XNA游戏开发之(四)——改变Draw频率
【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。