Gradle依赖的统一管理，解决依赖冲突

看见别人在用implementation rootProject.ext.dependencies["xxxx"]不知道是什么意思，上网查了一下，原来是为了解决或者说预防gradle依赖冲突的问题。

在项目开发中我们会经常引入多个Module,然而每个Module中又包含了V4、V7，为了升级新版本依赖包只用更改一次，我们决定采用Gradle依赖的统一管理，避免重复繁琐的劳动。

记录get到的新知识，用法如下：
1.在Project目录下新建config.gradle文件，文件名可自定义

具体内容如下：

 ext {

     android = [
             compileSdkVersion       : ,
             buildToolsVersion       : "27.0.0",
             minSdkVersion           : ,
             targetSdkVersion        : ,
             versionCode             : ,
             versionName             : "1.2.2",
             renderscriptTargetApi   :
     ]

     version = [
             supportLibraryVersion   : "26.1.1",
             okhttpVersion           : "3.9.0",
             retrofitVersion         : "2.3.0",
             glideVersion            : "4.0.0",
             butterknifeVersion      : "8.8.1",
             fragmentationVersion    : "1.1.9",
     ]

     dependencies = [
             //base
             "appcompat-v7"                      : "com.android.support:appcompat-v7:${version["supportLibraryVersion"]}",
             "cardview-v7"                       : "com.android.support:cardview-v7:${version["supportLibraryVersion"]}",
             "design"                            : "com.android.support:design:${version["supportLibraryVersion"]}",
             "constraint-layout"                 : "com.android.support.constraint:constraint-layout:1.0.2",

             //net
             "gson"                              : "com.google.code.gson:gson:2.8.2",
             "okhttp"                            : "com.squareup.okhttp3:okhttp:${version["okhttpVersion"]}",
             "logging-interceptor"               : "com.squareup.okhttp3:logging-interceptor:${version["okhttpVersion"]}",
             "retrofit"                          : "com.squareup.retrofit2:retrofit:${version["retrofitVersion"]}",
             "converter-gson"                    : "com.squareup.retrofit2:converter-gson:${version["retrofitVersion"]}",
             "adapter-rxjava2"                   : "com.squareup.retrofit2:adapter-rxjava2:${version["retrofitVersion"]}",

             //dao
             "greendao"                          : "org.greenrobot:greendao:3.2.2",

             //rx
             "rxjava"                            : "io.reactivex.rxjava2:rxjava:2.1.5",
             "rxandroid"                         : "io.reactivex.rxjava2:rxandroid:2.0.1",
             "rxbinding"                         : "com.jakewharton.rxbinding2:rxbinding:2.1.0",
             "rxpermissions"                     : "com.tbruyelle.rxpermissions2:rxpermissions:0.9.5@aar",

             //di
             "javax_annotation"                  : "org.glassfish:javax.annotation:10.0-b28",
             "butterknife"                       : "com.jakewharton:butterknife:${version["butterknifeVersion"]}",
             "butterknife-compiler"              : "com.jakewharton:butterknife-compiler:${version["butterknifeVersion"]}",

             //multidex
             "multidex"                          : "com.android.support:multidex:1.0.3",

             //kotlin
             "kotlin-stdlib"                     : "org.jetbrains.kotlin:kotlin-stdlib:1.2.10",

             //ui test
             "espresso-core"                     : "com.android.support.test.espresso:espresso-core:3.0.2",
             "espresso-idling-resource"          : "com.android.support.test.espresso:espresso-idling-resource:3.0.2",

             //unit test , 为了整合mockito和PowerMockito，mockito暂时最高只支持2.8.9
             "junit"                             : "junit:junit:4.12",
             "mockito"                           : "org.mockito:mockito-core:2.8.9",
             "powermock-module-junit4"           : "org.powermock:powermock-module-junit4:1.7.4"

     ]

 }

2.在Project的build.gradle中添加

apply from: "config.gradle"

3.在modle的build.gradle中添加引用

 apply plugin: 'com.android.application'

 android {
     compileSdkVersion rootProject.ext.android["compileSdkVersion"]
     buildToolsVersion rootProject.ext.android["buildToolsVersion"]

     defaultConfig {
         applicationId "json.chao.com.wanandroid"
         minSdkVersion rootProject.ext.android["minSdkVersion"]
         targetSdkVersion rootProject.ext.android["targetSdkVersion"]
         versionCode rootProject.ext.android["versionCode"]
         versionName rootProject.ext.android["versionName"]
         //AndroidJunitRunner必须要显示指定在defaultConfig中，使用Gradle依赖管理无法使其生效
         testInstrumentationRunner 'android.support.test.runner.AndroidJUnitRunner'
         renderscriptTargetApi rootProject.ext.android["renderscriptTargetApi"]
         renderscriptSupportModeEnabled true    // Enable RS support
         multiDexEnabled true
         vectorDrawables.useSupportLibrary = true
     }
 }

     dependencies {
         implementation fileTree(include: ['*.jar'], dir: 'libs')

         //base
         implementation rootProject.ext.dependencies["appcompat-v7"]
         implementation rootProject.ext.dependencies["cardview-v7"]
         implementation rootProject.ext.dependencies["design"]
         implementation rootProject.ext.dependencies["constraint-layout"]

         //net
         implementation rootProject.ext.dependencies["gson"]
         implementation rootProject.ext.dependencies["okhttp"]
         implementation rootProject.ext.dependencies["retrofit"]
         implementation rootProject.ext.dependencies["converter-gson"]
         implementation rootProject.ext.dependencies["adapter-rxjava2"]

         //dao
         implementation rootProject.ext.dependencies["greendao"]

         //rx
         implementation rootProject.ext.dependencies["rxjava"]
         implementation rootProject.ext.dependencies["rxandroid"]
         implementation rootProject.ext.dependencies["rxbinding"]
         implementation rootProject.ext.dependencies["rxpermissions"]

         //UI测试
         androidTestImplementation (rootProject.ext.dependencies["espresso-core"]) {
             exclude group: 'com.android.support', module: 'support-annotations'
         }
         implementation (rootProject.ext.dependencies["espresso-idling-resource"]) {
             exclude module: 'support-annotations'
         }
 }

（两个文件中有不对应的依赖方法，that's ok, 只是粘贴代码的时候删除了一些，知道用法就行了）