242 lines
11 KiB
Plaintext
242 lines
11 KiB
Plaintext
include ../../../../_includes/_util-fns
|
|
|
|
:marked
|
|
When the user clicks a link, pushes a button, or types on the keyboard
|
|
we want to know about it. These user actions all raise DOM events.
|
|
In this chapter we learn to bind to those events using the Angular Event Binding syntax.
|
|
|
|
[Live Example](/resources/live-examples/user-input/ts/plnkr.html).
|
|
|
|
:marked
|
|
## Binding to User Input Events
|
|
|
|
We can listen to [any DOM event](https://developer.mozilla.org/en-US/docs/Web/Events)
|
|
with an [Angular Event Binding](./template-syntax.html#event-binding).
|
|
|
|
The syntax is simple. We assign a template expression to the DOM event name, surrounded in parentheses.
|
|
A click Event Binding makes for a quick illustration.
|
|
+makeExample('user-input/ts/app/click-me.component.ts', 'click-me-button')(format=".", language="html")
|
|
|
|
<a id="click"></a>
|
|
:marked
|
|
The `(click)` to the left of the equal sign identifies the button's click event as the **target of the binding**.
|
|
The text within quotes on the right is the "**template expression**" in which we
|
|
respond to the click event by calling the component's `onClickMe` method. A [template expression](./template-syntax.html#template-expressions) is a subset
|
|
of JavaScript with a few added tricks.
|
|
|
|
When writing a binding we must be aware of a template expression's **execution context**.
|
|
The identifers appearing within an expression belong to a specific context object.
|
|
That object is usually the Angular component that controls the template ... which it definitely is
|
|
in this case because that snippet of HTML belongs to the following component:
|
|
|
|
+makeExample('user-input/ts/app/click-me.component.ts', 'click-me-component', 'app/click-me.component.ts')
|
|
:marked
|
|
The `onClickMe` in the template refers to the `onClickMe` method of the component.
|
|
When the user clicks the button, Angular calls the component's `onClickMe` method.
|
|
|
|
.l-main-section
|
|
:marked
|
|
## Get user input from the $event object
|
|
We can bind to all kinds of events. Let's bind to the "keyup" event of an input box and replay
|
|
what the user types back onto the screen.
|
|
|
|
This time we'll (1) listen to an event and (2) grab the user's input.
|
|
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-1-template', 'app/keyup.components.ts (template v.1)')
|
|
:marked
|
|
Angular makes an event object available in the **`$event`** variable
|
|
which we pass to the component's `onKey()` method.
|
|
The user data we want is in that variable somewhere.
|
|
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-1-class-no-type', 'app/keyup.components.ts (class v.1)')
|
|
:marked
|
|
The shape of the `$event` object is determined by whatever raises the event.
|
|
The `keyup` event comes from the DOM so `$event` must be a [standard DOM event object](https://developer.mozilla.org/en-US/docs/Web/API/Event).
|
|
The `$event.target` gives us the
|
|
[`HTMLInputElement`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement) which
|
|
has a `value` property and that's where we find our user input data.
|
|
|
|
With had this in mind when we passed `$event` to our `onKey()` component method where we extract the user's input and
|
|
concatenate it to the previous user data that we're accumulating in the component's' `values` property.
|
|
We then use [interpolation](./template-syntax.html#interpolation)
|
|
to display the accumulating `values` property back on screen.
|
|
|
|
Enter the letters "abc", backspace to remove them, and we should see:
|
|
code-example().
|
|
a | ab | abc | ab | a | |
|
|
figure.image-display
|
|
img(src='/resources/images/devguide/user-input/keyup1-anim.gif' alt="key up 1")
|
|
|
|
<a id="keyup1"></a>
|
|
.l-sub-section
|
|
:marked
|
|
We cast the `$event` as an `any` type which means we've abandoned strong typing
|
|
to simplify our code. We generally prefer the strong typing that TypeScript affords.
|
|
We can rewrite the method, casting to HTML DOM objects like this.
|
|
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-1-class', 'app/keyup.components.ts (class v.1 - strongly typed )')
|
|
:marked
|
|
<br>Strong typing reveals a serious problem with passing a DOM event into the method:
|
|
too much awareness of template details, too little separation of concerns.
|
|
|
|
We'll address this problem in our next try at processing user keystrokes.
|
|
:marked
|
|
|
|
.l-main-section
|
|
:marked
|
|
## Get user input from a local template variable
|
|
There's another way to get the user data without the `$event` variable.
|
|
|
|
Angular has syntax feature called [**local template variables**](./template-syntax.html#local-vars).
|
|
These variables grant us direct access to an element.
|
|
We declare a local template variable by preceding an identifier with a hash/pound character (#).
|
|
|
|
Let's demonstrate with a clever keystroke loopback in an ultra-simple template.
|
|
+makeExample('user-input/ts/app/loop-back.component.ts', 'loop-back-component', 'app/loop-back.component.ts')
|
|
:marked
|
|
We've declared a template local variable named `box` on the `<input>` element.
|
|
The `box` variable is a reference to the `<input>` element itself which means we can
|
|
grab the input element's `value` and display it
|
|
with interpolation between `<p>` tags.
|
|
|
|
The template is completely self contained. It doesn't bind to the component which does nothing.
|
|
|
|
Type in the input box and watch the display update with each keystroke. *Voila!*
|
|
|
|
figure.image-display
|
|
img(src='/resources/images/devguide/user-input/keyup-loop-back-anim.gif' alt="loop back")
|
|
.l-sub-section
|
|
:marked
|
|
**This won't work at all unless we bind to an event**.
|
|
|
|
Angular only updates the bindings (and therefore the screen)
|
|
if we do something in response to asynchronous events such as keystrokes.
|
|
|
|
That's why we bind the `keyup` event to an expression that does ... well, nothing.
|
|
We're binding to the number 0, the shortest expression we can think of.
|
|
That is all it takes to keep Angular happy. We said it would be clever!
|
|
:marked
|
|
That local template variable is intriguing. It's clearly easer to get to the textbox with that
|
|
variable than to go through the `$event` object. Maybe we can re-write our previous
|
|
"key-up" example using the variable to acquire the user's' input. Let's give it a try.
|
|
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-2' ,'app/keyup.components.ts (v2)')
|
|
:marked
|
|
That sure seems easier.
|
|
An especially nice aspect of this approach is that our component code gets clean data values from the view.
|
|
It no longer requires knowledge of the `$event` and its structure.
|
|
|
|
<a id="key-event"></a>
|
|
.l-main-section
|
|
:marked
|
|
## Key event filtering (with `key.enter`)
|
|
Perhaps we don't care about every keystroke.
|
|
We're only interested in the input box value when the user hits the "Enter" key. We'd like to ignore all other keys.
|
|
When we bind to the `(keyup)` event, our event handling expression hears *every key stroke*.
|
|
We could filter the keys first, examining every `$event.keyCode`, and update the `values` property only if the key is "Enter".
|
|
|
|
Angular can filter the key events for us. Angular has a special syntax for keyboard events.
|
|
We can listen for just the "Enter" key by binding to Angular's `keyup.enter` pseudo-event.
|
|
|
|
Only then do we update the component's `values` property ...
|
|
inside the event expression rather than in the component ...
|
|
because we *can* ... even if it is a dubious practice.
|
|
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-3' ,'app/keyup.components.ts (v3)')
|
|
:marked
|
|
Here's how it works.
|
|
figure.image-display
|
|
img(src='/resources/images/devguide/user-input/keyup3-anim.gif' alt="key up 3")
|
|
|
|
.l-main-section
|
|
:marked
|
|
## On blur
|
|
|
|
Our previous example won't transfer the current state of the input box if the user mouses away and clicks
|
|
elsewhere on the page. We only update the component's `values` property when the user presses "Enter"
|
|
inside the input box.
|
|
|
|
Let's fix that by listening to the input box's blur event as well.
|
|
|
|
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-4' ,'app/keyup.components.ts (v4)')
|
|
|
|
.l-main-section
|
|
:marked
|
|
## Put it all together
|
|
We learned how to [display data](./displaying-data.html) in the previous chapter.
|
|
We've acquired a small arsenal of event binding techniques in this chapter.
|
|
|
|
Let's put it all together in a micro-app
|
|
that can display a list of heroes and add new heroes to that list
|
|
by typing in the input box and hitting "Enter", clicking "Add", or clicking
|
|
elsewhere on the page.
|
|
|
|
figure.image-display
|
|
img(src='/resources/images/devguide/user-input/little-tour-anim.gif' alt="Little Tour of Heroes")
|
|
:marked
|
|
Below is the "Little Tour of Heroes" component.
|
|
We'll call out the highlights after we bask briefly in its minimalist glory.
|
|
|
|
+makeExample('user-input/ts/app/little-tour.component.ts', 'little-tour', 'app/little-tour.component.ts')
|
|
:marked
|
|
We've seen almost everything here before. A few things are new or bear repeating.
|
|
|
|
### *newHero* template variable
|
|
|
|
The *newHero* template variable refers to the `<input>` element.
|
|
|
|
We can access `newHero` from any sibling or child of the `<input>` element.
|
|
When the user clicks the button, we don't need a fancy css selector to
|
|
track down the textbox and extract its value.
|
|
|
|
### Extract the input box *value*
|
|
We could have passed the `newHero` into the component's `addHero()` method.
|
|
|
|
But that would require `addHero` to pick its way through the `<input>` DOM element,
|
|
something we learned to dislike in our first try at a [*KeyupComponent*](#keyup1).
|
|
|
|
Instead, we grab the input box *value* and pass *that* to `addHero()`.
|
|
The component knows nothing about HTML or DOM which is the way we like it.
|
|
|
|
### Don't let template expressions be complex
|
|
We bound `(blur)` to *two* JavaScript statements.
|
|
|
|
We like the first one that calls `addHero`.
|
|
We do not like the second one that assigns an empty string to the input box value.
|
|
|
|
We did it for a good reason. We have to clear the input box after adding the new hero to the list.
|
|
The component has no way to do that itself — because it has no access to the
|
|
input box (our design choice).
|
|
|
|
Although it *works*, we are rightly wary of JavaScript in HTML.
|
|
Template expressions are powerful. We're supposed to use them responsibly.
|
|
Complex JavaScript in HTML is irresponsible.
|
|
|
|
Should we reconsider our reluctance to pass the input box into the component?
|
|
|
|
There should be a better third way. And there is as we'll see when we learn about `NgModel` in the [Forms](forms.html) chapter.
|
|
.l-main-section
|
|
:marked
|
|
## Source code
|
|
|
|
Here is all the code we talked about in this chapter.
|
|
+makeTabs(`
|
|
user-input/ts/app/click-me.component.ts,
|
|
user-input/ts/app/keyup.components.ts,
|
|
user-input/ts/app/loop-back.component.ts,
|
|
user-input/ts/app/little-tour.component.ts
|
|
`,'',
|
|
`click-me.component.ts,
|
|
keyup.components.ts,
|
|
loop-back.component.ts,
|
|
little-tour.component.ts`)
|
|
|
|
.l-main-section
|
|
:marked
|
|
## Next Steps
|
|
|
|
We've mastered the basic primitives for responding to user input and gestures.
|
|
As powerful as these primitives are, they are a bit clumsy for handling
|
|
large amounts of user input. We're operating down at the low level of events when
|
|
we should be writing two-way bindings between data entry fields and model properties.
|
|
|
|
Angular has a two-way binding called `NgModel` and we learn about it
|
|
in the `Forms` chapter.
|
|
|
|
|