on top of the
child widget, in the Z-axis (will
be painted after).
Using AlignPositioned inside a Stack
Stack positions its children relative to the edges of its box. The
contains this text:
In particular, when using a Stack you can't position children relative to their size or the stack's own size.
However, by using
AlignPositioned you can do precisely that:
position (and size, rotate and transform) children relative to their size or the Stack's own size,
and consequently in relation to the other widgets inside the Stack.
If you recall how a
Stack works, each of its child widgets is either positioned or non-positioned.
The stack sizes itself to contain all the non-positioned children, which are positioned according to
alignment parameter. Next, the positioned children are laid out.
If you use the
AlignPositioned default constructor and put it inside a Stack, it will be a **
To create a positioned widget, use the
AlignPositioned.expand() factory. The
will then expand and fix itself to the corners of the
Stack. In other words, the
Stack will size
itself to their other non-positioned widgets, and then you can use the
AlignPositioned to lay out
its child in relation to the
Stack( children: [ Container(...), Positioner(child: Container(...)), AlignPositioned(...), AlignPositioned.expand(...), ... ]);
If you change the
AnimatedAlignPositioned parameters it will animate automatically:
return AnimatedAlignPositioned( duration: Duration(seconds: 3) alignment: Alignment.bottomCenter, rotateDegrees: isOk ? 0 : 180, child: AnimatedContainer( color: isOk ? Colors.yellow : Colors.red, duration: Duration(seconds: 2) ), );
How to Chain Implicit Animation Widgets
AnimChain widget lets you define a sequence of widgets, where each one will be displayed after
the previous after some wait time.
You can chain widgets which are totally different from one another. For example:
AnimChain(repeat: true, initialDelay: Duration(milliseconds: 150)) // Yellow box for 700 milliseconds .next( wait: Duration(milliseconds: 700), widget: Container(color: Colors.yellow, width: 95, height: 95, margin: const EdgeInsets.all(2.5))) // Red text for 1000 milliseconds .next( wait: Duration(milliseconds: 1000), widget: Container(child: Text("Hello world!", style: TextStyle(color: Colors.red, fontSize: 25)), width: 100, height: 100)) // Blue icon for 1200 milliseconds .next( wait: Duration(milliseconds: 1200), widget: Icon(Icons.accessibility, color: Colors.blue, size: 100));
Try running the example.
However, if you define implicitly animated widgets
then you can easily create a smooth animation.
return AnimChain(repeat: true, initialDelay: Duration(milliseconds: 150)) // Show the yellow box and wait 5 seconds. .next( wait: Duration(seconds: 5), widget: AnimatedAlignPositioned( alignment: Alignment.bottomCenter, rotateDegrees: 0, child: Container(color: Colors.yellow), ), ) // Rotate to the red box in 3 seconds. .next( wait: Duration(seconds: 3), widget: AnimatedAlignPositioned( duration: Duration(seconds: 3), rotateDegrees: 180, child: Container(color: Colors.red), ), ) // Finally, translate the blue in the vertical axis. .next( widget: AnimatedAlignPositioned( duration: Duration(seconds: 15), alignment: Alignment.bottomCenter, dy: 150, rotateDegrees: 180, child: Container(color: Colors.blue), ), ), );
Try running the example.
The Flutter packages I've authored:
My Medium Articles:
- Async Redux: Flutter’s non-boilerplate version of Redux ( versions: Português)
- i18n_extension ( versions: Português)
- Flutter: The Advanced Layout Rule Even Beginners Must Know ( versions: русский)
- The New Way to create Themes in your Flutter App
My article in the official Flutter documentation: