【原创】Alex
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 /// <summary>
17 /// This is the main type for your game
18 /// </summary>
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 /// <summary>
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 /// </summary>
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 /// <summary>
46 /// LoadContent will be called once per game and is the place to load
47 /// all of your content.
48 /// </summary>
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 /// <summary>
58 /// UnloadContent will be called once per game and is the place to unload
59 /// all content.
60 /// </summary>
61 protected override void UnloadContent()
62 {
63 // TODO: Unload any non ContentManager content here
64 }
65
66 /// <summary>
67 /// Allows the game to run logic such as updating the world,
68 /// checking for collisions, gathering input, and playing audio.
69 /// </summary>
70 /// <param name="gameTime">Provides a snapshot of timing values.</param>
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 /// <summary>
91 /// This is called when the game should draw itself.
92 /// </summary>
93 /// <param name="gameTime">Provides a snapshot of timing values.</param>
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。