内容提要：
podfile升级之后到最新版本，pod里的内容必须明确指出所用第三方库的target，否则会出现The dependency `` is not used in any concrete target这样的错误。
以下从三个步骤陈述：
1. Podfile升级
查看pod版本：
pod --version
pod 升级（此时是升级到1.0.0.beta.2版本）：
sudo gem install cocoapods --pre
2. 错误内容：
我的Podfile的内容是：
platform :ios, '7.0'
pod 'ReactiveCocoa', '2.1.8'
pod 'objectiveflickr', '2.0.4'
pod 'LinqToObjectiveC', '2.0.0'
pod 'SDWebImage', '3.6'
pod install后出现错误The dependency `` is not used in any concrete target
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pod install
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/universal-darwin14/rbconfig.rb:213: warning: Insecure world writable dir /Applications/Cocos/tools/ant/bin in PATH, mode 040777
Re-creating CocoaPods due to major version update.
Updating local specs repositories
Analyzing dependencies
[!] The dependency `ReactiveCocoa (= 2.1.8)` is not used in any concrete target.
The dependency `objectiveflickr (= 2.0.4)` is not used in any concrete target.
The dependency `LinqToObjectiveC (= 2.0.0)` is not used in any concrete target.
The dependency `SDWebImage (= 3.6)` is not used in any concrete target.
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3. 修改方法：
Podfile内容更改：
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platform :ios, '7.0'
target "RWTFlickrSearch" do
pod 'ReactiveCocoa', '2.1.8'
pod 'objectiveflickr', '2.0.4'
pod 'LinqToObjectiveC', '2.0.0'
pod 'SDWebImage', '3.6'
end
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这样再运行pod install，就会成功了。
