react-navigation/src/views/Header/HeaderStyleInterpolator.js at master · matzero/react-navigation · GitHub

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/* @flow */

import {
  I18nManager,
  Animated,
  type StyleDefinition,
  type AnimatedViewStylePropTypes,
} from 'react-native';

import type {
  NavigationSceneRendererProps,
  AnimatedViewStyleProp,
} from '../../TypeDefinition';

/**
 * Utility that builds the style for the navigation header.
 *
 * +-------------+-------------+-------------+
 * |             |             |             |
 * |    Left     |   Title     |   Right     |
 * |  Component  |  Component  | Component   |
 * |             |             |             |
 * +-------------+-------------+-------------+
 */

function forLeft(
  props: NavigationSceneRendererProps
): AnimatedViewStylePropTypes {
  const { position, scene } = props;
  const { index } = scene;
  return {
    opacity: position.interpolate({
      inputRange: [index - 1, index - 0.5, index, index + 0.5, index + 1],
      outputRange: ([0, 0, 1, 0, 0]: Array<number>),
    }),
  };
}

function forCenter(
  props: NavigationSceneRendererProps
): AnimatedViewStylePropTypes {
  const { position, scene } = props;
  const { index } = scene;
  return {
    opacity: position.interpolate({
      inputRange: [index - 1, index, index + 1],
      outputRange: ([0, 1, 0]: Array<number>),
    }),
    transform: [
      {
        translateX: position.interpolate({
          inputRange: [index - 1, index + 1],
          outputRange: I18nManager.isRTL
            ? ([-200, 200]: Array<number>)
            : ([200, -200]: Array<number>),
        }),
      },
    ],
  };
}

function forRight(
  props: NavigationSceneRendererProps
): AnimatedViewStylePropTypes {
  const { position, scene } = props;
  const { index } = scene;
  return {
    opacity: position.interpolate({
      inputRange: [index - 1, index, index + 1],
      outputRange: ([0, 1, 0]: Array<number>),
    }),
  };
}

export default {
  forLeft,
  forCenter,
  forRight,
};