Lập trình Androi part 26 pps

177 Launching Activities and Subactivities As you’ve learned, the theory behind the Android UI architecture is that developers should decompose their application into distinct activiti

177

Launching Activities

and Subactivities

As you’ve learned, the theory behind the Android UI architecture is that developers

should decompose their application into distinct activities Each activity is implemented

as an Activity, and each is reachable via intents, with a “main” activity being the one

launched by the Android launcher For example, a calendar application could have

activities for viewing the calendar, viewing a single event, editing an event (including

adding a new one), and so forth

This implies that one of your activities has the means to start up another activity For

example, if someone selects an event from the view-calendar activity, you might want to

show the event activity for that event So, you need to be able to cause the

view-event activity to launch and show a specific view-event (the one the user chose) This chapter

describes how to do that

NOTE: This chapter assumes that you know which activity you want to launch, probably because

it is another activity in your own application It’s also possible that you have a content Uri to do

something, and you want your users to be able to do something with it, but you do not know up

front what the options are This situation requires more advanced handling, which I cover in my

book The Busy Coders Guide to Advanced Android Development (CommonsWare, 2009)

Peers and Subs

One key question you need to answer when you decide to launch an activity is this:

Does your activity need to know when the launched activity ends?

For example, suppose you want to spawn an activity to collect authentication

information for some web service you are connecting to—maybe you need to

authenticate with OpenID (http://openid.net/) in order to use an OAuth service

18

(http://oauth.net/) In this case, your main activity will need to know when the

authentication is complete so it can start to use the web service

On the other hand, imagine an e-mail application in Android When the user elects to view an attachment, neither you nor the user necessarily expect the main activity to know when the user is finished viewing that attachment

In the first scenario, the launched activity is clearly subordinate to the launching activity

In that case, you probably want to launch the child as a subactivity, which means your activity will be notified when the child activity is complete

In the second scenario, the launched activity is more a peer of your activity, so you probably want to launch the child just as a regular activity Your activity will not be informed when the child is done, but, then again, your activity really doesn’t need to know

Start ’Em Up

The two pieces for starting an activity are an intent and your choice of how to start it up

Make an Intent

As discussed in the previous chapter, intents encapsulate a request, made to Android, for some activity or other intent receiver to do something

If the activity you intend to launch is one of your own, you may find it simplest to create

an explicit intent, naming the component you wish to launch For example, from within your activity, you could create an intent like this:

new Intent(this, HelpActivity.class);

This stipulates that you want to launch the HelpActivity This activity would need to be named in your AndroidManifest.xml file, though not necessarily with any intent filter, since you are trying to request it directly

Or you could put together an intent for some Uri, requesting a particular action:

Uri uri=Uri.parse("geo:"+lat.toString()+","+lon.toString());

Intent i=new Intent(Intent.ACTION_VIEW, uri);

Here, given that you have the latitude and longitude of some position (lat and lon, respectively) of type Double, you construct a geo scheme Uri and create an intent requesting to view this Uri (ACTION_VIEW)

Make the Call

Once you have your intent, you need to pass it to Android and get the child activity to launch You have two choices:

The simplest option is to call startActivity() with the Intent This

will cause Android to find the best-match activity and pass the intent

to it for handling Your activity will not be informed when the child

activity is complete

You can call startActivityForResult(), passing it the Intent and a

number (unique to the calling activity) Android will find the best-match

activity and pass the intent over to it Your activity will be notified when

the child activity is complete via the onActivityResult() callback

With startActivityForResult(), as noted, you can implement the onActivityResult()

callback to be notified when the child activity has completed its work The callback

receives the unique number supplied to startActivityForResult(), so you can

determine which child activity is the one that has completed You also get the following:

A result code, from the child activity calling setResult() Typically, this

is RESULT_OK or RESULT_CANCELLED, though you can create your own

return codes (pick a number starting with RESULT_FIRST_USER)

An optional String containing some result data, possibly a URL to

some internal or external resource For example, an ACTION_PICK intent

typically returns the selected bit of content via this data string

An optional Bundle containing additional information beyond the result

code and data string

To demonstrate launching a peer activity, take a peek at the Activities/Launch sample

application The XML layout is fairly straightforward: two fields for the latitude and

longitude, plus a button

<?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"

>

<TableLayout

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:stretchColumns="1,2"

>

<TableRow>

<TextView

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:paddingLeft="2dip"

android:paddingRight="4dip"

android:text="Location:"

/>

<EditText android:id="@+id/lat"

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:cursorVisible="true"

android:editable="true"

android:singleLine="true"

android:layout_weight="1"

/>

<EditText android:id="@+id/lon"

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:cursorVisible="true"

android:editable="true"

android:singleLine="true"

android:layout_weight="1"

/>

</TableRow>

</TableLayout>

<Button android:id="@+id/map"

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:text="Show Me!"

/>

</LinearLayout>

The button’s OnClickListener simply takes the latitude and longitude, pours them into a geo scheme Uri, and then starts the activity

package com.commonsware.android.activities;

import android.app.Activity;

import android.content.Intent;

import android.net.Uri;

import android.os.Bundle;

import android.view.View;

import android.widget.Button;

import android.widget.EditText;

public class LaunchDemo extends Activity {

private EditText lat;

private EditText lon;

@Override

public void onCreate(Bundle icicle) {

super.onCreate(icicle);

setContentView(R.layout.main);

Button btn=(Button)findViewById(R.id.map);

lat=(EditText)findViewById(R.id.lat);

lon=(EditText)findViewById(R.id.lon);

btn.setOnClickListener(new View.OnClickListener() {

public void onClick(View view) {

String _lat=lat.getText().toString();

String _lon=lon.getText().toString();

Uri uri=Uri.parse("geo:"+_lat+","+_lon);

startActivity(new Intent(Intent.ACTION_VIEW, uri));

}

});

}

}

The activity is not much to look at, as you can see in Figure 18–1

Figure 18–1 The LaunchDemo sample application, with a location filled in

If you fill in a location (e.g., 38.8891 latitude and -77.0492 longitude) and click the

button, the resulting map is more interesting, as shown in Figure 18–2

Figure 18–2 The map launched by LaunchDemo, showing the Lincoln Memorial in Washington DC

Note that this is the built-in Android map activity; we did not create our own activity to

display this map In Chapter 33, you will see how you can create maps in your own

activities, in case you need greater control over how the map is displayed

NOTE: This sample application may not work on an Android 2.0 AVD in the emulator, as the AVD

appears to lack the Maps application

Tabbed Browsing, Sort Of

One of the main features of the modern desktop web browser is tabbed browsing, where a single browser window can show several pages split across a series of tabs On

a mobile device, this may not make a lot of sense, given that you lose screen real estate for the tabs themselves In this book, however, we do not let little things like sensibility stop us, so let’s demonstrate a tabbed browser, using TabActivity and Intent objects

As you may recall from Chapter 9, a tab can have either a View or an Activity as its contents If you want to use an Activity as the content of a tab, you provide an Intent that will launch the desired Activity; Android’s tab-management framework will then pour the Activity’s UI into the tab

Your natural instinct might be to use an http: Uri the way we used a geo: Uri in the previous example:

Intent i=new Intent(Intent.ACTION_VIEW);

i.setData(Uri.parse("http://commonsware.com"));

That way, you could use the built-in browser application and get all of the features that it offers

Alas, this does not work You cannot host other applications’ activities in your tabs; only your own activities are allowed, for security reasons

So, we dust off our WebView demos from Chapter 13 and use those instead, repackaged

as Activities/IntentTab

Here is the source to the main activity, the one hosting the TabView:

public class IntentTabDemo extends TabActivity {

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

TabHost host=getTabHost();

host.addTab(host.newTabSpec("one")

setIndicator("CW")

setContent(new Intent(this, CWBrowser.class)));

host.addTab(host.newTabSpec("two")

setIndicator("Android")

setContent(new Intent(this, AndroidBrowser.class)));

}

}

As you can see, we are using TabActivity as the base class, and so we do not need our

own layout XML, since TabActivity supplies it for us All we do is get access to the

TabHost and add two tabs, each specifying an Intent that directly refers to another

class In this case, our two tabs will host a CWBrowser and an AndroidBrowser,

respectively

Those activities are simple modifications to the earlier browser demos:

public class CWBrowser extends Activity {

WebView browser;

@Override

public void onCreate(Bundle icicle) {

super.onCreate(icicle);

browser=new WebView(this);

setContentView(browser);

browser.loadUrl("http://commonsware.com");

}

}

public class AndroidBrowser extends Activity {

WebView browser;

@Override

public void onCreate(Bundle icicle) {

super.onCreate(icicle);

browser=new WebView(this);

setContentView(browser);

browser.loadUrl("http://www.android.com/");

}

}

They simply load a different URL into the browser: the CommonsWare home page in

one and the Android home page in the other

TIP: Using distinct subclasses for each targeted page is rather wasteful Instead, you could

package the URL to open as an “extra” in an Intent and used that Intent to spawn a

general-purpose BrowserTab activity, which would read the URL out of the Intent extra and use that

The resulting UI shows what tabbed browsing could look like on Android, as shown in

Figures 18–3 and 18–4

Figure 18–3 The IntentTabDemo sample application, showing the first tab

Figure 18–4 The IntentTabDemo sample application, showing the second tab