探索Android多媒体高级开发：图形、音乐与智能手机/平板应用

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"安卓多媒体高级编程是一本专门针对Android平台的高级开发指南，由Shawn Van Every编著，英文原版书名为《Pro Android Media》，由Apress出版社于2010年12月出版。该书深入探讨了如何利用Android媒体APIs来构建动态的智能手机和平板电脑应用，内容涵盖了图形、音乐、视频以及丰富的媒体应用开发。书中详细介绍了如何在Android环境中开发图形界面，包括用户界面设计、图像处理和动画技术。此外，它还着重讲解了音频和视频播放、捕捉、编辑和流媒体技术，这些都是移动设备上不可或缺的功能。对于音乐应用程序，作者可能讨论了如何集成音频播放控制、音效处理和音乐库管理。此外，作者强调了如何利用Android的多媒体能力来创建富媒体应用，这可能包括交互式内容、3D图形渲染和多媒体文件的解析与显示。通过这本书，开发者可以学习到如何优化资源加载，处理不同分辨率和设备间的兼容性问题，以及如何充分利用Android系统提供的各种多媒体框架和工具。《Pro Android Media》不仅提供理论知识，还包含了大量的实践案例和代码示例，使读者能够迅速掌握实际开发技巧。网络上有该书前三章的试读内容，得到了读者的好评，表明其具有较高的实用性和深度。版权方面，所有内容受版权保护，未经版权所有者及出版商事先书面许可，不得任何形式复制或传输。本书的纸质版和电子版分别拥有不同的ISBN号码，印刷和装订均在美国完成。书中提及的商标名、标识和图片仅用于编辑目的，并遵循商标使用规范。《安卓多媒体高级编程》是一本对Android开发者来说非常有价值的资源，无论你是想要提升现有应用的多媒体功能，还是想进入这个领域的初学者，都能从中获得宝贵的知识和灵感。"

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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