Summary of Chapter 12. Styles
In Xamarin.Forms, styles allow multiple views to share a collection of property settings. This reduces markup and enables maintaining consistent visual themes.
Styles are almost always defined and consumed in markup. An object of type
Style is instantiated in a resource dictionary and then set to the
Style property of a visual element using a
DynamicResource markup extension.
The basic Style
Style requires that its
TargetType be set to the type of the visual object it applies to. When a
Style is instantiated in a resource dictionary (as is common) it also requires an
In XAML the
Property setting is the name of a CLR property (such as the
Text property of
Button) but the styled property must be backed by a bindable property. Also, the property must be defined in the class indicated by the
TargetType setting, or inherited by that class.
You can specify the
Value setting using the property element
<Setter.Value>. This lets you set
Value to an object that can't be expressed in a text string, or to an
OnPlatform object, or to an object instantiated using
Value property can also be set with a
StaticResource expression to another item in the dictionary.
The BasicStyle program demonstrates the basic syntax and shows how to reference the
Style with a
StaticResource markup extension:
Style object and any object created in the
Style object as a
Value setting are shared among all views referencing that
Style cannot contain anything that cannot be shared, such as a
Event handlers cannot be set in a
GestureRecognizers property cannot be set in a
Style because it is not backed by a bindable property.
Styles in code
Although it's not common, you can instantiate and initialize
Style objects in code. This is demonstrated by the BasicStyleCode sample.
Style has a
BasedOn property that you can set to a
StaticResource markup extension referencing another style. This allows styles to inherit from previous styles, and add or replace property settings. The StyleInheritance sample demonstrates this.
Style2 is based on
Style2 must be the same as
Style1 or derived from
Style1. The resource dictionary in which
Style1 is stored must be the same resource dictionary as
Style2 or a resource dictionary higher in the visual tree.
Style in a resource dictionary does not have an
x:Key attribute setting, it is assigned a dictionary key automatically, and the
Style object becomes an implicit style. A view without a
Style setting and whose type matches the
TargetType exactly will find that style, as the ImplicitStyle sample demonstrates.
An implicit style can derive from a
Style with an
x:Key setting but not the other way around. You cannot explicitly reference an implicit style.
You can implement three types of hierarchy with styles and
- From styles defined on the
Pagedown to styles defined on layouts lower in the visual tree.
- From styles defined for base classes such as
Viewto styles defined for specific classes.
- From styles with explicit dictionary keys to implicit styles.
These hierarchies are demonstrated in the StyleHierarchy sample.
A style in a resource dictionary can be referenced by
DynamicResource rather than
StaticResource. This makes the style a dynamic style. If that style is replaced in the resource dictionary by another style with the same key, the views referencing that style with
DynamicResource automatically change. Also, the absence of a dictionary entry with the specified key will cause
StaticResource to raise an exception but not
You can use this technique to dynamically change styling or themes as the DynamicStyles sample demonstrates.
However, you cannot set the
BasedOn property to a
DynamicResource makeup extension because
BasedOn isn't backed by a bindable property. To derive a style dynamically, do not set
BasedOn. Instead, set the
BaseResourceKey property to the dictionary key of the style you want to derive from. The DynamicStylesInheritance sample demonstrates this technique.
Device.Styles nested class defines twelve static read-only fields for six styles with a
Label that you can use for common types of text usages.
Six of these fields are of type
Style that you can set directly to a
Style property in code:
The other six fields are of type
string and can be used as dictionary keys for dynamic styles:
BodyStyleKeyequal to "BodyStyle"
TitleStyleKeyequal to "TitleStyle"
SubtitleStyleKeyequal to "SubtitleStyle"
CaptionStyleKeyequal to "CaptionStyle"
ListItemTextStyleKeyequal to "ListItemTextStyle"
ListItemDetailTextStyleKeyequal to "ListItemDetailTextStyle"
These styles are illustrated by the DeviceStylesList sample.
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