Make empty objects and arrays collapsed in pretty print mode (#2506)

because it is a widespread convention.

Fixes #2502

2 files changed, 10 insertions, 1 deletions

diff --git a/formats/json/commonMain/src/kotlinx/serialization/json/internal/Composers.kt b/formats/json/commonMain/src/kotlinx/serialization/json/internal/Composers.kt
index c1ed8cca..abdd1c44 100644
--- a/formats/json/commonMain/src/kotlinx/serialization/json/internal/Composers.kt
+++ b/formats/json/commonMain/src/kotlinx/serialization/json/internal/Composers.kt
@@ -27,6 +27,10 @@ internal open class Composer(@JvmField internal val writer: InternalJsonWriter)
         writingFirst = false
     }
 
+    open fun nextItemIfNotFirst() {
+        writingFirst = false
+    }
+
     open fun space() = Unit
 
     fun print(v: Char) = writer.writeChar(v)
@@ -88,6 +92,11 @@ internal class ComposerWithPrettyPrint(
         repeat(level) { print(json.configuration.prettyPrintIndent) }
     }
 
+    override fun nextItemIfNotFirst() {
+        if (writingFirst) writingFirst = false
+        else nextItem()
+    }
+
     override fun space() {
         print(' ')
     }
diff --git a/formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonEncoder.kt b/formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonEncoder.kt
index 4f7b1ec7..cf562de5 100644
--- a/formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonEncoder.kt
+++ b/formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonEncoder.kt
@@ -96,7 +96,7 @@ internal class StreamingJsonEncoder(
     override fun endStructure(descriptor: SerialDescriptor) {
         if (mode.end != INVALID) {
             composer.unIndent()
-            composer.nextItem()
+            composer.nextItemIfNotFirst()
             composer.print(mode.end)
         }
     }