Implement -fs-page-break-min-height (#31)

* Implement -fs-pagebreak-min-height. This CSS property allows to force
a page break before an element if not the specified space is left on the
current page.

CSS orphans and page-break-inside:avoid are nice, but dont work always,
especially with complex tables.

This CSS property allows to workaround many page break problems.

Also adds a sample document which demonstrates the effect of
this property. Also the TestcaseRunner now prints onto the System.out
were the output PDF was written.

* Small typo fix.

Author: rototor

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/constants/CSSName.java

@@ -272,7 +272,7 @@ public final class CSSName implements Comparable {
                     NOT_INHERITED,
                     new PrimitivePropertyBuilders.FSCheckboxStyle()
             );
-    
+
     /**
      * Unique CSSName instance for CSS2 property.
      */
@@ -405,6 +405,19 @@ public final class CSSName implements Comparable {
                     new PrimitivePropertyBuilders.FSNamedDestination()
             );
 
+    /**
+     * Perform a page break before this element, if not at least the specified space is
+     * left on the current page.
+     */
+    public final static CSSName FS_PAGE_BREAK_MIN_HEIGHT =
+            addProperty(
+                    "-fs-page-break-min-height",
+                    PRIMITIVE,
+                    "0",
+                    INHERITS,
+                    new PrimitivePropertyBuilders.FSPageBreakMinHeight()
+            );
+
     /**
      * Unique CSSName instance for CSS2 property.
      */
@@ -1316,7 +1329,7 @@ public final class CSSName implements Comparable {
                     true,
                     new PrimitivePropertyBuilders.BorderBottomLeftRadius()
             );
-    
+
     /**
      * Unique CSSName instance for CSS2 property.
      */
@@ -1418,7 +1431,7 @@ public final class CSSName implements Comparable {
                     NOT_INHERITED,
                     new BackgroundPropertyBuilder()
             );
-    
+
 
     /**
      * Unique CSSName instance for CSS3 property.

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/parser/property/PrimitivePropertyBuilders.java

@@ -1256,6 +1256,9 @@ public static class MinHeight extends NonNegativeLengthLike {
     public static class MinWidth extends NonNegativeLengthLike {
     }
 
+    public static class FSPageBreakMinHeight extends NonNegativeLengthLike {
+    }
+
     public static class Orphans extends PlainInteger {
         protected boolean isNegativeValuesAllowed() {
             return false;

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/style/CalculatedStyle.java

@@ -1129,6 +1129,10 @@ public int getColSpan() {
         return result > 0 ? result : 1;
     }
 
+    public float getFSPageBreakMinHeight(CssContext c){
+        return getFloatPropertyProportionalTo(CSSName.FS_PAGE_BREAK_MIN_HEIGHT, 0, c);
+    }
+
     public Length asLength(CssContext c, CSSName cssName) {
         Length result = new Length();
 

openhtmltopdf-core/src/main/java/com/openhtmltopdf/layout/BlockBoxing.java

@@ -331,7 +331,8 @@ private static void repositionBox(LayoutContext c, BlockBox child, int trimmedPa
         }
         if (c.isPrint()) {
             boolean pageClear = child.isNeedPageClear() ||
-                                    child.getStyle().isForcePageBreakBefore();
+                                    child.getStyle().isForcePageBreakBefore() ||
+                                    child.isPageBreakNeededBecauseOfMinHeight(c);
             boolean needNewPageContext = child.checkPageContext(c);
 
             if (needNewPageContext && trimmedPageCount != NO_PAGE_TRIM) {

openhtmltopdf-core/src/main/java/com/openhtmltopdf/layout/LayoutUtil.java

@@ -105,7 +105,7 @@ public static FloatLayoutResult layoutFloated(
     private static void positionFloatOnPage(
             final LayoutContext c, LineBox currentLine, BlockBox block,
             boolean movedVertically) {
-        if (block.getStyle().isForcePageBreakBefore()) {
+        if (block.getStyle().isForcePageBreakBefore() || block.isPageBreakNeededBecauseOfMinHeight(c)) {
             block.forcePageBreakBefore(c, block.getStyle().getIdent(CSSName.PAGE_BREAK_BEFORE), false);
             block.calcCanvasLocation();
             resetAndFloatBlock(c, currentLine, block);

openhtmltopdf-core/src/main/java/com/openhtmltopdf/render/BlockBox.java

@@ -550,9 +550,27 @@ public void positionAbsolute(CssContext cssCtx, int direction) {
         calcChildLocations();
     }
 
+	/**
+     * Using the css:
+     *
+     * -fs-page-break-min-height: 5cm;
+     *
+     * on a block element you can force a pagebreak before this block, if not
+     * enough space (e.g. 5cm in this case) is remaining on the current page for the block.
+     *
+     * @return true if a pagebreak is needed before this block because
+     * there is not enough space left on the current page.
+     */
+    public boolean isPageBreakNeededBecauseOfMinHeight(LayoutContext context){
+        float minHeight = getStyle().getFSPageBreakMinHeight(context);
+        PageBox page = context.getRootLayer().getFirstPage(context, this);
+        return page != null && getAbsY() + minHeight > page.getBottom();
+    }
+
+
     public void positionAbsoluteOnPage(LayoutContext c) {
         if (c.isPrint() &&
-                (getStyle().isForcePageBreakBefore() || isNeedPageClear())) {
+                (getStyle().isForcePageBreakBefore() || isNeedPageClear() || isPageBreakNeededBecauseOfMinHeight(c))) {
             forcePageBreakBefore(c, getStyle().getIdent(CSSName.PAGE_BREAK_BEFORE), false);
             calcCanvasLocation();
             calcChildLocations();

openhtmltopdf-examples/src/main/java/com/openhtmltopdf/testcases/TestcaseRunner.java

@@ -29,6 +29,11 @@ public static void main(String[] args) throws Exception {
 		 */
 		runTestCase("RepeatedTableSample");
 
+		/*
+		 * This sample demonstrates the -fs-pagebreak-min-height css property
+		 */
+		runTestCase("FSPageBreakMinHeightSample");
+
 		/* Add additional test cases here. */
 	}
 
@@ -37,7 +42,8 @@ public static void runTestCase(String testCaseFile) throws Exception {
 				.getResourceAsStream("/testcases/" + testCaseFile + ".html"));
 		String html = new String(htmlBytes, Charsets.UTF_8);
 		String outDir = System.getProperty("OUT_DIRECTORY", ".");
-		FileOutputStream outputStream = new FileOutputStream(outDir + "/" + testCaseFile + ".pdf");
+		String testCaseOutputFile = outDir + "/" + testCaseFile + ".pdf";
+		FileOutputStream outputStream = new FileOutputStream(testCaseOutputFile);
 
 		try {
 			PdfRendererBuilder builder = new PdfRendererBuilder();
@@ -50,5 +56,6 @@ public static void runTestCase(String testCaseFile) throws Exception {
 		} finally {
 			outputStream.close();
 		}
+		System.out.println("Wrote " + testCaseOutputFile);
 	}
 }