ios资源加载策略

做了好几个月的ios，大框架都是别人搭好的，自己只是实现逻辑，很是失落。慢慢开始整理学习一些概念类的东西吧，希望自己能提高点。

cocos2d-x从cocos2d-2.0-x-2.0.2开始，考虑到自己是跨平台的引擎，并且Android有许多不同的分辨率，所以不再使用利用对图片加后缀hd ipad ipadhd 进行加载的策略。

本质上来说，资源的加载策略，就是为了应对不同平台的不同分辨率的。

新的加载策略如下：

 从CCFileUtils的setResourceDirectory设置的目录中去寻找。如果找不到则会在Resource/目录下查找。

一般情况下，我们在AppDelegate.cpp的applicationDidFinishLaunching函数中设置此目录。

 CCFileUtils::sharedFileUtils()->setResourceDirectory( "ipadhd" );

这样当你创建一个精灵时：

CCSprite * test = CCSprite::create("abc.png");

Cocos2d-x会首先在Resources/ipadhd 目录下查找abc.png文件，如果没有，则回到Resources 目录下寻找。
需要注意的是，使用setResourceDirectory设置时，默认父目录是Resource。

如果你是这样设置的

CCFileUtils::sharedFileUtils()->setResourceDirectory( "../ipadhd" );

那么，Cocos2d-x会首先在和Resources同级的目录ipadhd中查找文件，注意，是同级。如果没有，则回到Resources 目录下寻找。

有一个问题，不同的平台我们可以设置不同的资源文件夹。但是，有可能我们设置的查找资源文件夹可能是多个，而setResourceDirectory无法满足这个需求的。cocos2d-x官方管这个叫分布式策略。

所以cocos2d-x从2.1开始，已经不支持setResourceDirectory方法了，而是用新的方法setSearchResolutionsOrder替代。这样，我们可以根据不同的平台，设置cocos2d查找文件的路径为多个。注意看这个方法的参数，是vector类型的，这样满足了我们的需求了。其查找侧略还是没有变的。

virtual void setSearchResolutionsOrder(const std::vector<std::string>& searchResolutionsOrder);

cocos2d-x文档中对此方法的描述如下，是从http://blog.csdn.net/some_do/article/details/8914748 这里借鉴过来的，我也没有翻译，不过很简单，相信大家都能看懂：

1.1. Developer guilde

CCFileUtils::setSearchResolutionsOrder() is added to support distributed strategy. You can set searching resolutions order like this 

std::vector<std::string> resDirOrders;
if (resolution is ipad retina)
{
    resDirOrders.push_back("resources-ipadhd");
    resDirOrders.push_back("resources-ipad");
    resDirOrders.push_back("resources-iphonehd");
}
CCFileUtils::sharedFileUtils()->setSearchResolutionsOrder(resDirOrders);

After setting searching resolutions order, suppose we create a sprite like this

CCSprite *sprite = CCSprite::create("images/tex.png"); 

Engine will find tex.png in the following sequence
find it in images/resources-ipadhd/
if not found, find it in images/resources-ipad/
if not found, find it in images/resources-iphonehd/
if not found, find it in images/ （这个必须重点理解，其实是把目标图片的第一层父目录指为setSearchResolutionsOrder中设置的。）

1.2. Notes
This strategy is not suitable for multi-resolution adaption, because there are too many resolutions on Android. You can not provide all resources for all resolutions, then set searching order based on resolutions, such as

 std::vector<std::string> resDirOrders;
 if (resolution is reslution1)
 {
     resDirOrders->push_back("path1");
     resDirOrders->push_back("path2");
     ...
 }
 else if (resolution is resolution2)
 {
    resDirOrders->push_back("path-a");
    resDirOrders->push_back("path-b");
    ...
}
...

CCFileUtils::sharedFileUtils()->setSearchResolutionsOrder(resDirOrders);

2. Centralized strategy

2.1. Why use a different mechanism than cocos2d-iphone

Cocos2d-iphone uses -hd, -ipad, -ipadhd to determine which picture to load. This mechanism is good enough for iOS platform, but is not so suitable for Android, because it has many different resolutions. Cocos2d-x is a cross platform engine, so it should use another mechanism.

2.2. What is the new mechanism

Cocos2d-x uses the new mechanism to load a picture since version cocos2d-2.0-x-2.0.2. It does not use -hd, -ipad, -ipadhd suffixes to indicate images for different resolutions. The mechanism is: Try to find a picture in the paths set by CCFileUtils::setSearchPaths() firstly, if it's not found, then find the picture in Resources/

// set searching paths to "/mnt/sd/example" and "/data/data/org.cocos2dx.example" 
vector<string> searchPaths;
searchPaths.push_back("/mnt/sd/example");
searchPaths.push_back("/data/data/org.cocos2dx.example");
CCFileUtils::setSearchPaths(searchPaths);  
// engine will find "1.png" in /mnt/sd/example, if there it is not found, then engine will find "1.png" in /data/data/org.cocos2dx.example
// if not found, engine will find "1.png" in Resources/ (this path is platform dependent)
CCSprite *pSprite = CCSprite::create("1.png"); 

It is easy to add searching path to engine. Using this method, you can load resources into a path you know, then set this path to engine. Engine will find a resource in this path if needed.

2.3. Developer guide

Do not use -hd, -ipad, -ipadhd suffixes any more. Instead, put all hd files in a directory, then all ipad files in another directory, and so on, then set resource directory to tell the engine where to find a picture. If you want to share some resources between different resolutions, then you can put all shared resources in Resources/, and put resolution specified resources in different directories. You can refer to samples/HelloCpp for more information.

这个例子是我经过测试的：

 std::vector<std::string> pat;
    pat.push_back("i");
    CCFileUtils::sharedFileUtils()->setSearchResolutionsOrder(pat);
CCSprite * a = CCSprite::create("images/share/a.png");

如果此时，我的Resourecs/images/share/a.png是存在的，则不会报错。如果不存在，则报错。

如果 Resourecs/images/share/i/a.png是存在的，而Resourecs/images/share/a.png不存在，不会报错。

 目前我正在做的项目，是没有设置这个setSearchResolutionsOrder的。我们在创建一个图片精灵的时候直接使用:

CCSprite *m_pBackSprite = CCSprite::create( "Images/ipadhd/a.png" );

此时，cocos2d-x直接在Resources/Images/ipadhd/a.png 查找此文件，如果找不到，则会报错。

总结了一张图：