align_positioned #

Why is this widget an indispensable tool?

When your desired layout feels too complex for Columns and Rows, AlignPositioned widget is a real life saver. Flutter is very composable, which is good, but sometimes it's unnecessarily complex to translate some layout requirement into a composition of simpler widgets.

The AlignPositioned aligns, positions and optionally sizes its child in relation to both the container and the child itself. In other words, it lets you easily and declaratively define the position and size of some widget 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. Do you even know how to start doing this by composing basic Flutter widgets? Maybe, but with AlignPositioned it's much easier, and it takes a single widget.

The AlignPositioned widget is specially helpful for animations, since you can just calculate the final position and size you want for each frame. Without it you may find yourself having to animate a composition of widgets.

How it works #

Add align_positioned as a dependency in your pubspec.yaml file, then import it:

import 'package:align_positioned/align_positioned.dart';

Pass the AlignPositioned a child, and then one or more of the following parameters:

AlignPositioned(
   child: child,           
   alignment: ...,
   dx: ...,
   dy: ...,
   moveByChildWidth: ...,
   moveByChildHeight: ...,
   moveByContainerWidth: ...,
   moveByContainerHeight: ...,
   childWidth: ...,
   childHeight: ...,
   minChildWidth: ...,
   minChildHeight: ...,
   maxChildWidth: ...,
   maxChildHeight: ...,
   childWidthRatio: ...,
   childHeightRatio: ...,
   minChildWidthRatio: ...,
   minChildHeightRatio: ...,
   maxChildWidthRatio: ...,
   maxChildHeightRatio: ...,
   wins: ...,
   touch: ...,       
   ); 

Let's study each parameter in detail:

Align and Position parameters #

The alignment parameter works as expected. For example, Alignment.bottomRight represents the bottom right of the container, and Alignment(0.0, 0.0) 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.

If touch is Touch.inside, then alignment works just like the alignment for the Align widget, aligning the child inside of the container.

However, if touch is Touch.outside, then the alignment happens outside of the container.

As another example, if touch is Touch.inside, then 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).

But if touch is Touch.outside, then 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).

Parameters dx and dy can be positive or negative, and move the child horizontally and vertically, in pixels.

Parameters moveByChildWidth and 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.

Parameters moveByContainerWidth and moveByContainerHeight can be positive or negative, and move the child horizontally and vertically, but the unit here is not pixels, but container widths and heights.

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:

[alt text]

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:

[alt text]

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, check the example tab for the effects seen below:

[alt text]

Size Parameters #

Optionally, you can also define the child size:

• childWidth is the child width, in pixels.

• childHeight is the child height, in pixels.

• minChildWidth is the minimum width, in pixels. It has precedence over childWidth.

• minChildHeight is the minimum height, in pixels. It has precedence over childHeight.

• maxChildWidth is the maximum width, in pixels. It has precedence over childWidth.

• maxChildHeight is the maximum height, in pixels. It has precedence over childHeight.

• childWidthRatio is 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. You cannot define both childWidthRatio and childWidth at the same time.

• childHeightRatio is 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. You cannot define both childHeightRatio and childHeight at the same time.

• minChildWidthRatio is the minimum child width, as a fraction of the container width. It has precedence over childWidth. If both minChildWidth and minChildWidthRatio are 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 minChildHeight and minChildHeightRatio are defined, both will be applied (the minimum will be the larger one).

• maxChildWidthRatio is the maximum child width, as a fraction of the container width. It has precedence over childWidth. If both maxChildWidth and maxChildWidthRatio are defined, both will be applied (the maximum will be the smaller one).

• maxChildHeightRatio is the maximum child height, as a fraction of the container height. It has precedence over childHeight. If both maxChildHeight and maxChildHeightRatio are defined, both will be applied (the maximum will be the smaller one).

• wins decides what happens if the minimum size is larger then the maximum size. If wins is Wins.min, the default, the minimum size will be used.
  If wins is Wins.max, the maximum size will be used.

Using AlignPositioned inside of a Stack #

The Stack documentation 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) children relative to their size or the stack's own size.

To put an AlignPositioned inside of a Stack you must use AlignPositioned.expand(). The AlignPositioned will then expand and fix itself to the corners of the Stack. The Stack will size itself to their other non-positioned widgets, and then you can use the AlignPositioned to position its child in relation to the Stack.

Example:

Stack(
  children: [
    AlignPositioned.expand(child:..., childWidthRatio:...),
    ...
    ]);

You can put as many AlignPositioned inside of a Stack as you want.

Other Flutter packages I've authored:

• async_redux
• network_to_file_image
• back_button_interceptor
• indexed_list_view
• animated_size_and_fade