Merge pull request #17 from alfattack/page-break-inside

Re-perform layout on page break avoid

Author: mstancombe

Source/Demos/HtmlRenderer.Demo.Common/TestSamples/37. Breaking Pages 4 - Divs.htm

@@ -15,14 +15,14 @@
     <div style="page-break-inside: avoid; height: 1500px; background-color: blue; padding: 10px;">
         This is on page 4.
 
-        <div style="height: 500px; background-color: yellow;">
+        <div style="height: 200px; background-color: yellow;">
             This is also on page 4
         </div>
-        <div style="height: 100px; background-color: brown; page-break-inside: avoid;">
-            This is also on page 4.
-        </div>
 
-        <div style="height: 700px; background-color: aqua; page-break-inside: avoid;">
+        <div style="height: 300px; background-color: aqua; page-break-inside: avoid;">
+            This is on page 4
+        </div>
+        <div style="height:400px; background-color: orangered; page-break-inside: avoid;">
             This is on page 5
         </div>
     </div>

Source/Demos/HtmlRenderer.Demo.Console/Program.cs

@@ -24,7 +24,7 @@
 foreach (var htmlSample in samples)
 {
     ////Just doing one test here.  Comment this for all of them.
-    if (!htmlSample.FullName.Contains("37")) continue;
+    if (!htmlSample.FullName.Contains("37") && !htmlSample.FullName.Contains("34")) continue;
 
     //await skia.GenerateSampleAsync(htmlSample);
     //await svgSkia.GenerateSampleAsync(htmlSample);

Source/HtmlRenderer/Core/Dom/CssBox.cs

@@ -70,6 +70,12 @@ internal class CssBox : CssBoxProperties, IDisposable
         /// Flag that indicates that CssTable algorithm already made fixes on it.
         /// </remarks>
         internal bool _tableFixed;
+        
+        /// <summary>
+        /// Flag indicating that we're on a second pass for laying out the element after 
+        /// avoiding a page break.
+        /// </summary>
+        internal bool _pageBreakAvoided;
 
         protected bool _wordsSizeMeasured;
         private CssBox _listItemBox;
@@ -651,7 +657,9 @@ protected virtual async Task PerformLayoutImpAsync(RGraphics g)
                     Size = new RSize(width - ActualMarginLeft - ActualMarginRight, Size.Height);
                 }
 
-                if (Display != CssConstants.TableCell)
+                // Don't recalculate the element's position if we're 
+                // on our second pass after avoiding a page break.
+                if (Display != CssConstants.TableCell && !_pageBreakAvoided)
                 {
                     double left;
                     double top;
@@ -700,15 +708,6 @@ protected virtual async Task PerformLayoutImpAsync(RGraphics g)
                         {
                             this.BreakPage(true);
                         }
-                        else if (this.PageBreakInside == CssConstants.Avoid && prevSibling != null)
-                        {
-                            // handle page break avoiding.
-                            var pageLocationY = Location.Y % HtmlContainer.PageSize.Height;
-                            if (ActualHeight + pageLocationY > HtmlContainer.PageSize.Height)
-                            {
-                                this.BreakPage(true);
-                            }
-                        }
 
                         //Start with the assumption this is zero height.
                         ActualBottom = Location.Y;
@@ -754,6 +753,27 @@ protected virtual async Task PerformLayoutImpAsync(RGraphics g)
 
             if (!IsFixed && !IsAbsolute)
             {
+                if (PageBreakInside == CssConstants.Avoid 
+                    && prevSibling != null 
+                    && Display != CssConstants.TableCell
+                    && !_pageBreakAvoided)
+                {
+                    // handle page break avoiding.
+                    var pageLocationY = Location.Y % HtmlContainer.PageSize.Height;
+                    if (Size.Height + pageLocationY > HtmlContainer.PageSize.Height)
+                    {
+                        // if we break page, we'll do another pass at PerformLayoutAsync
+                        // so that child elements are re-positioned correctly.
+                        BreakPage(true);
+                        _pageBreakAvoided = true;
+                        await this.PerformLayoutAsync(g);
+
+                        // We'll set this flag back to false as this element
+                        // might need re-positioning by its parent.
+                        _pageBreakAvoided = false;
+                    }
+                }
+
                 var actualWidth = Math.Max(GetMinimumWidth() + GetWidthMarginDeep(this), Size.Width < 90999 ? ActualRight - HtmlContainer.Root.Location.X : 0);
                 HtmlContainer.ActualSize = CommonUtils.Max(HtmlContainer.ActualSize, new RSize(actualWidth, ActualBottom - HtmlContainer.Root.Location.Y));
             }