airbnb/caravel

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */
import isValidChild from './isValidChild';
import { TAB_TYPE, TABS_TYPE } from './componentTypes';

export const DROP_TOP = 'DROP_TOP';
export const DROP_RIGHT = 'DROP_RIGHT';
export const DROP_BOTTOM = 'DROP_BOTTOM';
export const DROP_LEFT = 'DROP_LEFT';
export const DROP_FORBIDDEN = 'DROP_FORBIDDEN';

// this defines how close the mouse must be to the edge of a component to display
// a sibling type drop indicator
const SIBLING_DROP_THRESHOLD = 20;
const NON_SHALLOW_DROP_THRESHOLD = 20;

// We cache the last recorded clientOffset per component in order to
// have access to it beyond the handleHover phase and into the handleDrop phase
// of drag-and-drop. we do not have access to it during drop because react-dnd's
// monitor.getClientOffset() returns null at this point
let CACHED_CLIENT_OFFSET = {};
export function clearDropCache() {
  CACHED_CLIENT_OFFSET = {};
}

export default function getDropPosition(monitor, Component) {
  const {
    depth: componentDepth,
    parentComponent,
    component,
    orientation,
    isDraggingOverShallow,
  } = Component.props;

  const draggingItem = monitor.getItem();

  // if dropped self on self, do nothing
  if (!draggingItem || draggingItem.id === component.id) {
    return null;
  }

  const validChild = isValidChild({
    parentType: component.type,
    parentDepth: componentDepth,
    childType: draggingItem.type,
  });

  const parentType = parentComponent && parentComponent.type;
  const parentDepth = // see isValidChild.js for why tabs don't increment child depth
    componentDepth +
    (parentType === TAB_TYPE || parentType === TABS_TYPE ? 0 : -1);

  const validSibling = isValidChild({
    parentType,
    parentDepth,
    childType: draggingItem.type,
  });

  if (!validChild && !validSibling) {
    return DROP_FORBIDDEN;
  }

  const hasChildren = (component.children || []).length > 0;
  const childDropOrientation =
    orientation === 'row' ? 'vertical' : 'horizontal';
  const siblingDropOrientation =
    orientation === 'row' ? 'horizontal' : 'vertical';

  if (validChild && !validSibling) {
    // easiest case, insert as child
    if (childDropOrientation === 'vertical') {
      return hasChildren ? DROP_RIGHT : DROP_LEFT;
    }
    return hasChildren ? DROP_BOTTOM : DROP_TOP;
  }

  const refBoundingRect = Component.ref.getBoundingClientRect();
  const clientOffset =
    monitor.getClientOffset() || CACHED_CLIENT_OFFSET[component.id];

  if (!clientOffset || !refBoundingRect) {
    return null;
  }

  CACHED_CLIENT_OFFSET[component.id] = clientOffset;
  const deltaTop = Math.abs(clientOffset.y - refBoundingRect.top);
  const deltaBottom = Math.abs(clientOffset.y - refBoundingRect.bottom);
  const deltaLeft = Math.abs(clientOffset.x - refBoundingRect.left);
  const deltaRight = Math.abs(clientOffset.x - refBoundingRect.right);

  // Most of the time we only want a drop indicator for shallow (top-level, non-nested) drop targets
  // However there are some cases where considering only shallow targets would result in NO drop
  // indicators which is a bad UX.
  // e.g.,
  //    when dragging row-a over a chart that's in another row-b, the chart is the shallow droptarget
  //    but row-a is not a valid child or sibling. in this case we want to show a sibling drop
  //    indicator for row-b, which is NOT a shallow drop target.
  // BUT if we ALWAYS consider non-shallow drop targets we may get multiple indicators shown at the
  // same time, which is also a bad UX. to prevent this we can enforce a threshold proximity of the
  // mouse to the edge of a non-shallow target
  if (
    !isDraggingOverShallow &&
    [deltaTop, deltaBottom, deltaLeft, deltaRight].every(
      delta => delta > NON_SHALLOW_DROP_THRESHOLD,
    )
  ) {
    return null;
  }

  // Drop based on mouse position relative to component center
  if (validSibling && !validChild) {
    if (siblingDropOrientation === 'vertical') {
      const refMiddleX =
        refBoundingRect.left +
        (refBoundingRect.right - refBoundingRect.left) / 2;
      return clientOffset.x < refMiddleX ? DROP_LEFT : DROP_RIGHT;
    }
    const refMiddleY =
      refBoundingRect.top + (refBoundingRect.bottom - refBoundingRect.top) / 2;
    return clientOffset.y < refMiddleY ? DROP_TOP : DROP_BOTTOM;
  }

  // either is valid, so choose location based on boundary deltas
  if (validSibling && validChild) {
    // if near enough to a sibling boundary, drop there
    if (siblingDropOrientation === 'vertical') {
      if (deltaLeft < SIBLING_DROP_THRESHOLD) return DROP_LEFT;
      if (deltaRight < SIBLING_DROP_THRESHOLD) return DROP_RIGHT;
    } else {
      if (deltaTop < SIBLING_DROP_THRESHOLD) return DROP_TOP;
      if (deltaBottom < SIBLING_DROP_THRESHOLD) return DROP_BOTTOM;
    }

    // drop as child
    if (childDropOrientation === 'vertical') {
      return hasChildren ? DROP_RIGHT : DROP_LEFT;
    }
    return hasChildren ? DROP_BOTTOM : DROP_TOP;
  }

  return null;
}