如何将MainActivity的字符串传入单例ServiceBuilder并修复Retrofit动态url失效问题

• Kotlin的object单例的非懒加载属性会在类首次被访问的时候一次性完成初始化，你的retrofit实例在你第一次调用ServiceBuilder的任何方法之前就已经用默认的url = "http://no-google.com"构建完成了
• 后续调用loadUrl()修改的只是普通变量url的值，已经构建好的Retrofit实例的baseUrl不会自动更新，所以设置不生效

根据业务场景可以选择两种不同的实现方案：

场景1：App存在多个固定的baseUrl，需要动态切换 #

推荐使用OkHttp拦截器修改请求地址的方案，不需要重复构建Retrofit实例，性能更高：

object ServiceBuilder {
    // 存储动态切换的baseUrl
    var dynamicBaseUrl: String = "http://no-google.com"

    private val baseUrlInterceptor = Interceptor { chain ->
        val originalRequest = chain.request()
        val originalUrl = originalRequest.url

        // 用dynamicBaseUrl替换原来的baseUrl部分
        val newUrl = originalUrl.newBuilder()
            .scheme(URL(dynamicBaseUrl).protocol)
            .host(URL(dynamicBaseUrl).host)
            .port(URL(dynamicBaseUrl).port)
            .build()

        val newRequest = originalRequest.newBuilder()
            .url(newUrl)
            .build()
        chain.proceed(newRequest)
    }

    val headerInterceptor = object: Interceptor {
        override fun intercept(chain: Interceptor.Chain): Response {
            var request = chain.request()
            request = request.newBuilder()
                .addHeader("x-device-type", Build.DEVICE)
                .addHeader("Accept-Language", Locale.getDefault().language)
                .build()
            return chain.proceed(request)
        }
    }

    private var logger = HttpLoggingInterceptor().setLevel(HttpLoggingInterceptor.Level.BODY)

    // OkHttp客户端全局复用
    private val okHttp = OkHttpClient.Builder()
        .callTimeout(5, TimeUnit.SECONDS)
        .addInterceptor(headerInterceptor)
        .addInterceptor(baseUrlInterceptor)
        .addInterceptor(logger)
        .build()

    // Retrofit只初始化一次，baseUrl后续会被拦截器替换
    private val retrofit = Retrofit.Builder()
        .baseUrl(dynamicBaseUrl)
        .addConverterFactory(GsonConverterFactory.create())
        .client(okHttp)
        .build()

    fun loadUrl(url: String): ServiceBuilder {
        this.dynamicBaseUrl = url
        return this
    }

    fun <T> buildService(serviceType: Class<T>): T {
        return retrofit.create(serviceType)
    }
}

场景2：baseUrl完全不固定，每次调用可能都不一样 #

这种场景需要每次设置新url之后重新构建Retrofit实例，修改代码如下：

object ServiceBuilder {
    var url = "http://no-google.com"

    fun loadUrl(url: String): ServiceBuilder{
        this.url = url
        return this
    }

    private var logger = HttpLoggingInterceptor().setLevel(HttpLoggingInterceptor.Level.BODY)

    val headerInterceptor = object: Interceptor {
        override fun intercept(chain: Interceptor.Chain): Response {
            var request = chain.request()
            request = request.newBuilder()
                .addHeader("x-device-type", Build.DEVICE)
                .addHeader("Accept-Language", Locale.getDefault().language)
                .build()
            return chain.proceed(request)
        }
    }

    // OkHttp客户端可以全局复用，不用每次重新构建
    private val okHttp = OkHttpClient.Builder()
        .callTimeout(5, TimeUnit.SECONDS)
        .addInterceptor(headerInterceptor)
        .addInterceptor(logger)
        .build()

    fun <T> buildService(serviceType: Class<T>): T {
        // 每次构建Service的时候用最新的url重新生成Retrofit实例
        val retrofit = Retrofit.Builder()
            .baseUrl(url)
            .addConverterFactory(GsonConverterFactory.create())
            .client(okHttp)
            .build()
        return retrofit.create(serviceType)
    }
}

内容的提问来源于stack exchange，提问作者Derek