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Log of migrating a GWT Mail sample app to Angular2 Dart

Prepare the project

Create the Angular Dart project

  • create an Anuglar2 Dart project with your usual tooling
  • add angular2_components: ^0.2.2 as a dependency in pubspec.yaml

Add GWT sources

Let's keep two copies of the GWT sources in the project.

The first copy in gwt/original will be unchanged.

The second copy in gwt/backlog will help us to keep track of the current backlog of the migration: files and logic that needs to be processed in one way or another.

Migrate the GWT app

Remove files that are relevant only for GWT

There are many files in our backlog that can be deleted easily:

- build.xml
- war/WEB-INF/*
- war/gradient_bg_th.png (duplicate)
- war/Mail.html
- c.g.g.s.m/Mail.gwt.xml

Before deleting, check the content in case it uses features that need to be addressed in the Dart code.

While checking war/Mail.html there are a few small touches that should be adapted (<meta>, <title>, <noscript>).

Move favicon.ico to web/

  • move war/favicon.ico -> web/favicon.ico

Set global CSS styles

We create a <link> reference in web/index.html that points to global.css with the few CSS styles that are outside of our main component's scope.

  • global.gss contains some references to body
  • sets margin: 0px and disables scrollbars on the outer window.

Migrate TopPanel

  • create the TopPanel component in lib/nav/top/top_panel.dart with its usual html and css files
  • add it to the app_component.html: <top-panel></top-panel>
  • reference it in the AppComponent's annotation: directives: const [TopPanel],
  • start processing TopPanel.ui.xml:
    • move logo.png to the lib/nav/top/ directory
    • move the styles from <ui:style> to top_panel.css
    • copy the structure inside <g:UIBinder> to top_panel.html

Migrating the logo:

  • the ui:image element becomes <img src="..."/>
  • to reference the logo, use src="packages/gwt_mail_sample/nav/top/logo.png"
  • add the CSS class logo to the element
  • remove the gwt-sprite: "logo"; from the CSS, there is no use for it

Migrating the g:HTMLPanel:

  • the logo div is not used anymore as we have the img element above

  • class="{style.statusDiv}" becomes class="statusDiv"

  • the g:Anchor reference becomes much simpler:

    in the template:

      <a href="" (click)="signOut($event)">Sign Out</a>
      <a href="" (click)="showAbout($event)">About</a>

    in the controller (after importing dart:html):

    void signOut(MouseEvent event) {
    void showAbout(MouseEvent event) {

Implementing the actions:

  • the signOut is simple: it calls window.alert()
  • the showAbout requires the about dialog, let's leave a TODO for now

There are a few tweaks to be made:

  • move the anchor-related (a) styles to global.css
  • in both method, add event.preventDefault();, because we don't want to have a place change when the user clicks on them

Migrate AboutDialog

  • create the AboutDialog component in lib/nav/about/about_dialog.dart with its usual html and css files
  • move gwt-logo.png to the same directory
  • move the .logo style from AboutDialog.ui.xml into about_dialog.css (without the gwt-sprite attribute)
  • the rest of the styling and Java code is better served with a clean re-implementation with the material modal dialog component

To reference the about dialog:

  • put <about-dialog></about-dialog> in top_panel.html and add the directive and a @ViewChild reference to the controller class:

      directives: const [AboutDialog],
    AboutDialog aboutDialog;

    This will inject a reference of the about dialog's controller into the top panel, making it straightforward to display it when needed.

To get started with the material model dialog, take a look into the Angular component examples. Using the headered dialog example provided 95% of the functionality, with the following tweaks:

  • the dialog's maximum width is set to 60% via CSS

  • *ngIf="visible" is used to lazily initialize the dialog

  • the gwt-logo.png is referenced by the packages/... resource path with the logo CSS class

    There is no conflict, the same CSS class name can be used in separate components (e.g. logo both here and in TopPanel), because Angular generates scoping rules for each of them.

Migrate Shortcuts

The left-side menu (Shortcuts) resembles the expansion panel component, however it requires several customization.

  • create the SidePanel component in lib/nav/side/side_panel.dart with its usual html and css files

  • move contactsgroup.png, mailboxesgroup.png, tasksgroup.png and gradient_bg_dark.png to the same directory

  • add the component to AppComponent:

    • <side-panel> to the template
    • SidePanel to the directives annotation
    • position it to the right (e.g. floating with max-width: 250px)
  • migrate the CSS styles from Shortcuts.ui.xml:

    • .shortcuts becomes :host
    • .stackHeader becomes :host-context header
    • add a bit margin for the .content
  • For each of the panels, a template like this covers the functionality, repeat it for tasks and contacts:

        [expanded]="selectedPanel == 'mailboxes'"
        (close)="close($event, 'mailboxes')">
      <div name>
        <img src="packages/gwt_mail_sample/nav/side/mailboxesgroup.png" />
      <div class="content">TODO: add tree of mailboxes.</div>
  • In the component:

      String selectedPanel = 'mailboxes';
      void open(String panel) {
        selectedPanel = panel;
      void close(AsyncAction action, String panel) {
        if (panel == selectedPanel) action.cancel();

<<<<<<< HEAD

  • String selectedPanel = 'mailboxes'; to keep track of the currently selected and active panel
  • the close event ensures that only the panel's self-close gets cancelled. =======

Migrate Tasks

The task list is a static component without any real binding. In the following code we will re-implement it in a way that the list becomes dynamic, and the checked state of the tasks are bound to a real field.

  • create the TaskList component in lib/task/task_list.dart with its usual html, but without any css files (no extra styling required)

  • reference it in SidePanel's directives list and template

  • the backing task object can be as simple as:

    class TaskItem {
      String label;
      bool isDone;
      TaskItem(this.label, {this.isDone: false});
  • put a list of these in our component:

    class TaskList {
      List<TaskItem> items = [
        new TaskItem('Get groceries'),
        new TaskItem('Walk the dog'),
      // ...
  • bind it with *ngFor in the template (using material checkbox):

    <div *ngFor="let item of items">

25433f9... Migrate Tasks

Migrate Contacts

  • create the ContactList component in lib/contact/contact_list.dart with its usual html and css files

  • move default_photo.jpg to the same directory

  • move the .contacts CSS styles from Contacts.ui.xml to contact_list.css

  • move the .popup, .photo, .right, and .email CSS styles from ContactPopup.ui.xml to contact_list.css (remove gwt-sprite: "photo";)

  • create a const to point to it:

    const String defaultPhotoUrl =
  • reference it in SidePanel's directives list and template

Similarly to the TaskList component, we'll create a backing component, we'll create a backing object to drive the template:

  • the backing contact object:

    class ContactItem {
      String name;
      String email;
      String photoUrl;
      ContactItem(,, {this.photoUrl: defaultPhotoUrl});
  • the list of these items in the main component:

    class ContactList {
      List<ContactItem> items = [
        new ContactItem('Benoit Mandelbrot', ''),
        new ContactItem('Albert Einstein', ''),
        // ...
  • bind it with *ngFor in the template

    <div class="contacts">
      <div *ngFor="let item of items">
        <a href="" (click)="showPopup($event, item)">{{}}</a>
  • implement the skeleton of the showPopup method:

      ContactItem selected;
      void showPopup(MouseEvent event, ContactItem item) {
        selected = item;

The material popup component requires a bit of preparation:

  • both the source and visible needs to be bound:


    Note: using the same popupVisible flag to create it has the added benefit of automatic cleanup after closing/hiding it.

  • the controller needs to initialize these fields:

      PopupSource popupSource;
      bool popupVisible = false;
      void showPopup(MouseEvent event, ContactItem item) {
        selected = item;
        popupVisible = true;
        Element element = event.currentTarget;
        Point p = new Point(element.offsetLeft + 14, element.offsetTop + 14);
        popupSource = new PopupSource.fromRectangle(new Rectangle.fromPoints(p, p));

    The positioning with (+14, +14) offset is coming from

  • migrate the template from ContactPopup.ui.xml and put it inside the <material-popup> element:

      <div class="popup">
        <img [src]="selected.photoUrl" class="photo"/>
        <div class="right">
          <div class="email">{{}}</div>

Migrate Mailboxes

  • create the MailFolder component in lib/mail/folder/mail_folder.dart with its usual html and css files
  • move the drafts.png, home.png, inbox.png, noimage.png, sent.png, templates.png, and trash.png to the same directory
  • reference it in SidePanel's directives list and template

While the GWT implementation uses a heavyweight tree component, the mailbox tree can be modeled and built on top of *ngFor with some additional logic:

  • a FolderItem needs to keep track of some fields:

    class FolderItem {
      String iconUrl;
      String label;
      bool isSelected = false;
  • to make it into a tree, one needs to keep track of the parent and children`

      bool isExpanded;
      FolderItem parent;
      List<FolderItem> children;
  • an item is visible if and only if all of its ancestors are visible and expanded:

      bool get isRoot => parent == null;
      bool get isVisible => isRoot || (parent.isVisible && parent.isExpanded);
  • open/close toggle buttons are visible only if it has children:

      bool get hasChildren => children?.isNotEmpty ?? false;
      bool get expandVisible => hasChildren && !isExpanded;
      bool get collapseVisible => hasChildren && isExpanded;
  • to preserve the tree-like visual layout, one needs to indent based on the depth of the item:

      int get depth => parent == null ? 0 : parent.depth + 1;
      int get indentPx => depth * 16;
  • build the tree of the sample inbox folders:

    FolderItem root = new FolderItem('',
        iconUrl: '$baseUrl/home.png',
        children: [
          new FolderItem('Inbox', iconUrl: '$baseUrl/inbox.png'),
          new FolderItem('Drafts', iconUrl: '$baseUrl/drafts.png'),
          new FolderItem('Templates', iconUrl: '$baseUrl/templates.png'),
          new FolderItem('Sent', iconUrl: '$baseUrl/sent.png'),
          new FolderItem('Trash', iconUrl: '$baseUrl/trash.png'),
          new FolderItem('custom-1', children: [
            new FolderItem('custom-1-1'),
            new FolderItem('custom-1-2'),
            new FolderItem('custom-1-3'),

    Note: make sure the constructor sets the parent field of the children.

  • traverse the tree into a flattened list:

    List<FolderItem> items = [];
    void _traverseItems(FolderItem item) {
  • the template should be a combination of list iteration with visibility check:

    <div *ngFor="let item of items">
      <div *ngIf="item.isVisible" class="item">
        <!-- here comes the content -->
  • the .item CSS style is a flexbox which renders the following blocks as they were table rows

  • indentation is a simple padding:

    <div [style.width.px]="item.indentPx">&nbsp;</div>
  • the tree controls (+ and -) is handled by the following template:

    <div class="toggle">
      <span *ngIf="item.expandVisible" (click)="item.toggle()">&#x2795;</span>
      <span *ngIf="item.collapseVisible" (click)="item.toggle()">	&#x2796;</span>

    and code:

    void toggle() {
      isExpanded = !isExpanded;
  • put there an icon

    <img [src]="item.iconUrl" class="icon"/>

    and the label:

    <div [class.selected]="item.isSelected"
  • implement the selectFolder method:

    • deselect the previously selected item
    • store the current one and set its isSelected flag

With that, we have a simplified tree component that:

  • looks and works like a tree (indentation and tree controls)
  • supports custom icons and styles
  • keep track of a single selected item (which can later trigger the loading the list of messages for that e-mail folder)

Handle e-mail data (MailItems)

In the GWT application, the list of e-mails is stored in as a static field, and it is generated in a consistent way. The mail-related components access it though the static methods.

This approach makes testing, mocking and encapsulation harder, and instead, we'll use an injected service to achieve better coupling.

  • create lib/mail/mail_service.dart

  • migrate and create the following data holder class:

    class MailItem {
      String sender;
      String email;
      String subject;
      String body;
      MailItem(this.sender,, this.subject, this.body);
  • create the service interface:

    abstract class MailService {
  • add folder selection:

      String get selectedFolder;
      Future selectFolder(String label);

    selectFolder is an asynchronous method that returns with a Future, which will complete once the service completes loading the first page.

  • add information about the current folder:

      int get mailCount;
      List<MailItem> get pageItems;
  • add pagination information and page switching methods:

      int get pageIndex;
      int get pageCount;
      int get pageSize;
      Future nextPage();
      Future prevPage();

    Similarly to the above, the methods complete when the service finishes loading the mail items.

The next step is to create a mock MailService implementation:

  • create lib/mail/mock_mail_service.dart and start implementing the previously create interface:

    • keep track of the selectedFolder, mailCount, pageIndex, pageCount and pageItems properties as private fields
    • hardcode int get pageSize => 20 for simplicity
  • copy the prepared values from into private top-level fields (some values need to be fixed, e.g. escape $ signs)

  • redirect all async methods to a single mail item generator:

      Future nextPage() => _generateItems(selectedFolder, pageIndex + 1);
      Future prevPage() => _generateItems(selectedFolder, pageIndex - 1);
      Future selectFolder(String label) => _generateItems(label, 0);
      Future _generateItems(String label, int newPageIndex) async {

To implement the _generateItems method, follow similar logic as in MailItems.createFakeMail(), with the following additions:

  • initialize e-mail count based on the label's hash
  • check if page is valid for that count and reset to 0 if needed
  • calculate how many items needs to be on the page and generate them
  • use label, _pageIndex, and index` (on page) to initialize the "random" pointers when creating a new mail item

To make it available for components through injection, register it in main.dart as a provided service. Note that the we register the abstract interface, and provide a concrete instance that implements it:

main() {
  bootstrap(AppComponent, [
    new Provider(MailService, useValue: new MockMailService()),

Injecting it into the MailFolder component requires a small change in the constructor:

class MailFolder {
  final MailService mailService;
  MailFolder(this.mailService) {
  // ...

Add its use to the selectFolder method:

  void selectFolder(FolderItem item) {
    // ...

Migrate NavBar

Assuming e-mail is always ordered by descending date, the older > and < newer buttons in the NavBar can be easily matched with the MailService's nextPage() and prevPage() methods:

  • create the MailNavBar component in lib/mail/list/mail_nav_bar.dart with its usual html and css files

  • add the component to AppComponent:

    • <mail-navbar> to the template
    • MailNavBar to the directives annotation
  • inject the MailService into this new component

  • create a very simple template like:

    <material-button dense *ngIf="hasNewer" (click)="newer()">&lt; newer</material-button>
    {{start}}-{{end}} of {{total}}
    <material-button dense [disabled]="!hasOlder" (click)="older()">older &gt;</material-button>

    Note: while the newer button becomes visible only after the first page, the older button is always there, but may be disabled. That way the height of the component doesn't change if neither of them is visible.

  • implement the above field and methods like the following:

      int get total => mailService.mailCount;
      bool get hasNewer => mailService.pageIndex > 0;
      bool get hasOlder => end < total;
      void newer() {
  • adapt the .anchor CSS style from NavBar.ui.xml and apply it on material-button

Migrate MailList

As we don't have a full-blown open source material table yet, we will fill in the gap with *ngFor and some styling.

  • create the MailList component in lib/mail/list/mail_list.dart with its usual html and css files
  • inject the MailService into this new component

Add the component to AppComponent, and at the same time remove the MailNavBar from it (we'll add it to the header of the list):

  • <mail-list> to the template

  • MailList to the directives annotation

  • use a new CSS style in a div to position the mail list component:

    .right-side {
      margin-left: 260px;

Adapt the style properties for the table, mixing new ones with MailList.ui.xml:

  • create .table style to set the outer border

  • use .row with flexbox to create tabular layout

  • set fixed width for the sender and email columns, restrict their size increase (flex-grow: 0)

  • set padding for the columns

  • set background shade and gradient for the .header

  • set different background for hovering and for the selected row

  • introduce a content area where rows are going to be scrolled:

    .content {
      height: 200px;
      overflow: auto;
      cursor: pointer;

Create the base layout of the table:

<div class="table">
  <div class="header">
    <div class="row">
      <div class="col sender">Sender</div>
      <div class="col email">Email</div>
      <div class="col subject">
  <div class="content">
    <div *ngFor="let item of items"
      <div class="col sender">{{item.sender}}</div>
      <div class="col email">{{}}</div>
      <div class="col subject">{{item.subject}}</div>

The items can be a simple pass-through:

  List<MailItem> get items => mailService.pageItems;

To align the mail-nav-bar to the right, use some additional styling:

mail-nav-bar {
  display: block;
  text-align: right;
  flex-grow: 1;

In the simplest case isSelectedRow and selectRow could be implemented by introducing a new MailItem selected field in the component, but we know that we want to expose the same instance in the MailDetail, a component that we'll build in the next step.

One way to share the data between the two is to place the field inside MailService, and update it each time the mailbox folder or the pagination changes:

  void selectRow(MailItem item) {
    mailService.selectedItem = item;

  bool isSelectedRow(MailItem item) => mailService.selectedItem == item;

Migrate MailDetail

  • create the MailDetail component in lib/mail/detail/mail_detail.dart with its usual html and css files

  • inject the MailService into this new component

  • add the component to AppComponent:

    • <mail-detail> to the template
    • MailDetail to the directives annotation
  • copy most of the styling from MailDetail.ui.xml

  • migrate the template, it will be roughly as simple as:

    <div class="detail">
      <div class="header">
        <div class="headerItem">{{subject}}</div>
        <div class="headerItem"><b>From: </b>{{sender}}</div>
        <div class="headerItem"><b>To: </b>{{recipient}}</div>
      <div class="body" [innerHTML]="body"></div>
  • the fields shall be delegates:

      String get subject => mailService.selectedItem?.subject;
      String get sender => mailService.selectedItem?.sender;
      String get recipient => '';
      String get body => mailService.selectedItem?.body;
  • add margin and overflow CSS properties

Clean GWT backlog

Check the remaining files, but everything should have been migrated and they are safe to remove.

  • replace the GWT logos with the Dart logo

Make the design fresh

  • update borders with a light-gray version (rgba(0,0,0,0.12))
  • remove the dark gradient background from headers, make it much lighter
  • remove text-shadow styles
  • increase global font size (11pt)
  • use more whitespace and a nicer hovering effect in side menus
  • add ripple to the mail item selection
  • don't hide the 'newer' button, disable instead
  • add proper site name to the top nav, remove logo overlap
  • add link to the source code in the top nav
  • replace icons in side panel with material glyphs
  • replace icons in mail folder with material glyphs (and migrate to the new list widget)
  • let the side panel to be collapsed

Resize panels on dragging the resizer between them

In the absence of layout- or docking panels, we can keep track of our panels' desired dimension and bind it to the appropriate CSS property.

  • in MailList component create a new property:

      int height = 200;
  • bind it to the height CSS:

    <div class="content" [style.height.px]="height">

    Note: at this point the height: 200px can be removed from the CSS.

  • create three flex-based panel list in AppComponent:

    • one for the top (with fixed height) and the rest
    • one for the side panel (with a resizer) and the rest of mail panels
    • one for the separation of mail-list and mail-detail
  • create new fields for the dimensions:

      int sideWidthPx = 250;
      int mailHeightPx = 250;
  • bind their values to the components:

    <side-panel [style.flex-basis.px]="sideWidthPx"></side-panel>
    <mail-list [height]="mailHeightPx"></mail-list>
  • Handling the resize can be implemented by listening on mousedown events, and on each of these, track the mousemove and mouseup on the document Element. For example:

      <div class="side-resizer" (mousedown)="resizeSide($event)"></div>
      void resizeSide(MouseEvent down) {
        int originX = down.client.x;
        int originWidth = sideWidthPx;
        StreamSubscription subscription =
            document.onMouseMove.listen((MouseEvent move) {
          int newWidth = originWidth + move.client.x - originX;
          sideWidthPx = max(200, min(newWidth, 500));
        document.onMouseUp.first.then((MouseEvent up) {
  • set the appropriate mouse cursor on the -resizer styles

Stretch the components to fill the available space

The internal scroll handling (overflow:auto) of the components doesn't make it easy to stretch the components to fill the screen. Instead, we can monitor the layout with DomService and act on any event the affects the layout.

The following guide is for MailDetail, implement a similar mechanism for SidePanel:

  • Annotate the bottommost div with #bottom (or create a new one at the end of the html template):

    <div #bottom></div>
  • Make sure it is injected into the component:

      ElementRef bottomRef;
  • Initialize a height value for the content area, and calculate the gap (the difference that it needs to add to the value):

      int heightPx = 200;
      int _calculateGap() {
        Element element = bottomRef.nativeElement;
        int bottom = element.offsetTop + element.offsetHeight;
        return window.innerHeight - bottom;
  • Import and inject DomService. It is a useful utility that enables very efficient (and forced relayout-free) tracking of changes on the UI.

  • Implement AfterContentInit and OnDestroy on the component class. On initialization we subscribe to layout tracking, and on destroy the subscription needs be cleared up:

      StreamSubscription _layoutSubscription;
      ngAfterContentInit() {
        _layoutSubscription =
            domService.trackLayoutChange(_calculateGap, (int gap) {
          heightPx = max(10, heightPx + gap);
        }, runInAngularZone: true);
      ngOnDestroy() {
        _layoutSubscription = null;
  • Bind the height value in the template:

    <div class="body" [innerHTML]="body" [style.height.px]="heightPx"></div>

Trim down the generated code size

Developing the application and working with the material components was was easy, because we did import everything available, like the following code:

import 'package:angular_components/angular_components.dart';

  // ...
  directives: const [materialDirectives],
  providers: const [materialProviders],

But the convenience has it price: it prevents proper tree-shaking, and the dart2js compiler won't be able to decide which components need to be included in final build. To make it easier for the tools, as a last step, we shall clear up our dependencies, and only import the directly used ones:

import 'package:angular_components/src/components/material_popup/material_popup.dart';
import 'package:angular_components/src/laminate/popup/popup.dart';

  // ...
  directives: const [MaterialPopupComponent],

Guiding tree-shaking with the above approach helps to remove a good chunk of unused code. In addition to that, we can fine-tune the dart2js compilation with additional command line attributes in the pubspec.yaml:

- $dart2js:
     commandLineOptions: [--trust-type-annotations --trust-primitives]