基于手势的图片预览与缩放

场景一：对图片进行放大、缩小、拖拽移动，且放大过程中也可同时进行拖拽操作

方案

1、使用组合手势GestureGroup，同时绑定捏合手势PinchGesture和滑动手势PanGesture，设置组合手势识别模式为并行识别模式：Parallel，并行识别组合手势中注册的手势将同时进行识别，直到所有手势识别结束，并行识别手势组合中的手势进行识别时互不影响。

2、在对图片进行双指捏合时，优先触发绑定的PinchGesture手势，对图片进行缩放操作；当滑动拖拽图片时，识别绑定的PanGesture手势，对图片进行拖拽移动。

核心代码

1、绑定组合手势GestureGroup，设置为并行识别模式，添加捏合手势PinchGesture和滑动手势PanGesture。

@Styles
onImageGesture(){
  .gesture(
    GestureGroup(GestureMode.Parallel,
      // 双指捏合手势
      PinchGesture({ fingers: 2, distance: 0.1 })
        .onActionUpdate((event: GestureEvent) => {
          this.onPinchGestureActionUpdate(event);
        })
        .onActionEnd(() => {
          this.onPinchGestureActionEnd();
        }),
      // 拖动手势
      PanGesture(this.panOption)
        .onActionUpdate((event?: GestureEvent) => {
          this.onPanGestureActionUpdate(event);
        })
        .onActionEnd(() => {
          this.onPanGestureActionEnd();
        })
    )
  )
}

2、在捏合手势的onActionUpdate和onActionEnd回调中修改scale参数，进行图片缩放处理。

// 当捏合手势触发时，可以通过回调函数获取缩放比例，从而修改组件的缩放比例
onPinchGestureActionUpdate(event: GestureEvent) {
  const SCALE_VALUE = this.pinchValue * event.scale;
  if (SCALE_VALUE <= this.imageMeta.MAX_SCALE_VALUE && SCALE_VALUE >= this.MIN_SCALE) {
    this.scaleValue = SCALE_VALUE;
    this.pinchX = event.pinchCenterX;
    this.pinchY = this.getPinchY(event.pinchCenterY);
    if (this.scaleValue > 1) {
      this.offsetX = 0;
    }
  }
}

getPinchY(pinchCenterY: number | string) {
  let pinchY = pinchCenterY;
  if (this.scaleValue > 1) {
    // 当为横图时，在留白处放大，图片位置不居中，手动计算pinchY，并移动Y轴中心点，即offsetY
    if (this.imageMeta.IMAGE_WIDTH >= this.imageMeta.IMAGE_HEIGHT) {
      const SCREEN_HEIGHT = px2vp(display.getDefaultDisplaySync().height);
      const MIN_PINCH_Y = SCREEN_HEIGHT / 2 - this.imageMeta.IMAGE_HEIGHT / 2;
      const MAX_PINCH_Y = SCREEN_HEIGHT / 2 + this.imageMeta.IMAGE_HEIGHT / 2;
      const MIDDLE_PINCH_Y = SCREEN_HEIGHT / 2;
      if (pinchY < MIN_PINCH_Y) {
        pinchY = MIN_PINCH_Y;
      }
      if (pinchY > MAX_PINCH_Y) {
        pinchY = MAX_PINCH_Y;
      }
      if (pinchY < MIDDLE_PINCH_Y && typeof this.pinchY === 'number') {
        this.offsetY = (Number(pinchY) - MIDDLE_PINCH_Y) * (this.scaleValue - 1);
      }
      if (pinchY > MIDDLE_PINCH_Y && typeof this.pinchY === 'number') {
        this.offsetY = (Number(pinchY) - MIDDLE_PINCH_Y) * (this.scaleValue - 1);
      }
    }
  }
  return pinchY;
}

// 当捏合手势结束时，计算当前的isScaling
// 如果缩放比例小于1，弹簧效果重置成原比例
onPinchGestureActionEnd() {
  this.pinchValue = this.scaleValue;
  if (this.pinchValue > 1) {
    this.scaleEnable();
  } else {
    this.scaleUnEnable();
    if (this.pinchValue < 1) {
      this.reset();
    }
  }
}

场景二：图片在放大的情况下也可切换到后一张图或者前一张图

方案

1、在panGesture手势的onActionUpdate回调中获取偏移位置，计算图片放大后分别往左或者往右拖动时是否到达边界，记录向左或向右边界的状态。

2、在onTouch事件中识别手指滑动方向，并判断边界是翻页还是拖动，方向为左右且到达边界，执行scaleUnEnable方法，将panGesture手势方向置为none，执行翻页；反之执行滑动手势，不翻页。

核心代码

1、在PanGesture手势的onActionUpDate回调中获取偏移位置，计算拖拽过程中左右两边分别是否已到边界。

// 当拖动手势触发时，可以通过回调函数获取偏移位置，并计算是否已到边界，不可拖出边界
onPanGestureActionUpdate(event: GestureEvent) {
  if (!event || this.scaleValue <= 1) {
    return;
  }
  let boundaryOffset: BoundaryOffset = {
    left: 0,
    right: 0,
    top: 0,
    bottom: 0
  };
  this.xBoundary = HorizontalBoundary.MIDDLE;
  this.yBoundary = VerticalBoundary.MIDDLE;

  const CENTER_X_RELATIVE = typeof this.pinchX === 'number' ? this.pinchX / this.imageMeta.IMAGE_WIDTH : 0.5;
  boundaryOffset.left = this.imageMeta.IMAGE_WIDTH * (this.scaleValue - 1) * CENTER_X_RELATIVE;
  boundaryOffset.right = -this.imageMeta.IMAGE_WIDTH * (this.scaleValue - 1) * (1 - CENTER_X_RELATIVE);
  const NEWOFFSET_X = this.positionX + event.offsetX;
  // 更新x轴偏移位置
  if (NEWOFFSET_X <= boundaryOffset.left && newOffsetX >= boundaryOffset.right) {
    // 在中间
    this.offsetX = NEWOFFSET_X;
  } else {
    // 在边界
    if (this.fingerDirection === FingerDirection.LEFT && this.offsetX !== boundaryOffset.right) {
      this.offsetX = boundaryOffset.right;
    }
    if (this.fingerDirection === FingerDirection.RIGHT && this.offsetX !== boundaryOffset.left) {
      this.offsetX = boundaryOffset.left;
    }
  }
  // 在左边界
  if (NEWOFFSET_X >= boundaryOffset.left) {
    this.xBoundary = HorizontalBoundary.LEFT;
  }
  // 在右边界
  if (NEWOFFSET_X <= boundaryOffset.right) {
    this.xBoundary = HorizontalBoundary.RIGHT;
  }
  if (this.imageMeta.PAN_DIRECTION === PanDirection.All || this.imageMeta.PAN_DIRECTION === PanDirection.Vertical||this.imageMeta.PAN_DIRECTION === PanDirection.Horizontal) {
    // 更新y轴偏移量
    const NEWOFFSET_Y = this.positionY + event.offsetY;
    this.offsetY = NEWOFFSET_Y;
  }
}

2、通过绑定onTouch事件，在滑动的过程中根据手指方向和是否到达边界通过isPageChangeOnBoundary方法判断后分别执行scaleEnable和scaleUnEnable方法进行翻页或者拖动处理。

// 识别手指滑动方向，并判断边界是翻页还是拖动
@Styles
onImageTouch(){
  .onTouch((event: TouchEvent) => {
    if (this.scaleValue > 1) {
      if (event.type === TouchType.Down) {
        this.touchPos.x = event.touches[0].x;
        this.touchPos.y = event.touches[0].y;
      }
      if (event.type === TouchType.Move) {
        this.fingerDirection = getMoveFingerDirection(this.touchPos, event, this.fingerDirection);
        if (!isPageChangeOnBoundary(this.fingerDirection, this.xBoundary)) {
          if (!this.isScaling) {
            this.touchFlag = 1;
            this.scaleEnable();
            this.xBoundary = HorizontalBoundary.MIDDLE;
            this.temp = this.offsetX;
            this.tempY = this.offsetY;
          }
        }
      }
      if (event.type === TouchType.Up) {
        if (this.touchFlag == 1) {
          this.onPanGestureActionEnd();
        }
        this.touchFlag = 0;
        this.temp = 0;
        this.tempY = 0;
        if (isPageChangeOnBoundary(this.fingerDirection, this.xBoundary) || this.scaleValue === 1) {
          this.scaleUnEnable();
        } else {
          this.scaleEnable();
        }
      }
    }
  })
}

// 拖动，不翻页
scaleEnable() {
  if (this.isScaling) {
    return;
  }
  this.panOption.setDirection(this.imageMeta.PAN_DIRECTION);
  this.isScaling = true;
}

// 不能拖动，翻页
scaleUnEnable() {
  if (!this.isScaling) {
    return;
  }
  this.panOption.setDirection(PanDirection.None);
  this.isScaling = false;
}

3、在isPageChangeOnBoundary 方法中根据传入的手指滑动方向和拖动到达的边界方向来判断返回，之后再分别调用不同场景下的方法处理逻辑，进行翻页或者拖动。

// 在边界进行翻页
export const isPageChangeOnBoundary = (fingerDirection: FingerDirection, xBoundary: HorizontalBoundary) => {
  return (fingerDirection === FingerDirection.LEFT && xBoundary === HorizontalBoundary.RIGHT ||
    fingerDirection === FingerDirection.RIGHT && xBoundary === HorizontalBoundary.LEFT);
}