Add a placeholderText property to QPlainTextEdit

Setting this property makes the editor display a grayed-out
placeholder text as long as the document() is empty.

Change-Id: I997edb867419613ff7cedc760a87c684a2ded711
Reviewed-by: David Faure <david.faure@kdab.com>

src/widgets/widgets/qplaintextedit.cpp

@@ -1319,6 +1319,35 @@ QTextDocument *QPlainTextEdit::document() const
     return d->control->document();
 }
 
+/*!
+    \since 5.3
+
+    \property QPlainTextEdit::placeholderText
+    \brief the editor placeholder text
+
+    Setting this property makes the editor display a grayed-out
+    placeholder text as long as the document() is empty.
+
+    By default, this property contains an empty string.
+
+    \sa document()
+*/
+void QPlainTextEdit::setPlaceholderText(const QString &placeholderText)
+{
+    Q_D(QPlainTextEdit);
+    if (d->placeholderText != placeholderText) {
+        d->placeholderText = placeholderText;
+        if (d->control->document()->isEmpty())
+            d->viewport->update();
+    }
+}
+
+QString QPlainTextEdit::placeholderText() const
+{
+    Q_D(const QPlainTextEdit);
+    return d->placeholderText;
+}
+
 /*!
     Sets the visible \a cursor.
 */
@@ -1945,7 +1974,16 @@ void QPlainTextEdit::paintEvent(QPaintEvent *e)
             }
 
 
-            layout->draw(&painter, offset, selections, er);
+            if (!placeholderText().isEmpty() && document()->isEmpty()) {
+              Q_D(QPlainTextEdit);
+              QColor col = d->control->palette().text().color();
+              col.setAlpha(128);
+              painter.setPen(col);
+              const int margin = int(document()->documentMargin());
+              painter.drawText(r.adjusted(margin, 0, 0, 0), Qt::AlignTop | Qt::TextWordWrap, placeholderText());
+            } else {
+              layout->draw(&painter, offset, selections, er);
+            }
             if ((drawCursor && !drawCursorAsBlock)
                 || (editable && context.cursorPosition < -1
                     && !layout->preeditAreaText().isEmpty())) {

src/widgets/widgets/qplaintextedit.h

@@ -83,6 +83,7 @@ class Q_WIDGETS_EXPORT QPlainTextEdit : public QAbstractScrollArea
     Q_PROPERTY(int maximumBlockCount READ maximumBlockCount WRITE setMaximumBlockCount)
     Q_PROPERTY(bool backgroundVisible READ backgroundVisible WRITE setBackgroundVisible)
     Q_PROPERTY(bool centerOnScroll READ centerOnScroll WRITE setCenterOnScroll)
+    Q_PROPERTY(QString placeholderText READ placeholderText WRITE setPlaceholderText)
 public:
     enum LineWrapMode {
         NoWrap,
@@ -96,6 +97,9 @@ public:
     void setDocument(QTextDocument *document);
     QTextDocument *document() const;
 
+    void setPlaceholderText(const QString &placeholderText);
+    QString placeholderText() const;
+
     void setTextCursor(const QTextCursor &cursor);
     QTextCursor textCursor() const;
 

src/widgets/widgets/qplaintextedit_p.h

@@ -179,6 +179,7 @@ public:
     void _q_modificationChanged(bool);
 
     int originalOffsetY;
+    QString placeholderText;
 };
 
 QT_END_NAMESPACE