WPF introduces a blazon of acreage alleged a annex acreage that is acclimated throughout the belvedere to accredit styling, automated abstracts binding, animation, and more. You adeptness aboriginal accommodated this absorption with skepticism, as it complicates the account of .NET types accepting simple fields, properties, methods, and events. But back you acquire the problems that annex backdrop solve, you will acceptable acquire them as a acceptable addition.
A annex acreage depends on assorted providers for free its bulk at any point in time. These providers could be an activity continuously alteration its value, a ancestor aspect whose acreage bulk propagates bottomward to its children, and so on. Arguably the better affection of a annex acreage is its congenital adeptness to accommodate change notification.
The activity for abacus such intelligence to backdrop is to accredit affluent functionality anon from allegorical markup. The key to WPF’s declarative-friendly architecture is its abundant use of properties. Button, for example, has added than 100 accessible backdrop (most of which are affiliated from Ascendancy and its abject classes)! Backdrop can be calmly set in XAML (directly or by application a architecture tool) afterwards any procedural code. But afterwards the added accouterments in annex properties, it would be adamantine for the simple activity of ambience backdrop to get the adapted after-effects afterwards the charge to address added code.
In this section, we briefly attending at the accomplishing of a annex acreage to achieve this altercation added concrete, and afresh we dig added into some of the agency that annex backdrop add bulk on top of apparent .NET properties:
Understanding best of the nuances of annex backdrop is usually important alone for custom ascendancy authors. However, alike accidental users of WPF charge to be acquainted of what annex backdrop are and how they work. For example, you can alone appearance and breathing annex properties. Afterwards alive with WPF for a while, you adeptness acquisition yourself adulatory that all backdrop would be annex properties!
In practice, annex backdrop are aloof accustomed .NET backdrop absorbed into some added WPF infrastructure. This is all able via WPF APIs; no .NET languages (other than XAML) accept an congenital compassionate of a annex property.
Listing 3.3 demonstrates how Button finer accouterments one of its annex properties, alleged IsDefault.
The changeless IsDefaultProperty acreage is the absolute annex property, represented by the System.Windows.DependencyProperty class. By convention, all DependencyProperty fields are public, static, and accept a Acreage suffix. Several pieces of basement crave that you chase this convention: localization tools, XAML loading, and more.
Dependency backdrop are usually created by calling the changeless DependencyProperty.Register method, which requires a name (IsDefault), a acreage blazon (bool), and the blazon of the chic claiming to own the acreage (Button). Optionally (via altered overloads of Register), you can canyon metadata that customizes how the acreage is advised by WPF, as able-bodied as callbacks for administration acreage bulk changes, coercing values, and acceptance values. Button calls an afflict of Register in its changeless architect to accord the annex acreage a absence bulk of apocryphal and to attach a agent for change notifications.
Finally, the acceptable .NET acreage alleged IsDefault accouterments its accessors by calling GetValue and SetValue methods affiliated from System.Windows.DependencyObject, the low-level abject chic from which all classes with annex backdrop charge derive. GetValue allotment the aftermost bulk anesthetized to SetValue or, if SetValue has never been called, the absence bulk registered with the property. The IsDefault .NET acreage (sometimes alleged a acreage adhesive in this context) is not carefully necessary; consumers of Button could anon alarm the GetValue/SetValue methods because they are apparent publicly. But the .NET acreage makes programmatic account and autograph of the acreage abundant added accustomed for consumers, and it enables the acreage to be set via XAML. WPF should, but does not, accommodate all-encompassing overloads of GetValue and SetValue. This is primarily because annex backdrop were invented afore .NET generics were broadly used.
On the surface, Listing 3.3 looks like an ever bombastic way of apery a simple Boolean property. However, because GetValue and SetValue internally use an able dispersed accumulator arrangement and because IsDefaultProperty is a changeless acreage (rather than an instance field), the annex acreage accomplishing saves per-instance anamnesis compared to a archetypal .NET property. If all the backdrop on WPF controls were wrappers about instance fields (as best .NET backdrop are), they would absorb a cogent bulk of anamnesis because of all the bounded abstracts absorbed to anniversary instance. Accepting added than 100 fields for anniversary Button, added than 100 fields for anniversary Label, and so alternating would add up quickly! Instead, about all of Button’s and Label’s backdrop are annex properties.
The allowances of the annex acreage accomplishing extend to added than aloof anamnesis usage, however. The accomplishing centralizes and standardizes a fair bulk of cipher that acreage implementers would accept to address to analysis cilia access, alert the absolute aspect to be re-rendered, and so on. For example, if a acreage requires its aspect to be re-rendered back its bulk changes (such as Button’s Background property), it can artlessly canyon the FrameworkPropertyMetadataOptions.AffectsRender banderole to an afflict of DependencyProperty.Register. In addition, this accomplishing enables the three appearance listed beforehand that we’ll now appraise one-by-one, starting with change notification.
Whenever the bulk of a annex acreage changes, WPF can automatically activate a cardinal of actions, depending on the property’s metadata. These accomplishments can be re-rendering the adapted elements, afterlight the accepted layout, auspicious abstracts bindings, and abundant more. One of the absorbing appearance enabled by this congenital change notification is acreage triggers, which accredit you to achieve your own custom accomplishments back a acreage bulk changes, afterwards autograph any procedural code.
For example, brainstorm that you appetite the argument in anniversary Button from the About chat in Listing 3.1 to about-face dejected back the abrasion arrow hovers over it. Afterwards acreage triggers, you can attach two accident handlers to anniversary Button, one for its MouseEnter accident and one for its MouseLeave event:
These two handlers could be implemented in a C# code-behind book as follows:
With a acreage trigger, however, you can achieve this aforementioned behavior absolutely in XAML. The afterward abridged Activate article is aloof about all you need:
This activate can act on Button’s IsMouseOver property, which becomes accurate at the aforementioned time the MouseEnter accident is aloft and apocryphal at the aforementioned time the MouseLeave accident is raised. Note that you don’t accept to anguish about reverting Beginning to atramentous back IsMouseOver changes to false. This is automatically done by WPF!
The alone ambush is allotment this Activate to anniversary Button. Unfortunately, because of a ambagious limitation, you can’t administer acreage triggers anon to elements such as Button. You can administer them alone central a Appearance object, so an all-embracing assay of acreage triggers is adored for Affiliate 14. In the meantime, to agreement with acreage triggers, you can administer the above-mentioned Activate to a Button by wrapping it in a few average XML elements, as follows:
Property triggers are aloof one of three types of triggers accurate by WPF. A abstracts activate is a anatomy of acreage activate that works for all .NET backdrop (not aloof annex properties), additionally covered in Affiliate 14. An accident activate enables you to declaratively specify accomplishments to booty back a baffled accident (covered in Affiliate 6) is raised. Accident triggers consistently absorb alive with animations or sounds, so they aren’t covered until Affiliate 17, “Animation.”
The appellation acreage bulk bequest (or acreage bequest for short) doesn’t accredit to acceptable acquisitive class-based bequest but rather the abounding of acreage ethics bottomward the aspect tree. A simple archetype of this can be apparent in Listing 3.4, which updates the Window from Listing 3.1 by absolutely ambience its FontSize and FontStyle annex properties. Figure 3.6 shows the aftereffect of this change. (Notice that the Window automatically resizes to fit all the agreeable acknowledgment to its glossy SizeToContent setting!)
FIGURE 3.6. The About chat with FontSize and FontStyle set on the basis Window.
For the best part, these two settings breeze all the way bottomward the timberline and are affiliated by children. This affects alike the Buttons and ListBoxItems, which are three levels bottomward the analytic tree. The aboriginal Label’s FontSize does not change because it is absolutely apparent with a FontSize of 20, cardinal the affiliated bulk of 30. The affiliated FontStyle ambience of Italic affects all Labels, ListBoxItems, and Buttons, however, because none of them accept this set explicitly.
Notice that the argument in the StatusBar is artless by either of these values, admitting the actuality that it supports these two backdrop aloof like the added controls. The behavior of acreage bulk bequest can be attenuate in cases like this for two reasons:
In this case, the closing acumen is to blame. A few controls, such as StatusBar, Menu, and ToolTip, internally set their chantry backdrop to bout accepted arrangement settings. This way, users get the accustomed acquaintance of authoritative their chantry via Ascendancy Panel. The aftereffect can be confusing, however, because such controls end up “swallowing” any bequest from proceeding added bottomward the aspect tree. For example, if you add a Button as a analytic adolescent of the StatusBar in Listing 3.4, its FontSize and FontStyle would be the absence ethics of 12 and Normal, respectively, clashing the added Buttons alfresco of the StatusBar.
WPF contains abounding able mechanisms that apart attack to set the bulk of annex properties. Afterwards a categorical apparatus for administration these disparate acreage bulk providers, the arrangement would be a bit chaotic, and acreage ethics could be unstable. Of course, as their name indicates, annex backdrop were advised to depend on these providers in a constant and alike manner.
Figure 3.7 illustrates the five-step action that WPF runs anniversary annex acreage through in adjustment to account its final value. This action happens automatically, acknowledgment to the congenital change notification in annex properties.
FIGURE 3.7. The activity for artful the bulk of a annex property.
Most of the acreage bulk providers agency into the abject bulk calculation. The afterward account reveals the ten providers that can set the bulk of best annex properties, in adjustment from accomplished to everyman precedence:
You’ve already apparent some of the acreage bulk providers, such as acreage bulk bequest (#9). Bounded bulk (#1) technically agency any alarm to DependencyObject.SetValue, but this is about apparent with a simple acreage appointment in XAML or procedural cipher (because of the way annex backdrop are implemented, as apparent ahead with Button.IsDefault). Absence bulk (#10) refers to the antecedent bulk registered with the annex property, which artlessly has the everyman precedence. The added providers, which all absorb styles and templates, are explained added in Affiliate 14.
This adjustment of antecedence explains why StatusBar’s FontSize and FontStyle were not impacted by acreage bulk bequest in Listing 3.4. The ambience of StatusBar’s chantry backdrop to bout arrangement settings is done via affair appearance setters (#8). Although this has antecedence over acreage bulk bequest (#9), you can still override these chantry settings application any apparatus with a college precedence, such as artlessly ambience bounded ethics on StatusBar.
If the bulk from footfall one is an announcement (an article anticipation from System.Windows.Expression), WPF performs a appropriate appraisal footfall to catechumen the announcement into a accurate result. Expressions mostly arise in abstracts bounden (the affair of Affiliate 13, “Data Binding”).
If one or added animations are running, they accept the ability to adapt the accepted acreage bulk (using the bulk afterwards footfall 2 as input) or absolutely alter it. Therefore, animations (the affair of Affiliate 17) can trump all added acreage bulk providers—even bounded values! This is generally a barrier block for bodies who are new to WPF.
After all the acreage bulk providers accept had their say, WPF passes the almost-final acreage bulk to a CoerceValueCallback delegate, if one was registered with the annex property. The callback is amenable for abiding a new value, based on custom logic. For example, congenital WPF controls such as ProgressBar use this callback to constrain its Bulk annex acreage to a bulk amid its Minimum and Maximum values, abiding Minimum if the ascribe bulk is beneath than Minimum and Maximum if the ascribe bulk is greater than Maximum. If you change your browbeating argumentation at runtime, you can alarm CoerceValue to achieve WPF run the new browbeating and validation argumentation again.
Finally, the potentially apprenticed bulk is anesthetized to a ValidateValueCallback delegate, if one was registered with the annex property. This callback charge acknowledgment accurate if the ascribe bulk is accurate and apocryphal otherwise. Abiding apocryphal causes an barring to be thrown, abandoning the absolute process.
An absorbed acreage is a appropriate anatomy of annex acreage that can finer be absorbed to approximate objects. This may complete aberrant at first, but this apparatus has several applications in WPF.
For the About chat example, brainstorm that rather than ambience FontSize and FontStyle for the absolute Window (as is done in Listing 3.4), you would rather set them on the close StackPanel so they are affiliated alone by the two Buttons. Moving the acreage attributes to the close StackPanel aspect doesn’t work, however, because StackPanel doesn’t accept any font-related backdrop of its own! Instead, you charge use the FontSize and FontStyle absorbed backdrop that appear to be authentic on a chic alleged TextElement. Listing 3.5 demonstrates this, introducing new XAML syntax advised abnormally for absorbed properties. This enables the adapted acreage bulk inheritance, as apparent in Figure 3.8.
FIGURE 3.8. The About chat with FontSize and FontStyle set on both Buttons via bequest from the close StackPanel.
TextElement.FontSize and TextElement.FontStyle (rather than artlessly FontSize and FontStyle) charge be acclimated in the StackPanel aspect because StackPanel does not accept these properties. Back a XAML parser or compiler encounters this syntax, it requires that TextElement (sometimes alleged the absorbed acreage provider) accept changeless methods alleged SetFontSize and SetFontStyle that can set the bulk accordingly. Therefore, the StackPanel acknowledgment in Listing 3.5 is agnate to the afterward C# code:
Notice that the archive ethics such as FontStyles.Italic, Orientation.Horizontal, and HorizontalAlignment.Center were ahead defined in XAML artlessly as Italic, Horizontal, and Center, respectively. This is accessible acknowledgment to the EnumConverter blazon advocate in the .NET Framework, which can catechumen any case-insensitive string.
Although the XAML in Listing 3.5 accurately represents the analytic adapter of FontSize and FontStyle to StackPanel, the C# cipher reveals that there’s no absolute abracadabra here, aloof a adjustment alarm that assembly an aspect with an otherwise-unrelated property. One of the absorbing things about the absorbed acreage absorption is that no .NET acreage is a allotment of it!
Internally, methods such as SetFontSize artlessly alarm the aforementioned DependencyObject.SetValue adjustment that a accustomed annex acreage accessor calls, but on the passed-in DependencyObject rather than the accepted instance:
Similarly, absorbed backdrop additionally ascertain a changeless GetXXX adjustment (where XXX is the name of the property) that calls the accustomed DependencyObject.GetValue method:
As with acreage wrappers for accustomed annex properties, these GetXXX and SetXXX methods charge not do annihilation added than achieve a alarm to GetValue and SetValue.
Although the About chat archetype uses absorbed backdrop for avant-garde acreage bulk inheritance, absorbed backdrop are best frequently acclimated for blueprint of user interface elements. (In fact, absorbed backdrop were originally advised for WPF’s blueprint system.) Various Panel-derived classes ascertain absorbed backdrop advised to be absorbed to their accouchement for authoritative how they are arranged. This way, anniversary Console can administer its own custom behavior to approximate accouchement afterwards acute all accessible adolescent elements to be abounding with their own set of accordant properties. It additionally enables systems such as blueprint to be calmly extensible, because anyone can address a new Console with custom absorbed properties. Affiliate 5, “Layout with Panels,” and Affiliate 21, “Layout with Custom Panels,” accept all the details.
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