XNA游戏开发之(四)——改变Draw频率

当运行XNA游戏时，XNA会以尽可能大的频率调用Draw方法

Draw的频率大于屏幕刷新频率时，Draw设置失效。

如果屏幕刷新频率只有每秒100次，每秒绘制110次是无用的。在PC和Xbox360平台上，，屏幕刷新率是由PC屏幕和它的设置决定的，Zune 30每秒刷新60次，其他Zune设备每秒刷新30次。

Update方法每秒调用60次。如果游戏计算量太大，Draw方法调用次数会变少以保证Update方法可以每秒调用60次。

在某些情况中，以最大频率调用Draw方法是有用的，需要在XNA游戏中使用的最大帧频率时，可以将graphics.SynchronizeWithVerticalRetrace变量设置为true。

     必须在Game的构造函数顶部加入这行代码，因为XNA需要在创建GraphicsDevice 之前知道这个设置

  1 using System;
  2  using System.Collections.Generic;
  3  using System.Linq;
  4 using Microsoft.Xna.Framework;
  5 using Microsoft.Xna.Framework.Audio;
  6 using Microsoft.Xna.Framework.Content;
  7 using Microsoft.Xna.Framework.GamerServices;
  8 using Microsoft.Xna.Framework.Graphics;
  9 using Microsoft.Xna.Framework.Input;
 10 using Microsoft.Xna.Framework.Media;
 11 using Microsoft.Xna.Framework.Net;
 12 using Microsoft.Xna.Framework.Storage;
 13 
 14 namespace Alex
 15 {
 16     /// 
 17     /// This is the main type for your game
 18     /// 
 19     public class AlexGame : Microsoft.Xna.Framework.Game
 20     {
 21         GraphicsDeviceManager graphics;
 22         SpriteBatch spriteBatch;
 23 
 24         public AlexGame()
 25         {
 26             graphics = new GraphicsDeviceManager(this);
 27             Content.RootDirectory = "Content";
 28             //将Draw刷新频率设置为最大
 29             graphics.SynchronizeWithVerticalRetrace = true;
 30         }
 31 
 32         /// 
 33         /// Allows the game to perform any initialization it needs to before starting to run.
 34         /// This is where it can query for any required services and load any non-graphic
 35         /// related content.  Calling base.Initialize will enumerate through any components
 36         /// and initialize them as well.
 37         /// 
 38         protected override void Initialize()
 39         {
 40             // TODO: Add your initialization logic here
 41             this.TargetElapsedTime = TimeSpan.FromSeconds(1.0f / 100.0f);
 42              base.Initialize();
 43         }
 44 
 45         /// 
 46         /// LoadContent will be called once per game and is the place to load
 47         /// all of your content.
 48         /// 
 49         protected override void LoadContent()
 50         {
 51             // Create a new SpriteBatch, which can be used to draw textures.
 52             spriteBatch = new SpriteBatch(GraphicsDevice);
 53 
 54             // TODO: use this.Content to load your game content here
 55         }
 56 
 57         /// 
 58         /// UnloadContent will be called once per game and is the place to unload
 59         /// all content.
 60         /// 
 61         protected override void UnloadContent()
 62         {
 63             // TODO: Unload any non ContentManager content here
 64         }
 65 
 66         /// 
 67         /// Allows the game to run logic such as updating the world,
 68         /// checking for collisions, gathering input, and playing audio.
 69         /// 
 70         /// 
Provides a snapshot of timing values.
 71         protected override void Update(GameTime gameTime)
 72         {
 73             // Allows the game to exit
 74             if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
 75             {
 76                 this.Exit();
 77             }
 78             if (Keyboard.GetState().IsKeyDown(Keys.Escape))
 79             {
 80                 this.Exit();
 81             }
 82 
 83             this.Window.Title = gameTime.IsRunningSlowly.ToString();
 84 
 85             // TODO: Add your update logic here
 86 
 87             base.Update(gameTime);
 88         }
 89 
 90         /// 
 91         /// This is called when the game should draw itself.
 92         /// 
 93         /// Provides a snapshot of timing values.
 94         protected override void Draw(GameTime gameTime)
 95         {
 96             GraphicsDevice.Clear(Color.CornflowerBlue);
 97 
 98             // TODO: Add your drawing code here
 99 
100             base.Draw(gameTime);
101         }
102     }
103 }
104 
 

Update和Draw方法调用频率的重要性

更新逻辑放置在Update方法中，Update频率的减少会导致游戏中的所有物体变慢。

当Draw调用频率小于屏幕刷新频率时，只有游戏的视觉表现会暂时受影响，游戏帧频率暂时由每秒100帧降到80帧不容易察觉。

如果必要，XNA会降低Draw的频率以保证Update能以每秒60帧的频率调用。