public GUITexture graphic; public Texture2D standard; public Texture2D downgfx; public Texture2D upgfx; public Texture2D heldgfx; void Start() { graphic.texture = standard; } void Update () { bool down = Input.GetKeyDown(KeyCode.Space); bool held = Input.GetKey(KeyCode.Space); bool up = Input.GetKeyUp(KeyCode.Space); if(down) { graphic.texture = downgfx; } else if(held) { graphic.texture = heldgfx; } else if(up) { graphic.texture = upgfx; } else { graphic.texture = standard; } guiText.text = " " + down + "\n " + held + "\n " + up; }解释这段代码

这段代码是关于在Unity中改变GUITexture的纹理。其中包括四种不同的纹理（standard、downgfx、upgfx和heldgfx），根据用户的输入（是否按下、按住或松开空格键）来切换这些纹理显示。同时，在屏幕上也显示了每个状态的文字。