Change the default for the measure_func field of ContentSize to N…

…one. (#9346)

# Objective

The default for `ContentSize` should have the `measure_func` field set
to `None`, instead of a fixed size of zero. This means that until a
measure func is set the size of the UI node will be determined by its
`Style` constraints. This is preferable as it allows users to specify
the space the Node should take up in the layout while waiting for
content to load.

## Solution

Derive `Default` for `ContentSize`.

The PR also adds a `fixed_size` helper function to make it a bit easier
to access the old behaviour.

## Changelog
* Derived `Default` for `ContentSize`
* Added a `fixed_size` helper function to `ContentSize` that creates a
new `ContentSize` with a `MeasureFunc` that always returns the same
value, regardless of layout constraints.

## Migration Guide
The default for `ContentSize` now sets its `measure_func` to `None`,
instead of a fixed size measure that returns `Vec2::ZERO`.
The helper function `fixed_size` can be called with
`ContentSize::fixed_size(Vec2::ZERO)` to get the previous behaviour.

crates/bevy_ui/src/measurement.rs

@@ -1,7 +1,7 @@
 use bevy_ecs::prelude::Component;
 use bevy_ecs::reflect::ReflectComponent;
 use bevy_math::Vec2;
-use bevy_reflect::Reflect;
+use bevy_reflect::{std_traits::ReflectDefault, Reflect};
 use std::fmt::Formatter;
 pub use taffy::style::AvailableSpace;
 use taffy::{node::MeasureFunc, prelude::Size as TaffySize};
@@ -46,8 +46,8 @@ impl Measure for FixedMeasure {
 
 /// A node with a `ContentSize` component is a node where its size
 /// is based on its content.
-#[derive(Component, Reflect)]
-#[reflect(Component)]
+#[derive(Component, Reflect, Default)]
+#[reflect(Component, Default)]
 pub struct ContentSize {
     /// The `Measure` used to compute the intrinsic size
     #[reflect(ignore)]
@@ -66,12 +66,11 @@ impl ContentSize {
         };
         self.measure_func = Some(MeasureFunc::Boxed(Box::new(measure_func)));
     }
-}
 
-impl Default for ContentSize {
-    fn default() -> Self {
-        Self {
-            measure_func: Some(MeasureFunc::Raw(|_, _| TaffySize::ZERO)),
-        }
+    /// Creates a `ContentSize` with a `Measure` that always returns given `size` argument, regardless of the UI layout's constraints.
+    pub fn fixed_size(size: Vec2) -> ContentSize {
+        let mut content_size = Self::default();
+        content_size.set(FixedMeasure { size });
+        content_size
     }
 }