我正在开发一个需要查看用户存储的图像的 Android 应用程序。问题是如果用户通过 USB 电缆安装了 sdcard，我无法读取磁盘上的图像列表。
有没有人知道一种方法来判断 USB 是否已安装，以便我可以弹出一条消息通知用户它无法工作？
【问题讨论】：
如果您尝试访问设备上的图像，最好的方法是使用MediaStore content provider。以content provider 的形式访问它可以让您查询存在的图像，并将content:// URL 映射到设备上适当的文件路径。
如果您仍需要访问 SD 卡，Camera 应用程序包含一个 ImageUtils 类，用于检查 SD 卡是否已安装，如下所示：
static public boolean hasStorage(boolean requireWriteAccess) {
//TODO: After fix the bug, add "if (VERBOSE)" before logging errors.
String state = Environment.getExternalStorageState();
Log.v(TAG, "storage state is " + state);
if (Environment.MEDIA_MOUNTED.equals(state)) {
if (requireWriteAccess) {
boolean writable = checkFsWritable();
Log.v(TAG, "storage writable is " + writable);
return writable;
} else {
return true;
}
} else if (!requireWriteAccess && Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
return true;
}
return false;
}
【讨论】：
checkFsWritable();在哪里？
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/> 才能使其在外部存储上运行？
这是jargonjustin帖子中缺少的checkFsWritable函数
private static boolean checkFsWritable() {
// Create a temporary file to see whether a volume is really writeable.
// It's important not to put it in the root directory which may have a
// limit on the number of files.
String directoryName = Environment.getExternalStorageDirectory().toString() + "/DCIM";
File directory = new File(directoryName);
if (!directory.isDirectory()) {
if (!directory.mkdirs()) {
return false;
}
}
return directory.canWrite();
}
【讨论】：
对于发布非 Android 方式的做法，我深表歉意，希望有人可以使用 Android API 提供答案。
您可以列出 sdcard 根目录下的文件。如果没有，则 sdcard 要么完全空白（不寻常，但可能），要么已卸载。如果您尝试在 sdcard 上创建一个空文件但失败，这意味着您试图在 sdcard 的挂载点创建一个文件，由于权限问题，该文件将被拒绝，因此您会知道 sdcard 不是已安装。
是的，我知道这很丑……
【讨论】：
public static boolean isSdPresent() {
return android.os.Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED);
}
【讨论】：
关于 jargonjustin 的帖子：
方法hasStorage -->
public static boolean hasStorage(boolean requireWriteAccess) {
String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state)) {
if (requireWriteAccess) {
boolean writable = checkFsWritable();
return writable;
} else {
return true;
}
} else if (!requireWriteAccess
&& Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
return true;
}
return false;
}
方法checkFsWritable -->
private static boolean checkFsWritable() {
// Create a temporary file to see whether a volume is really writeable.
// It's important not to put it in the root directory which may have a
// limit on the number of files.
String directoryName =
Environment.getExternalStorageDirectory().toString() + "/DCIM";
File directory = new File(directoryName);
if (!directory.isDirectory()) {
if (!directory.mkdirs()) {
return false;
}
}
File f = new File(directoryName, ".probe");
try {
// Remove stale file if any
if (f.exists()) {
f.delete();
}
if (!f.createNewFile()) {
return false;
}
f.delete();
return true;
} catch (IOException ex) {
return false;
}
}
【讨论】：
我使用光标从 sd 卡中检索图像，当设备中没有插入 sd 卡时，光标为空。 实际上，这是通过从设备中物理移除卡来卸载 sdcard 卷的情况。这是我使用的代码：
Cursor mCursor = this.getContentResolver().query(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, null, null, null, null);
if (mCursor == null || !mCursor .moveToFirst()) {
/**
*mCursor == null:
* - query failed; the app don't have access to sdCard; example: no sdCard
*
*!mCursor.moveToFirst():
* - there is no media on the device
*/
} else {
// process the images...
mCursor.close();
}
更多信息：http://developer.android.com/guide/topics/media/mediaplayer.html#viacontentresolver
【讨论】：
在对外部存储进行任何操作之前，您应该始终调用 getExternalStorageState() 来检查媒体是否可用。媒体可能已安装到计算机、丢失、只读或处于其他状态。例如，您可以使用以下几种方法来检查可用性：
/* Checks if external storage is available for read and write */
public boolean isExternalStorageWritable() {
String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state)) {
return true;
}
return false;
}
/* Checks if external storage is available to at least read */
public boolean isExternalStorageReadable() {
String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state) ||
Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
return true;
}
return false;
}
来源： http://developer.android.com/guide/topics/data/data-storage.html
【讨论】：
Cool....Check it out...
try {
File mountFile = new File("/proc/mounts");
usbFoundCount=0;
sdcardFoundCount=0;
if(mountFile.exists())
{
Scanner usbscanner = new Scanner(mountFile);
while (usbscanner.hasNext()) {
String line = usbscanner.nextLine();
if (line.startsWith("/dev/fuse /storage/usbcard1")) {
usbFoundCount=1;
Log.i("-----USB--------","USB Connected and properly mounted---/dev/fuse /storage/usbcard1" );
}
}
}
if(mountFile.exists()){
Scanner sdcardscanner = new Scanner(mountFile);
while (sdcardscanner.hasNext()) {
String line = sdcardscanner.nextLine();
if (line.startsWith("/dev/fuse /storage/sdcard1")) {
sdcardFoundCount=1;
Log.i("-----USB--------","USB Connected and properly mounted---/dev/fuse /storage/sdcard1" );
}
}
}
if(usbFoundCount==1)
{
Toast.makeText(context,"USB Connected and properly mounted", 7000).show();
Log.i("-----USB--------","USB Connected and properly mounted" );
}
else
{
Toast.makeText(context,"USB not found!!!!", 7000).show();
Log.i("-----USB--------","USB not found!!!!" );
}
if(sdcardFoundCount==1)
{
Toast.makeText(context,"SDCard Connected and properly mounted", 7000).show();
Log.i("-----SDCard--------","SDCard Connected and properly mounted" );
}
else
{
Toast.makeText(context,"SDCard not found!!!!", 7000).show();
Log.i("-----SDCard--------","SDCard not found!!!!" );
}
}catch (Exception e) {
e.printStackTrace();
}
【讨论】：