Correctly handle UI hierarchy without a camera

[salvadorcarvalhinho]

Objective

Add support so bevy_ui can correctly handle an UI hierarchy without a camera present.

• Fixes Panic with UI hierarchy when no camera is present #12184

Solution

As there was no default behavior for what should happen when a camera is not present in a UI hierarchy, the solution
was based in defining that default behavior and improving the overall handling of this "exception".

Changelog

• Create default values to be used in upsert_node
• Add flag to control warnings about no camera present
• Create unit test no_camera_ui (to test if ui handles no camera present)

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[StrikeForceZero]

IMO the no_camera flag is probably not needed and adds an extra layer of state to manage. I tested it with this diff below applied to your branch and it still passed the test. But the else upsert_node below with the default style and layout is a good catch. 👍

If we are worried about spamming the console we could use a warn_once! instead.

Index: crates/bevy_ui/src/layout/mod.rs
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/crates/bevy_ui/src/layout/mod.rs b/crates/bevy_ui/src/layout/mod.rs
--- a/crates/bevy_ui/src/layout/mod.rs	(revision 8bbf91532c1fcdef9949ae021f3c3531c970c865)
+++ b/crates/bevy_ui/src/layout/mod.rs	(date 1711922950048)
@@ -63,7 +63,6 @@
     camera_entity_to_taffy: EntityHashMap<EntityHashMap<taffy::node::Node>>,
     camera_roots: EntityHashMap<Vec<RootNodePair>>,
     taffy: Taffy,
-    no_camera: bool,
 }
 
 fn _assert_send_sync_ui_surface_impl_safe() {
@@ -91,7 +90,6 @@
             camera_entity_to_taffy: Default::default(),
             camera_roots: Default::default(),
             taffy,
-            no_camera: false,
         }
     }
 }
@@ -340,7 +338,6 @@
                     );
                     continue;
                 };
-                ui_surface.no_camera = false;
                 let layout_info = camera_layout_info
                     .entry(camera_entity)
                     .or_insert_with(|| calculate_camera_layout_info(camera));
@@ -348,10 +345,7 @@
             }
             None => {
                 if cameras.is_empty() {
-                    if !ui_surface.no_camera {
-                        ui_surface.no_camera = true;
-                        warn!("No camera found to render UI to. To fix this, add at least one camera to the scene.");
-                    }
+                    warn!("No camera found to render UI to. To fix this, add at least one camera to the scene.");
                 } else {
                     warn!(
                         "Multiple cameras found, causing UI target ambiguity. \

[salvadorcarvalhinho]

That makes perfect sense, I'll make the commit with that changes! Thank you for your help!

[james7132]

This should be a Default trait impl.

[salvadorcarvalhinho]

Yes, that make sense! Added those changes in the newest commit!

[james7132]

No, as in LayoutContext should implement Default instead of a default method like this.

[StrikeForceZero]

impl Default for LayoutContext {
   fn default() -> Self {
        Self {
            scale_factor: 1.0,
            physical_size: Vec2::ZERO,
            min_size: 0.0,
            max_size: 0.0,
        }
   }
}

[salvadorcarvalhinho]

Oh, I misunderstood your sugestion, thanks for clarifying!
The last commit should already have those changes implemented.

[StrikeForceZero]

The merge of the main into the PR's branch makes me nervous but I think all PR's get squashed so it shouldn't matter. Everything else looks good.