feat: add page-by-page preview in the GUI after building (Closes #18) (#21)

.plans/issue-18-page-preview.md

@@ -0,0 +1,44 @@
+# Issue #18: Add page-by-page preview in the GUI after building
+
+## Summary
+
+Add a PDF preview panel to the GUI that appears after a successful build. It renders PDF pages as images using Apache PDFBox and displays them with previous/next navigation and a page counter. The preview loads pages lazily for performance and updates automatically on new builds.
+
+## AC Verification Checklist
+
+1. After a successful build, a preview panel appears showing generated pages
+2. Users can navigate between pages (previous/next buttons)
+3. Current page number and total count displayed (e.g., "Seite 3 / 42")
+4. Preview renders actual PDF pages as images (PDF-to-image via PDFBox)
+5. Preview panel can be closed/hidden to return to normal view
+6. Preview updates automatically when a new build completes
+7. GUI remains responsive while preview is loading (async rendering)
+
+## Implementation Steps
+
+### Step 1: Add PDFBox dependency to gui module
+
+Add `org.apache.pdfbox:pdfbox:3.0.4` to gui/build.gradle.kts.
+
+### Step 2: Create PdfPreviewState class
+
+A state holder for the preview: current page index, total pages, rendered page images (cached), loading state. Pages are rendered lazily — only the current page is rendered at a time.
+
+### Step 3: Create PdfPreviewPanel composable
+
+A Compose panel with:
+- An Image composable showing the current page
+- Navigation row: "< Prev" button | "Seite X / Y" label | "Next >" button
+- A close/hide button
+- Loading indicator while page is rendering
+
+### Step 4: Integrate preview into App composable
+
+After a successful build:
+- Show a "Vorschau" button in the action buttons row
+- When clicked, show the preview panel (replacing or overlaying the song list area)
+- When a new build succeeds, update the preview automatically
+
+### Step 5: Lazy page rendering
+
+Render pages on demand using coroutines on Dispatchers.IO to keep the UI responsive.

gui/build.gradle.kts

@@ -11,6 +11,7 @@ dependencies {
     implementation(compose.desktop.currentOs)
     implementation("io.github.microutils:kotlin-logging-jvm:3.0.5")
     implementation("ch.qos.logback:logback-classic:1.5.16")
+    implementation("org.apache.pdfbox:pdfbox:3.0.4")
 }
 
 compose.desktop {

gui/src/main/kotlin/de/pfadfinder/songbook/gui/App.kt

@@ -48,6 +48,7 @@ fun App() {
     var statusMessages by remember { mutableStateOf<List<StatusMessage>>(emptyList()) }
     var isRunning by remember { mutableStateOf(false) }
     var lastBuildResult by remember { mutableStateOf<BuildResult?>(null) }
+    val previewState = remember { PdfPreviewState() }
 
     val scope = rememberCoroutineScope()
 
@@ -132,48 +133,57 @@ fun App() {
 
                 Spacer(modifier = Modifier.height(16.dp))
 
-                // Song list
-                Text(
-                    text = "Lieder (${songs.size}):",
-                    fontWeight = FontWeight.Medium
-                )
-                Spacer(modifier = Modifier.height(4.dp))
-
-                Box(modifier = Modifier.weight(1f).fillMaxWidth()) {
-                    val listState = rememberLazyListState()
-                    LazyColumn(
-                        state = listState,
-                        modifier = Modifier.fillMaxSize().padding(end = 12.dp)
-                    ) {
-                        if (songs.isEmpty() && projectPath.isNotBlank()) {
-                            item {
-                                Text(
-                                    "Keine Lieder gefunden. Bitte Projektverzeichnis prüfen.",
-                                    color = Color.Gray,
-                                    modifier = Modifier.padding(8.dp)
-                                )
-                            }
-                        } else if (projectPath.isBlank()) {
-                            item {
-                                Text(
-                                    "Bitte ein Projektverzeichnis auswählen.",
-                                    color = Color.Gray,
-                                    modifier = Modifier.padding(8.dp)
-                                )
-                            }
-                        }
-                        items(songs) { song ->
-                            Row(modifier = Modifier.fillMaxWidth().padding(vertical = 2.dp, horizontal = 8.dp)) {
-                                Text(song.title, modifier = Modifier.weight(1f))
-                                Text(song.fileName, color = Color.Gray, fontSize = 12.sp)
-                            }
-                            Divider()
-                        }
-                    }
-                    VerticalScrollbar(
-                        modifier = Modifier.align(Alignment.CenterEnd).fillMaxHeight(),
-                        adapter = rememberScrollbarAdapter(listState)
+                // Central content area: song list or preview panel
+                if (previewState.isVisible) {
+                    // Show preview panel
+                    PdfPreviewPanel(
+                        state = previewState,
+                        modifier = Modifier.weight(1f)
                     )
+                } else {
+                    // Song list
+                    Text(
+                        text = "Lieder (${songs.size}):",
+                        fontWeight = FontWeight.Medium
+                    )
+                    Spacer(modifier = Modifier.height(4.dp))
+
+                    Box(modifier = Modifier.weight(1f).fillMaxWidth()) {
+                        val listState = rememberLazyListState()
+                        LazyColumn(
+                            state = listState,
+                            modifier = Modifier.fillMaxSize().padding(end = 12.dp)
+                        ) {
+                            if (songs.isEmpty() && projectPath.isNotBlank()) {
+                                item {
+                                    Text(
+                                        "Keine Lieder gefunden. Bitte Projektverzeichnis prüfen.",
+                                        color = Color.Gray,
+                                        modifier = Modifier.padding(8.dp)
+                                    )
+                                }
+                            } else if (projectPath.isBlank()) {
+                                item {
+                                    Text(
+                                        "Bitte ein Projektverzeichnis auswählen.",
+                                        color = Color.Gray,
+                                        modifier = Modifier.padding(8.dp)
+                                    )
+                                }
+                            }
+                            items(songs) { song ->
+                                Row(modifier = Modifier.fillMaxWidth().padding(vertical = 2.dp, horizontal = 8.dp)) {
+                                    Text(song.title, modifier = Modifier.weight(1f))
+                                    Text(song.fileName, color = Color.Gray, fontSize = 12.sp)
+                                }
+                                Divider()
+                            }
+                        }
+                        VerticalScrollbar(
+                            modifier = Modifier.align(Alignment.CenterEnd).fillMaxHeight(),
+                            adapter = rememberScrollbarAdapter(listState)
+                        )
+                    }
                 }
 
                 Spacer(modifier = Modifier.height(16.dp))
@@ -222,6 +232,11 @@ fun App() {
                                     }
                                 }
                                 isRunning = false
+
+                                // Automatically load preview after successful build
+                                if (result.success && result.outputFile != null) {
+                                    previewState.loadPdf(result.outputFile!!)
+                                }
                             }
                         },
                         enabled = !isRunning && projectPath.isNotBlank()
@@ -283,6 +298,26 @@ fun App() {
                         ) {
                             Text("PDF öffnen")
                         }
+
+                        // Show/hide preview button
+                        Button(
+                            onClick = {
+                                if (previewState.isVisible) {
+                                    previewState.isVisible = false
+                                } else {
+                                    scope.launch {
+                                        if (previewState.totalPages == 0) {
+                                            lastBuildResult?.outputFile?.let { previewState.loadPdf(it) }
+                                        } else {
+                                            previewState.isVisible = true
+                                        }
+                                    }
+                                }
+                            },
+                            enabled = !isRunning
+                        ) {
+                            Text(if (previewState.isVisible) "Vorschau ausblenden" else "Vorschau")
+                        }
                     }
 
                     if (isRunning) {

gui/src/main/kotlin/de/pfadfinder/songbook/gui/PdfPreviewPanel.kt

@@ -0,0 +1,221 @@
+package de.pfadfinder.songbook.gui
+
+import androidx.compose.foundation.Image
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.*
+import androidx.compose.material.*
+import androidx.compose.runtime.*
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.ImageBitmap
+import androidx.compose.ui.graphics.toComposeImageBitmap
+import androidx.compose.ui.layout.ContentScale
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
+import org.apache.pdfbox.Loader
+import org.apache.pdfbox.pdmodel.PDDocument
+import org.apache.pdfbox.rendering.PDFRenderer
+import java.awt.image.BufferedImage
+import java.io.File
+
+/**
+ * State holder for the PDF preview. Manages the current page, total page count,
+ * and a cache of rendered page images.
+ */
+class PdfPreviewState {
+    var pdfFile: File? by mutableStateOf(null)
+        private set
+    var totalPages: Int by mutableStateOf(0)
+        private set
+    var currentPage: Int by mutableStateOf(0)
+        private set
+    var isLoading: Boolean by mutableStateOf(false)
+        private set
+    var currentImage: ImageBitmap? by mutableStateOf(null)
+        private set
+    var isVisible: Boolean by mutableStateOf(false)
+
+    private var document: PDDocument? = null
+    private var renderer: PDFRenderer? = null
+    private val pageCache = mutableMapOf<Int, ImageBitmap>()
+
+    /**
+     * Load a new PDF file for preview. Resets state and renders the first page.
+     */
+    suspend fun loadPdf(file: File) {
+        close()
+        pdfFile = file
+        currentPage = 0
+        pageCache.clear()
+        currentImage = null
+
+        withContext(Dispatchers.IO) {
+            try {
+                val doc = Loader.loadPDF(file)
+                document = doc
+                renderer = PDFRenderer(doc)
+                totalPages = doc.numberOfPages
+            } catch (_: Exception) {
+                totalPages = 0
+            }
+        }
+
+        if (totalPages > 0) {
+            isVisible = true
+            renderCurrentPage()
+        }
+    }
+
+    suspend fun goToPage(page: Int) {
+        if (page < 0 || page >= totalPages) return
+        currentPage = page
+        renderCurrentPage()
+    }
+
+    suspend fun nextPage() {
+        goToPage(currentPage + 1)
+    }
+
+    suspend fun previousPage() {
+        goToPage(currentPage - 1)
+    }
+
+    private suspend fun renderCurrentPage() {
+        val cached = pageCache[currentPage]
+        if (cached != null) {
+            currentImage = cached
+            return
+        }
+
+        isLoading = true
+        try {
+            val image = withContext(Dispatchers.IO) {
+                try {
+                    val pdfRenderer = renderer ?: return@withContext null
+                    // Render at 150 DPI for a good balance of quality and speed
+                    val bufferedImage: BufferedImage = pdfRenderer.renderImageWithDPI(currentPage, 150f)
+                    bufferedImage.toComposeImageBitmap()
+                } catch (_: Exception) {
+                    null
+                }
+            }
+            if (image != null) {
+                pageCache[currentPage] = image
+                currentImage = image
+            }
+        } finally {
+            isLoading = false
+        }
+    }
+
+    fun close() {
+        try {
+            document?.close()
+        } catch (_: Exception) {
+            // ignore
+        }
+        document = null
+        renderer = null
+        pageCache.clear()
+        currentImage = null
+        totalPages = 0
+        currentPage = 0
+        isVisible = false
+    }
+}
+
+@Composable
+fun PdfPreviewPanel(
+    state: PdfPreviewState,
+    modifier: Modifier = Modifier
+) {
+    val scope = rememberCoroutineScope()
+
+    Column(modifier = modifier.fillMaxWidth()) {
+        // Header with title and close button
+        Row(
+            modifier = Modifier.fillMaxWidth().padding(bottom = 8.dp),
+            horizontalArrangement = Arrangement.SpaceBetween,
+            verticalAlignment = Alignment.CenterVertically
+        ) {
+            Text(
+                text = "Vorschau",
+                fontSize = 16.sp,
+                fontWeight = FontWeight.Bold
+            )
+            Button(
+                onClick = { state.isVisible = false },
+                colors = ButtonDefaults.buttonColors(backgroundColor = Color.LightGray)
+            ) {
+                Text("Schliessen")
+            }
+        }
+
+        // Page image area
+        Box(
+            modifier = Modifier
+                .fillMaxWidth()
+                .weight(1f)
+                .background(Color(0xFFE0E0E0)),
+            contentAlignment = Alignment.Center
+        ) {
+            if (state.isLoading) {
+                CircularProgressIndicator()
+            } else if (state.currentImage != null) {
+                Image(
+                    bitmap = state.currentImage!!,
+                    contentDescription = "Seite ${state.currentPage + 1}",
+                    modifier = Modifier.fillMaxSize().padding(4.dp),
+                    contentScale = ContentScale.Fit
+                )
+            } else {
+                Text(
+                    "Keine Vorschau verfuegbar",
+                    color = Color.Gray
+                )
+            }
+        }
+
+        // Navigation row
+        Row(
+            modifier = Modifier.fillMaxWidth().padding(top = 8.dp),
+            horizontalArrangement = Arrangement.Center,
+            verticalAlignment = Alignment.CenterVertically
+        ) {
+            Button(
+                onClick = {
+                    scope.launch { state.previousPage() }
+                },
+                enabled = state.currentPage > 0 && !state.isLoading
+            ) {
+                Text("<")
+            }
+
+            Spacer(modifier = Modifier.width(16.dp))
+
+            Text(
+                text = "Seite ${state.currentPage + 1} / ${state.totalPages}",
+                fontWeight = FontWeight.Medium,
+                textAlign = TextAlign.Center,
+                modifier = Modifier.widthIn(min = 120.dp)
+            )
+
+            Spacer(modifier = Modifier.width(16.dp))
+
+            Button(
+                onClick = {
+                    scope.launch { state.nextPage() }
+                },
+                enabled = state.currentPage < state.totalPages - 1 && !state.isLoading
+            ) {
+                Text(">")
+            }
+        }
+    }
+}