KTOR-1219 Make Android logger log to Logcat on Android if the SLF4J b…

ktor-client/ktor-client-plugins/ktor-client-logging/jvm/src/io/ktor/client/plugins/logging/LoggerJvm.kt

@@ -6,6 +6,7 @@ package io.ktor.client.plugins.logging
 
 import io.ktor.client.*
 import org.slf4j.*
+import org.slf4j.helpers.NOPLoggerFactory
 
 public actual val Logger.Companion.DEFAULT: Logger
     get() = object : Logger {
@@ -16,11 +17,50 @@ public actual val Logger.Companion.DEFAULT: Logger
     }
 
 /**
- * Android [Logger]: breaks up long log messages that would be truncated by Android's max log
- * length of 4068 characters
+ * Android [Logger]: Logs to the Logcat on Android if the SLF4J provider isn't found.
+ * Otherwise, uses the [Logger.Companion.DEFAULT].
+ * Breaks up long log messages that would be truncated by Android's max log
+ * length of 4068 characters.
  */
-public val Logger.Companion.ANDROID: Logger
-    get() = MessageLengthLimitingLogger()
+public val Logger.Companion.ANDROID: Logger by lazy { getAndroidLogger() }
+
+private fun getAndroidLogger(): Logger {
+    val logger = Logger.DEFAULT
+
+    val logClass = try {
+        Class.forName("android.util.Log")
+    } catch (_: ClassNotFoundException) {
+        return MessageLengthLimitingLogger(delegate = logger)
+    }
+
+    if (LoggerFactory.getILoggerFactory() !is NOPLoggerFactory) {
+        return MessageLengthLimitingLogger(delegate = logger)
+    }
+
+    return MessageLengthLimitingLogger(delegate = LogcatLogger(logClass, logger))
+}
+
+private class LogcatLogger(logClass: Class<*>, private val fallback: Logger) : Logger {
+    private val tag = "Ktor Client"
+
+    private val method = try {
+        logClass.getDeclaredMethod("i", String::class.java, String::class.java)
+    } catch (_: Throwable) {
+        null
+    }
+    override fun log(message: String) {
+        if (method == null) {
+            fallback.log(message)
+            return
+        }
+
+        try {
+            method.invoke(null, tag, message)
+        } catch (_: Throwable) {
+            fallback.log(message)
+        }
+    }
+}
 
 /**
  * A [Logger] that breaks up log messages into multiple logs no longer than [maxLength]