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NativeScript Core

Layout Process

NativeScript provides a recursive layout system that sizes and positions views on the screen. Layout is the process of measuring and positioning of Layout containers and their child views. Layout is an intensive process whose speed and performance depend on the count of the children and the complexity of the layout container. For example, a simple layout container such as AbsoluteLayout might perform better than a more complex layout container, such as GridLayout.

Layout completes in two passes—a measure pass and a layout pass. To this end, each View provides a measure and layout methods. Additionally, each layout container provides its own onMeasure and onLayout to achieve its own specific layout.

Measure Pass

During the measure pass, each View is measured to retrieve its desired size. The measure pass evaluates the following properties:

  • width
  • height
  • minWidth
  • minHeight
  • visibility
  • marginTop
  • marginRight
  • marginBottom
  • marginLeft

Layout Pass

During the layout pass, each View is placed in a specific layout slot. This slot is determined by the desired size of the view (retrieved from the measure pass) and the following properties:

  • marginTop
  • marginRight
  • marginBottom
  • marginLeft
  • horizontalAlignment
  • verticalAlignment

Layout Properties

Margins

The four margin properties (marginTop, marginRight, marginBottom and marginLeft) describe the distance between a view and its parent.

When you set margins through XML, you can choose between the following approaches.

  • Set one value: Provide a single value that will be applied on all sides of the view.
  • Set two values: Provide two values. The first value is applied to the top side, the second value is applied to the right side. Next, the first value is applied to the bottom and the second value to the left side (in that order).
  • Set four values: Provide four values for each margin. The first value is applied to the top, the second value is applied to the right, the third value is applied to the bottom and the fourth value is applied to the left side (in that order).

Paddings

The four padding properties (paddingTop, paddingRight, paddingBottom and paddingLeft) describe the distance between the layout container and its children.

When you set paddings through XML, you can choose between the following approaches.

  • Set one value: Provide a single value that will be applied on all sides of the view.
  • Set two values: Provide two values. The first value is applied to the top side, the second value is applied to the right side. Next, the first value is applied to the bottom and the second value to the left side (in that order).
  • Set four values: Provide four values for each padding. The first value is applied to the top, the second value is applied to the right, the third value is applied to the bottom and the fourth value is applied to the left side (in that order).

Alignments

Layout applies horizontal and vertical alignment only when an element is allocated more size than it needs.

The following table shows the valid values of horizontalAlignment.

Member Description
left The view is aligned to the left of the layout slot of the parent element.
center The view is aligned to the center of the layout slot of the parent element.
right The view is aligned to the right of the layout slot of the parent element.
stretch The view is stretched to fill the layout slot of the parent element; width takes precedence, if set.

The following table shows the valid values of verticalAlignment.

Member Description
top The view is aligned to the top of the layout slot of the parent element.
center The view is aligned to the center of the layout slot of the parent element.
bottom The view is aligned to the bottom of the layout slot of the parent element.
stretch The view is stretched to fill the layout slot of the parent element; height takes precedence, if set.

Percentage Support

NativeScript supports percentage values for width, height and margin. When a layout pass begins, first the percent values are calculated based on parent available size. This means that on vertical StackLayout if you place two Buttons with height='50%' they will get all the available height (e.g., they will fill the StackLayout vertically.). The same applies for margin properties. For example, if you set marginLeft='5%', the element will have a margin that corresponds to 5% of the parent's available width.

iOS Safe Area Support

The iOS Safe Area is a term that Apple introduced in iOS 11. It is the area of the screen that is free to use and won’t be obstructed by hardware and software parts of the system. The safe area is not a constant. It is affected by the notch, the rounded corners of the screen, the status bar and the home indicator, but also from parts of your application like the action bar and the tab bar. To get a better understanding refer to the Apple docs.

Since version 5.0 NativeScript provides a default handling mechanism for the iOS Safe Area. The default behavior is that certain container View components (these that can have children) overflow the safe area and are laid out to the edges of the screen. These container components are:

  • Layouts
  • ListView
  • ScrollView
  • WebView
  • Repeater

Internally, the workflow is as follows:

  1. Measure pass - all components are measured in the safe area portion of the screen.
  2. Layout pass - all components are laid out in full screen, but are inset to the safe area boundaries.
  3. Layout pass - if the component borders the safe area, it is adjusted and expanded to the edges of the screen.

NOTE: The above workflow can lead to containers being laid out with a bigger size than initially declared in the markup. You can prevent this behavior by setting the iosOverflowSafeArea property below to false.

iosOverflowSafeArea Property

The above default behavior should provide good UX out of the box. Additionally, NativeScript 5.0 exposes a property iosOverflowSafeArea that can control how components handle the iOS Safe Area. Set this property value to true if you want the component to expand to the edges of the screen when it borders the safe area. Set it to false to explicitly prevent this behavior. The default value for container components is true. All other components are considered content that should be constrained to the safe area and default to false.

Layouts

LayoutBase is the base class for all views that provide positioning of child elements.

You can use the various layout containers to position elements. They evaluate the base properties of View such as width, height, minWidth and alignments, and expose additional specific properties for positioning child views.

Predefined Layouts

The following table shows predefined layouts that NativeScript provides.

Layouts Description Screenshot
[FlexboxLayout][FlexboxLayout] This layout is a non-conforming implementation of the CSS Flexible Box Layout FlexboxLayout android
[AbsoluteLayout][AbsoluteLayout] This layout lets you set exact locations (left/top coordinates) for its children. AbsoluteLayout android
[DockLayout][DockLayout] This layout arranges its children at its outer edges and allows its last child to take up the remaining space. DockLayout android
[GridLayout][GridLayout] This layout defines a rectangular layout area that consists of columns and rows. GridLayout android
[StackLayout][StackLayout] This layout arranges its children horizontally or vertically. The direction is set with the orientation property. StackLayout android
[WrapLayout][WrapLayout] This layout positions its children in rows or columns, based on the orientation property, until the space is filled and then wraps them on a new row or column. WrapLayout android