Preliminary implementation of DragGestureDetector

Bug: 124533040
Test: DragGestureDetectorTest

Change-Id: I936869fe9f52f85f63f7c4e54cb1e771ac276fba

diff --git a/ui/framework/src/main/java/androidx/ui/core/gesture/constants.kt b/ui/framework/src/main/java/androidx/ui/core/gesture/constants.kt
new file mode 100644
index 0000000..714bd82
--- /dev/null
+++ b/ui/framework/src/main/java/androidx/ui/core/gesture/constants.kt

@@ -0,0 +1,143 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package androidx.ui.core.gesture
+
+import androidx.ui.core.Duration
+import androidx.ui.core.dp
+import androidx.ui.core.milliseconds
+
+/**
+ * Modeled after Android's ViewConfiguration:
+ * https://github.com/android/platform_frameworks_base/blob/master/core/java/android/view/ViewConfiguration.java
+ */
+
+/**
+ * The time that must elapse before a tap gesture sends onTapDown, if there's
+ * any doubt that the gesture is a tap.
+ */
+val PressTimeout: Duration = 100.milliseconds
+
+/**
+ * Maximum length of time between a tap down and a tap up for the gesture to be
+ * considered a tap. (Currently not honored by the TapGestureRecognizer.)
+ */
+// TODO(shepshapard): Remove this, or implement a hover-tap gesture recognizer which
+// uses this.
+val HoverTapTimeout: Duration = 150.milliseconds
+
+/**
+ * Maximum distance between the down and up pointers for a tap. (Currently not
+ * honored by the [TapGestureRecognizer]; [PrimaryPointerGestureRecognizer],
+ * which TapGestureRecognizer inherits from, uses [kTouchSlop].)
+ */
+// TODO(shepshapard): Remove this or implement it correctly.
+val HoverTapSlop = 20.dp
+
+/** The time before a long press gesture attempts to win. */
+val LongPressTimeout: Duration = 500.milliseconds
+
+/**
+ * The maximum time from the start of the first tap to the start of the second
+ * tap in a double-tap gesture.
+ */
+// TODO(shepshapard): In Android, this is actually the time from the first's up event
+// to the second's down event, according to the ViewConfiguration docs.
+val DoubleTapTimeout: Duration = 300.milliseconds
+
+/**
+ * The minimum time from the end of the first tap to the start of the second
+ * tap in a double-tap gesture. (Currently not honored by the
+ * DoubleTapGestureRecognizer.)
+ */
+// TODO(shepshapard): Either implement this or remove the constant.
+val DoubleTapMinTime: Duration = 40.milliseconds
+
+/**
+ * The distance a touch has to travel for the framework to be confident that
+ * the gesture is a scroll gesture, or, inversely, the maximum distance that a
+ * touch can travel before the framework becomes confident that it is not a
+ * tap.
+ */
+// This value was empirically derived. We started at 8.0 and increased it to
+// 18.0 after getting complaints that it was too difficult to hit targets.
+val TouchSlop = 18.dp
+
+/**
+ * The maximum distance that the first touch in a double-tap gesture can travel
+ * before deciding that it is not part of a double-tap gesture.
+ * DoubleTapGestureRecognizer also restricts the second touch to this distance.
+ */
+val DoubleTapTouchSlop = TouchSlop
+
+/**
+ * Distance between the initial position of the first touch and the start
+ * position of a potential second touch for the second touch to be considered
+ * the second touch of a double-tap gesture.
+ */
+val DoubleTapSlop = 100.dp
+
+/**
+ * The time for which zoom controls (e.g. in a map interface) are to be
+ * displayed on the screen, from the moment they were last requested.
+ */
+val ZoomControlsTimeout: Duration = 3000.milliseconds
+
+/**
+ * The distance a touch has to travel for the framework to be confident that
+ * the gesture is a paging gesture. (Currently not used, because paging uses a
+ * regular drag gesture, which uses kTouchSlop.)
+ */
+// TODO(shepshapard): Create variants of HorizontalDragGestureRecognizer et al for
+// paging, which use this constant.
+val PagingTouchSlop = TouchSlop * 2.dp
+
+/**
+ * The distance a touch has to travel for the framework to be confident that
+ * the gesture is a panning gesture.
+ */
+val PanSlop = TouchSlop * 2.dp
+
+/**
+ * The distance a touch has to travel for the framework to be confident that
+ * the gesture is a scale gesture.
+ */
+val ScaleSlop = TouchSlop
+
+/**
+ * The margin around a dialog, popup menu, or other window-like widget inside
+ * which we do not consider a tap to dismiss the widget. (Not currently used.)
+ */
+// TODO(shepshapard): Make ModalBarrier support this.
+val WindowTouchSlop = 16.dp
+
+/**
+ * The minimum velocity for a touch to consider that touch to trigger a fling
+ * gesture.
+ */
+// TODO(shepshapard): Make sure nobody has their own version of this.
+val MinFlingVelocity = 50.dp // Logical pixels / second
+
+/** Drag gesture fling velocities are clipped to this value. */
+// TODO(shepshapard): Make sure nobody has their own version of this.
+val MaxFlingVelocity = 8000.dp // Logical pixels / second
+
+/**
+ * The maximum time from the start of the first tap to the start of the second
+ * tap in a jump-tap gesture.
+ */
+// TODO(shepshapard): Implement jump-tap gestures.
+val JumpTapTimeout: Duration = 500.milliseconds