Game Objects: UI vs. 2D Sprite
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It is instructive to consider 2 different categories of GameObjects that we'll work with in this course. UI elements vs. 2D sprites, which we need to understand how we can define custom sorting layers to control rendering layer ordering and also how to define custom tags to help easily identify gameObjects within a game scene.
UI gameObjects are a relatively new addition to Unity, they provide easy to customize User-Interaction elements. UI Elements should always be children of a Canvas GameObject, and there should also be an EventSystem GameObject as a child of the Canvas, in order to have UI events active on UI elements. UI elements include Buttons, Text, Sliders, and can be useful in creating in-game menus.
2D Sprites are the other main category of gameObjects that we will work with in this course. 2D sprites are used to add images assets to your game. 2D sprites are often configured to have physics behaviors.
Layers provide a means to control physics interactions between objects.
Tags provide a method to add categories to gameObjects that can be used to identify gameObjects within a custom script. Tags are also created using the Layer editor, by selecting the 'Edit Layers' option, you are provided with an interface where you can create custom tags. Then, a tag can be set for a gameObject at the top of the inspector panel for that gameObject.
Sorting layers provide a method to control the layer order of sprite rendering. Sorting Layer are is attribute of the Sprite Renderer component for 2D sprite gameObjects. Sorting Layers are created using the Layer inspector panel, at the upper left of the editor window, see image below. Select the bottom choice 'Edit Layers' to add new Sorting Layers to your game. Although the Layer editor is the same for Layers, Tags, and Sorting Layers, it's important to realize that Layers and Tags refer to the GameObject, while Sorting Layers refer only to the Rendering Components. There is no ability to set Sorting-layer for individual UI elements, there is a single Sorting-Layer that can be set for the entire Canvas. So, to control layer ordering of Canvas UI elements, the Hierarchy order controls layering, lower elements in the Hierarchy are considered to be closer to the camera.
