Promptless AI is here soon - Production-ready contextual code. Don't just take our word for it. Know more
Know More

How to Build Responsive Apps using Constraints in Flutter?

No items found.

Nidhi Sorathiya

August 28, 2023

Nidhi Sorathiya

August 28, 2023

In our journey of app development with Flutter, understanding the layout mechanism becomes crucial. It is the basis of how we arrange our widgets on the screen and define our app's UI. One key concept in this layout mechanism is 'Constraints'. To efficiently layout our widgets in a Flutter app, we need to understand the concept of 'constraints in Flutter'.

The Basics of Constraints in Flutter

So, what exactly are these constraints? In Flutter, each widget is bound by some limitations in terms of Height and Width. These are referred to as ‘constraints’. While laying out the widgets, each parent widget gives its child widget a set of constraints to adhere to. Nevertheless, it is worth noting that Flutter's layout mechanism is vastly different from its HTML counterpart.

The Golden Rule of Constraints in Flutter

Demystifying Flutter constraints requires to imbibe a fundamental rule:

  • Constraints go down
  • Sizes go up
  • Parent sets positions

In other words, the parent widget passes down constraints to its child widgets. These child widgets then decide their sizes within their respective constraints and pass them back up to their parent. Lastly, the parent widget sets their children's positions on the screen.

This 'Flutter constraints' example demonstrates how the Container (parent) passes the constraints (width and height) to the Center (child). Here, the child widget is allowed to occupy a maximum width and height of 100 pixels, but it decides to fill lesser space considering its positioning.

Detailed Overview of Constraints in Flutter

Let's take a more in-depth look at how Flutter handles constraints. The process involves several steps:

Widget and its parent

Firstly, a widget asks its parent about the constraints it must adhere to. These constraints encompass a minimum and maximum width and height.

Widget and its children

In the next step, the widget iteratively passes these constraints to its children, potentially modifying them if necessary. Upon receiving the constraints, each child widget decides its own size.

Positioning of children

The parent widget then places its children one by one on the screen. While doing so, it respects the size preferences of the children and positions them both horizontally (along the x-axis) and vertically (along the y-axis).

Widget and its own size

In the final step, the parent widget expresses its size to its own parent, taking the original constraints into account.

In this 'Flutter constraints' example, the overall Container widget (parent) envelops two child Container widgets and sets the positioning of children after considering their sizes. The red box has a width and height of 100 each, while the green box settles for a width and height of 50 each. Both respect the overall constraints of width 200 and height 200 passed down by the parent widget.

Example Scenario to Understand Constraints Handling

To further quench our thirst for knowledge about 'constraints in Flutter', let’s employ examples. Visual learning always cements the understanding of concepts. So, let's walk through the negotiation that happens in a typical scenario where a widget wants to layout its two children.

  1. The widget initiates the negotiation by asking the parent about the constraints.
  2. The parent defines a minimum and maximum width and height for the widget.
  3. Widget defining constraints for its children and ordering their sizes.
  4. Widget positioning its children and ultimately informing the parent about its chosen size and position.

In this 'Flutter BoxConstraints example', the ConstrainedBox (parent widget) defines a set of constraints for its child Container. The ConstrainedBox tells the Container that it must have a width and height of anywhere between 70 and 150 pixels. It cannot go beyond these limits. Despite the Container desiring to be 1000 pixels in width and height, it must respect the constraints and settle for the maximum permissible size of 150 pixels each.

Limitations of Flutter’s Layout Engine

While Flutter's layout engine is efficient, a few limitations result from its one-pass layout process.

  1. Widget size: A widget can decide its own size but within the constraints given to it by its parent. This implies a widget usually can't have any size it desires.
  2. Widget position: A widget doesn’t have knowledge of its position on the screen. It is the parent widget's duty to define the position of the widget.
  3. Dependency on parent's size and position: The parent widget's size and position depend on its own parent. Hence, it's impractical to define the precise size and position of any widget without considering the whole tree.
  4. Child's size might be ignored: If a child desires a different size from its parent and the parent lacks the information to align it, the child’s size might be ignored.

Understanding Flutter Boxes and Their Types

In Flutter, every widget is rendered by underlying RenderBox objects. These boxes dictate how the widget handles constraints and aligns its children. We generally encounter three kinds of boxes when dealing with constraints:

Boxes that try to be as big as possible

These boxes try to occupy as much space as they can. Center and ListView widgets are prime examples of this kind.

Boxes that try to be the same size as their children

Such boxes size themselves to be as large as their children. Transform and Opacity widgets follow this behavior.

Boxes that try to be a particular size

These boxes aim to maintain a specific size, like Image and Text widgets. They do not adjust their size based on the child widget or parent widget.

Exploring Widgets in terms of Constraints

It is critical to understand how different widgets handle constraints in Flutter. Widgets like Container, Row, and Column have specific behaviors and rules for managing constraints.

Container Widget

Let's take the Container widget, one of the most commonly used widgets in Flutter. By default, it tries to be as big as possible. However, if you provide it width or height, it tries to honor that and be that specific size.

In this 'Flutter BoxConstraints example', the Container widget is constrained to be 200 pixels wide and 200 pixels high. It doesn't matter how much space is available to the widget; it will stick to the provided size unless constraints from its parent object force it to update.

Other Widgets

While Row and Column widgets, also known as flex boxes, have their behavior determined based on the constraints they receive. They behave differently depending on whether the constraints are bounded or unbounded, which we will explore more in later sections.

Constraints: Tight vs. Loose

Flutter constraints can be broadly categorized into two types based on the flexibility they offer - Tight and Loose constraints.

Tight Constraints

Tight constraints are essentially bindings that provide one exact size to follow. They have their minimum width equal to the maximum width, and the minimum height equal to the maximum height. For instance, an App widget, which is encapsulated by the RenderView class, is given a constraint to exactly fill the application's content area, typically the whole screen.

In this 'Flutter BoxConstraints example', BoxConstraints.tight provides a constraint that has an exact size of 100 pixels in width and height, contributing towards tight constraints.

Loose Constraints

On the other hand, Loose constraints have a minimum of zero and a maximum non-zero. These are the constraints that a parent widget provides to its child when it allows the child to decide its size. For instance, the Center allows its child to be any size it wants, up to the screen size. It imposes only the maximum width and the maximum height constraints, which too equal to the parent size.

In this 'Flutter BoxConstraints example', the Center widget allows the Container to define its own width and height within the loose constraints.

Understanding Unbounded Constraints

While dealing with constraints in Flutter, another type that comes into play is unbounded or infinite constraints. These come into effect when either the maximum width or the maximum height is set to double.infinity.

Infinite Unbounded Constraints

A certain category of widgets that attempt to be as large as possible, wouldn't be able to function well given an unbounded constraint, and in debug mode, it throws an exception. The most common scenario where you encounter unbounded constraint is within flex boxes, such as Row or Column, and within a scrollable regime, like ListView and other ScrollView subclasses.

Consider this Flutter box constraints example:

In this 'Flutter BoxConstraints example', the Container widget attempts to take up infinite width, leading to an unbounded width constraint.

Handling Unbounded Constraints

It's crucial to effectively manage unbounded constraints to avoid "overflow warning". For instance, when a child widget fails to comply with the constraints from its parent, the parent widget can choose to wrap it with a Flexible widget. This flexibility solves the overflow issues by adjusting the size of the child widget to fit within the available space.

In order to develop robust and responsive layouts, it's vital to understand and handle unbounded constraints effectively.

Flexibility in Flutter

Flexibility in Flutter

Flexibility plays a key role in managing space distribution among children along the main axis. In the world of 'constraints in Flutter', Row and Column widgets reflect this concept, known as flex boxes.

A flex box behaves differently based on whether its constraint is bounded or unbounded along the main axis. In bounded constraints, it tries to be as spacious as possible while in unbounded constraints, it sizes itself to fit its children.

Furthermore, a flex box's cross direction (width for Column and height for Row) must never be unbounded, as it doesn’t make sense for a flex box to have unbounded constraints, because it wouldn't know how to size and align its children along that axis.

Let's demonstrate how flex affects 'Flutter constraints' with this final example:

In this 'Flutter constraints' example, the Row widget has two child Container widgets. The Expanded widget wraps both containers with a given flex factor. This factor determines how much space they should occupy in the Row. The red container takes up more space than the blue one as it has a larger flex factor.

Concluding Thoughts

Through this deep exploration, we've decoded the concept of 'constraints in Flutter'. We began with understanding the basic philosophy behind Flutter constraints, surveying their influence on widgets, and their categories: tight, loose, and unbounded. We dived into examples to understand how different widgets handle these constraints and uncovered the pivotal role of the flex property in managing space distribution among widgets inside a flex container (Row or Column).

This understanding forms the foundation to handle the alignment, sizing, and positioning of widgets in complex Flutter layouts efficiently. As we develop our Flutter applications, keeping these constraints principles in mind will ensure a responsive and crisp UI/UX!

Frequently asked questions

How do you set constraints in Flutter?

Constraints are typically set by the parent widgets in Flutter. For example, you can use widgets like Container, ConstrainedBox, or SizedBox where you define the width and height properties to set the constraints for the child widget.

What is the difference between ConstrainedBox and LimitedBox?

The ConstrainedBox allows you to directly control the constraints of a child while a LimitedBox limits its size only if it's unconstrained. If the LimitedBox is given specific constraints, it passes them to the child and doesn't impose limits.

How do you set maximum width in Flutter?

Maximum width can be set using several widgets such as Container, SizedBox, and ConstrainedBox. You can define the width property in a Container or SizedBox, or set constraints: BoxConstraints(maxWidth: value) in a ConstrainedBox

What is a FlutterBox?

In Flutter, every widget is rendered by an underlying RenderBox object which measures its layout. FlutterBox is not a specific class, instead it refers to the boxes used by Flutter during the layout phase.

How do you use box constraints?

You can use box constraints within widgets such as ConstrainedBox, where you can set minimum and maximum widths and heights for the child widget. Example:

Frequently asked questions

No items found.