Add minimum constraint on custom fees field

src/qt/bitcoinamountfield.cpp

@@ -28,6 +28,7 @@ public:
         singleStep(COIN) // koinu
     {
         setAlignment(Qt::AlignRight);
+        min = CAmount(0);
 
         connect(lineEdit(), SIGNAL(textEdited(QString)), this, SIGNAL(valueChanged()));
     }
@@ -64,12 +65,27 @@ public:
         emit valueChanged();
     }
 
+    void setMinimum(const int min)
+    {
+        this->min = min * COIN;
+        bool valid = false;
+        CAmount val = value(&valid);
+        if (val < this->min) {
+            setValue(this->min);
+        }
+    }
+
+    int minimum()
+    {
+        return min / COIN;
+    }
+
     void stepBy(int steps)
     {
         bool valid = false;
         CAmount val = value(&valid);
         val = val + steps * singleStep;
-        val = qMin(qMax(val, CAmount(0)), BitcoinUnits::maxMoney());
+        val = qMin(qMax(val, min), BitcoinUnits::maxMoney());
         setValue(val);
     }
 
@@ -126,6 +142,7 @@ public:
 
 private:
     int currentUnit;
+    CAmount min;
     CAmount singleStep;
     mutable QSize cachedMinimumSizeHint;
 
@@ -140,7 +157,7 @@ private:
         bool valid = BitcoinUnits::parse(currentUnit, text, &val);
         if(valid)
         {
-            if(val < 0 || val > BitcoinUnits::maxMoney())
+            if(val < min || val > BitcoinUnits::maxMoney())
                 valid = false;
         }
         if(valid_out)
@@ -227,6 +244,16 @@ void BitcoinAmountField::setText(const QString &text)
         amount->setValue(QLocale().toDouble(text));
 }
 
+void BitcoinAmountField::setMinimum(const int min)
+{
+    amount->setMinimum(min);
+}
+
+int BitcoinAmountField::minimum()
+{
+    return amount->minimum();
+}
+
 void BitcoinAmountField::clear()
 {
     amount->clear();

src/qt/bitcoinamountfield.h

@@ -33,6 +33,9 @@ public:
     CAmount value(bool *value=0) const;
     void setValue(const CAmount& value);
 
+    void setMinimum(const int min);
+    int minimum();
+
     /** Set single step in satoshis **/
     void setSingleStep(const CAmount& step);
 

src/qt/forms/sendcoinsdialog.ui

@@ -882,7 +882,14 @@
                   </widget>
                  </item>
                  <item>
-                  <widget class="BitcoinAmountField" name="customFee"/>
+                  <widget class="BitcoinAmountField" name="customFee">
+                   <property name="minimum">
+                    <number>1</number>
+                   </property>
+                   <property name="value">
+                    <number>1</number>
+                   </property>
+                  </widget>
                  </item>
                  <item>
                   <spacer name="horizontalSpacer_6">