How to finish a task before launching a System web app

Prerequisites

System Web App, WebUI

Problem

I wanted to defer the launch of my SWA until a task is completed. For example, for the feedback tool SWA, I need to take a screenshot for all screens first before showing the UI to users.

Solution

• Step 1: Implement a method that will complete a task async and once done will invoke the callback passed in.

// Take a screenshot and invoke the callback when completed or failed.
void OsFeedbackScreenshotManager::TakeScreenshot(ScreenshotCallback callback) {
  // SKip taking if one has been taken already. Although the feedback tool only
  // allows one instance, the user could request it multiple times while one
  // instance is still running.
  if (screenshot_png_data_ == nullptr) {
    // In unit tests, shell is not created.
    aura::Window* primary_window = ash::Shell::HasInstance()
                                       ? ash::Shell::GetPrimaryRootWindow()
                                       : nullptr;
    if (primary_window) {
      gfx::Rect rect = primary_window->bounds();
      ui::GrabWindowSnapshotAsPNG(
          primary_window, rect,
          base::BindOnce(&OsFeedbackScreenshotManager::OnScreenshotTaken,
                         weak_ptr_factory_.GetWeakPtr(), std::move(callback)));
    } else {
      std::move(callback).Run(false);
    }
  } else {
    std::move(callback).Run(false);
  }
}

• Step 2: Override the LaunchAndNavigateSystemWebApp of the class ash::SystemWebAppDelegate
  • If not there yet, add a class 'OSFeedbackAppDelegate' header file in chrome/browser/ash/web_applications/os_feedback_system_web_app_info.h that extends ash::SystemWebAppDelegate.
  • Override the LaunchAndNavigateSystemWebApp

#include "chrome/browser/profiles/profile.h"
#include "chrome/browser/web_applications/system_web_apps/system_web_app_delegate.h"
#include "chrome/browser/web_applications/system_web_apps/system_web_app_types.h"
#include "ui/gfx/geometry/rect.h"

class Browser;
struct WebAppInstallInfo;

namespace web_app {
class WebAppProvider;
}  // namespace web_app

class OSFeedbackAppDelegate : public web_app::SystemWebAppDelegate {
 public:
  explicit OSFeedbackAppDelegate(Profile* profile);

  // web_app::SystemWebAppDelegate overrides:
  std::unique_ptr<WebAppInstallInfo> GetWebAppInfo() const override;
  bool IsAppEnabled() const override;
  bool ShouldAllowScriptsToCloseWindows() const override;
  bool ShouldCaptureNavigations() const override;
  bool ShouldAllowMaximize() const override;
  bool ShouldAllowResize() const override;
  gfx::Rect GetDefaultBounds(Browser*) const override;
  Browser* LaunchAndNavigateSystemWebApp(
      Profile* profile,
      web_app::WebAppProvider* provider,
      const GURL& url,
      const apps::AppLaunchParams& params) const override;
};

// Returns a WebAppInstallInfo used to install the app.
std::unique_ptr<WebAppInstallInfo> CreateWebAppInfoForOSFeedbackSystemWebApp();

// Returns the default bounds.
gfx::Rect GetDefaultBoundsForOSFeedbackApp(Browser*);

• Step 3: Implement the LaunchAndNavigateSystemWebApp in chrome/browser/ash/web_applications/os_feedback_system_web_app_info.cc

1. Launch and hide the SWA.
2. Start the task with a callback to show the SWA.

Browser* OSFeedbackAppDelegate::LaunchAndNavigateSystemWebApp(
    Profile* profile,
    web_app::WebAppProvider* provider,
    const GURL& url,
    const apps::AppLaunchParams& params) const {
  // This check is needed to enforce the policy no matter how and from where the
  // feedback tool is to be launched.
  if (IsUserFeedbackAllowed(profile)) {
    Browser* browser = SystemWebAppDelegate::LaunchAndNavigateSystemWebApp(
        profile, provider, url, params);
    if (browser && browser->window()) {
      // Hide the app, take a screenshot async, then show it afterward.
      browser->window()->Hide();
      ash::OsFeedbackScreenshotManager::GetInstance()->TakeScreenshot(
          base::BindOnce(&OSFeedbackAppDelegate::OnScreenshotTaken,
                         weak_ptr_factory_.GetWeakPtr(), browser));
    }
    return browser;
  }
  return nullptr;
}

void OSFeedbackAppDelegate::OnScreenshotTaken(Browser* browser,
                                              bool status) const {
  if (browser && browser->window()) {
    browser->window()->Show();
  }
}

Example CL

• https://crrev.com/c/3645699
  • Parent change: https://crrev.com/c/3642641

Comment/Discussion

Note: Simply fires an async operation when starting to launch the SWA may not work because we have no control over the timing. If the UI is being drawn while the screenshot taking is in progress, the scrennshot will include some of the UI elements of the feedback tool.