Make Grid3D plugin more flexible

[chapulina]

This fixes gazebosim/gz-sim#312.

Trying to load ogre when ogre2 is already loaded explodes the whole application. With this change, when loading the plugin from the menu, it now defaults to the first loaded engine / scene instead ogre.

[codecov]

Codecov Report

Merging #172 (2cad85a) into ign-gui3 (e09a4e3) will decrease coverage by 1.19%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##           ign-gui3     #172      +/-   ##
============================================
- Coverage     60.31%   59.12%   -1.20%     
============================================
  Files            20       21       +1     
  Lines          2510     2608      +98     
============================================
+ Hits           1514     1542      +28     
- Misses          996     1066      +70     

Impacted Files	Coverage Δ
src/plugins/grid_3d/Grid3D.cc	28.57% <0.00%> (ø)

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[ahcorde]

One issue that I found appears when the gui.config file contains a 3DScene and a Grid3D

<!-- 3D scene -->
<plugin filename="Scene3D" name="3D View">
  <ignition-gui>
    <title>3D View</title>
    <property type="bool" key="showTitleBar">false</property>
    <property type="string" key="state">docked</property>
  </ignition-gui>

  <engine>ogre2</engine>
  <scene>scene</scene>
  <ambient_light>0.4 0.4 0.4</ambient_light>
  <background_color>0.8 0.8 0.8</background_color>
  <camera_pose>6 0 6 0 0.5 3.14</camera_pose>
</plugin>


<plugin filename="Grid3D" name="3D Grid">
  <engine>ogre2</engine>
  <scene>scene</scene>
</plugin>

This is the trace of ign-gui

[GUI] [Msg] Loading config [gui.config]
[GUI] [Dbg] [Application.cc:305] Loading plugin [Scene3D]
[GUI] [Msg] Added plugin [3D View] to main window
[GUI] [Msg] Loaded plugin [Scene3D] from path [/home/ahcorde/ignition_citadel/install/lib/ign-gui-3/plugins/libScene3D.so]
[GUI] [Dbg] [Application.cc:305] Loading plugin [Grid3D]
[GUI] [Msg] Added plugin [3D Grid] to main window
[GUI] [Msg] Loaded plugin [Grid3D] from path [/home/ahcorde/ignition_citadel/install/lib/ign-gui-3/plugins/libGrid3D.so]
[GUI] [Dbg] [Application.cc:266] Loading window config
[GUI] [Dbg] [Application.cc:421] Applying config
[GUI] [Msg] Loading plugin [ignition-rendering-ogre2]
[GUI] [Wrn] [Grid3D.cc:233] Engine "ogre2" has no scene, Grid plugin won't work.
[GUI] [Msg] Loading plugin [ignition-rendering-ogre2]
[GUI] [Dbg] [Scene3D.cc:941] Create scene [scene]

The Scene created by the 3DScene is create after the Grid3D is trying to access it. I'm getting a segfault. Is there any way to wait to other plugin to be loaded?

If I add to the config file only the 3DScene and then manually a added with the interface the Grid 3D I'm not getting the segfault

[chapulina]

Is there any way to wait to other plugin to be loaded?

Nice catch, addressed in 2cad85a. Also added the grid to the example scene.