深入浅出Android媒体与富媒体开发

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"这是一本关于Android媒体API的专业书籍，名为《Pro Android Media》。作者Shawn Van Every深入探讨了如何在智能手机和平板电脑上开发图形、音乐、视频和丰富的媒体应用。书中涵盖了Android基础开发、驱动开发以及应用开发的相关知识。" 在Android开发中，媒体API扮演着至关重要的角色，它们允许开发者创建具有动态功能的移动应用程序。《Pro Android Media》这本书详细介绍了如何利用这些API来实现以下功能： 1. **图形开发**：Android提供了多种图形库，如OpenGL ES，用于构建高性能的2D和3D图形。书中的内容可能包括如何绘制图形，创建动画，以及使用Canvas和Bitmap对象进行基本的图像处理。 2. **音乐开发**：通过Android的MediaPlayer类和AudioTrack类，开发者可以播放、暂停、控制音轨，并实现音乐流媒体功能。书可能涵盖如何加载和播放音频文件，处理音频效果，以及与其他媒体组件集成的方法。 3. **视频开发**：Android的VideoView和MediaRecorder类使得视频播放和录制变得简单。书中可能讲解了如何处理视频流，创建自定义视频播放器，以及如何进行视频编辑和处理。 4. **富媒体应用开发**：这可能包括集成社交媒体分享，实时流媒体，以及AR（增强现实）技术的应用。开发者将学习如何利用Android的相机API，处理多媒体数据，以及如何构建用户友好的交互界面。 5. **驱动开发**：虽然书籍主要关注应用层，但Android的驱动开发也是基础开发的一部分。这部分可能涉及对硬件设备的理解，例如传感器、摄像头等，以及如何编写HAL（硬件抽象层）代码来与底层硬件交互。 6. **应用开发基础**：除了媒体API，书籍也可能覆盖Android应用的基本架构，如Activity生命周期，Intent通信，以及如何使用Android Studio进行项目设置和调试。 7. **版权和商标信息**：书中还提到了版权和商标的使用规定，强调在开发应用时尊重知识产权的重要性。《Pro Android Media》是一本全面的指南，适合希望提升Android媒体应用开发技能的开发者。它不仅提供了理论知识，还包含实际案例和最佳实践，帮助读者掌握在Android平台上创建引人入胜、功能丰富的多媒体应用所需的技术。

CHAPTER 1: Introduction to Android Imaging

offer image capture capabilities on a par with the Camera application without having to

build their own custom capture routines.

An intent filter is a means for a programmer of an application to specify that their

application offers a specific capability. Specifying an intent filter in the

AndroidManifest.xml file of an application tells Android that this application and, in

particular, the activity that contains the intent filter will perform the specified task, on

command.

The Camera application has the following intent filter specified in its manifest file. The

intent filter shown here is contained within the “Camera” activity tags.

<intent-filter>

</intent-filter>

In order to utilize the Camera application via an intent, we simply have to construct an

intent that will be caught by the foregoing filter.

Intent i = new Intent("android.media.action.IMAGE_CAPTURE");

In practice, we probably don’t want to create the intent with that action string directly. In

this case, a constant is specified in the MediaStore class, ACTION_IMAGE_CAPTURE. The

reason we should use the constant rather than the string itself is that if the string

happens to change, it is likely that the constant will change as well, thereby making our

call a bit more future-proof than it would otherwise be.

Intent i = new Intent(android.provider.MediaStore.ACTION_IMAGE_CAPTURE);

startActivity(i);

Using this intent in a basic Android activity will cause the default Camera application to

launch in still picture mode, as shown in Figure 1–1.

Figure 1–1. The built-in Camera application as called from an intent shown running in an emulator

CHAPTER 1: Introduction to Android Imaging

<ImageView android:id="@+id/ReturnedImageView" android:layout_width="wrap_content"

android:layout_height="wrap_content"></ImageView>

</LinearLayout>

To complete the foregoing example, here are the contents of AndroidManifest.xml.

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

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

android:versionCode="1"

android:versionName="1.0" package="com.apress.proandroidmedia.ch1.cameraintent">

<activity android:name=".CameraIntent"

android:label="@string/app_name">

<intent-filter>

</intent-filter>

</activity>

</application>

<uses-sdk android:minSdkVersion="4" />

</manifest>

In this example, the image is returned from the Camera application in an extra passed

through the intent that is sent to our calling activity in the onActivityResult method. The

name of the extra is "data" and it contains a Bitmap object, which needs to be cast from

a generic Object.

// Get Extras from the intent

Bundle extras = intent.getExtras();

// Get the returned image from that extra

Bitmap bmp = (Bitmap) extras.get("data");

In our layout XML (layout/main.xml) file, we have an ImageView. An ImageView is an

extension of a generic View, which supports the display of images. Since we have an

ImageView with the id ReturnedImageView specified, in our activity we need to obtain a

reference to that and set its Bitmap through its setImageBitmap method to be our

returned image. This enables the user of our application to view the image that was

captured.

To get a reference to the ImageView object, we use the standard findViewById method

specified in the Activity class. This method allows us to programmatically reference

elements specified in the layout XML file that we are using via setContentView by

passing in the id of the element. In the foregoing example, the ImageView object is

specified in the XML as follows:

<ImageView android:id="@+id/ReturnedImageView" android:layout_width="wrap_content"

android:layout_height="wrap_content"></ImageView>

To reference the ImageView and tell it to display the Bitmap from the Camera, we use

the following code.

imv = (ImageView) findViewById(R.id.ReturnedImageView);imv.setImageBitmap(bmp);

When you run this example, you’ll probably notice that the resulting image is small. (On

my phone, it is 121 pixels wide by 162 pixels tall. Other devices have different default

CHAPTER 1: Introduction to Android Imaging

NOTE: The foregoing code snippet for creating the URI to the image file could be simplified to the

following: imageFileUri =

Uri.parse("file:///sdcard/myfavoritepicture.jpg");

In practice, though, using the method shown will be more device-independent and future-proof

should the SD card–naming conventions or the URI syntax for the local filesystem change.

Displaying Large Images

Loading and displaying an image has significant memory usage implications. For

instance, the HTC G1 phone has a 3.2-megapixel camera. A 3.2-megapixel camera

typically captures images at 2048 pixels by 1536 pixels. Displaying a 32-bit image of

that size would take more than100663kb or approximately 13MB of memory. While this

may not guarantee that our application will run out of memory, it will certainly make it

more likely.

Android offers us a utility class called BitmapFactory, which provides a series of static

methods that allow the loading of Bitmap images from a variety of sources. For our

needs, we’ll be loading it from a file to display in our original activity. Fortunately, the

methods available in BitmapFactory take in a BitmapFactory.Options class, which

allows us to define how the Bitmap is read into memory. Specifically, we can set the

sample size that the BitmapFactory should use when loading an image. Indicating the

inSampleSize parameter in BitmapFactory.Options indicates that the resulting Bitmap

image will be that fraction of the size once loaded. For instance, setting the

inSampleSize to 8 as I do here would yield an image that is 1/8 the size of the original

image.

BitmapFactory.Options bmpFactoryOptions = new BitmapFactory.Options();

bmpFactoryOptions.inSampleSize = 8;

Bitmap bmp = BitmapFactory.decodeFile(imageFilePath, bmpFactoryOptions);

imv.setImageBitmap(bmp);

This is a quick way to load up a large image but doesn’t really take into account the

image’s original size nor the size of the screen. It would be better if we scaled the image

to something that would fit nicely on our screen.

The segments of code that follow illustrate how to use the dimensions of the display to

determine the amount of down sampling that should occur when loading the image.

When we use these methods, the image is assured of filling the bounds of the display as

much as possible. If, however, the image is only going to be shown at 100 pixels in any

one dimension, that value should be used instead of the display dimensions, which we

obtain as follows.

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