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Dart 与 HTML 关联

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This tutorial is the first of a series on basic, low-level web programming with the dart:html library. If you use a web framework, some of these concepts might be useful, but you might not need to use the dart:html library at all.

What's the point?

  • DartPad lets you write a simple Dart web app without HTML boilerplate.
  • A Dart web app has Dart, HTML, and (usually) CSS code.
  • Compile a web app’s Dart code to JavaScript to run the app in any modern browser.
  • An HTML file hosts your Dart code in a browser page.
  • The DOM models a browser page in a tree/node structure.
  • Use querySelector() with an ID to get an element from the DOM.
  • CSS selectors are patterns used to select matching elements in the DOM.
  • Use CSS rules to style elements.

To write a low-level web app with Dart, you need to understand several topics—the DOM tree, nodes, elements, HTML, and the Dart language and libraries.

The interdependencies are circular, but we have to begin somewhere, so we begin with a simple HTML file, which introduces the DOM tree and nodes. From there, you build a bare bones, stripped-down Dart app that contains just enough code to dynamically put text on the page from the Dart side.

Though simple, this example shows you how to connect a Dart app to an HTML page and one way that a Dart app can interact with items on the page. These concepts provide the foundation for more interesting and useful web apps.

About the Dart, HTML, and CSS triumvirate

If you’ve used DartPad, you may have already seen the Dart, HTML, and CSS tabs that let you write the code for a web app. Each of these three languages is responsible for a different aspect of the web app.

Language Purpose
Dart Implements the interactivity and dynamic behavior of the web app
HTML Describes the content of the web app’s page (the elements in the document and the structure)
CSS Governs the appearance of page elements

A Dart program can respond to events such as mouse clicks, manipulate the elements on a web page dynamically, and save information. Before the web app is deployed, the Dart code must be compiled into JavaScript code.

HTML is a language for describing web pages. Using tags, HTML sets up the initial page structure, puts elements on the page, and embeds any scripts for page interactivity. HTML sets up the initial document tree and specifies element types, classes, and IDs, which allow HTML, CSS, and Dart programs to refer to the same elements.

CSS, which stands for Cascading Style Sheets, describes the appearance of the elements within a document. CSS controls many aspects of formatting: type face, font size, color, background color, borders, margins, and alignment, to name a few.

About the DOM

The Document Object Model (DOM) represents the structure of a web document as a tree of nodes. When an HTML file is loaded into a browser, the browser interprets the HTML and displays the document in a window. The following diagram shows a simple HTML file and the resulting web browser page in Chrome.

A simple HTML file and its resulting web page

HTML uses tags to describe the document. For example, the simple HTML code above uses the <title> tag for the page title, <h1> for a level-one header, and <p> for a paragraph. Some tags in the HTML code, such as <head> and <body>, are not visible on the web page, but do contribute to the structure of the document.

In the DOM, the document object sits at the root of the tree (it has no parent). Different kinds of nodes in the tree represent different kinds of objects in the document. For example, the tree has page elements, text nodes, and attribute nodes. Here is the DOM tree for the simple HTML file above.

A Dart dynamically changing the DOM

Notice that some tags, such as the <p> paragraph tag, are represented by multiple nodes. The paragraph itself is an element node. The text within the paragraph is a text node (and in some cases, might be a subtree containing many nodes). And the ID is an attribute node.

Except for the root node, each node in the tree has exactly one parent. Each node can have many children.

An HTML file defines the initial structure of a document. Dart or JavaScript can dynamically modify that document by adding, deleting, and modifying the nodes in the DOM tree. When the DOM is changed, the browser immediately re-renders the window.

A Dart program can dynamically change the DOM

The diagram shows a small Dart program that makes a modest change to the DOM by dynamically changing a paragraph’s text. A program could add and delete nodes, or even insert an entire subtree of nodes.

Create a new Dart app

  1. Go to the DartPad.
  2. Click the New Pad button to undo any changes you might have made the last time you visited DartPad.
  3. Click Dart.
  4. Select HTML (below Dart), so you can edit HTML and CSS in DartPad.

Edit the HTML source code

  1. Click HTML, at the upper left of DartPad. The view switches from Dart code to the (currently non-existent) HTML code.

  2. Add the following HTML code:

    <p id="RipVanWinkle">
  RipVanWinkle paragraph.
</p>

  3. Expand the output pane to see how a browser would render your HTML.

About the HTML source code

This HTML code is similar to the HTML code in the various diagrams earlier in this tutorial, but it’s even simpler.

In DartPad you need only the tags you really care about—in this case: <p>. You don’t need surrounding tags such as <html> and <body>. Because DartPad knows where your Dart code is, you don’t need a <script> tag.

The paragraph tag has the identifier RipVanWinkle. The Dart code you create in the next step uses this ID to get the paragraph element.

Edit the Dart source code

  1. Click Dart at the upper right of DartPad. The view switches from HTML code to Dart code.

  2. Change the Dart code to the following:

    import 'dart:html';

void main() {
  querySelector('#RipVanWinkle')!.text = 'Wake up, sleepy head!';
}

  3. Click Run to execute your code.

The text in the output pane changes to “Wake up, sleepy head!”

About the Dart source code

Let’s step through the Dart code.

Importing libraries

The import directive imports the specified library, making all the classes and functions in that library available to your program.

The following import statement imports Dart’s HTML library, which contains key classes and functions for programming the DOM:

import 'dart:html';

This program imports Dart’s HTML library, which contains key classes and functions for programming the DOM. Key classes include:

Dart class Description
Node Implements a DOM node.
Element A subclass of Node; implements a web page element.
Document Another subclass of Node; implements the document object.

The dart:core library, which is automatically imported, contains many other useful classes, such as: List, a parameterized class that can specify the type of its members. An instance of Element keeps its list of child Element objects in a List<Element>.

Using the querySelector() function

This app’s main() function contains a single line of code that is a little like a run-on sentence with multiple things happening one after another. Let’s deconstruct it.

querySelector() is a top-level function provided by the dart:html library that gets an Element object from the DOM.

querySelector('#RipVanWinkle')!.text = 'Wake up, sleepy head!';

The argument to querySelector() is a string containing a CSS selector that identifies the object. Most commonly CSS selectors specify classes, identifiers, or attributes. We’ll look at these in more detail later, when we add a CSS file to the mini app. In this case, RipVanWinkle is the unique ID for a paragraph element declared in the HTML file, and #RipVanWinkle specifies that ID.

Another useful function for getting elements from the DOM is querySelectorAll(), which returns multiple Element objects via a list of elements—List<Element>—all of which match the provided selector.

Handling nullable elements

Because DOM elements might be missing, the querySelector() function returns a nullable result, as indicated by the ? in the function’s return type (Element?). A null return value means that no element matches the specified CSS selector.

To use the text property of a returned Element?, the type must be promoted to Element. Because we wrote the HTML and know that the element is always present, we can use the null assertion operator (!) when referring to the element’s properties:

querySelector('#RipVanWinkle')!.text = 'Wake up, sleepy head!';

If the element with the #RipVanWinkle ID isn’t guaranteed to be present in the DOM, you can instead use the conditional member access operator (?.) to set text only if the returned element is not null:

querySelector('#RipVanWinkle')?.text = 'Wake up, sleepy head!';

If you plan to access the element multiple times and know its type, another option is to typecast the queried element to the expected type:

final paragraph = querySelector('#RipVanWinkle') as ParagraphElement;
paragraph.text = 'Wake up, sleepy head!';

To learn more about nullable types and null safety in general, see Sound null safety.

Setting the text of an Element

In the DOM, the text of a page element is contained in a child node, specifically, a text node. In the following diagram, the node containing the string “RipVanWinkle paragraph.” is a text node.

DOM tree for a paragraph element

More complex text, such as text with style changes or embedded links and images, would be represented with a subtree of text nodes and other objects.

In Dart, you can simply use the Element text property, which has a getter and setter that walk the subtree of nodes for you and extract or set their text:

querySelector('#RipVanWinkle')!.text = 'Wake up, sleepy head!';

However, if the text node has styles (and thus a subtree), getting text and then setting it immediately is likely to change the DOM, as a result of losing subtree information. Often, as with our RipVanWinkle example, this simplification has no adverse effects.

The assignment operator (=) sets the text of the Element returned by the querySelector() function to the string “Wake up, sleepy head!”.

querySelector('#RipVanWinkle')!.text = 'Wake up, sleepy head!';

This causes the browser to immediately re-render the browser page containing this app,
thus dynamically displaying the text on the browser page.

HTML and Dart connections

The Dart web app changed the text in the browser window dynamically at runtime. Of course, placing text on a browser page and doing nothing else could be accomplished with straight HTML. This little app only shows you how to make a connection from a Dart app to a browser page.

In DartPad, the only visible connection between the Dart code and the HTML code is the RipVanWinkle ID.

The RipVanWinkle ID is used by both Dart and HTML

To run your app outside DartPad, you need to compile your Dart code to JavaScript. Use the webdev build command to compile your app to deployable JavaScript. Then you need to make another connection between the HTML and generated JavaScript: you must add a <script> tag to the HTML to tell the browser where to find the compiled Dart code.

Here’s the full HTML code for this app, assuming that the Dart code is in a file named main.dart:

<!DOCTYPE html>

<html>
  <head>
    <title>A Minimalist App</title>
    <script defer src="main.dart.js"></script>
  </head>
  <body>
    <p id="RipVanWinkle">
      RipVanWinkle paragraph.
    </p>
  </body>
</html>

The <script> element specifies the location of the compiled Dart code.

Give the app some style with CSS

Most HTML uses cascading style sheets (CSS) to define styles that control the appearance of page elements. Let’s customize the CSS for the mini app.

  1. Click CSS at the upper left of DartPad. The view switches from Dart code to the (non-existent) CSS code.

  2. Add the following CSS code:

    #RipVanWinkle {
  font-size: 20px;
  font-family: 'Roboto', sans-serif;
  text-align: center;
  margin-top: 20px;
  background-color: SlateBlue;
  color: Yellow;
}

The display in the output pane immediately changes to reflect the new styles, which apply only to the page element that has the ID RipVanWinkle.

About CSS selectors

IDs, classes, and other information about elements are established in HTML. Your Dart code can use this information to get elements using a CSS selector—a pattern used to select matching elements in the DOM. CSS selectors allow the CSS, HTML, and Dart code to refer to the same objects. Commonly, a selector specifies an ID, an HTML element type, a class, or an attribute. Selectors can also be nested.

CSS selectors are important in Dart programs because you use them with querySelector() and querySelectorAll() to get matching elements from the DOM. Most often Dart programs use ID selectors with querySelector() and class selectors with querySelectorAll().

Here are some examples of CSS selectors:

Selector type Example Description
ID selector #RipVanWinkle Matches a single, unique element
HTML element p Matches all paragraphs
HTML element h1 Matches all level-one headers
Class .classname Matches all items with the class classname
Asterisk * Matches all elements
Attribute input[type="button"] Matches all button input elements

Let’s look at the CSS code for the mini app. The CSS file for the mini app has one CSS rule in it. A CSS rule has two main parts: a selector and a set of declarations.

The parts of a CSS rule

In the mini app, the selector #RipVanWinkle is an ID selector, as signaled by the hashtag (#); it matches a single, unique element with the specified ID, our now tired RipVanWinkle paragraph element. RipVanWinkle is the ID in the HTML file. It is referred to in the CSS file and in the Dart code using a hashtag(#). Classnames are specified in the HTML file without a period (.) and referred to in the CSS file and in Dart code with a period (.).

Between the curly brackets of a CSS rule is a list of declarations, each of which ends in a semi-colon (;). Each declaration specifies a property and its value. Together the set of declarations define the style sheet for all matching elements. The style sheet is used to set the appearance of the matching element(s) on the web page.

A declaration specifies an attribute and its value

The CSS rule for the RipVanWinkle paragraph specifies several properties; for example, it sets the text color to Yellow.

Other resources

What next?

The next tutorial, Add elements to the DOM, shows you how to dynamically change the HTML page by adding elements to the DOM.