最近换了工作，新工作是负责用qml做qt安卓开发。
工作中遇到一个问题：安卓设备有USB口，需要插入一个U盘在程序里读写U盘中的文件，由于安卓系统的安全性的问题导致QFile、c++的文件操作相关方法都不能读写成功，想要读写成功只能调用java代码，在java代码里面使用安卓的DocumentFile库。
经过一番探索，成功解决了问题。qt如何添加java代码不说了，网上有。
下面是具体的java代码：
package com.example.myapplication;
import android.annotation.TargetApi;
import android.content.Context;
import android.content.SharedPreferences;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.os.ParcelFileDescriptor;
import android.preference.PreferenceManager;
import android.provider.DocumentsContract;
import android.util.Log;
import androidx.documentfile.provider.DocumentFile;
import java.io.File;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.RandomAccessFile;
import java.util.ArrayList;
import java.util.List;
public class DocumentsUtils {
private static final String TAG = DocumentsUtils.class.getSimpleName();
public static final int OPEN_DOCUMENT_TREE_CODE = 8000;
public static String as;
private static List<String> sExtSdCardPaths = new ArrayList<>();
private DocumentsUtils() {
}
public static void cleanCache() {
sExtSdCardPaths.clear();
}
/**
* Get a list of external SD card paths. (Kitkat or higher.)
*
* @return A list of external SD card paths.
*/
@TargetApi(Build.VERSION_CODES.KITKAT)
private static String[] getExtSdCardPaths(Context context) {
if (sExtSdCardPaths.size() > 0) {
return sExtSdCardPaths.toArray(new String[0]);
}
for (File file : context.getExternalFilesDirs("external")) {
if (file != null && !file.equals(context.getExternalFilesDir("external"))) {
int index = file.getAbsolutePath().lastIndexOf("/Android/data");
if (index < 0) {
Log.w(TAG, "Unexpected external file dir: " + file.getAbsolutePath());
} else {
String path = file.getAbsolutePath().substring(0, index);
try {
path = new File(path).getCanonicalPath();
} catch (IOException e) {
// Keep non-canonical path.
}
sExtSdCardPaths.add(path);
}
}
}
if (sExtSdCardPaths.isEmpty()) sExtSdCardPaths.add("/storage/sdcard1");
return sExtSdCardPaths.toArray(new String[0]);
}
/**
* Determine the main folder of the external SD card containing the given file.
*
* @param file the file.
* @return The main folder of the external SD card containing this file, if the file is on an SD
* card. Otherwise,
* null is returned.
*/
@TargetApi(Build.VERSION_CODES.KITKAT)
private static String getExtSdCardFolder(final File file, Context context) {
String[] extSdPaths = getExtSdCardPaths(context);
try {
for (int i = 0; i < extSdPaths.length; i++) {
if (file.getCanonicalPath().startsWith(extSdPaths[i])) {
return extSdPaths[i];
}
}
} catch (IOException e) {
return null;
}
return null;
}
/**
* Determine if a file is on external sd card. (Kitkat or higher.)
*
* @param file The file.
* @return true if on external sd card.
*/
@TargetApi(Build.VERSION_CODES.KITKAT)
public static boolean isOnExtSdCard(final File file, Context c) {
return getExtSdCardFolder(file, c) != null;
}
/**
* Get a DocumentFile corresponding to the given file (for writing on ExtSdCard on Android 5).
* If the file is not
* existing, it is created.
*
* @param file The file.
* @param isDirectory flag indicating if the file should be a directory.
* @return The DocumentFile
*/
public static DocumentFile getDocumentFile(final File file, final boolean isDirectory,
Context context ) {
if (Build.VERSION.SDK_INT <= Build.VERSION_CODES.KITKAT) {
return DocumentFile.fromFile(file);
}
String baseFolder = getExtSdCardFolder(file, context);
// Log.i(TAG,"lum_ baseFolder " + baseFolder);
boolean originalDirectory = false;
if (baseFolder == null) {
return null;
}
String relativePath = null;
try {
String fullPath = file.getCanonicalPath();
if (!baseFolder.equals(fullPath)) {
relativePath = fullPath.substring(baseFolder.length() + 1);
} else {
originalDirectory = true;
}
} catch (IOException e) {
return null;
} catch (Exception f) {
originalDirectory = true;
//continue
}
// String as = PreferenceManager.getDefaultSharedPreferences(context).getString(baseFolder, null);
//as = androidx.preference.PreferenceManager.getDefaultSharedPreferences(context).getString(baseFolder, null);
String st = as;
Uri treeUri = null;
if (as != null) treeUri = Uri.parse(as);
if (treeUri == null) {
return null;
}
// start with root of SD card and then parse through document tree.
DocumentFile document = DocumentFile.fromTreeUri(context, treeUri);
if (originalDirectory) return document;
String[] parts = relativePath.split("/");
for (int i = 0; i < parts.length; i++) {
DocumentFile nextDocument = document.findFile(parts[i]);
if (nextDocument == null) {
if ((i < parts.length - 1) || isDirectory) {
nextDocument = document.createDirectory(parts[i]);
} else {
nextDocument = document.createFile("image", parts[i]);
}
}
document = nextDocument;
}
return document;
}
public static boolean mkdirs(Context context, File dir) {
boolean res = dir.mkdirs();
if (!res) {
if (DocumentsUtils.isOnExtSdCard(dir, context)) {
DocumentFile documentFile = DocumentsUtils.getDocumentFile(dir, true, context);
res = documentFile != null && documentFile.canWrite();
}
}
return res;
}
public static boolean delete(Context context, File file) {
boolean ret = file.delete();
if (!ret && DocumentsUtils.isOnExtSdCard(file, context)) {
DocumentFile f = DocumentsUtils.getDocumentFile(file, false, context);
if (f != null) {
ret = f.delete();
}
}
return ret;
}
public static boolean canWrite(File file) {
boolean res = file.exists() && file.canWrite();
if (!res && !file.exists()) {
try {
if (!file.isDirectory()) {
res = file.createNewFile() && file.delete();
} else {
res = file.mkdirs() && file.delete();
}
} catch (IOException e) {
e.printStackTrace();
}
}
return res;
}
public static boolean canWrite(Context context, File file) {
boolean res = canWrite(file);
if (!res && DocumentsUtils.isOnExtSdCard(file, context)) {
DocumentFile documentFile = DocumentsUtils.getDocumentFile(file, true, context);
res = documentFile != null && documentFile.canWrite();
}
return res;
}
/**
* 重命名
* @param context
* @param src
* @param dest
* @return
*/
public static boolean renameTo(Context context, File src, File dest) {
boolean res = src.renameTo(dest);
if (!res && isOnExtSdCard(dest, context)) {
DocumentFile srcDoc;
if (isOnExtSdCard(src, context)) {
srcDoc = getDocumentFile(src, false, context);
} else {
srcDoc = DocumentFile.fromFile(src);
}
DocumentFile destDoc = getDocumentFile(dest.getParentFile(), true, context);
if (srcDoc != null && destDoc != null) {
try {
Log.i("renameTo", "src.getParent():" + src.getParent() + ",dest.getParent():" + dest.getParent());
if (src.getParent().equals(dest.getParent())) {//同一目录
res = srcDoc.renameTo(dest.getName());
} else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {//不同一目录
Uri renameSrcUri = DocumentsContract.renameDocument(context.getContentResolver(),//先重命名
srcDoc.getUri(), dest.getName());
res = DocumentsContract.moveDocument(context.getContentResolver(),//再移动
renameSrcUri,
srcDoc.getParentFile().getUri(),
destDoc.getUri()) != null;
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
return res;
}
public static InputStream getInputStream(Context context, File destFile) {
InputStream in = null;
try {
if (!canWrite(destFile) && isOnExtSdCard(destFile, context)) {
DocumentFile file = DocumentsUtils.getDocumentFile(destFile, false, context);
if (file != null && file.canWrite()) {
in = context.getContentResolver().openInputStream(file.getUri());
}
} else {
in = new FileInputStream(destFile);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
return in;
}
public static OutputStream getOutputStream(Context context, File destFile) {
OutputStream out = null;
try {
if (!canWrite(destFile) && isOnExtSdCard(destFile, context)) {
DocumentFile file = DocumentsUtils.getDocumentFile(destFile, false, context);
if (file != null && file.canWrite()) {
out = context.getContentResolver().openOutputStream(file.getUri());
}
} else {
out = new FileOutputStream(destFile);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
return out;
}
/**
* 获取文件流
* @param context
* @param destFile 目标文件
* @param mode May be "w", "wa", "rw", or "rwt".
* @return
*/
public static OutputStream getOutputStream(Context context, File destFile, String mode) {
OutputStream out = null;
try {
if (!canWrite(destFile) && isOnExtSdCard(destFile, context)) {
DocumentFile file = DocumentsUtils.getDocumentFile(destFile, false, context);
if (file != null && file.canWrite()) {
out = context.getContentResolver().openOutputStream(file.getUri(), mode);
}
} else {
out = new FileOutputStream(destFile, mode.equals("rw") || mode.equals("wa"));
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
return out;
}
public static FileDescriptor getFileDescriptor(Context context, File destFile) {
FileDescriptor fd = null;
try {
if (/*!canWrite(destFile) && */isOnExtSdCard(destFile, context)) {
DocumentFile file = DocumentsUtils.getDocumentFile(destFile, false, context);
if (file != null && file.canWrite()) {
ParcelFileDescriptor out = context.getContentResolver().openFileDescriptor(file.getUri(), "rw");
fd = out.getFileDescriptor();
}
} else {
RandomAccessFile file = null;
try {
file = new RandomAccessFile(destFile, "rws");
file.setLength(0);
fd = file.getFD();
} catch (Exception e){
e.printStackTrace();
} finally {
if (file != null) {
try {
file.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
return fd;
}
public static boolean saveTreeUri(Context context, String rootPath, Uri uri) {
DocumentFile file = DocumentFile.fromTreeUri(context, uri);
if (file != null && file.canWrite()) {
SharedPreferences perf = PreferenceManager.getDefaultSharedPreferences(context);
perf.edit().putString(rootPath, uri.toString()).apply();
Log.e(TAG, "save uri" + rootPath);
return true;
} else {
Log.e(TAG, "no write permission: " + rootPath);
}
return false;
}
/**
* 返回true表示没有权限
* @param context
* @param rootPath
* @return
*/
public static boolean checkWritableRootPath(Context context, String rootPath) {
File root = new File(rootPath);
if (!root.canWrite()) {
Log.e(TAG,"sd card can not write:" + rootPath + ",is on extsdcard:" + DocumentsUtils.isOnExtSdCard(root, context));
if (DocumentsUtils.isOnExtSdCard(root, context)) {
DocumentFile documentFile = DocumentsUtils.getDocumentFile(root, true, context);
if (documentFile != null) {
Log.i(TAG, "get document file:" + documentFile.canWrite());
}
return documentFile == null || !documentFile.canWrite();
} else {
SharedPreferences perf = PreferenceManager.getDefaultSharedPreferences(context);
String documentUri = perf.getString(rootPath, "");
Log.i(TAG,"lum_2 get perf documentUri:" + documentUri);
if (documentUri == null || documentUri.isEmpty()) {
return true;
} else {
DocumentFile file = DocumentFile.fromTreeUri(context, Uri.parse(documentUri));
if (file != null)
Log.i(TAG,"lum get perf documentUri:" + file.canWrite());
return !(file != null && file.canWrite());
}
}
}else{
Log.e(TAG,"sd card can write...");
}
return false;
}
}在网上搜索DocumentFile就会搜出一段处理处理DocumentFile的java代码，都一样的抄来抄去，这段代码也是根据那段代码改的，经过修改可直接放到qt工程里使用。
程序第一次运行时调用（用的qt5）：
QAndroidJniObject androidJinObject = QtAndroid::androidActivity();
androidJinObject.callMethod<void>("showOpenDocumentTree");这会弹出申请读写设备权限的弹窗。这个弹窗只弹出一次，再次执行也不会弹出除非卸载应用重新安装。
之后就可以读写了，读写也只能用DocumentFile，QFile依旧不行。
注意这里获取QAndroidJniObject 使用的是QtAndroid::androidActivity()，直接创建一个QAndroidJniObject 对象调用java的static方法可以，非static方法会崩溃，原因未知。
复制文件的例子（不完整）：
File dest;
if(dest.exists())
{
dest.delete();
}
File source;
InputStream input = null;
OutputStream output = null;
try
{
input = getInputStream(this,source);//通过DocumentFile获取流
output = getOutputStream(this,dest);
byte[] buf = new byte[1024];
int bytesRead;
while ((bytesRead = input.read(buf)) > 0)
{
output.write(buf, 0, bytesRead);
}
}
finally
{
input.close();
output.close();
}java导入库后，qt编译时如果提示库文件不存在那么把库文件添加到build.gradle文件中，如：文章来源：https://www.toymoban.com/news/detail-461361.html
dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar', '*.aar'])
implementation "androidx.documentfile:documentfile:1.0.1"
implementation 'androidx.appcompat:appcompat:1.3.0'
implementation 'com.google.android.material:material:1.4.0'
implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
implementation 'androidx.navigation:navigation-fragment:2.3.5'
implementation 'androidx.navigation:navigation-ui:2.3.5'
implementation 'androidx.preference:preference:1.1.1'
}此文件编译时会自动生成，若要往build.gradle文件添加内容需要先把这个文件添加到qt工程中，那么再次编译时就用的工程中的此文件。文章来源地址https://www.toymoban.com/news/detail-461361.html
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