Reference of meta data placeholder

Reference blog ：
https://blog.csdn.net/weixin_30378311/article/details/97071152
https://blog.csdn.net/tushiba/article/details/96862824
https://android.zhizuobiao.com/android-18091800052/
In the last article about About how to package through multiple channels In, we use variables ${channelName}, Now let's talk about Android Basic use of placeholders .

1. stay app Of build.gradle In the definition of manifestPlaceholders Of key and val, Choose and define the location according to your needs , For example, in defaultConfig perhaps buildTypes Or as in the packaging above productFlavors.
   
   Now we will defaultConfig Definition channelNum For example , Show the basic use of placeholders .
   
2. stay AndroidManifest.xml Reference variables in
   
3. Generate apk, Verify variable values
   
   Be careful ： because manifestPlaceholders yes map type , So in order to avoid values being overwritten, it is best to use += Otherwise it will be overwritten , Prompt error when quoting