Android Questions （问题集锦）

开发过程中，解决问题集锦~

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Android Framework Questions

Android External Storage Questions

happen W/ContextImpl: Failed to ensure directory
不要使用 context.getExternalFilesDir( String type) Api ,顺源码查看，发现此API，存在多个挂载区但不可用时，会打印警告信息。使用 Environment.getExternalStorageDirectory() 则不会
I follow the getExternalFilesDir() source

/** * Ensure that given directories exist, trying to create them if missing. If * unable to create, they are filtered by replacing with {@code null}. */private File[] ensureExternalDirsExistOrFilter(File[] dirs) {    File[] result = new File[dirs.length];    for (int i = 0; i < dirs.length; i++) {        File dir = dirs[i];        if (!dir.exists()) {            if (!dir.mkdirs()) {                // recheck existence in case of cross-process race                if (!dir.exists()) {                    // Failing to mkdir() may be okay, since we might not have                    // enough permissions; ask vold to create on our behalf.                    final IMountService mount = IMountService.Stub.asInterface(                            ServiceManager.getService("mount"));                    try {                        final int res = mount.mkdirs(getPackageName(), dir.getAbsolutePath());                        if (res != 0) {                            Log.w(TAG, "Failed to ensure " + dir + ": " + res);                            dir = null;                        }                    } catch (Exception e) {                        Log.w(TAG, "Failed to ensure " + dir + ": " + e);                        dir = null;                    }                }            }        }        result[i] = dir;    }    return result;

how to get ExternalFilesDir

public final class StorageUtil {public static final String DIR_ANDROID = "Android";private static final String DIR_DATA = "data";private static final String DIR_FILES = "files";private static final String DIR_CACHE = "cache";@Nullablepublic static synchronized File getExternalStorageAppFilesFile(Context context, String fileName) {    if (context == null) return null;    if (fileName == null) return null;    if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {        File dirs = buildExternalStorageAppFilesDirs(Environment.getExternalStorageDirectory().getAbsolutePath(), context.getPackageName());        return new File(dirs, fileName);    }    return null;}public synchronized static File buildExternalStorageAppFilesDirs(String externalStoragePath, String packageName) {    return buildPath(externalStoragePath, DIR_ANDROID, DIR_DATA, packageName, DIR_FILES);}public synchronized static File buildPath(String base, String... segments) {    File cur = new File(base);    for (String segment : segments) {        cur = new File(cur, segment);    }    return cur;}

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Android Image Load Libraries

Glide

1. happenCaused by: java.lang.IllegalArgumentException: You cannot start a load for a destroyed activity
加上 Glide.with(context).isPaused() 去调用Glide.with(context).pauseRequests();
即可

public static void resumeRequests(@NonNull final Context context) {    if (!isContextDestroyed(context) && isPaused(context)) {        Glide.with(context).resumeRequests();    }}public static void pauseRequests(@NonNull final Context context) {    if (!isContextDestroyed(context) && !isPaused(context)) {        Glide.with(context).pauseRequests();    }}public static boolean isPaused(@NonNull final Context context) {    return isContextDestroyed(context) || Glide.with(context).isPaused();}private static boolean isContextDestroyed(@NonNull final Context context) {    if (context instanceof Activity) {        Activity activity = (Activity) context;        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1 && activity.isDestroyed()) {            return true;        }    }    return false;}

Fragment

popBackStack 和 popBackStackImmediate 之坑

在复杂的Fragment 交互中，popBackStack 偶尔抛出java.lang.IllegalStateException Fragment already added
popBackStackImmediate 则偶尔抛出java.lang.IllegalStateException Can not perform this action after onSaveInstanceState。
跟踪它们源码发现，popBackStack 是Handler异步出栈，popBackStackImmediate 是马上出栈的。我推荐用
popBackStackImmediate，问题来了如何解决上面问题呢

package android.support.v4.app;/** * Created by lazy on 2017/8/23. */public class VendingFragmentManager {    /**     * make sure no throws java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState ,HA HA     *     * @param fragmentManager     */    public static void allowStateLoss(FragmentManager fragmentManager) {        ((FragmentManagerImpl) fragmentManager).noteStateNotSaved();    }}

FragmentManager fragmentManager = getSupportFragmentManager();VendingFragmentManager.allowStateLoss(fragmentManager);fragmentManager.popBackStackImmediate();addMainFragment();

FragmentManagerImpl 类是用默认访问修饰符修饰的，新建android.support.v4.app 包就可以投机取巧访问了。

android studio not find usage

File->Invalidate and Restart