刚下好Android Studio，想改个sdk路径，一直改不了，没法下载sdk
见过的错误提示：
android studio first run unable to access.
unable to access android sdk add-on list.
the android sdk location cannot be at the filesystem root.
android studio required component missing.
SDK emulator directory is missing.
尝试：换了代理、改了路径，都无法在下载时勾选 sdk(unavailable)
解决：重启电脑，就能勾选了
（无语，搞了一早上，就这么解决了，生气）
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新的问题：项目无法通过代理下载gradle
解决：我做了以下几步
1、添加镜像
项目根目录下的build.gradle中在repositories下添加以下代码
maven { allowInsecureProtocol = true url 'https://maven.aliyun.com/nexus/content/groups/public/' } maven { allowInsecureProtocol = true url 'https://maven.aliyun.com/nexus/content/repositories/jcenter' } maven { allowInsecureProtocol = true url 'https://maven.aliyun.com/nexus/content/repositories/google' } maven { allowInsecureProtocol = true url 'https://maven.aliyun.com/nexus/content/repositories/gradle-plugin' }
如图：
2、更新3个文件中的gradle版本配置
1：
2：
3：
