Android API23培训速览：从入门到交互与数据管理

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本资源是一份详尽的Android Training API23官方培训文档，涵盖了从入门到进阶的多个关键知识点，旨在帮助开发者更好地理解和掌握Android应用开发。以下是部分章节的详细说明： 1. **GettingStarted**：这部分主要介绍了如何构建第一个Android项目，包括创建项目、运行应用以及构建基本用户界面。学习者将学会设置项目结构、运行应用程序，并通过发送按钮启动新活动，展示基本交互流程。 - **Creating an Android Project**：讲解了如何在Android Studio中创建一个新的项目，配置所需的环境和组件。 - **Running Your App**：涉及安装应用并查看在模拟器或真实设备上的运行效果，强调调试和用户体验的基础。 - **Building a Simple User Interface**：教授布局管理器（如LinearLayout和RelativeLayout）的使用，以及如何设计和实现基本的UI元素。 1.2章节讲述了**ActionBar**的使用，它在Android应用中扮演导航和控制中心的角色。内容包括设置ActionBar、添加动作按钮、定制样式以及可能的层级关系（如与顶部栏的叠加）。 1.3章重点在于**Supporting Different Devices**，开发者需要了解如何适应不同语言、屏幕尺寸和平台版本，确保应用兼容性。这涉及到资源管理、国际化和适配策略。 1.4章深入探讨**Managing the Activity Lifecycle**，包括启动、暂停、恢复和销毁活动的生命周期管理，这对于保持应用性能和用户体验至关重要。 1.5章节是关于**Building a Dynamic UI with Fragments**，学习如何使用Fragment来构建可复用、动态扩展的用户界面，以及如何在Fragment之间进行通信。 1.6节则讲解**Saving Data**的方法，包括保存键值对、文件、SQLite数据库等，这些对于数据持久化和应用状态管理是必不可少的。 2.0章节转向**Building Apps with Content Sharing**，介绍了如何共享简单数据，比如向其他应用发送数据，这是跨应用交互的基础。这份Android Training API23文档提供了全面的开发教程，不仅适合初学者入门，也适合有一定经验的开发者提升技能和深化理解。通过阅读和实践这些内容，开发者可以构建出兼容多设备、功能丰富的Android应用，并熟练掌握内容分享等高级功能。

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easier to find and update. Externalizing the strings also allows you to localize your app

to different languages by providing alternative definitions for each string resource.

For more information about using string resources to localize your app for other

languages, see the Supporting Different Devices class.

Add a Button

1. In Android Studio, from the res/layout directory, edit the activity_my.xml file.

2. Within the <LinearLayout> element, define a <Button> element immediately following the

<EditText> element.

3. Set the button's width and height attributes to "wrap_content" so the button is only as big

as necessary to fit the button's text label.

4. Define the button's text label with the android:text attribute; set its value to the

button_send string resource you defined in the previous section.

Your <LinearLayout> should look like this:

res/layout/activity_my.xml

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

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:orientation="horizontal" >

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

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:hint="@string/edit_message" />

<Button

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="@string/button_send" />

</LinearLayout>

Note: This button doesn't need the android:id attribute, because it won't be

referenced from the activity code.

The layout is currently designed so that both the EditText and Button widgets are only

as big as necessary to fit their content, as shown in figure 2.

Figure 2. The EditText and Button widgets have their widths set to "wrap_content".

This works fine for the button, but not as well for the text field, because the user might

type something longer. It would be nice to fill the unused screen width with the text

field. You can do this inside a LinearLayout with the weight property, which you can

specify using the android:layout_weight attribute.

The weight value is a number that specifies the amount of remaining space each view

should consume, relative to the amount consumed by sibling views. This works kind of

like the amount of ingredients in a drink recipe: "2 parts soda, 1 part syrup" means

two-thirds of the drink is soda. For example, if you give one view a weight of 2 and

another one a weight of 1, the sum is 3, so the first view fills 2/3 of the remaining

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space and the second view fills the rest. If you add a third view and give it a weight of

1, then the first view (with weight of 2) now gets 1/2 the remaining space, while the

remaining two each get 1/4.

The default weight for all views is 0, so if you specify any weight value greater than 0

to only one view, then that view fills whatever space remains after all views are given

the space they require.

Make the Input Box Fill in the Screen Width

To fill the remaining space in your layout with the EditText element, do the following:

1. In the activity_my.xml file, assign the <EditText> element's layout_weight attribute a

value of 1.

2. Also, assign <EditText> element's layout_width attribute a value of 0dp.

res/layout/activity_my.xml

<EditText

android:layout_weight="1"

android:layout_width="0dp"

... />

To improve the layout efficiency when you specify the weight, you should

change the width of the EditText to be zero (0dp). Setting the width to zero

improves layout performance because using "wrap_content" as the width

requires the system to calculate a width that is ultimately irrelevant because the

weight value requires another width calculation to fill the remaining space.

Figure 3 shows the result when you assign all weight to the EditText element.

Figure 3. The EditText widget is given all the layout weight, so it fills the

remaining space in the LinearLayout.

Here‟s how your complete activity_my.xmllayout file should now look:

res/layout/activity_my.xml

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

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

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:orientation="horizontal">

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

android:layout_weight="1"

android:layout_width="0dp"

android:layout_height="wrap_content"

android:hint="@string/edit_message" />

<Button

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="@string/button_send" />

</LinearLayout>

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Run Your App

This layout is applied by the default Activity class that the SDK tools generated when

you created the project. Run the app to see the results:

 In Android Studio, from the toolbar, click Run .

 Or from a command line, change directories to the root of your Android project and

execute:

ant debug

adb install bin/MyFirstApp-debug.apk

Continue to the next lesson to learn how to respond to button presses, read content

from the text field, start another activity, and more.

 Starting Another Activity

After completing the previous lesson, you have an app that shows an activity (a single

screen) with a text field and a button. In this lesson, you‟ll add some code to

MyActivity that starts a new activity when the user clicks the Send button.

Respond to the Send Button

1. In Android Studio, from the res/layout directory, edit the activity_my.xml file.

2. To the <Button> element, add the android:onClick attribute.

res/layout/activity_my.xml

<Button

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="@string/button_send"

android:onClick="sendMessage" />

The android:onClick attribute‟s value, "sendMessage", is the name of a method

in your activity that the system calls when the user clicks the button.

3. In the java/com.mycompany.myfirstapp directory, open the MyActivity.java file.

4. Within the MyActivity class, add the sendMessage() method stub shown below.

java/com.mycompany.myfirstapp/MyActivity.java

/** Called when the user clicks the Send button */

public void sendMessage(View view) {

// Do something in response to button

}

In order for the system to match this method to the method name given to

android:onClick, the signature must be exactly as shown. Specifically, the

method must:

o Be public

o Have a void return value

o Have a View as the only parameter (this will be the View that was clicked)

Next, you‟ll fill in this method to read the contents of the text field and deliver that text

to another activity.

Build an Intent

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1. In MyActivity.java, inside the sendMessage() method, create an Intent to start an

activity called DisplayMessageActivity with the following code:

java/com.mycompany.myfirstapp/MyActivity.java

public void sendMessage(View view) {

Intent intent = new Intent(this, DisplayMessageActivity.class);

}

Intents

An Intent is an object that provides runtime binding between separate

components (such as two activities). The Intent represents an app‟s "intent to

do something." You can use intents for a wide variety of tasks, but most often

they‟re used to start another activity. For more information, see Intents and

Intent Filters.

Note: The reference to DisplayMessageActivity will raise an error if you‟re

using an IDE such as Android Studio because the class doesn‟t exist yet.

Ignore the error for now; you‟ll create the class soon.

The constructor used here takes two parameters:

o A Context as its first parameter (this is used because the Activity class is a

subclass of Context)

o The Class of the app component to which the system should deliver the Intent (in

this case, the activity that should be started)

Android Studio indicates that you must import the Intent class.

2. At the top of the file, import the Intent class:

java/com.mycompany.myfirstapp/MyActivity.java

import android.content.Intent;

Tip: In Android Studio, press Alt + Enter (option + return on Mac) to import

missing classes.

3. Inside the sendMessage() method, use findViewById() to get the EditText element.

java/com.mycompany.myfirstapp/MyActivity.java

public void sendMessage(View view) {

Intent intent = new Intent(this, DisplayMessageActivity.class);

EditText editText = (EditText) findViewById(R.id.edit_message);

}

4. At the top of the file, import the EditText class.

In Android Studio, press Alt + Enter (option + return on Mac) to import missing

classes.

5. Assign the text to a local message variable, and use the putExtra() method to add its text

value to the intent.

java/com.mycompany.myfirstapp/MyActivity.java

public void sendMessage(View view) {

Intent intent = new Intent(this, DisplayMessageActivity.class);

EditText editText = (EditText) findViewById(R.id.edit_message);

String message = editText.getText().toString();

intent.putExtra(EXTRA_MESSAGE, message);

}

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An Intent can carry data types as key-value pairs called extras. The putExtra()

method takes the key name in the first parameter and the value in the second

parameter.

6. At the top of the MyActivity class, add the EXTRA_MESSAGE definition as follows:

java/com.mycompany.myfirstapp/MyActivity.java

public class MyActivity extends ActionBarActivity {

public final static String EXTRA_MESSAGE =

"com.mycompany.myfirstapp.MESSAGE";

...

}

For the next activity to query the extra data, you should define the key for your

intent's extra using a public constant. It's generally a good practice to define

keys for intent extras using your app's package name as a prefix. This ensures

the keys are unique, in case your app interacts with other apps.

7. In the sendMessage() method, to finish the intent, call the startActivity() method, passing

it the Intent object created in step 1.

With this new code, the complete sendMessage() method that's invoked by the Send

button now looks like this:

java/com.mycompany.myfirstapp/MyActivity.java

/** Called when the user clicks the Send button */

public void sendMessage(View view) {

Intent intent = new Intent(this, DisplayMessageActivity.class);

EditText editText = (EditText) findViewById(R.id.edit_message);

String message = editText.getText().toString();

intent.putExtra(EXTRA_MESSAGE, message);

startActivity(intent);

}

The system receives this call and starts an instance of the Activity specified by the

Intent. Now you need to create the DisplayMessageActivity class in order for this to

work.

Create the Second Activity

All subclasses of Activity must implement the onCreate() method. This method is

where the activity receives the intent with the message, then renders the message.

Also, the onCreate() method must define the activity layout with the setContentView()

method. This is where the activity performs the initial setup of the activity components.

Create a new activity using Android Studio

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