在Android项目中使用OKHttp3怎么实现一个下载功能

apply plugin: 'com.android.application'  android {   compileSdkVersion 24   buildToolsVersion "24.0.3"    defaultConfig {     applicationId "com.lanou3g.downdemo"     minSdkVersion 15     targetSdkVersion 24     versionCode 1     versionName "1.0"     testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"     //为了开启Java8     jackOptions{       enabled true;     }   }   buildTypes {     release {       minifyEnabled false       proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'     }   }    //开启Java1.8 能够使用lambda表达式   compileOptions{     sourceCompatibility JavaVersion.VERSION_1_8     targetCompatibility JavaVersion.VERSION_1_8   } }  dependencies {   compile fileTree(dir: 'libs', include: ['*.jar'])   androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {     exclude group: 'com.android.support', module: 'support-annotations'   })   compile 'com.android.support:appcompat-v7:24.1.1'   testCompile 'junit:junit:4.12'    //OKHttp   compile 'com.squareup.okhttp3:okhttp:3.6.0'   //RxJava和RxAndroid 用来做线程切换的   compile 'io.reactivex.rxjava2:rxandroid:2.0.1'   compile 'io.reactivex.rxjava2:rxjava:2.0.1' }