qt: Improve BitcoinAmountField class

This adds functions for specifing a min/max value for a
BitcoinAmountField. These options only affect user input, so it's still
possible to use setValue to set values outside of the min/max range. The
existing value will not be changed when calling these functions even if
it's out of range. The min/max range will be reinforced when the field
loses focus.
This also adds `SetAllowEmpty` function which specifies if the field is
allowed to be left empty by the user. If set to false the field will be
set to the minimum allowed value if it's empty when focus is lost.

src/qt/bitcoinamountfield.cpp

@@ -23,9 +23,7 @@ class AmountSpinBox: public QAbstractSpinBox
 
 public:
     explicit AmountSpinBox(QWidget *parent):
-        QAbstractSpinBox(parent),
-        currentUnit(BitcoinUnits::BTC),
-        singleStep(100000) // satoshis
+        QAbstractSpinBox(parent)
     {
         setAlignment(Qt::AlignRight);
 
@@ -44,10 +42,19 @@ public:
 
     void fixup(QString &input) const
     {
-        bool valid = false;
-        CAmount val = parse(input, &valid);
-        if(valid)
-        {
+        bool valid;
+        CAmount val;
+
+        if (input.isEmpty() && !m_allow_empty) {
+            valid = true;
+            val = m_min_amount;
+        } else {
+            valid = false;
+            val = parse(input, &valid);
+        }
+
+        if (valid) {
+            val = qBound(m_min_amount, val, m_max_amount);
             input = BitcoinUnits::format(currentUnit, val, false, BitcoinUnits::separatorAlways);
             lineEdit()->setText(input);
         }
@@ -64,12 +71,27 @@ public:
         Q_EMIT valueChanged();
     }
 
+    void SetAllowEmpty(bool allow)
+    {
+        m_allow_empty = allow;
+    }
+
+    void SetMinValue(const CAmount& value)
+    {
+        m_min_amount = value;
+    }
+
+    void SetMaxValue(const CAmount& value)
+    {
+        m_max_amount = value;
+    }
+
     void stepBy(int steps)
     {
         bool valid = false;
         CAmount val = value(&valid);
         val = val + steps * singleStep;
-        val = qMin(qMax(val, CAmount(0)), BitcoinUnits::maxMoney());
+        val = qBound(m_min_amount, val, m_max_amount);
         setValue(val);
     }
 
@@ -125,9 +147,12 @@ public:
     }
 
 private:
-    int currentUnit;
-    CAmount singleStep;
+    int currentUnit{BitcoinUnits::BTC};
+    CAmount singleStep{CAmount(100000)}; // satoshis
     mutable QSize cachedMinimumSizeHint;
+    bool m_allow_empty{true};
+    CAmount m_min_amount{CAmount(0)};
+    CAmount m_max_amount{BitcoinUnits::maxMoney()};
 
     /**
      * Parse a string into a number of base monetary units and
@@ -174,11 +199,10 @@ protected:
         StepEnabled rv = 0;
         bool valid = false;
         CAmount val = value(&valid);
-        if(valid)
-        {
-            if(val > 0)
+        if (valid) {
+            if (val > m_min_amount)
                 rv |= StepDownEnabled;
-            if(val < BitcoinUnits::maxMoney())
+            if (val < m_max_amount)
                 rv |= StepUpEnabled;
         }
         return rv;
@@ -275,6 +299,21 @@ void BitcoinAmountField::setValue(const CAmount& value)
     amount->setValue(value);
 }
 
+void BitcoinAmountField::SetAllowEmpty(bool allow)
+{
+    amount->SetAllowEmpty(allow);
+}
+
+void BitcoinAmountField::SetMinValue(const CAmount& value)
+{
+    amount->SetMinValue(value);
+}
+
+void BitcoinAmountField::SetMaxValue(const CAmount& value)
+{
+    amount->SetMaxValue(value);
+}
+
 void BitcoinAmountField::setReadOnly(bool fReadOnly)
 {
     amount->setReadOnly(fReadOnly);

src/qt/bitcoinamountfield.h

@@ -31,6 +31,15 @@ public:
     CAmount value(bool *value=0) const;
     void setValue(const CAmount& value);
 
+    /** If allow empty is set to false the field will be set to the minimum allowed value if left empty. **/
+    void SetAllowEmpty(bool allow);
+
+    /** Set the minimum value in satoshis **/
+    void SetMinValue(const CAmount& value);
+
+    /** Set the maximum value in satoshis **/
+    void SetMaxValue(const CAmount& value);
+
     /** Set single step in satoshis **/
     void setSingleStep(const CAmount& step);