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docs(user-input): copyedit TS user-input 

closes #665
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Kathy Walrath 2016-01-07 14:48:15 -08:00 committed by Ward Bell
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@ -1,26 +1,27 @@
include ../../../../_includes/_util-fns
:marked
When the user clicks a link, pushes a button, or types on the keyboard
When the user clicks a link, pushes a button, or enters text
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.
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).
[Run the live example](/resources/live-examples/user-input/ts/plnkr.html)
:marked
## Binding to User Input Events
## 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).
We can use [Angular event bindings](./template-syntax.html#event-binding)
to respond to [any DOM event](https://developer.mozilla.org/en-US/docs/Web/Events).
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.
As an example, here's an event binding that implements a click handler:
+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
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.
@ -29,37 +30,37 @@ include ../../../../_includes/_util-fns
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')
+makeExample('user-input/ts/app/click-me.component.ts', 'click-me-component', 'app/click-me.component.ts')(format=".")
: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 $event object
We can bind to all kinds of events. Let's bind to the "keyup" event of an input box and replay
We can bind to all kinds of events. Let's bind to the keyup event of an input box and replay
what the user types back onto the screen.
This time we'll (1) listen to an event and (2) grab the user's input.
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-1-template', 'app/keyup.components.ts (template v.1)')
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-1-template', 'app/keyup.components.ts (template v.1)')(format=".")
:marked
Angular makes an event object available in the **`$event`** variable
Angular makes an event object available in the **`$event`** variable,
which we pass to the component's `onKey()` method.
The user data we want is in that variable somewhere.
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-1-class-no-type', 'app/keyup.components.ts (class v.1)')
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-1-class-no-type', 'app/keyup.components.ts (class v.1)')(format=".")
:marked
The shape of the `$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.
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 an
[`HTMLInputElement`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement), which
has a `value` property that contains our user input data.
We had this in mind when we passed `$event` to our `onKey()` component method. This is where we extract the user's input and
concatenate it to the previous user data that we're accumulating in the component's `values` property.
The `onKey()` component method is where we extract the user's input
from the event object, adding that input to the list of user data that we're accumulating in the component's `values` property.
We then use [interpolation](./template-syntax.html#interpolation)
to display the accumulating `values` property back on screen.
Enter the letters "abc", backspace to remove them, and we should see:
Enter the letters "abc", and then backspace to remove them.
Here's what the UI displays:
code-example().
a | ab | abc | ab | a | |
figure.image-display
@ -68,10 +69,10 @@ figure.image-display
<a id="keyup1"></a>
.l-sub-section
:marked
We cast the `$event` as an `any` type which means we've abandoned strong typing
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 )')
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-1-class', 'app/keyup.components.ts (class v.1 - strongly typed )')(format=".")
: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.
@ -88,17 +89,19 @@ figure.image-display
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')
Here's an example of using a local template variable
to implement 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')(format=".")
: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
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.
The template is completely self contained. It doesn't bind to the component,
and the component does nothing.
Type in the input box and watch the display update with each keystroke. *Voila!*
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")
@ -114,9 +117,9 @@ figure.image-display
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 easier 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)')
variable than to go through the `$event` object. Maybe we can rewrite our previous
keyup example so that it uses the variable to get the user's input. Let's give it a try.
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-2' ,'app/keyup.components.ts (v2)')(format=".")
:marked
That sure seems easier.
An especially nice aspect of this approach is that our component code gets clean data values from the view.
@ -127,17 +130,17 @@ figure.image-display
:marked
## Key event filtering (with `key.enter`)
Perhaps we don't care about every keystroke.
We're only interested in the input box value when the user hits the "Enter" key. We'd like to ignore all other keys.
When we bind to the `(keyup)` event, our event handling expression hears *every key stroke*.
We could filter the keys first, examining every `$event.keyCode`, and update the `values` property only if the key is "Enter".
Maybe we're only interested in the input box value when the user presses Enter, and we'd like to ignore all other keys.
When we bind to the `(keyup)` event, our event handling expression hears *every keystroke*.
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.
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)')
Only then do we update the component's `values` property. (In this example,
the update happens inside the event expression. A better practice
would be to put the update code in the component.)
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-3' ,'app/keyup.components.ts (v3)')(format=".")
:marked
Here's how it works.
figure.image-display
@ -148,12 +151,12 @@ figure.image-display
## 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.
elsewhere on the page. We update the component's `values` property only when the user presses Enter
while the focus is 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)')
+makeExample('user-input/ts/app/keyup.components.ts', 'key-up-component-4' ,'app/keyup.components.ts (v4)')(format=".")
.l-main-section
:marked
@ -162,9 +165,9 @@ figure.image-display
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.
that can display a list of heroes and add new heroes to that list.
The user can add a hero by first typing in the input box and then
pressing Enter, clicking the Add button, 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")
@ -172,44 +175,46 @@ figure.image-display
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')
+makeExample('user-input/ts/app/little-tour.component.ts', 'little-tour', 'app/little-tour.component.ts')(format=".")
:marked
We've seen almost everything here before. A few things are new or bear repeating.
### *newHero* template variable
### Use template variables to refer to elements
The *newHero* template variable refers to the `<input>` element.
The `newHero` template variable refers to the `<input>` element.
We can use `newHero` from any sibling or child of 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.
Getting the element from a template variable makes the button click handler
simpler. Without the variable, we'd have to use a fancy CSS selector
to find the input element.
### Extract the input box *value*
We could have passed the `newHero` into the component's `addHero()` method.
### Pass values, not elements
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).
something we learned to dislike in our first try at a [keyup component](#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.
Instead, we grab the input box *value* and pass *that* to `addHero`.
The component knows nothing about HTML or the DOM, which is the way we like it.
### Don't let template expressions be complex
### Keep template expressions simple
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 like the first one, which calls `addHero`.
We do not like the second one, which 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 &mdash; because it has no access to the
The second statement exists 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.
Although the example *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.
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
@ -228,12 +233,12 @@ figure.image-display
.l-main-section
:marked
## Next Steps
## Summary
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
Angular has a two-way binding called `NgModel`, which we'll learn about
in the `Forms` chapter.