Dali 3D User Interface Engine
Size Negotiation for Controls


This document details how to create controls using the size negotiation API and is intended for UI control writters. For an introduction to size negotiation please see the Size Negotiation Programming Guide.

The topics covered are:

The Relayout Controller


The RelayoutController is an object that is private in DALi Core. It's main job is to take relayout requests from actors. It can be enabled or disabled internally. If disabled, then all relayout requests are ignored. By default the relayout controller is disabled until just after the initial application initialize. This allows the scene for an application to be created without generating many relayout requests. After the application has initialized the scene, then the relayout controller is automatically enabled and a relayout request is called on the root of the scene. This request spreads down the scene hierarchy and requests relayout on all actors that have size negotiation enabled.

Relayout requests are put in automatically when a property is changed on an actor or a change to the stage hierarchy is made and manual requests are usually not necessary.

Resize Policies

In addition to the resize policies detailed in the Size Negotiation Programming Guide there is one additional policy available to control writers:

Creating a Control: Popups


Size negotiation is enabled on controls by default. If a control is desired to not have size negotiation enabled then simply pass in the DISABLE_SIZE_NEGOTIATION flag into the Control constructor.

The other step to perform is to set default resize policies for width and height.

A Simple Example: Popup

This example shows how to set up a popup for use with size negotiation. The popup contains a layer to raise it above all other controls, a semi-transparent full-screen backing image to dim the screen, a background image with a shadow border, and buttons that are positioned and resized by the popup. The following screen shot shows an example popup.


The first step is to set the default resize policies. This is done in the OnInitialize method. In the following snippet the popup is set to have a height resize policy of ResizePolicy::FIT_TO_CHILDREN. This assumes that the width of the popup will be specified by the user of the popup and that the desired behaviour is to fit the height of the popup to the size of its content.

void Popup::OnInitialize()
Actor self = Self();

The popup will use a layer to place its content in. The layer is created and specified to fill the whole screen by using the following command.

A half transparent backing image is added to the layer and told to fill the layer with the following.

The popup control is added to the layer and a background image is specified to fill the size of the popup and add a border by the following.

Vector3 border( mPopupStyle->backgroundOuterBorder.x, mPopupStyle->backgroundOuterBorder.z, 0.0f );
mBackgroundImage.SetSizeModeFactor( border );

A table view is added to the popup to specify layout. It will fill to the width of the popup and expand/contract around its children cell heights.

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

Override the OnRelayout method to position and resize the buttons.

void Popup::OnRelayout( const Vector2& size, RelayoutContainer& container )

Another aspect to the popup is that depending which resize policies are active on it then the inner table view requires different resize policies itself. OnSetResizePolicy can be overridden to receive notice that the resize policy has changed on the control and action can be taken.

void Popup::OnSetResizePolicy( ResizePolicy::Type policy, Dimension::Type dimension )
// Make content fit to children
mPopupLayout.SetResizePolicy( ResizePolicy::USE_NATURAL_SIZE, dimension );
if( dimension & Dimension::HEIGHT )
mPopupLayout.SetFitHeight( 1 );
mPopupLayout.SetResizePolicy( ResizePolicy::FILL_TO_PARENT, dimension );
// Make the content cell fill the whole of the available space
if( dimension & Dimension::HEIGHT )
mPopupLayout.SetRelativeHeight( 1, 1.0f );

Popup also implements the following methods for use with the relevant resize policies:

Size Negotiation API

Base Class Methods

The base class methods are used to call functionality held in Actor and are defined in CustomActorImpl.

There is a RelayoutRequest method defined. This method is available for deriving controls to call when they would like themselves to be relaid out.

void RelayoutRequest()

Overridable Methods

These overridable methods in control provide customization points for the size negotiation algorithm.

Responding to the Change of Size on a Control

OnRelayout is called during the relayout process at the end of the frame immediately after the new size has been set on the actor. If the actor has calculated the size of child actors then add them to container with their desired size and set the ResizePolicy::USE_ASSIGNED_SIZE resize policy on them. At this point the size of the actor has been calculated so it is a good place to calculate positions of child actors etc.

virtual void OnRelayout( const Vector2& size, RelayoutContainer& container )

The OnRelayoutSignal signal is raised after SetSize and OnRelayout have been called during the relayout processing at the end of the frame. If the control is deriving from Control then the OnRelayout virtual is preferred over this signal. The signal is provided for instance when custom code needs to be run on the children of an actor that is not a control.

OnRelayoutSignalType& OnRelayoutSignal()

The OnCalculateRelayoutSize is called right before the size is calculated for an actor's dimension during the size negotiation phase. At this point all other actors this actor is dependent on have been negotiated so calculations depending on these actors can be performed before the size for this actor is calculated. Useful for container size calculations.

virtual void OnCalculateRelayoutSize( Dimension::Type dimension )

OnLayoutNegotiated is called right after the size in a given dimension has been negotiated for an actor. This allows calculations to be performed in response to the change in a given dimension but before OnRelayout is called.

virtual void OnLayoutNegotiated( float size, Dimension::Type dimension )

Calculating Sizes

Calculate the natural size for this control. This will be called when a control's resize policy is set to USE_NATURAL_SIZE. For example, TableView will calculated the size of the table given its various cell properties.

virtual Vector3 GetNaturalSize()

Given an input width return the correct height for this control. This will be called when the resize policy is set to ResizePolicy::DIMENSION_DEPENDENCY and height has a dependency on width.

virtual float GetHeightForWidth( float width )

Given the input height return the correct width for this control. This will be called when the resize policy is set to ResizePolicy::DIMENSION_DEPENDENCY and width has a dependency on height.

virtual float GetWidthForHeight( float height )

Relayout Dependencies

Return true from this method if this control is dependent on any of its children to calculate its own size. All relayout containers that can be dependent on their children for their own size need to return true from this.

virtual bool RelayoutDependentOnChildren( Dimension::Type dimension = Dimension::ALL_DIMENSIONS )

This will be called by children when they are using the ResizePolicy::FILL_TO_PARENT resize policy. It is the parent's responsibility to calculate the child's correct size.

virtual float CalculateChildSize( const Dali::Actor& child, Dimension::Type dimension )


OnSetResizePolicy is called when the resize policy is set on an actor. Allows deriving actors to respond to changes in resize policy.

virtual void OnSetResizePolicy( ResizePolicy::Type policy, Dimension::Type dimension )

Creating a Control: TableView

This section demonstrates how size negotiation may be used when creating a table view.

First we define some policies for how table row and columns may resize. These are:

A data structure is defined to hold information for each row and column regarding their cell size policy and their assigned and calculated sizes.

We need to be able to calculate the fixed sizes of all actors placed into table cells. The place to do this is in OnCalculateRelayoutSize. When this is called every actor the table view is dependent on has already had their sizes calculated. Calculations can be made that the main calculation for the actor can then use.

void TableView::OnCalculateRelayoutSize( Dimension::Type dimension )
if( dimension & Dimension::WIDTH )
CalculateFixedSizes( mColumnData, Dimension::WIDTH );
mFixedTotals.width = CalculateTotalFixedSize( mColumnData );
if( dimension & Dimension::HEIGHT )
CalculateFixedSizes( mRowData, Dimension::HEIGHT );
mFixedTotals.height = CalculateTotalFixedSize( mRowData );

An important override is GetNaturalSize. This will simply return the total sum of the fixed cells for each row and column.

Vector3 TableView::GetNaturalSize()
return Vector3( mFixedTotals.width, mFixedTotals.height, 1.0f );

When the time comes to calculate the size of each child in the table cells the following method will be called.

float TableView::CalculateChildSize( const Actor& child, Dimension::Type dimension )
// Use cell data to calculate child size

The table view is dependent on its children if its size policy is set to USE_NATURAL_SIZE or a row or column is set to "fit" an actor. The following code shows calling the base class RelayoutDependentOnChildren to check the resize policy and then searches for fit row or columns.

bool TableView::RelayoutDependentOnChildren( Dimension::Type dimension )
if ( Control::RelayoutDependentOnChildren( dimension ) )
return true;
return FindFit( mRowData ) || FindFit( mColumnData );

With the cell sizes already calculated, the job of OnRelayout is to position all the actors in the table view in their respective positions.

void TableView::OnRelayout( const Vector2& size, RelayoutContainer& container )
// Find each actor and position it, taking padding into account
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