Delay applying a changed filter value

When changing a filter value delay the application of the new value on
the table for a couple of milliseconds. This makes the filter process
much smoother for fast typing users working on large tables.

See issue #415.

src/FilterLineEdit.cpp

@@ -0,0 +1,70 @@
+#include "FilterLineEdit.h"
+
+#include <QTimer>
+#include <QKeyEvent>
+
+FilterLineEdit::FilterLineEdit(QWidget* parent, QList<FilterLineEdit*>* filters, int columnnum) : QLineEdit(parent), filterList(filters), columnNumber(columnnum)
+{
+    setPlaceholderText(tr("Filter"));
+#if QT_VERSION >= QT_VERSION_CHECK(5, 2, 0)
+    setClearButtonEnabled(true);
+#endif
+    setProperty("column", columnnum);            // Store the column number for later use
+
+    // Introduce a timer for delaying the signal triggered whenever the user changes the filter value.
+    // The idea here is that the textChanged() event isn't connected to the update filter slot directly anymore
+    // but instead there this timer mechanism in between: whenever the user changes the filter the delay timer
+    // is (re)started. As soon as the user stops typing the timer has a chance to trigger and call the
+    // delayedSignalTimerTriggered() method which then stops the timer and emits the delayed signal.
+    delaySignalTimer = new QTimer(this);
+    delaySignalTimer->setInterval(300);         // This is the milliseconds of not-typing we want to wait before triggering
+    connect(this, SIGNAL(textChanged(QString)), delaySignalTimer, SLOT(start()));
+    connect(delaySignalTimer, SIGNAL(timeout()), this, SLOT(delayedSignalTimerTriggered()));
+
+    // Immediately emit the delayed filter value changed signal if the user presses the enter or the return key or
+    // the line edit widget loses focus
+    connect(this, SIGNAL(editingFinished()), this, SLOT(delayedSignalTimerTriggered()));
+}
+
+void FilterLineEdit::delayedSignalTimerTriggered()
+{
+    // Stop the timer first to avoid triggering in intervals
+    delaySignalTimer->stop();
+
+    // Emit the delayed signal using the current value
+    emit delayedTextChanged(text());
+}
+
+void FilterLineEdit::keyReleaseEvent(QKeyEvent* event)
+{
+    if(event->key() == Qt::Key_Tab)
+    {
+        if(columnNumber < filterList->size() - 1)
+        {
+            filterList->at(columnNumber + 1)->setFocus();
+            event->accept();
+        }
+    } else if(event->key() == Qt::Key_Backtab) {
+        if(columnNumber > 0)
+        {
+            filterList->at(columnNumber - 1)->setFocus();
+            event->accept();
+        }
+    }
+}
+
+void FilterLineEdit::clear()
+{
+    // When programatically clearing the line edit's value make sure the effects are applied immediately, i.e.
+    // bypass the delayed signal timer
+    QLineEdit::clear();
+    delayedSignalTimerTriggered();
+}
+
+void FilterLineEdit::setText(const QString& text)
+{
+    // When programatically setting the line edit's value make sure the effects are applied immediately, i.e.
+    // bypass the delayed signal timer
+    QLineEdit::setText(text);
+    delayedSignalTimerTriggered();
+}