src/Tizen.NUI.Components/Controls/ScrollableBase.cs

   1 /* Copyright (c) 2019 Samsung Electronics Co., Ltd.
   2  *
   3  * Licensed under the Apache License, Version 2.0 (the "License");
   4  * you may not use this file except in compliance with the License.
   5  * You may obtain a copy of the License at
   6  *
   7  * http://www.apache.org/licenses/LICENSE-2.0
   8  *
   9  * Unless required by applicable law or agreed to in writing, software
  10  * distributed under the License is distributed on an "AS IS" BASIS,
  11  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  12  * See the License for the specific language governing permissions and
  13  * limitations under the License.
  14  *
  15  */
  16 using System;
  17 using Tizen.NUI.BaseComponents;
  18 using System.ComponentModel;
  19 using System.Diagnostics;
  20 namespace Tizen.NUI.Components
  21 {
  22     /// <summary>
  23     /// [Draft] This class provides a View that can scroll a single View with a layout. This View can be a nest of Views.
  24     /// </summary>
  25     /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API.
  26     [EditorBrowsable(EditorBrowsableState.Never)]
  27     public class ScrollableBase : Control
  28     {
  29             static bool LayoutDebugScrollableBase = false; // Debug flag
  30         private Direction mScrollingDirection = Direction.Vertical;
  31
  32         private class ScrollableBaseCustomLayout : LayoutGroup
  33         {
  34             protected override void OnMeasure(MeasureSpecification widthMeasureSpec, MeasureSpecification heightMeasureSpec)
  35             {
  36                 Extents padding = Padding;
  37                 float totalHeight = padding.Top + padding.Bottom;
  38                 float totalWidth = padding.Start + padding.End;
  39
  40                 MeasuredSize.StateType childWidthState = MeasuredSize.StateType.MeasuredSizeOK;
  41                 MeasuredSize.StateType childHeightState = MeasuredSize.StateType.MeasuredSizeOK;
  42
  43                 Direction scrollingDirection = Direction.Vertical;
  44                 ScrollableBase scrollableBase = this.Owner as ScrollableBase;
  45                 if (scrollableBase)
  46                 {
  47                    scrollingDirection = scrollableBase.ScrollingDirection;
  48                 }
  49
  50                 // measure child, should be a single scrolling child
  51                 foreach( LayoutItem childLayout in LayoutChildren )
  52                 {
  53                     if (childLayout != null)
  54                     {
  55                         // Get size of child
  56                         // Use an Unspecified MeasureSpecification mode so scrolling child is not restricted to it's parents size in Height (for vertical scrolling)
  57                         // or Width for horizontal scrolling
  58                         MeasureSpecification unrestrictedMeasureSpec = new MeasureSpecification( heightMeasureSpec.Size, MeasureSpecification.ModeType.Unspecified);
  59
  60                         if (scrollingDirection == Direction.Vertical)
  61                         {
  62                             MeasureChild( childLayout, widthMeasureSpec, unrestrictedMeasureSpec );  // Height unrestricted by parent
  63                         }
  64                         else
  65                         {
  66                             MeasureChild( childLayout, unrestrictedMeasureSpec, heightMeasureSpec );  // Width unrestricted by parent
  67                         }
  68
  69                         float childWidth = childLayout.MeasuredWidth.Size.AsDecimal();
  70                         float childHeight = childLayout.MeasuredHeight.Size.AsDecimal();
  71
  72                         // Determine the width and height needed by the children using their given position and size.
  73                         // Children could overlap so find the left most and right most child.
  74                         Position2D childPosition = childLayout.Owner.Position2D;
  75                         float childLeft = childPosition.X;
  76                         float childTop = childPosition.Y;
  77
  78                         // Store current width and height needed to contain all children.
  79                         Extents childMargin = childLayout.Margin;
  80                         totalWidth = childWidth + childMargin.Start + childMargin.End;
  81                         totalHeight = childHeight + childMargin.Top + childMargin.Bottom;
  82
  83                         if (childLayout.MeasuredWidth.State == MeasuredSize.StateType.MeasuredSizeTooSmall)
  84                         {
  85                             childWidthState = MeasuredSize.StateType.MeasuredSizeTooSmall;
  86                         }
  87                         if (childLayout.MeasuredWidth.State == MeasuredSize.StateType.MeasuredSizeTooSmall)
  88                         {
  89                             childHeightState = MeasuredSize.StateType.MeasuredSizeTooSmall;
  90                         }
  91                     }
  92                 }
  93
  94
  95                 MeasuredSize widthSizeAndState = ResolveSizeAndState(new LayoutLength(totalWidth), widthMeasureSpec, MeasuredSize.StateType.MeasuredSizeOK);
  96                 MeasuredSize heightSizeAndState = ResolveSizeAndState(new LayoutLength(totalHeight), heightMeasureSpec, MeasuredSize.StateType.MeasuredSizeOK);
  97                 totalWidth = widthSizeAndState.Size.AsDecimal();
  98                 totalHeight = heightSizeAndState.Size.AsDecimal();
  99
 100                 // Ensure layout respects it's given minimum size
 101                 totalWidth = Math.Max( totalWidth, SuggestedMinimumWidth.AsDecimal() );
 102                 totalHeight = Math.Max( totalHeight, SuggestedMinimumHeight.AsDecimal() );
 103
 104                 widthSizeAndState.State = childWidthState;
 105                 heightSizeAndState.State = childHeightState;
 106
 107                 SetMeasuredDimensions( ResolveSizeAndState( new LayoutLength(totalWidth), widthMeasureSpec, childWidthState ),
 108                                        ResolveSizeAndState( new LayoutLength(totalHeight), heightMeasureSpec, childHeightState ) );
 109
 110             }
 111
 112             protected override void OnLayout(bool changed, LayoutLength left, LayoutLength top, LayoutLength right, LayoutLength bottom)
 113             {
 114                 foreach( LayoutItem childLayout in LayoutChildren )
 115                 {
 116                     if( childLayout != null )
 117                     {
 118                         LayoutLength childWidth = childLayout.MeasuredWidth.Size;
 119                         LayoutLength childHeight = childLayout.MeasuredHeight.Size;
 120
 121                         Position2D childPosition = childLayout.Owner.Position2D;
 122                         Extents padding = Padding;
 123                         Extents childMargin = childLayout.Margin;
 124
 125                         LayoutLength childLeft = new LayoutLength(childPosition.X + childMargin.Start + padding.Start);
 126                         LayoutLength childTop = new LayoutLength(childPosition.Y + childMargin.Top + padding.Top);
 127
 128                         childLayout.Layout( childLeft, childTop, childLeft + childWidth, childTop + childHeight );
 129                     }
 130                 }
 131             }
 132         } //  ScrollableBaseCustomLayout
 133
 134         /// <summary>
 135         /// The direction axis to scroll.
 136         /// </summary>
 137         /// <since_tizen> 6 </since_tizen>
 138         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API.
 139         [EditorBrowsable(EditorBrowsableState.Never)]
 140         public enum Direction
 141         {
 142             /// <summary>
 143             /// Horizontal axis.
 144             /// </summary>
 145             /// <since_tizen> 6 </since_tizen>
 146             Horizontal,
 147
 148             /// <summary>
 149             /// Vertical axis.
 150             /// </summary>
 151             /// <since_tizen> 6 </since_tizen>
 152             Vertical
 153         }
 154
 155         /// <summary>
 156         /// [Draft] Configurable speed threshold that register the gestures as a flick.
 157         /// If the flick speed less than the threshold then will not be considered a flick.
 158         /// </summary>
 159         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API.
 160         [EditorBrowsable(EditorBrowsableState.Never)]
 161         public float FlickThreshold { get; set; } = 0.2f;
 162
 163         /// <summary>
 164         /// [Draft] Configurable duration modifer for the flick animation.
 165         /// Determines the speed of the scroll, large value results in a longer flick animation. Range (0.1 - 1.0)
 166         /// </summary>
 167         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 168         [EditorBrowsable(EditorBrowsableState.Never)]
 169         public float FlickAnimationSpeed { get; set; } = 0.4f;
 170
 171         /// <summary>
 172         /// [Draft] Configurable modifer for the distance to be scrolled when flicked detected.
 173         /// It a ratio of the ScrollableBase's length. (not child's length).
 174         /// First value is the ratio of the distance to scroll with the weakest flick.
 175         /// Second value is the ratio of the distance to scroll with the strongest flick.
 176         /// Second > First.
 177         /// </summary>
 178         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 179         [EditorBrowsable(EditorBrowsableState.Never)]
 180         public Vector2 FlickDistanceMultiplierRange { get; set; } = new Vector2(0.6f, 1.8f);
 181
 182         /// <summary>
 183         /// [Draft] Scrolling direction mode.
 184         /// Default is Vertical scrolling.
 185         /// </summary>
 186         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 187         [EditorBrowsable(EditorBrowsableState.Never)]
 188         public Direction ScrollingDirection
 189         {
 190             get
 191             {
 192                 return mScrollingDirection;
 193             }
 194             set
 195             {
 196                 if(value != mScrollingDirection)
 197                 {
 198                     mScrollingDirection = value;
 199                     mPanGestureDetector.RemoveDirection(value == Direction.Horizontal ? PanGestureDetector.DirectionVertical : PanGestureDetector.DirectionHorizontal);
 200                     mPanGestureDetector.AddDirection(value == Direction.Horizontal ? PanGestureDetector.DirectionHorizontal : PanGestureDetector.DirectionVertical);
 201                 }
 202             }
 203         }
 204
 205         /// <summary>
 206         /// [Draft] Pages mode, enables moving to the next or return to current page depending on pan displacement.
 207         /// Default is false.
 208         /// </summary>
 209         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 210         [EditorBrowsable(EditorBrowsableState.Never)]
 211         public bool SnapToPage { set; get; } = false;
 212
 213         /// <summary>
 214         /// [Draft] Pages mode, Number of pages.
 215         /// </summary>
 216         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 217         [EditorBrowsable(EditorBrowsableState.Never)]
 218         public int NumberOfPages { set; get; } = 1;
 219
 220         /// <summary>
 221         /// [Draft] Duration of scroll animation.
 222         /// </summary>
 223         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 224         [EditorBrowsable(EditorBrowsableState.Never)]
 225         public int ScrollDuration { set; get; } = 125;
 226
 227         /// <summary>
 228         /// [Draft] Width of the Page.
 229         /// </summary>
 230         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 231         [EditorBrowsable(EditorBrowsableState.Never)]
 232         public int PageWidth { set; get; } = 1080; // Temporary use for prototype, should get ScrollableBase width
 233
 234         /// <summary>
 235         /// ScrollEventArgs is a class to record scroll event arguments which will sent to user.
 236         /// </summary>
 237         /// <since_tizen> 6 </since_tizen>
 238         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 239         [EditorBrowsable(EditorBrowsableState.Never)]
 240         public class ScrollEventArgs : EventArgs
 241         {
 242         }
 243
 244         /// <summary>
 245         /// An event emitted when the scrolling starts, user can subscribe or unsubscribe to this event handler.<br />
 246         /// </summary>
 247         /// <since_tizen> 6 </since_tizen>
 248         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 249         [EditorBrowsable(EditorBrowsableState.Never)]
 250         public event EventHandler<ScrollEventArgs> ScrollStartedEvent;
 251
 252         /// <summary>
 253         /// An event emitted when the scrolling ends, user can subscribe or unsubscribe to this event handler.<br />
 254         /// </summary>
 255         /// <since_tizen> 6 </since_tizen>
 256         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 257         [EditorBrowsable(EditorBrowsableState.Never)]
 258         public event EventHandler<ScrollEventArgs> ScrollEndedEvent;
 259
 260         private Animation scrollAnimation;
 261         private float maxScrollDistance;
 262         private float childTargetPosition = 0.0f;
 263         private PanGestureDetector mPanGestureDetector;
 264         private TapGestureDetector mTapGestureDetector;
 265         private View mScrollingChild;
 266         private float multiplier =1.0f;
 267         private bool scrolling = false;
 268         private float ratioOfScreenWidthToCompleteScroll = 0.5f;
 269         private float totalDisplacementForPan = 0.0f;
 270
 271         // If false then can only flick pages when the current animation/scroll as ended.
 272         private bool flickWhenAnimating = false;
 273
 274         private int currentPage = 0;
 275
 276         /// <summary>
 277         /// [Draft] Constructor
 278         /// </summary>
 279         /// <since_tizen> 6 </since_tizen>
 280         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 281         [EditorBrowsable(EditorBrowsableState.Never)]
 282         public ScrollableBase() : base()
 283         {
 284             mPanGestureDetector = new PanGestureDetector();
 285             mPanGestureDetector.Attach(this);
 286             mPanGestureDetector.Detected += OnPanGestureDetected;
 287
 288             mTapGestureDetector = new TapGestureDetector();
 289             mTapGestureDetector.Attach(this);
 290             mTapGestureDetector.Detected += OnTapGestureDetected;
 291
 292             ClippingMode = ClippingModeType.ClipToBoundingBox;
 293
 294             mScrollingChild = new View();
 295
 296             Layout = new ScrollableBaseCustomLayout();
 297         }
 298
 299         /// <summary>
 300         /// Called after a child has been added to the owning view.
 301         /// </summary>
 302         /// <param name="view">The child which has been added.</param>
 303         /// <since_tizen> 6 </since_tizen>
 304         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 305         [EditorBrowsable(EditorBrowsableState.Never)]
 306         public override void OnChildAdd(View view)
 307         {
 308             mScrollingChild = view;
 309             {
 310             if (Children.Count > 1)
 311                 Log.Error("ScrollableBase", $"Only 1 child should be added to ScrollableBase.");
 312             }
 313         }
 314
 315         /// <summary>
 316         /// Called after a child has been removed from the owning view.
 317         /// </summary>
 318         /// <param name="view">The child which has been removed.</param>
 319         /// <since_tizen> 6 </since_tizen>
 320         /// This may be public opened in tizen_6.0 after ACR done. Before ACR, need to be hidden as inhouse API
 321         [EditorBrowsable(EditorBrowsableState.Never)]
 322         public override void OnChildRemove(View view)
 323         {
 324             mScrollingChild = new View();
 325         }
 326
 327         private void OnScrollStart()
 328         {
 329             ScrollEventArgs eventArgs = new ScrollEventArgs();
 330             ScrollStartedEvent?.Invoke(this, eventArgs);
 331         }
 332
 333         private void OnScrollEnd()
 334         {
 335             ScrollEventArgs eventArgs = new ScrollEventArgs();
 336             ScrollEndedEvent?.Invoke(this, eventArgs);
 337         }
 338
 339         private void StopScroll()
 340         {
 341             if (scrollAnimation != null)
 342             {
 343                 if (scrollAnimation.State == Animation.States.Playing)
 344                 {
 345                     Debug.WriteLineIf(LayoutDebugScrollableBase, "StopScroll Animation Playing");
 346                     scrollAnimation.Stop(Animation.EndActions.Cancel);
 347                     OnScrollEnd();
 348                 }
 349                 scrollAnimation.Clear();
 350             }
 351         }
 352
 353         // static constructor registers the control type
 354         static ScrollableBase()
 355         {
 356             // ViewRegistry registers control type with DALi type registry
 357             // also uses introspection to find any properties that need to be registered with type registry
 358             CustomViewRegistry.Instance.Register(CreateInstance, typeof(ScrollableBase));
 359         }
 360
 361         internal static CustomView CreateInstance()
 362         {
 363             return new ScrollableBase();
 364         }
 365
 366         private void AnimateChildTo(int duration, float axisPosition)
 367         {
 368             Debug.WriteLineIf(LayoutDebugScrollableBase, "AnimationTo Animation Duration:" + duration + " Destination:" + axisPosition);
 369
 370             StopScroll(); // Will replace previous animation so will stop existing one.
 371
 372             if (scrollAnimation == null)
 373             {
 374                 scrollAnimation = new Animation();
 375                 scrollAnimation.Finished += ScrollAnimationFinished;
 376             }
 377
 378             scrollAnimation.Duration = duration;
 379             scrollAnimation.DefaultAlphaFunction = new AlphaFunction(AlphaFunction.BuiltinFunctions.EaseOutSine);
 380             scrollAnimation.AnimateTo(mScrollingChild, (ScrollingDirection == Direction.Horizontal) ? "PositionX" : "PositionY", axisPosition);
 381             scrolling = true;
 382             OnScrollStart();
 383             scrollAnimation.Play();
 384         }
 385
 386         private void ScrollBy(float displacement, bool animate)
 387         {
 388             if (GetChildCount() == 0 || displacement == 0)
 389             {
 390                 return;
 391             }
 392
 393             float childCurrentPosition = (ScrollingDirection == Direction.Horizontal) ? mScrollingChild.PositionX: mScrollingChild.PositionY;
 394
 395             Debug.WriteLineIf(LayoutDebugScrollableBase, "ScrollBy childCurrentPosition:" + childCurrentPosition +
 396                                                    " displacement:" + displacement,
 397                                                    " maxScrollDistance:" + maxScrollDistance );
 398
 399             childTargetPosition = childCurrentPosition + displacement; // child current position + gesture displacement
 400             childTargetPosition = Math.Min(0,childTargetPosition);
 401             childTargetPosition = Math.Max(-maxScrollDistance,childTargetPosition);
 402
 403             Debug.WriteLineIf( LayoutDebugScrollableBase, "ScrollBy currentAxisPosition:" + childCurrentPosition + "childTargetPosition:" + childTargetPosition);
 404
 405             if (animate)
 406             {
 407                 // Calculate scroll animaton duration
 408                 float scrollDistance = 0.0f;
 409                 if (childCurrentPosition < childTargetPosition)
 410                 {
 411                     scrollDistance = Math.Abs(childCurrentPosition + childTargetPosition);
 412                 }
 413                 else
 414                 {
 415                     scrollDistance = Math.Abs(childCurrentPosition - childTargetPosition);
 416                 }
 417
 418                 int duration = (int)((320*FlickAnimationSpeed) + (scrollDistance * FlickAnimationSpeed));
 419                 Debug.WriteLineIf(LayoutDebugScrollableBase, "Scroll Animation Duration:" + duration + " Distance:" + scrollDistance);
 420
 421                 AnimateChildTo(duration, childTargetPosition);
 422             }
 423             else
 424             {
 425                 // Set position of scrolling child without an animation
 426                 if (ScrollingDirection == Direction.Horizontal)
 427                 {
 428                     mScrollingChild.PositionX = childTargetPosition;
 429                 }
 430                 else
 431                 {
 432                     mScrollingChild.PositionY = childTargetPosition;
 433                 }
 434             }
 435         }
 436
 437         /// <summary>
 438         /// you can override it to clean-up your own resources.
 439         /// </summary>
 440         /// <param name="type">DisposeTypes</param>
 441         /// <since_tizen> 6 </since_tizen>
 442         /// This will be public opened in tizen_5.5 after ACR done. Before ACR, need to be hidden as inhouse API.
 443         [EditorBrowsable(EditorBrowsableState.Never)]
 444         protected override void Dispose(DisposeTypes type)
 445         {
 446             if (disposed)
 447             {
 448                 return;
 449             }
 450
 451             if (type == DisposeTypes.Explicit)
 452             {
 453                 StopScroll();
 454
 455                 if (mPanGestureDetector != null)
 456                 {
 457                     mPanGestureDetector.Detected -= OnPanGestureDetected;
 458                     mPanGestureDetector.Dispose();
 459                     mPanGestureDetector = null;
 460                 }
 461
 462                 if (mTapGestureDetector != null)
 463                 {
 464                     mTapGestureDetector.Detected -= OnTapGestureDetected;
 465                     mTapGestureDetector.Dispose();
 466                     mTapGestureDetector = null;
 467                 }
 468             }
 469             base.Dispose(type);
 470         }
 471
 472         private float CalculateDisplacementFromVelocity(float axisVelocity)
 473         {
 474             // Map: flick speed of range (2.0 - 6.0) to flick multiplier of range (0.7 - 1.6)
 475             float speedMinimum = FlickThreshold;
 476             float speedMaximum = FlickThreshold + 6.0f;
 477             float multiplierMinimum = FlickDistanceMultiplierRange.X;
 478             float multiplierMaximum = FlickDistanceMultiplierRange.Y;
 479
 480             float flickDisplacement = 0.0f;
 481
 482             float speed = Math.Min(4.0f,Math.Abs(axisVelocity));
 483
 484             Debug.WriteLineIf(LayoutDebugScrollableBase, "ScrollableBase Candidate Flick speed:" + speed);
 485
 486             if (speed > FlickThreshold)
 487             {
 488                 // Flick length is the length of the ScrollableBase.
 489                 float flickLength = (ScrollingDirection == Direction.Horizontal) ?CurrentSize.Width:CurrentSize.Height;
 490
 491                 // Calculate multiplier by mapping speed between the multiplier minimum and maximum.
 492                 multiplier =( (speed - speedMinimum) / ( (speedMaximum - speedMinimum) * (multiplierMaximum - multiplierMinimum) ) )+ multiplierMinimum;
 493
 494                 // flick displacement is the product of the flick length and multiplier
 495                 flickDisplacement = ((flickLength * multiplier) * speed) / axisVelocity;  // *speed and /velocity to perserve sign.
 496
 497                 Debug.WriteLineIf(LayoutDebugScrollableBase, "Calculated FlickDisplacement[" + flickDisplacement +"] from speed[" + speed + "] multiplier:"
 498                                                         + multiplier);
 499             }
 500             return flickDisplacement;
 501         }
 502
 503         private float CalculateMaximumScrollDistance()
 504         {
 505             int scrollingChildLength = 0;
 506             int scrollerLength = 0;
 507             if (ScrollingDirection == Direction.Horizontal)
 508             {
 509                 Debug.WriteLineIf(LayoutDebugScrollableBase, "Horizontal");
 510
 511                 scrollingChildLength = (int)mScrollingChild.Layout.MeasuredWidth.Size.AsRoundedValue();
 512                 scrollerLength = CurrentSize.Width;
 513             }
 514             else
 515             {
 516                 Debug.WriteLineIf(LayoutDebugScrollableBase, "Vertical");
 517                 scrollingChildLength = (int)mScrollingChild.Layout.MeasuredHeight.Size.AsRoundedValue();
 518                 scrollerLength = CurrentSize.Height;
 519             }
 520
 521             Debug.WriteLineIf(LayoutDebugScrollableBase, "ScrollBy maxScrollDistance:" + (scrollingChildLength - scrollerLength) +
 522                                                    " parent length:" + scrollerLength +
 523                                                    " scrolling child length:" + scrollingChildLength);
 524
 525             return scrollingChildLength - scrollerLength;
 526         }
 527
 528         private void PageSnap()
 529         {
 530             Debug.WriteLineIf(LayoutDebugScrollableBase, "PageSnap with pan candidate totalDisplacement:" + totalDisplacementForPan +
 531                                                                 " currentPage[" + currentPage + "]" );
 532
 533             //Increment current page if total displacement enough to warrant a page change.
 534             if (Math.Abs(totalDisplacementForPan) > (PageWidth * ratioOfScreenWidthToCompleteScroll))
 535             {
 536                 if (totalDisplacementForPan < 0)
 537                 {
 538                     currentPage = Math.Min(NumberOfPages-1, ++currentPage);
 539                 }
 540                 else
 541                 {
 542                     currentPage = Math.Max(0, --currentPage);
 543                 }
 544             }
 545
 546             // Animate to new page or reposition to current page
 547             int destinationX = -(currentPage * PageWidth);
 548             Debug.WriteLineIf(LayoutDebugScrollableBase, "Snapping to page[" + currentPage + "] to:"+ destinationX + " from:" + mScrollingChild.PositionX);
 549             AnimateChildTo(ScrollDuration, destinationX);
 550         }
 551
 552         private void Flick(float flickDisplacement)
 553         {
 554           if (SnapToPage)
 555           {
 556               if ( ( flickWhenAnimating && scrolling == true) || ( scrolling == false) )
 557               {
 558                   if(flickDisplacement < 0)
 559                   {
 560                       currentPage = Math.Min(NumberOfPages - 1, currentPage + 1);
 561                       Debug.WriteLineIf(LayoutDebugScrollableBase, "Snap - to page:" + currentPage);
 562                   }
 563                   else
 564                   {
 565                       currentPage = Math.Max(0, currentPage - 1);
 566                       Debug.WriteLineIf(LayoutDebugScrollableBase, "Snap + to page:" + currentPage);
 567                   }
 568                   float targetPosition = -(currentPage* PageWidth); // page size
 569                   Debug.WriteLineIf(LayoutDebugScrollableBase, "Snapping to :" + targetPosition);
 570                   AnimateChildTo(ScrollDuration,targetPosition);
 571               }
 572           }
 573           else
 574           {
 575               ScrollBy(flickDisplacement, true); // Animate flickDisplacement.
 576           }
 577         }
 578
 579         private void OnPanGestureDetected(object source, PanGestureDetector.DetectedEventArgs e)
 580         {
 581             if (e.PanGesture.State == Gesture.StateType.Started)
 582             {
 583                 Debug.WriteLineIf(LayoutDebugScrollableBase, "Gesture Start");
 584                 if (scrolling && !SnapToPage)
 585                 {
 586                     StopScroll();
 587                 }
 588                 maxScrollDistance = CalculateMaximumScrollDistance();
 589                 totalDisplacementForPan = 0.0f;
 590             }
 591             else if (e.PanGesture.State == Gesture.StateType.Continuing)
 592             {
 593                 if (ScrollingDirection == Direction.Horizontal)
 594                 {
 595                     ScrollBy(e.PanGesture.Displacement.X, false);
 596                     totalDisplacementForPan += e.PanGesture.Displacement.X;
 597                 }
 598                 else
 599                 {
 600                     ScrollBy(e.PanGesture.Displacement.Y, false);
 601                     totalDisplacementForPan += e.PanGesture.Displacement.Y;
 602                 }
 603                 Debug.WriteLineIf(LayoutDebugScrollableBase, "OnPanGestureDetected Continue totalDisplacementForPan:" + totalDisplacementForPan);
 604
 605             }
 606             else if (e.PanGesture.State == Gesture.StateType.Finished)
 607             {
 608                 float axisVelocity = (ScrollingDirection == Direction.Horizontal) ? e.PanGesture.Velocity.X : e.PanGesture.Velocity.Y;
 609                 float flickDisplacement = CalculateDisplacementFromVelocity(axisVelocity);
 610
 611                 Debug.WriteLineIf(LayoutDebugScrollableBase, "FlickDisplacement:" + flickDisplacement + "TotalDisplacementForPan:" + totalDisplacementForPan);
 612
 613                 if (flickDisplacement > 0 | flickDisplacement < 0)// Flick detected
 614                 {
 615                     Flick(flickDisplacement);
 616                 }
 617                 else
 618                 {
 619                     // End of panning gesture but was not a flick
 620                     if (SnapToPage)
 621                     {
 622                         PageSnap();
 623                     }
 624                 }
 625                 totalDisplacementForPan = 0;
 626             }
 627         }
 628
 629         private void OnTapGestureDetected(object source, TapGestureDetector.DetectedEventArgs e)
 630         {
 631             if (e.TapGesture.Type == Gesture.GestureType.Tap)
 632             {
 633                 // Stop scrolling if tap detected (press then relase).
 634                 // Unless in Pages mode, do not want a page change to stop part way.
 635                 if(scrolling && !SnapToPage)
 636                 {
 637                     StopScroll();
 638                 }
 639             }
 640         }
 641
 642         private void ScrollAnimationFinished(object sender, EventArgs e)
 643         {
 644             scrolling = false;
 645             OnScrollEnd();
 646         }
 647
 648     }
 649
 650 } // namespace