Android Activity

First of all, what is an activity?
Let me paint you a picture, suppose that you are a director, and you are directing a play.
Now, you need to design all the scenes that you want to show to your audiences.
Here are go, remember that every scene is an Activity.
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So, an Activity is a picture that you want to show in the monitor, and you use it to make interaction with your users.
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And then, you should arrange your stage properties.
You can decide which actor(actress) should be appear in you scene, and how many actors(actresses) you need. When you find a actor(actress),
you should tell him(her) what does he(she) should do in the stage, and you can decide where to put them in the stage.
Ok, let's make a conclusion:
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You are a director.
An application(a software) is a play, is a show, it contains many scenes to show to your audiences.
An activity is a single scene, in Java language, it is a Class which must extend from the super class named Activity.
An UI widget is a actor(actress), it's performance is manipulated by you.
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Now, let's begin to direct your play.
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Step1: You need a manifest to list all your actors(actresses).
In java language they are classes, for instance, Activities, Intents, Services, Content Providers.
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In Android, the manifest is named as AndroidManifest.xml, here is a manifest.

<?xml version="1.0" encoding="utf-8"?><manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.vhow" android:versionCode="1" android:versionName="1.0"> <application android:icon="@drawable/icon" android:label="@string/app_name"><!-- AndroidManifest.xml is a manifest, it list all the classes that you will use in you application.For instance, you should list all the Activities, Intents, Services, Content Providers and so on that you will use in you application.In this manifest, we have only one scene, namely, this application only paints a picture in the monitor and show it to the user. --> <activity android:name=".ActivityDemo" android:label="@string/app_name"> <!-- When the application is launched, this activity is launched first --> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> </application> <uses-sdk android:minSdkVersion="8" /></manifest>

 

Step2: You need to select actors(actresses) to arrange your scene.
In Android, you arrange your widgets for one Activity in a XML file in a folder named 'layout'.
Here is a layout, main.xml.
By the way, you can use "DroidDraw" to design your layout.
You can download it at http://www.droiddraw.org/

<?xml version="1.0" encoding="utf-8"?><!-- Since an activity is a scene, you arrange the scene here --><!-- LinearLayout, you want to put your widget in a line, from north to south. --><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent" ><!-- TextView is a actor, is a widget, you want it to appear in the stage. --> <TextView android:id="@+id/TextView01" android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="@string/lines" /><Buttonandroid:id="@+id/Button01"android:layout_width="wrap_content"android:layout_height="wrap_content"android:text="Tell TextView to change lines"></Button> </LinearLayout>

 

Step3: Design the lines that you want your widgets to show in the stage.

<?xml version="1.0" encoding="utf-8"?><resources><!-- In Android, the lines are stored in strings.xml, you can prepare strings_in_Chinese.xml and strings_in_English.xml, When your audiences are English speakers, you can ask you actors to say English, When your audiences are Chinese speakers, you can ask you actors to say Chinese.--> <string name="lines">Good evening!</string> <string name="new_lines">Goodbye!</string> <string name="app_name">ActivityDemo</string></resources>

 

Step4: Tell your actors what to do in the stage. Namely, you should design the activity here.

package com.vhow;import android.app.Activity;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.TextView;public class ActivityDemo extends Activity { // list your actors private Button btn; private TextView tv; private static int flag; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); System.out.println("here"); // setContentView, set the scene you want to put in the stage setContentView(R.layout.main); // find you actors to manipulate btn = (Button)findViewById(R.id.Button01); tv = (TextView)findViewById(R.id.TextView01); // set the lister of btn btn.setOnClickListener(new ListenOfButton01()); } class ListenOfButton01 implements OnClickListener {@Overridepublic void onClick(View v) {if (flag % 2 == 0){tv.setText(R.string.new_lines);flag++;} else {tv.setText(R.string.lines);flag--;}} };}