Dali 3D User Interface Engine
Size Negotiation

Overview

Size negotiation, also known as layout management, is responsible for allocating sizes to all actors on the stage based on rules of dependency between the actors. Requests for relayout on actors are collected during the frame with the actual relayout performed at the end of the frame.

This document details how to use the size negotiation API and is intended for application writters.

The topics covered are:

Dimensions

The notion of width and height is generalised into the concept of a Dimension. Several methods take a Dimension parameter.

The Dimension enum specifies the available dimensions as bitfields:

If a method can process width and height at the same time then the Dimension::ALL_DIMENSIONS mask can be specified.

Resize Policies

Policies

The ResizePolicy enum specifies a range of options for controlling the way actors resize. These are powerful rules that enable much automatic resizing behaviour. They are as following:

ResizePolicies.png

Actor

This section details how an actor may be used with size negotiation.

Enabling Size Negotiation

Text and image actors have relayout enabled by default, while a plain Actor is disabled unless a call to SetResizePolicy is made.

Specifying Size Policies

Actors have different size policies by default. For example ImageActor is set to USE_NATURAL_SIZE. This ensures that when an image actor is placed on the stage it will use its natural size by default. However if the user calls SetSize with non-zero sizes on the image actor then the current size policy is overridden by the FIXED size policy and the actor will take on the size specified.

The next step is to specify how an actor will be size negotiated. The resize policies for an actor may be specified by the following method:

void SetResizePolicy( ResizePolicy::Type policy, Dimension::Type dimension )

It is common to specifiy different policies for the different dimensions of width and height to achive different layouts. Different actors have different resize policies specified by default. For example ImageActors are set to use USE_NATURAL_SIZE.

The following example code snippet shows rootActor having its width policy set to ResizePolicy::FILL_TO_PARENT and its height policy set to ResizePolicy::FIT_TO_CHILDREN. It has an ImageActor added to it with an explicit call to USE_NATURAL_SIZE in both dimensions called on it. This will make an actor that will fill up the space of its parent in the width dimension and fit to its child in the height dimension. As the image actor child is using natural size the height of the root actor will fit to the height of the child image.

Actor rootActor = Actor::New();
rootActor.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
rootActor.SetResizePolicy( ResizePolicy::FIT_TO_CHILDREN, Dimension::HEIGHT );
ImageActor image = ImageActor::New( Image::New( MY_IMAGE_PATH ) );
rootActor.Add( image );

The following images show the before and after layouts for this code example.

Before:

SizeNegotiationExample_Before.png

After:

SizeNegotiationExample_After.png

This example shows an actor rootActor set to expand to its parent's width and contract/expand around its child's height. The child image actor is set to natural size which means it will display at the acutal size of the image.

To specify that a dimension has a dependency on another dimension use ResizePolicy::DIMENSION_DEPENDENCY. For example if dimension is Dimension::HEIGHT and dependency is Dimension::WIDTH then there is a height-for-width dependency in effect. The classic use case for this is a text view that wraps its text. The following example snippet shows a text view that expands its width to the size of its parent, wraps its contents and then determines its height based on the width.

TextLabel text = TextLabel::New( "Example" );
text.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
text.SetResizePolicy( ResizePolicy::DIMENSION_DEPENDENCY, Dimension::HEIGHT );

Specifying Sizes and Size Limits

When wanting a specific fixed size for an actor then specify the resize policy to be FIXED and set the desired, or preferred size using SetSize. If only one dimension is FIXED then the other value in the size parameter will be ignored, so it is safe to set it to zero.

To constrain the final negotiated size of an actor, set the following for minimum and maximum sizes respectively.

void SetMinimumSize( const Vector2& size )
void SetMaximumSize( const Vector2& size )

Altering Negotiated Size

When an actor is required to maintain the aspect ratio of its natural size the following method can be used. This is useful for size negotiating images to ensure they maintain their aspect ratio while still fitting within the bounds they have been allocated. This can be one of SizeScalePolicy::USE_SIZE_SET, SizeScalePolicy::FIT_WITH_ASPECT_RATIO or SizeScalePolicy::FILL_WITH_ASPECT_RATIO. The first is the default. The second will fit the actor within the bounds it has been allocated while maintaining aspect ratio. The third will fill all available space, potentially overflowing its bounds, while maintaining apsect ratio.

void SetSizeScalePolicy( SizeScalePolicy::Type policy )

Using Actors in Containers

When laying out actors in containers such as TableView it is useful to be able to specify padding that surrounds the actor. E.g. You may want some white space around an image actor placed in a table cell. The padding specifies the left, right, bottom and top padding values.

void SetPadding( const Padding& padding )

An Example

This section shows a more complex example of how to configure size negotiation. It creates a popup and adds a table view to it with a text view, an image and a sub-table. The sub-table contains a checkbox and another text view.

mPopup = CreatePopup();
mPopup.SetTitle( "Warning" );
// Content
Toolkit::TableView content = Toolkit::TableView::New( 2, 2 );
content.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
content.SetResizePolicy( ResizePolicy::USE_NATURAL_SIZE, Dimension::HEIGHT );
content.SetFitHeight( 0 );
content.SetFitHeight( 1 );
content.SetPadding( Padding( 20.0f, 20.0f, 20.0f, 0.0f ) );
// Text
Toolkit::TextLabel text = Toolkit::TextLabel::New( "Do you really want to quit?" );
text.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
text.SetResizePolicy( ResizePolicy::DIMENSION_DEPENDENCY, Dimension::HEIGHT );
content.AddChild( text, Toolkit::TableView::CellPosition( 0, 0 ) );
// Image
ImageActor image = ImageActor::New( ResourceImage::New( IMAGE1 ) );
image.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
image.SetResizePolicy( ResizePolicy::DIMENSION_DEPENDENCY, Dimension::HEIGHT );
image.SetPadding( Padding( 20.0f, 0.0f, 0.0f, 0.0f ) );
content.AddChild( image, Toolkit::TableView::CellPosition( 0, 1 ) );
// Checkbox and text
Toolkit::TableView root = Toolkit::TableView::New( 1, 2 );
root.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
root.SetResizePolicy( ResizePolicy::USE_NATURAL_SIZE, Dimension::HEIGHT );
root.SetFitHeight( 0 );
root.SetFitWidth( 0 );
root.SetPadding( Padding( 0.0f, 0.0f, 0.0f, 20.0f ) );
Dali::Image unchecked = Dali::ResourceImage::New( CHECKBOX_UNCHECKED_IMAGE );
Dali::Image checked = Dali::ResourceImage::New( CHECKBOX_CHECKED_IMAGE );
Toolkit::CheckBoxButton checkBox = Toolkit::CheckBoxButton::New();
checkBox.SetBackgroundImage( unchecked );
checkBox.SetSelectedImage( checked );
checkBox.SetSize( Vector2( 48, 48 ) );
root.AddChild( checkBox, Toolkit::TableView::CellPosition( 0, 0 ) );
Toolkit::TextLabel text2 = Toolkit::TextLabel::New( "Don't show again" );
text2.SetPadding( Padding( 20.0f, 0.0f, 0.0f, 10.0f ) );
root.AddChild( text2, Toolkit::TableView::CellPosition( 0, 1 ) );
content.AddChild( root, Toolkit::TableView::CellPosition( 1, 0, 0, 2 ) ); // Column span 2
mPopup.Add( content );

The resulting popup with additional buttons added is shown below.

Popup.png

The key things to pick out from this example are the use of the size negotiation API. The content table view is set to ResizePolicy::FILL_TO_PARENT for its width and USE_NATURAL_SIZE for its height. This will result in the table view expanding its width to fit the available space in the popup while also expanding/contracting its height based on the size of the contents in its cells.

content.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
content.SetResizePolicy( ResizePolicy::USE_NATURAL_SIZE, Dimension::HEIGHT );

To add a little space around the left, right and bottom of the table view, some padding is added.

content.SetPadding( Padding( 20.0f, 20.0f, 20.0f, 0.0f ) );

The first text view has its width set to ResizePolicy::FILL_TO_PARENT and its height has a dimension dependency on its width. This will result in a text view that fills up its width to available space in the table cell and then then calculates its height based on its new width. The table view will then fit its height taking the height of the text view into account.

text.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
text.SetResizePolicy( ResizePolicy::DIMENSION_DEPENDENCY, Dimension::HEIGHT );

The image view performs a similar relayout. It fits its width to the size of the cell and calculates its height based on the new width. Some padding is added to the left of it as well to center it more.

image.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
image.SetResizePolicy( ResizePolicy::DIMENSION_DEPENDENCY, Dimension::HEIGHT );
image.SetPadding( Padding( 20.0f, 0.0f, 0.0f, 0.0f ) );

The sub table view is similar as well in that it expands its width to the size of its cell. When it is added to the table view it will span two columns. Its height is set to natural size so that it will grow or shrink based on its children cells. Note that for a container like table view, USE_NATURAL_SIZE acts in a similar manner to ResizePolicy::FIT_TO_CHILDREN in that the size of the container could grow or shrink based on the sizes of the child actors.

root.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, Dimension::WIDTH );
root.SetResizePolicy( ResizePolicy::USE_NATURAL_SIZE, Dimension::HEIGHT );

The checkbox is set to have a fixed size.

checkBox.SetResizePolicy( ResizePolicy::FIXED, Dimension::ALL_DIMENSIONS );

The second text view has not specified a resize policy so will use its default of USE_NATURAL_SIZE.

Debugging

When constructing large scenes using interacting resize policies it is useful to be able to debug the relayout process. The following sections describe a pitfall to avoid when creating scenes and a way to print debug output about the actors.

Infinite Dependency Loops

Despite the power of the resize rules there is one pitfall to be aware of: infinite dependency loops. The most simplest form of this is shown by a parent actor with resize policy set to ResizePolicy::FIT_TO_CHILDREN with a child that has a resize policy of ResizePolicy::FILL_TO_PARENT. Who should determine the size in this case? A more complex loop occurs when ResizePolicy::DIMENSION_DEPENDENCY comes into play. Say a parent has a width policy of ResizePolicy::DIMENSION_DEPENDENCY with height and a height policy of ResizePolicy::FIT_TO_CHILDREN. The parent has a single child with a height policy ResizePolicy::DIMENSION_DEPENDENCY with width. If the child's width policy is ResizePolicy::FILL_TO_PARENT then a loop will occur. These are two simple examples but the loops could occur over larger spreads of parent child relationships. These loops are detected by the relayout algorithm with the result being that actors will receive zero sizes. These loops are not common but are still something to watch out for.

Inspecting Actor Relayout Properties

To get a print out of the stage hierarchy before and after negotiation, with a list of actors that were negotiated set the LOG_RELAYOUT_CONTROLLER environment variable to 3,true.

E.g. On desktop run:

$ LOG_RELAYOUT_CONTROLLER=3,true dali-demo

Example output from the logging is as follows:

PushButton, OKAY_BUTTON - Pos: [185, 0, 0.1] Size: [165, 76, 76], Dirty: (FALSE,FALSE), Negotiated: (TRUE,TRUE), Enabled: TRUE, (0x1649850)

The format is as follows:

[Actor type], [Actor name] ? Pos:[X, Y, Z] Size[Dimension::WIDTH, Dimension::HEIGHT, DEPTH], Dirty:(Dimension::WIDTH, Dimension::HEIGHT), Negotiated: (Dimension::WIDTH, Dimension::HEIGHT), Enabled: BOOLEAN, (Object address)

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