Reflective discovery of styleable properties
Nir Lisker
nlisker at gmail.com
Wed Dec 6 10:37:22 UTC 2023
I thought about the option of reflection, but I opted to propose
annotations instead. The following is my reasoning.
Firstly, reflection is very magic-y. The author of the class has no
indication of what side effects happen due to the code they write, the
output (css handling in this case) comes out of nowhere from their
perspective. As with other reflection cases, it is a "pull" rather than
"push" approach - you don't write what should happen, you let someone else
decide that. For writers of skin/control classes, this means that they need
to know exactly what constitutes a hook for the reflection mechanism, or
face surprises. There is no compile time check that tells you whether you
have declared your styleable property properly or not (without an external
ad-hoc checker).
We do this somewhat with properties - any method of the form
"...property()" gets special treatment, but this is for the docs. I don't
think we have code that depends on this other than in tests.
Secondly, the proposed mechanism depends on the runtime type, not the
declared type. As a user, I see no indication in the API whether a property
is styleable or not. This is also (what I would consider) a problem with
the current state. When I thought about using reflection to solve this, I
at least thought to specify the declared type of the property as styleable,
like StyleableBooleanProperty instead of BooleanProperty (restricting the
returned type is backwards compatible). A downside of this is that it gives
access to the methods of StyleableProperty, which are not useful for the
user, I think, but maybe someone has a use for them.
Thirdly, maybe I want to declare a styleable property not to be used
automatically. I can't think off the top of my head when I would want to do
that, but I'm also not a heavy css user. Are we sure that just initializing
a property with a styleable runtime type should *always* be caught by this
process?
To compare, annotations have the following benefits:
Firstly, they are declarative, which means no surprises for the class
author (WYSIWYG). This also allows more flexibility/control over which
properties get special treatment via an opt-in mechanism.
Secondly, They can also be displayed in the JavaDocs (via @Documented) with
their assigned values. For example, the padding property of Region can be
annotated with @Styleable(property="-fx-padding"), informing the user both
that this value can be set by css, and how to do it. Interestingly, the
annotation doesn't need to be public API to be displayed, so we are not
bound by contracts.
In terms of similarities:
In both the reflection and the annotation proposals, the steps are:
1. Create styleable properties.
2. That's it.
It's just that step 1 also adds an annotation to the creation of the
property (which was/is a 2-step process anyway, declaring the property and
its css metadata).
Annotations also require a processor to read the data from their values and
target (the field/method). This is a bit of work, but
Michael's CssMetaDataCache class is basically that - read the data from the
class (via reflection or annotations) and store it in a map. The logic
should be the same, just the method to obtain the data is different. Both,
as a result, have the benefits of handling control/skin combinations (what
I mentioned in the point "Usable both in controls and in skins (or other
classes)").
The benefit of co-locating the property and its css metadata in the class
itself also remains.
To summarize, both approaches eliminate all the clutter of writing
styleable properties (John, will you like to create styleable properties
now? [1] :) ), both apply the flexibility of caching per class, both allow
better structuring of the class, but they read the properties differently
and have a different level of declarativness.
[1] https://mail.openjdk.org/pipermail/openjfx-dev/2023-December/044010.html
On Tue, Dec 5, 2023 at 11:21 PM Andy Goryachev <andy.goryachev at oracle.com>
wrote:
> I like the idea.
>
>
>
> I wonder if it is possible to reduce the amount of boilerplate code? For
> example, a CssMetaData can have a
>
>
>
> setGetter(Function<S, StyleableProperty<V>> getter)
>
>
>
> method which supplies the property reference? This way
> CssMetaData.isSettable(Node) and CssMetaData.getStyleableProperty(Node) can
> be implemented in the base class (there are more complicated cases, so
> perhaps setIsSettable(Predicate<Node>) would also be required).
>
>
>
> Example:
>
>
>
> CssMetaData.<ControlExample,Font>of("-fx-font", Font.getDefault(), (n) ->
> n.font)
>
>
>
> Just a thought. What do you think?
>
>
>
> -andy
>
>
>
>
>
> *From: *openjfx-dev <openjfx-dev-retn at openjdk.org> on behalf of Michael
> Strauß <michaelstrau2 at gmail.com>
> *Date: *Sunday, December 3, 2023 at 22:02
> *To: *openjfx-dev <openjfx-dev at openjdk.org>
> *Subject: *Reflective discovery of styleable properties
>
> Following up the discussion around the CssMetaData API, I'd like to
> chime in with yet another idea. To recap, here's Nir's summary of the
> current API [0]:
>
> "Let's look at what implementation is required from a user who wants
> to write their own styleable control:
> 1. Create styleable properties.
> 2. Create a list of these properties to be passed on.
> 3. Create a public static method that returns the concatenation of
> this list with the one of its parent. (This method happens to be
> poorly documented, as mstr said.)
> 4. Create a public non-static method that calls the static method in a
> forced-override pattern because otherwise you will be calling the
> wrong static method. (This method's docs seem to be just wrong because
> you don't always want to delegate to Node's list.)"
>
>
> I think this could reasonably be replaced with the following
> implementation requirements:
> 1. Create styleable properties.
> 2. That's it.
>
> Let's look at what we're actually trying to do: create a list of
> CSS-styleable property metadata of a class. But we can easily do that
> without all of the boilerplate code.
>
> When ´Node.getCssMetaData()` is invoked, all public methods of the
> class are reflectively enumerated, and metadata is retrieved from
> `Property` and `StyleableProperty` getters. This is a price that's
> only paid once for any particular class (i.e. not for every instance).
> The resulting metadata list is cached and reused for all instances of
> that particular class.
>
> As a further optimization, metadata lists are also cached and
> deduplicated for Control/Skin combinations (currently every Control
> instance has its own copy of the metadata list).
>
> Another benefit of this approach is that the CssMetaData can now be
> co-located with the property implementation, and not be kept around in
> other parts of the source code file. Here's how that looks like when a
> new "myValue" property is added to MyClass:
>
> StyleableDoubleProperty myValue =
> new SimpleStyleableDoubleProperty(this, "myValue") {
>
> static final CssMetaData<MyClass, Number> METADATA =
> new CssMetaData<MyClass, Number>(
> "-fx-my-value",
> SizeConverter.getInstance(),
> USE_COMPUTED_SIZE) {
> @Override
> public boolean isSettable(MyClass node) {
> return !node.myValue.isBound();
> }
>
> @Override
> public StyleableProperty getStyleableProperty(
> MyClass node) {
> return node.myValue;
> }
> };
>
> @Override
> public CssMetaData getCssMetaData() {
> return METADATA;
> }
> };
>
> public final DoubleProperty myValueProperty() {
> return myValue;
> }
>
> It is not required to override the `getCssMetaData()` method, nor is
> it required to redeclare a new static `getClassCssMetaData()` method.
> It is also not required to manually keep the list of styleable
> properties in sync with the list of CSS metadata.
>
> I've prototyped this concept for the `Node`, `Region` and `Control`
> classes [1].
>
> [0]
> https://mail.openjdk.org/pipermail/openjfx-dev/2023-December/044046.html
> [1] https://github.com/openjdk/jfx/pull/1299
>
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