AndroidStudio Unknown attribute in xml and Namespace not found（xmL 无代码提示）

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编写xml的时候提示：Unknown attribute in xml and Namespace not found

解决方案

1、

File --->power save mode--->取消勾选

2、

删除.idea和.iml，然后ReBuild

Finally, deleting .idea folder and all .iml files from the project, restarting Android Studio and rebuilding gradle did the trick.
Autocomplete in XML support library is working again.
Probably checking out files from Version Control or copying all the source files in a new project would do the trick aswell.
这是stackoverflow上大神的回复，就是删除.idea文件夹和所有的.iml文件，然后重启as，重新build project，经测试时有效果的。

3、卸载已有的api版本，重新安装（耗时可能会比较长）