Android Preview M：授予权限后重新创建活动

Question

我使用 Preview M 并在上面测试我的应用程序，尤其是“将文件保存到外部存储”部分。 在下载/保存过程开始之前，我要求

  Manifest.permission.WRITE_EXTERNAL_STORAGE

开发者页面上描述的权限：https://developer.android.com/preview/features/runtime-permissions.html

对话框按预期出现：

"Allow <AppName> to access photos, media, and files on your device?" Deny / Allow

如果我点击“拒绝”按钮，则调用活动的 onRequestPermissionsResult 方法。

如果我按下“允许”按钮，则首先重新创建活动，然后调用 onRequestPermissionsResult 方法。我认为这是授予权限的结果。

但是第二种情况比较棘手，因为我想触发一个开始下载的回调，但是此时这个对象为空：

public void onRequestPermissionsResult(int requestCode,
                                       String permissions[], int[] grantResults) {

    getLogger().error("onRequestPermissionsResult ( " + requestCode + ", " + permissions + ", " + grantResults + " )");
    switch (requestCode) {


        //permission for saving files?
        case PermissionCode.WRITE_EXTERNAL_STORAGE: {


            if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                //HERE IS THE NULL-OBJECT 
                if (controller != null) {

                    controller.triggerCallback();
                }
            }
            break;
        }
        default: {

            super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }
}

所以我的问题是：

1. 我可以避免这种消遣吗？
2. 如果不是，我该如何重新设计我的代码来解决问题 - 我现在完全没有想法

编辑 1：

我尝试使用处理程序和 postDelayed 来解决问题 - 但我跳过了它。

编辑 2：

我初步解决了这个问题，并向用户展示了一个 Toast，请求再次按下下载按钮（在获得许可的情况下）。但是谷歌：你是认真的吗？

编辑 3：

最新的 SDK 6.0（第 23 版）没有任何乐趣 - 也许有人听到了我在山景城的哭泣 :-)

Answer 1

您可以随时在onCreate() 中重新创建活动后立即检查下载情况：

static final int MY_PERMISSION_REQUEST_WRITE_EXTERNAL_STORAGE = 0;
boolean initiateDownload = false;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if(savedInstanceState != null) {
        initiateDownload = savedInstanceState.getBoolean("toDownload");
    }
}

@Override
public void onResume() {
    super.onResume();
    final boolean hasPermission = checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE)
            == PackageManager.PERMISSION_GRANTED;
    if(initiateDownload && hasPermission) {
        // start download here...
    } else {
        requestPermissions(new String[]{WRITE_EXTERNAL_STORAGE},
                MainActivity.MY_PERMISSION_REQUEST_WRITE_EXTERNAL_STORAGE);
    }
}

@Override
public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    if(requestCode == MY_PERMISSION_REQUEST_WRITE_EXTERNAL_STORAGE) {
        if(grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            initiateDownload = true;
        } else {
            // denied permission...
        }
    }
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putBoolean("toDownload", initiateDownload);
}

Answer 2

出于多种原因，Android 可能会破坏并重新创建您的 Activity。请求权限的用例也不例外。您可以使用您喜欢的方式来保存和恢复 UI 状态。如果你的状态是可包裹的，你可以像下面的例子那样做。为什么Request App Permissions 或Android RuntimePermissions Sample 中没有提到这一点我不知道。

查看更多内容 Saving and restoring transient UI state 和 Saving UI States

public class ExampleActivity extends AppCompatActivity {
    private static final String CURRENT_DOWNLOAD_ITEM = "CURRENT_DOWNLOAD_ITEM";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (savedInstanceState != null)
            currentDownloadItem = savedInstanceState.getParcelable(CURRENT_DOWNLOAD_ITEM);
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        outState.putParcelable(CURRENT_DOWNLOAD_ITEM, currentDownloadItem);
        super.onSaveInstanceState(outState);
    }

    public void downloadItem(Parcelable item) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M &&
                checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
            currentDownloadItem = item;
            requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
        } else
            perfornDownload(item);
    }

    private Parcelable currentDownloadItem;

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        switch (requestCode) {
            case 1:
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    if (currentDownloadItem != null) {
                        perfornDownload(currentDownloadItem);
                        currentDownloadItem = null;
                    } else
                        Toast.makeText(this, "Please select download again now that we have permission to save it", Toast.LENGTH_LONG).show();
                } else
                    Toast.makeText(this, "Download cancelled, as we don\'t have permission to save the downloaded files", Toast.LENGTH_LONG).show();
                break;
        }
    }

    private void perfornDownload(Parcelable item) {
        // Here we are sure we have the permission
    }
}