E: Package ‘xxx‘ has no installation candidate

例如换了清华源后，安装 ‘libgtk2.0-dev’，出现：

Package libgtk2.0-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'libgtk2.0-dev' has no installation candidate

解决方法：

reload
然后执行：

sudo apt-get update

再安装就可以了

sudo apt-get install libgtk2.0-dev