WTableView: preloadMarginRows

Author: RockinRoel

src/Wt/WTableView.C

@@ -58,6 +58,7 @@ WTableView::WTableView()
     dropEvent_(impl_, "dropEvent"),
     scrolled_(impl_, "scrolled"),
     itemTouchSelectEvent_(impl_, "itemTouchSelectEvent"),
+    preloadMarginRows_(-1),
     firstColumn_(-1),
     lastColumn_(-1),
     viewportLeft_(0),
@@ -685,10 +686,14 @@ void WTableView::renderTable(const int fr, const int lr,
   assert(lastRow() == lr && firstRow() == fr);
   assert(lastColumn() == lc && firstColumn() == fc);
 
+  const int marginHeight = preloadMarginRows_ == -1 ?
+        viewportHeight_ / 2 :
+        static_cast<int>(preloadMarginRows_ * rowHeight().toPixels()) / 2;
+
   int scrollX1 = std::max(0, viewportLeft_ - viewportWidth_ / 2);
   int scrollX2 = viewportLeft_ + viewportWidth_ / 2;
-  int scrollY1 = std::max(0, viewportTop_ - viewportHeight_ / 2);
-  int scrollY2 = viewportTop_ + viewportHeight_ / 2;
+  int scrollY1 = std::max(0, viewportTop_ - marginHeight);
+  int scrollY2 = viewportTop_ + marginHeight;
 
   WStringStream s;
 
@@ -1640,22 +1645,25 @@ void WTableView::computeRenderedArea()
 
     if (viewportHeight_ != -1) {
       /* row range */
-      int top = std::min(viewportTop_,
+      const int top = std::min(viewportTop_,
 			 static_cast<int>(canvas_->height().toPixels()));
 
-      int height = std::min(viewportHeight_,
+      const int height = std::min(viewportHeight_,
 			    static_cast<int>(canvas_->height().toPixels()));
 
-      int renderedRows = static_cast<int>(height / rowHeight().toPixels()
+      const int renderedRows = static_cast<int>(height / rowHeight().toPixels()
 					  + 0.5);
 
       renderedFirstRow_ = static_cast<int>(top / rowHeight().toPixels());
 
+      const int marginRows = preloadMarginRows_ == -1 ? renderedRows + borderRows : preloadMarginRows_;
+
       renderedLastRow_
-	= std::min(renderedFirstRow_ + renderedRows * 2 + borderRows,
-		   modelHeight - 1);
+        = static_cast<int>(std::min(static_cast<long long>(renderedFirstRow_) + renderedRows + marginRows,
+                   static_cast<long long>(modelHeight - 1)));
       renderedFirstRow_
-	= std::max(renderedFirstRow_ - renderedRows - borderRows, 0);
+        = static_cast<int>(std::max(static_cast<long long>(renderedFirstRow_) - marginRows,
+                                    0LL));
     } else {
       renderedFirstRow_ = 0;
       renderedLastRow_ = modelHeight - 1;
@@ -2110,6 +2118,15 @@ void WTableView::setOverflow(Overflow overflow,
     contentsContainer_->setOverflow(overflow, orientation);
 }
 
+void WTableView::setPreloadMarginRows(int rows)
+{
+  preloadMarginRows_ = rows;
+
+  computeRenderedArea();
+
+  scheduleRerender(RenderState::NeedAdjustViewPort);
+}
+
 void WTableView::setRowHeaderCount(int count)
 {
   WAbstractItemView::setRowHeaderCount(count);

src/Wt/WTableView.h

@@ -123,6 +123,25 @@ class WT_API WTableView : public WAbstractItemView
 		   WFlags<Orientation> orientation 
 		   = (Orientation::Horizontal | Orientation::Vertical));
 
+  /*! \brief Sets the amount of rows to preload.
+   *
+   * Configures the amount of rows before and after the currently visible
+   * range that should be loaded. If N rows are shown, and C is the margin,
+   * then N + 2C rows are loaded: N visible, C rows before, and C rows after.
+   *
+   * Set to 0 if you don't want to load more rows than are currently visible.
+   *
+   * Set to -1 if you want to keep default behaviour. This means that if N rows
+   * are visible, then 3N rows are loaded: N visible, N rows before, and N rows after.
+   */
+  void setPreloadMarginRows(int rows);
+
+  /*! \brief Get the preload margin.
+   *
+   * \sa setPreloadMarginRows()
+   */
+  int preloadMarginRows() const { return preloadMarginRows_; }
+
   virtual void setHidden(bool hidden,
 			 const WAnimation& animation = WAnimation()) override;
 
@@ -176,6 +195,8 @@ class WT_API WTableView : public WAbstractItemView
   Signals::connection touchMoveConnection_;
   Signals::connection touchEndConnection_;
 
+  int preloadMarginRows_;
+
   /* Ajax only: First and last columns rendered (this somewhat
    * redundant with the state contained in the widgets, but because
    * columns are variable width, we cache these values as well). The