Android_Learning_Notes_Part 2

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本文档详细介绍了Android应用开发的基础知识，包括AndroidManifest.xml配置、常用UI组件与布局、屏幕尺寸获取方法、J2ME应用移植到Android平台的方法、权限设置、线程使用技巧及代码示例等内容。

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Android Learning Notes

Part II – Android API Framework & Programming Skills

-- Last revised on Nov 30, 2008

1. AndroidManifest.xml

1) Reference page:

/android-sdk-windows-1.0_r1/docs/devel/bblocks-manifest.html

2) Sample:

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

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

package="com.my_domain.app.helloactivity">

<intent-filter>

</intent-filter>

</activity>

</application>

</manifest>

l xmlns:android -- 这是XML命名空间（name space）的声明,它是告诉Android的工具, 你将要涉及到公共的属性?? 已被定义在XML命名空间。在每一个Android的布局文件(包括manifest, layout等) 的最外边的标签必须有这个属性。

l <action android:name="android.intent.action.MAIN" />

-- Primary attributes

action -- The general action to be performed, such as ACTION_VIEW, ACTION_EDIT, ACTION_MAIN, etc.

data -- The data to operate on, such as a person record in the contacts database, expressed as a Uri.

l <category android:name="android.intent.category.LAUNCHER"/>

-- Secondary attributes

category --

1. CATEGORY_LAUNCHER means it should appear in the Launcher as a top-level application.

2. CATEGORY_ALTERNATIVE means it should be included in a list of alternative actions the user can perform on a piece of data.

type – ???

component – ???

extras -- This is a Bundle of any additional information.

l More information about intent please refer to Android’s document at: /android-sdk-windows-1.0_r1/docs/reference/android/content/Intent.html

Note: Every Activity must have an <activity> tag in the manifest whether it is exposed to the world or intended for use only within its own package. If an Activity has no matching tag in the manifest, you won't be able to launch it.

When startActivity(Intent) is called, Android compares the information in an Intent object with the Intent-Filter exposed by every application and finds the activity most appropriate to handle the data or action specified by the caller. Then system picks the Activity whose Intent-Filter best matches intent and launch it.

2. Some classes and API in Android

1) Views

• TextView, EditText, Button, ImageView, Checkbox, Lists, etc

2) Layouts: can be nested

• FrameLayout : Each Child a Layer

• LinearLayout : Single Row / Column

• RelativeLayout : Relative to Parent / Other Views

• TableLayout : Rows and Columns - HTML like

• AbsoluteLayout : <x,y> Coords – Discouraged

3) int screenHeight = getWindowManager().getDefaultDisplay().getHeight();

3. Porting J2ME application to Android platform

1) Replacement classes in Android API for classes in J2ME

J2ME Class		Android Class
javax.microedition.lcdui.*	Canvas	android.view.View
	Canvas.repaint()	View.invalidate()
	Graphics	android.graphics.Canvas
	Graphics.fillRect(startX, startY, width, height)	Canvas.drawRect(startX, startY, endX, endY)
	Graphics.drawRect(startX, startY, width, height)	Canvas.drawLine(startX, startY, startX, endY) Canvas.drawLine(startX, startY, endX, startY) Canvas.drawLine(startX, endY, endX, endY) Canvas.drawLine(endX, startY, endX, endY)
	Image	Bitmap, BitmapFactory
	Display.setCurrent(Displayable)	Activity. setContentView(View)
	Canvas.keyPressed / keyReleased	View.onKeyDown()
	Left/Right Softkey	View.onTouchEvent() or Button to simulate












HttpConnection		HttpURLConnection

Thread		Handler.post(Runnable())
		Typeface mFace = Typeface.createFromAsset(getContext().getAssets(),"fonts/samplefont.ttf")

4. Programming skills and Tips

1) Do as little work as possible in the constructor of your class that is extended from Activity class, but do most of initialization work in the onCreate() method.

2) The construct of main Activity MUST be modified by public. ???

3) Call View.setFocusable(true) to enable View to have focus and receive user key events.

4) J2ME draws text from left to right and from up downwards to bottom; Android draws text from left to right but from bottom upwards to top, as the following diagram shows.

For image, shape and etc, Android draws in the same way as J2ME.

5) Use the android.util.Log class to log messages in Android.

l Log.v() - Send a VERBOSE log message.

l Log.d() - Send a DEBUG log message.

l Log.i() - Send an INFO log message.

l Log.w() - Send a WARN log message.

l Log.e() - Send an ERROR log message.

6) You should NOT call Activity.getCurrentFocus() in Activity.onStart() / onResume() / onCreate() method, which will return null instance.

7) Resource files' name should not be initialized with digital. Because resource files’ name will be compiled into R.java as an int variable, which should not be initialized with digital. – Further, resource file name must contain ONLY [a-z0-9_.].

8) Asdfasdfd

9) Asdfasdfd

10)

5. Use permissions in AndroidManifest.xml

Permission Type	Use Cases
android.permission.INTERNET	Access Internet using Http or other protocols
android.permission.READ_CONTACTS android.permission.WRITE_CONTACTS	Access Contacts (Phone Book) in the handset