topmostLineMonitor.ts

/*---------------------------------------------------------------------------------------------
  *  Licensed under the MIT License. See License.txt in the project root for license information.
 *--------------------------------------------------------------------------------------------*/

import * as vscode from 'vscode'
import { disposeAll } from '../util/dispose'
import { isAsciidocFile } from './file'

export class AsciidocFileTopmostLineMonitor {
  private readonly disposables: vscode.Disposable[] = [];

  private readonly pendingUpdates = new Map<string, number>();

  private readonly throttle = 50;

  constructor () {
    vscode.window.onDidChangeTextEditorVisibleRanges((event) => {
      if (isAsciidocFile(event.textEditor.document)) {
        const line = getVisibleLine(event.textEditor)
        if (typeof line === 'number') {
          this.updateLine(event.textEditor.document.uri, line)
        }
      }
    }, null, this.disposables)
  }

  dispose () {
    disposeAll(this.disposables)
  }

  private readonly _onDidChangeTopmostLineEmitter = new vscode.EventEmitter<{ resource: vscode.Uri, line: number }>();
  public readonly onDidChangeTopmostLine = this._onDidChangeTopmostLineEmitter.event;

  private updateLine (
    resource: vscode.Uri,
    line: number
  ) {
    const key = resource.toString()
    if (!this.pendingUpdates.has(key)) {
      // schedule update
      setTimeout(() => {
        if (this.pendingUpdates.has(key)) {
          this._onDidChangeTopmostLineEmitter.fire({
            resource,
            line: this.pendingUpdates.get(key) as number,
          })
          this.pendingUpdates.delete(key)
        }
      }, this.throttle)
    }

    this.pendingUpdates.set(key, line)
  }
}

/**
 * Get the top-most visible range of `editor`.
 *
 * Returns a fractional line number based the visible character within the line.
 * Floor to get real line number
 */
export function getVisibleLine (
  editor: vscode.TextEditor
): number | undefined {
  if (!editor.visibleRanges.length) {
    return undefined
  }

  const firstVisiblePosition = editor.visibleRanges[0].start
  const lineNumber = firstVisiblePosition.line
  const line = editor.document.lineAt(lineNumber)
  const progress = firstVisiblePosition.character / (line.text.length + 2)
  return lineNumber + progress
}