RFR: 8301302: Platform preferences API [v2]

[Michael Strauß]

On Tue, 31 Jan 2023 20:18:05 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> 2. ObservableMap.  Similarly to Node.getProperties(), I wonder if there might be a better way to observe the changes.  May be a different metaphor (subscription?), like adding a value change listener to a specific key.  We do need a set of keys (perhaps that can be an ObservableSet).  Having said that, ObservableMap is good enough solution, and forgive me for stating the obvious, it should not initialize anything if the platform properties have not been requested by the application code.

I've pulled on that string a little more:

#### 1. `Optional` API + listeners
This would remove the `ObservableMap` implementation, and add the following methods instead:

interface Preferences {
    ...
    void addListener(String key, InvalidationListener listener);
    void removeListener(String key, InvalidationListener listener);

    void addListener(String key, ChangeListener<?> listener);
    void removeListener(String key, ChangeListener<?> listener);

    Optional<Integer> getInteger(String key);
    Optional<Double> getDouble(String key);
    ...
}

I don't quite like this idea though, since it would allow developers to either add listeners to non-existent keys, or require developers to probe whether a key exists before adding a listener for it (maybe by also adding a `boolean keyExists(String key)` method. It also doesn't allow developers to enumerate the keys that are available on a given platform.
If we added a set of keys (maybe as an `ObservableSet`), the result is basically a map. We're better off implementing `ObservableMap` in this case.

#### 2. `Property` API

interface Preferences {
    ...
    ReadOnlyIntegerProperty getIntegerProperty(String key);
    ReadOnlyDoubleProperty getDoubleProperty(String key);
    ...
}

With this idea, instead of having manual listener management and an API to query value mappings, we'd expose read-only properties for keys. We might also need a method like `boolean keyExists(String key)`, but we can't enumerate all platform preferences with this approach.

#### 3. `Optional` + `Property` API

interface Preferences {
    ...
    Optional<ReadOnlyIntegerProperty> getIntegerProperty(String key);
    Optional<ReadOnlyDoubleProperty> getDoubleProperty(String key);
    ...
}

This API combines all aspects into a single method call. We also can't enumerate the platform preferences.


I still think that implementing `ObservableMap` is preferable to all of these alternatives.

-------------

PR Comment: https://git.openjdk.org/jfx/pull/1014#issuecomment-1500535347