Fix tst_qquickpincharea::pan with highdpi and document it

The test was expanded slightly and documented, so that it's easier to
understand the strangeness of the center.
On my display the dragThreshold is returned as 16 while the test assued
it's always <= 10.
Explain the math and the coordinate changes due to everything being in
item coordinates.

Change-Id: I4bf72271895ecd9e248154c08bf36eab376c6d79
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

2 files changed, 54 insertions, 13 deletions

diff --git a/tests/auto/quick/qquickpincharea/data/pinchproperties.qml b/tests/auto/quick/qquickpincharea/data/pinchproperties.qml
index 37d706fc8e..c5daf6cbfe 100644
--- a/tests/auto/quick/qquickpincharea/data/pinchproperties.qml
+++ b/tests/auto/quick/qquickpincharea/data/pinchproperties.qml
@@ -2,7 +2,7 @@ import QtQuick 2.0
 Rectangle {
     id: whiteRect
     property variant center
-    property real scale
+    property real scale: -1.0
     property int pointCount: 0
     property bool pinchActive: false
     width: 240; height: 320
diff --git a/tests/auto/quick/qquickpincharea/tst_qquickpincharea.cpp b/tests/auto/quick/qquickpincharea/tst_qquickpincharea.cpp
index aee35b4b90..1d7273f6df 100644
--- a/tests/auto/quick/qquickpincharea/tst_qquickpincharea.cpp
+++ b/tests/auto/quick/qquickpincharea/tst_qquickpincharea.cpp
@@ -286,6 +286,7 @@ void tst_QQuickPinchArea::pan()
     QPoint p1(80, 80);
     QPoint p2(100, 100);
     {
+        const int dragThreshold = QGuiApplication::styleHints()->startDragDistance();
         QTest::QTouchEventSequence pinchSequence = QTest::touchEvent(window, device);
         pinchSequence.press(0, p1, window).commit();
         QQuickTouchUtils::flush(window);
@@ -293,23 +294,63 @@ void tst_QQuickPinchArea::pan()
         // we have to reuse the same pinchSequence object.
         pinchSequence.stationary(0).press(1, p2, window).commit();
         QQuickTouchUtils::flush(window);
-        p1 += QPoint(10,10);
-        p2 += QPoint(10,10);
-        pinchSequence.move(0, p1,window).move(1, p2,window).commit();
+        QVERIFY(!root->property("pinchActive").toBool());
+        QCOMPARE(root->property("scale").toReal(), -1.0);
+
+        p1 += QPoint(dragThreshold - 1, 0);
+        p2 += QPoint(dragThreshold - 1, 0);
+        pinchSequence.move(0, p1, window).move(1, p2, window).commit();
         QQuickTouchUtils::flush(window);
+        // movement < dragThreshold: pinch not yet active
+        QVERIFY(!root->property("pinchActive").toBool());
+        QCOMPARE(root->property("scale").toReal(), -1.0);
 
-        QCOMPARE(root->property("scale").toReal(), 1.0);
+        // exactly the dragThreshold: pinch starts
+        p1 += QPoint(1, 0);
+        p2 += QPoint(1, 0);
+        pinchSequence.move(0, p1, window).move(1, p2, window).commit();
+        QQuickTouchUtils::flush(window);
         QVERIFY(root->property("pinchActive").toBool());
+        QCOMPARE(root->property("scale").toReal(), 1.0);
 
-        p1 += QPoint(10,10);
-        p2 += QPoint(10,10);
-        pinchSequence.move(0, p1,window).move(1, p2,window).commit();
+        // Calculation of the center point is tricky at first:
+        // center point of the two touch points in item coordinates:
+        // scene coordinates: (80, 80) + (dragThreshold, 0), (100, 100) + (dragThreshold, 0)
+        //                    = ((180+dT)/2, 180/2) = (90+dT, 90)
+        // item  coordinates: (scene) - (50, 50) = (40+dT, 40)
+        QCOMPARE(root->property("center").toPointF(), QPointF(40 + dragThreshold, 40));
+        // pan started, but no actual movement registered yet:
+        // blackrect starts at 50,50
+        QCOMPARE(blackRect->x(), 50.0);
+        QCOMPARE(blackRect->y(), 50.0);
+
+        p1 += QPoint(10, 0);
+        p2 += QPoint(10, 0);
+        pinchSequence.move(0, p1, window).move(1, p2, window).commit();
         QQuickTouchUtils::flush(window);
-    }
+        QCOMPARE(root->property("center").toPointF(), QPointF(40 + 10 + dragThreshold, 40));
+        QCOMPARE(blackRect->x(), 60.0);
+        QCOMPARE(blackRect->y(), 50.0);
 
-    QCOMPARE(root->property("center").toPointF(), QPointF(60, 60)); // blackrect is at 50,50
-    QCOMPARE(blackRect->x(), 60.0);
-    QCOMPARE(blackRect->y(), 60.0);
+        p1 += QPoint(0, 10);
+        p2 += QPoint(0, 10);
+        pinchSequence.move(0, p1, window).move(1, p2, window).commit();
+        QQuickTouchUtils::flush(window);
+        // next big surprise: the center is in item local coordinates and the item was just
+        // moved 10 to the right... which offsets the center point 10 to the left
+        QCOMPARE(root->property("center").toPointF(), QPointF(40 + 10 - 10 + dragThreshold, 40 + 10));
+        QCOMPARE(blackRect->x(), 60.0);
+        QCOMPARE(blackRect->y(), 60.0);
+
+        p1 += QPoint(10, 10);
+        p2 += QPoint(10, 10);
+        pinchSequence.move(0, p1, window).move(1, p2, window).commit();
+        QQuickTouchUtils::flush(window);
+        // now the item moved again, thus the center point of the touch is moved in total by (10, 10)
+        QCOMPARE(root->property("center").toPointF(), QPointF(50 + dragThreshold, 50));
+        QCOMPARE(blackRect->x(), 70.0);
+        QCOMPARE(blackRect->y(), 70.0);
+    }
 
     // pan x beyond bound
     p1 += QPoint(100,100);
@@ -318,7 +359,7 @@ void tst_QQuickPinchArea::pan()
     QQuickTouchUtils::flush(window);
 
     QCOMPARE(blackRect->x(), 140.0);
-    QCOMPARE(blackRect->y(), 160.0);
+    QCOMPARE(blackRect->y(), 170.0);
 
     QTest::touchEvent(window, device).release(0, p1, window).release(1, p2, window);
     QQuickTouchUtils::flush(window);