Player Controller - Pro Unity Game Development with C# - page 174

Listing 5-8. Displaying Textures
110 //------------------------------------------------
111 //ON GUI Function to show texture
112 void OnGUI()
113 {
114 if( ShowDamage ){GUI.DrawTexture(ScreenRect,DamageTexture);}
115 }
//[…]
132 //------------------------------------------------
133 void Update()
134 {
135 //Build screen rect on update (in case screen size changes)
136 ScreenRect.x = ScreenRect.y = 0;
137 ScreenRect.width = Screen.width;
138 ScreenRect.height = Screen.height;
139 }
140 //------------------------------------------------
Here, the Update function is used to size a rectangle structure ( ScreenRect ) to the screen dimensions
(in pixels). This rect is updated on each frame, instead of being generated at application-start, since
it's possible, in some circumstances, for the display size to change during gameplay. For example,
the user could change the screen resolution or resize the game window. The OnGUI function is where
the texture is drawn or flashed to the display for a few seconds while damage is taken, using a call
to GUI.DrawTexture . The OnGUI function is called implicitly by Unity several times per frame . This
means OnGUI is usually called more regularly than Update , making it one of the most computationally
expensive events and a frequent source of performance problems. In short, you'll almost never want
to do anything in OnGUI, except draw graphical elements using the native GUI class. And there are
even developers who recommend never using OnGUI at all, even for GUIs . For my part, while the GUI
class and OnGUI events can be useful for drawing limited GUIs (such as showing flashing red textures),
I almost never use it for GUIs, because I find it often causes performance issues on mobile devices.
Sometimes workarounds can be used, but later in this topic we'll see an alternative method for GUIs.
In this case, however, OnGUI can be used profitably to display a damage animation.
Note Don't just take my word that this code works. Test it for yourself. Temporarily edit the Update event,
for example, so that a key press triggers the ApplyDamage event, to see the damage functionality in action!