Tự học Begin Android 4 - Trial version

7/31/2019 T hc Begin Android 4 - Trial version

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android:theme="@style/AppTheme">

The Activity base class defines a series of events that govern the life cycle

of an activity. The Activity class defines the following events:onCreate() Called when the activity is first createdonStart() Called when the activity becomes visible to the useronResume() Called when the activity starts interacting with the useronPause() Called when the current activity is being paused and the previous

activity is being resumedonStop() Called when the activity is no longer visible to the useronDestroy() Called before the activity is destroyed by the system (either manually

or by the system to conserve memory)onRestart() Called when the activity has been stopped and is restarting again


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By default,

the activity

created for

you contains

the

onCreate()

event. Within

this event

handler is the

code that

helps to

display the

UI elements

of your

screen.

Understanding the Life Cycle of an Activity by adding the following statements in bold in

the Activity101Activity.javafile:publicclass Activity101Activity extends Activity {

String tag = "Lifecycle";

/** Called when the activity is first created. */@Overridepublicvoid onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);// This activity load its UI using main.xml defined in res/layoutsetContentView(R.layout.main);Log.d(tag, "In the onCreate() event");


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Click the Backbutton ( ) on the Android Emulator, the following is printed

(below figure):

Click the Home button ( ) and hold it there. Click the Activities icon and observe

the following:

Click the Phone button ( ) on the Android emulator so that the activity is pushed

to the background. Observe the output in the LogCat window:


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Notice that the onDestroy()event is not called, indicating that the activity is still in

memory. Exit the phone dialer by clicking the Back button. The activity is now visible

again. Observe the output in the LogCat window:

Applying Styles and Themes to an ActivityBy default, an activity occupies the entire screen. However, you can apply a dialog theme

to an activity so that it is displayed as a floating dialog. To apply a dialog theme to anactivity, simply modify the element in the AndroidManifest.xmlfile by

adding the android:themeattribute:


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Hiding the Activity Title

I can also hide the title of an activity if desired (such as when you just want to display a

status update to the user). To do so, use the requestWindowFeature()method and pass it

the Window.FEATURE_NO_TITLEconstant, like this in Activity101Activity.javafile:publicvoid onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

// hides the title barrequestWindowFeature(Window.FEATURE_NO_TITLE);// This activity load its UI using main.xml defined in res/layoutsetContentView(R.layout.main);Log.d(tag, "In the onCreate() event");

}This will hide the title bar, as shown in below figure:

Displaying a Dialog Window


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There are times when you need to display a dialog window to get a confirmation from the

user. In this case, you can override the onCreateDialog()protected method defined in the

Activity base class to display a dialog window. The following shows us how (in the

main.xml):

This be shown in below figure

We fix codes in the Activity101Activity.javafile:publicclass Activity101Activity extends Activity {

CharSequence[] items = {"Google", "Apple", "Microsoft"};boolean[] itemsChecked = newboolean[items.length];

/** Called when the activity is first created. */@Overridepublicvoid onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);setContentView(R.layout.main);

}

// OnCreateDialog method is called when we call the showDialog method// by clicking on button which name is "Click to display a dialog"publicvoid onClick(View v) {

showDialog(0);}

// Display a dialog@Overrideprotected Dialog onCreateDialog(int id) {

switch (id) {case 0:

returnnew AlertDialog.Builder(this).setIcon(R.drawable.ic_launcher).setTitle("This is a dialog with some simple text...").setPositiveButton("OK",

new DialogInterface.OnClickListener() {

publicvoid onClick(DialogInterfacedialog, int which) {


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// TODO Auto-generated method stubToast.makeText(getBaseContext(),

"OK clicked", Toast.LENGTH_SHORT).show();}

}).setNegativeButton("Cancel",

newDialogInterface.OnClickListener() {

publicvoid onClick(DialogInterfacedialog, int which) {

// TODO Auto-generated method stubToast.makeText(getBaseContext(),

"Cancel clicked", Toast.LENGTH_SHORT).show();}

}).setMultiChoiceItems(items, itemsChecked,

newDialogInterface.OnMultiChoiceClickListener() {

publicvoidonClick(DialogInterface dialog, int which, boolean isChecked) {

// TODO Auto-generatedmethod stub

Toast.makeText(getBaseContext(), items[which] + (isChecked ? "checked!":" unchecked!"), Toast.LENGTH_SHORT).show();

}}).create();

}returnnull;

}

@Overridepublicboolean onCreateOptionsMenu(Menu menu) {

getMenuInflater().inflate(R.menu.main, menu);returntrue;

}}Or rewrite onCreateDialogmethod in the way:@Override

protected Dialog onCreateDialog(int id) {switch(id) {case 0:

returnnew AlertDialog.Builder(this)

.setIcon(R.drawable.ic_launcher)

.setTitle("This is a dialog with some simple text...")

.setPositiveButton("OK",new DialogInterface.OnClickListener() {

publicvoid onClick(DialogInterface dialog, intwhichButton)

{Toast.makeText(getBaseContext(), "OK clicked!",

Toast.LENGTH_SHORT).show();}


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}).setNegativeButton("Cancel",

new DialogInterface.OnClickListener() {publicvoid onClick(DialogInterface dialog, int

whichButton){

Toast.makeText(getBaseContext(), "Cancelclicked!", Toast.LENGTH_SHORT).show();

}}

).setMultiChoiceItems(items, itemsChecked,

new DialogInterface.OnMultiChoiceClickListener() {publicvoid onClick(DialogInterface dialog,int which, boolean isChecked) {

Toast.makeText(getBaseContext(),items[which] + (isChecked ? " checked!":"

unchecked!"),Toast.LENGTH_SHORT).show();

}}

).create();}returnnull;

}Run app and click button, this is shown in:


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If we clickOKbutton, this is shown in:

Or if we clickCancel Button, this is shown in:

Displaying a Process (Please Wait) DialogWe add the following statements in highlight to the main.xmlfile:


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android:layout_width="fill_parent"android:layout_height="wrap_content"android:text="Click to display a dialog"android:onClick="onClick"/>

and add the following statements in highlight in Activity101Activity.javafile:publicvoid onClick(View v) {

showDialog(0);}

publicvoid onClick2(View v) {

/* to create a progress dialog, you created an instance of the* ProgressDialog class and called its show() method*/

final ProgressDialog dialog = ProgressDialog.show(this, "Doingsomething", "Please wait...", true);

/* To perform a long-running task in the background, you created aThread

* using a Runnable block*/

new Thread(new Runnable() {publicvoid run() {

try {// Simulate doing lengthy

(5 seconds)Thread.sleep(5000);

// dismiss the dialogdialog.dismiss();

} catch (InterruptedException e){

e.printStackTrace();}

}}).start();

}When we click button name Click to display a progress dialog, we have a dialog

appear with title is Doing something and content is Please wait. The circleanimation icon near Please wait will rotate until elapse 5 seconds and then, this

dialog disappear, we back to main layout of application.

Displaying a More Sophisticated Progress Dialog


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We add the following lines in highlight to the main.xml file:

and add the following statements in highlight to the

Activity101Activity.java file:

CharSequence[] items = {"Google", "Apple", "Microsoft"};boolean[] itemsChecked = newboolean[items.length];

ProgressDialog progressDialog;

/** Called when the activity is first created. */@Overridepublicvoid onCreate(Bundle savedInstanceState) {...}

publicvoid onClick(View v) {showDialog(0);

}

publicvoid onClick2(View v) {...}

publicvoid onClick3(View v) {showDialog(1);progressDialog.setProgress(0);new Thread(new Runnable(){

publicvoid run() {for (int i=1; i


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}Run app, click button name Click to display a

detailed progress dialog which will appear

dialog as below figure It will count from 1 to 15, with a delay of onesecond between each number and each number

correspond to 6%. The

incrementProgressBy() method increments the

counter in the progress dialog. When the

progress dialog reaches 100%, it is dismissed.

Linking Activities using IntentsWhen your application has more than one activity, you often need to navigate from one to

another. In Android, you navigate between activities through what is known as an intent.The first of all, we create a new class in Activity101Activity project. As shown in below

figure:

It appear the New Java Class dialog and we fill name of new class in Name textbox and

clickFinish button, as shown in below figure (next page):Next, we add the following statements to AndroidManifest.xml:


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android:label="Second Activity">

We continue create secondactivity.xmlin res/layoutby copy main.xmlin same folder.

Open secondactivity.xmlfile and add the following in highlight:


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Open SecondActivity.java file and add the following:publicclass SecondActivity extends Activity {

@Overridepublicvoid onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.secondactivity);}

}Note:If we create secondactivity.xmlfile in other way,R.layoutstatement in

setContentView() method will not understand secondactivity value (when write

R.layout.secondactivity, Eclipse program will give error in paragraph contain this

statement.Finish, we modify Activity101Activity.javafile:@Override

publicvoid onCreate(Bundle savedInstanceState) {...}

publicvoid onClick(View v) {startActivity(new Intent("net.learn2develop.SecondActivity"));

}Debug application (Ctrl + F11). We show in below figure:

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Tự học Begin Android 4 - Trial version