Lập trình Androi part 21 pptx

Raising Toasts A Toast is a transient message, meaning that it displays and disappears on its own without user interaction.. The Toast class offers a static makeText that accepts a Stri

149

Showing Pop-Up

Messages

Sometimes, your activity (or other piece of Android code) will need to speak up

Not every interaction with Android users will be tidy and containable in activities

composed of views Errors will crop up Background tasks may take much longer than

expected Something asynchronous may occur, such as an incoming message In these

and other cases, you may need to communicate with the user outside the bounds of the

traditional UI

Of course, this is nothing new Error messages in the form of dialog boxes have been

around for a very long time More subtle indicators also exist—from task tray icons to

bouncing dock icons to vibrating cell phones

Android has quite a few systems for letting you alert your users outside the bounds of an

Activity-based UI One, notifications, is tied heavily into intents and services and, as

such, is covered in Chapter 31 In this chapter, you will learn about two means of raising

pop-up messages: toasts and alerts

Raising Toasts

A Toast is a transient message, meaning that it displays and disappears on its own

without user interaction Moreover, it does not take focus away from the currently active

Activity, so if the user is busy writing the next Great Programming Guide, his

keystrokes will not be “eaten” by the message

Since a Toast is transient, you have no way of knowing if the user even notices it You

get no acknowledgment, nor does the message stick around for a long time to pester

the user Hence, the Toast is mostly for advisory messages, such as indicating a

long-running background task is completed, the battery has dropped to a low (but not too

low) level, and so on

14

Making a Toast is fairly easy The Toast class offers a static makeText() that accepts a String (or string resource ID) and returns a Toast instance The makeText() method also needs the Activity (or other Context) plus a duration The duration is expressed in the form of the LENGTH_SHORT or LENGTH_LONG constants to indicate, on a relative basis, how long the message should remain visible

If you would prefer your Toast be made out of some other View, rather than be a boring old piece of text, simply create a new Toast instance via the constructor (which takes a Context), and then call setView() to supply it with the view to use and setDuration() to set the duration

Once your Toast is configured, call its show() method, and the message will be

displayed

Alert! Alert!

If you would prefer something in the more classic dialog box style, what you want is an AlertDialog As with any other modal dialog box, an AlertDialog pops up, grabs the focus, and stays there until closed by the user You might use this for a critical error, a validation message that cannot be effectively displayed in the base activity UI, or some other situation where you are sure that the user needs to see the message and needs to see it now

The simplest way to construct an AlertDialog is to use the Builder class Following in true builder style, Builder offers a series of methods to configure an AlertDialog, each method returning the Builder for easy chaining At the end, you call show() on the builder to display the dialog

Commonly used configuration methods on Builder include the following:

setMessage(): Sets the “body” of the dialog to be a simple textual message, from either a supplied String or a supplied string resource ID

setTitle() and setIcon(): Configure the text and/or icon to appear in the title bar of the dialog

setPositiveButton(), setNeutralButton(), and setNegativeButton():

Indicate which button(s) should appear across the bottom of the dialog, where they should be positioned (left, center, or right, respectively), what their captions should be, and what logic should be invoked when the button is clicked (besides dismissing the dialog)

If you need to configure the AlertDialog beyond what the builder allows, instead of calling show(), call create() to get the partially built AlertDialog instance, configure it the rest of the way, and then call one of the flavors of show() on the AlertDialog itself Once show() is called, the dialog will appear and await user input

Checking Them Out

To see how these work in practice, take a peek at Messages/Message, containing the

following layout:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:orientation="vertical"

android:layout_width="fill_parent"

android:layout_height="fill_parent" >

<Button

android:id="@+id/alert"

android:text="Raise an alert"

android:layout_width="fill_parent"

android:layout_height="wrap_content"/>

<Button

android:id="@+id/toast"

android:text="Make a toast"

android:layout_width="fill_parent"

android:layout_height="wrap_content"/>

</LinearLayout>

Here’s the Java code:

public class MessageDemo extends Activity implements View.OnClickListener {

Button alert;

Button toast;

@Override

public void onCreate(Bundle icicle) {

super.onCreate(icicle);

setContentView(R.layout.main);

alert=(Button)findViewById(R.id.alert);

alert.setOnClickListener(this);

toast=(Button)findViewById(R.id.toast);

toast.setOnClickListener(this);

}

public void onClick(View view) {

if (view==alert) {

new AlertDialog.Builder(this)

.setTitle("MessageDemo")

.setMessage("eek!")

.setNeutralButton("Close", new DialogInterface.OnClickListener() {

public void onClick(DialogInterface dlg, int sumthin) {

// do nothing – it will close on its own

}

})

.show();

}

else {

Toast

.makeText(this, "<clink, clink>", Toast.LENGTH_SHORT)

.show();

}

}

}

The layout is unremarkable—just a pair of buttons to trigger the alert and the toast When the Raise an alert button is clicked, we use a builder (new Builder(this)) to set the title (setTitle("MessageDemo")), message (setMessage("eek!")), and neutral button (setNeutralButton("Close", new OnClickListener() ) before showing the dialog When the Close button is clicked, the OnClickListener callback does nothing; the mere fact that the button was pressed causes the dialog to be dismissed However, you could update information in your activity based on the user action, particularly if you have multiple buttons for the user to choose from The result is a typical dialog, as shown in Figure 14–1

Figure 14–1 The MessageDemo sample application, after clicking the Raise an alert button

When the Make a toast button is clicked, the Toast class makes us a text-based toast (makeText(this, "<clink, clink>", LENGTH_SHORT)), which we then show() The result

is a short-lived, noninterrupting message, as shown in Figure 14–2

Figure 14–2 The same application, after clicking the Make a toast button