一、相关api介绍

        1、requestPermissions

/**
 * Requests permissions to be granted to this application. These permissions
 * must be requested in your manifest, they should not be granted to your app,
 * and they should have protection level {@link
 * android.content.pm.PermissionInfo#PROTECTION_DANGEROUS dangerous}, regardless
 * whether they are declared by the platform or a third-party app.
 * <p>
 * Normal permissions {@link android.content.pm.PermissionInfo#PROTECTION_NORMAL}
 * are granted at install time if requested in the manifest. Signature permissions
 * {@link android.content.pm.PermissionInfo#PROTECTION_SIGNATURE} are granted at
 * install time if requested in the manifest and the signature of your app matches
 * the signature of the app declaring the permissions.
 * </p>
 * <p>
 * Call {@link #shouldShowRequestPermissionRationale(String)} before calling this API to
 * check if the system recommends to show a rationale UI before asking for a permission.
 * </p>
 * <p>
 * If your app does not have the requested permissions the user will be presented
 * with UI for accepting them. After the user has accepted or rejected the
 * requested permissions you will receive a callback on {@link
 * #onRequestPermissionsResult(int, String[], int[])} reporting whether the
 * permissions were granted or not.
 * </p>
 * <p>
 * Note that requesting a permission does not guarantee it will be granted and
 * your app should be able to run without having this permission.
 * </p>
 * <p>
 * This method may start an activity allowing the user to choose which permissions
 * to grant and which to reject. Hence, you should be prepared that your activity
 * may be paused and resumed. Further, granting some permissions may require
 * a restart of you application. In such a case, the system will recreate the
 * activity stack before delivering the result to {@link
 * #onRequestPermissionsResult(int, String[], int[])}.
 * </p>
 * <p>
 * When checking whether you have a permission you should use {@link
 * #checkSelfPermission(String)}.
 * </p>
 * <p>
 * You cannot request a permission if your activity sets {@link
 * android.R.styleable#AndroidManifestActivity_noHistory noHistory} to
 * <code>true</code> because in this case the activity would not receive
 * result callbacks including {@link #onRequestPermissionsResult(int, String[], int[])}.
 * </p>
 * <p>
 * The <a href="https://github.com/android/permissions-samples">
 * RuntimePermissions</a> sample apps demonstrate how to use this method to
 * request permissions at run time.
 * </p>
 *
 * @param permissions The requested permissions. Must me non-null and not empty.
 * @param requestCode Application specific request code to match with a result
 *    reported to {@link #onRequestPermissionsResult(int, String[], int[])}.
 *    Should be >= 0.
 *
 * @throws IllegalArgumentException if requestCode is negative.
 *
 * @see #onRequestPermissionsResult(int, String[], int[])
 * @see #checkSelfPermission(String)
 * @see #shouldShowRequestPermissionRationale(String)
 */
public final void requestPermissions(@NonNull String[] permissions, int requestCode) {
    if (requestCode < 0) {
        throw new IllegalArgumentException("requestCode should be >= 0");
    }

    if (mHasCurrentPermissionsRequest) {
        Log.w(TAG, "Can request only one set of permissions at a time");
        // Dispatch the callback with empty arrays which means a cancellation.
        onRequestPermissionsResult(requestCode, new String[0], new int[0]);
        return;
    }

    if (!getAttributionSource().getRenouncedPermissions().isEmpty()) {
        final int permissionCount = permissions.length;
        for (int i = 0; i < permissionCount; i++) {
            if (getAttributionSource().getRenouncedPermissions().contains(permissions[i])) {
                throw new IllegalArgumentException("Cannot request renounced permission: "
                        + permissions[i]);
            }
        }
    }

    final Intent intent = getPackageManager().buildRequestPermissionsIntent(permissions);
    startActivityForResult(REQUEST_PERMISSIONS_WHO_PREFIX, intent, requestCode, null);
    mHasCurrentPermissionsRequest = true;
}

注释翻译如下：

        该函数作用是请求授予应用程序权限。不管这些权限是否被平台或三方应用声明过，他们都必须在manifest清单中进行申请，他们应该没有被授予给应用程序，并且这些权限应该具有危险的保护级别。

        如果在manifest中声明了普通权限（正常保护级别），则在安装应用的时就会授予这些普通权限给应用。如果签名权限（签名保护级别）在manifest中声明了，并app的签名和app声明权限的签名匹配，则在安装应用程序时会授予签名权限。

        在调用requestPermissions()函数之前调用shouldShowRequestPermissionRationale(String)来检查在询问权限前，系统是否推荐展示请求权限原因的UI。

        如果你的app没有这些被请求的权限，为了接受这些权限，将会展示UI给用户。在用户接受或拒绝这些请求的权限之后，你将收到一个回调函数onRequestPermissionsResult(int, String[], int[])来报告这些权限是否被授予或没有被授予。

        注意，请求一个权限不能保证权限被授予，并且你的应用app应该有能力在不具有这些权限的情况下正常运行。

        该函数可能启动一个activity允许用户选择哪个权限被授予，哪个权限被拒绝。因此，您应该预料到您的activity可能会被暂停和resumed。更进一步，授予某些权限可能需要应用程序进行重启。在这种情况下，在把结果交付给onRequestPermissionsResult(int, String[], int[])前，系统将重建activity栈。

        当检查是否你具有某个权限，你应该使用checkSelfPermission(String)。

        如果你的activity设置了 android.R.styleable#AndroidManifestActivity_noHistory noHistory=true，你将不能请求权限，因为在这种情况下activity将不会接受onRequestPermissionsResult(int, String[], int[])中的回调结果。

        链接https://github.com/android/permissions-samples的例子RuntimePermissions演示了在运行时，如何使用该方法请求权限。

demo演示

package com.example.helloworld.activity.camera;

import android.Manifest;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.view.View;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;

import com.example.helloworld.R;
import com.google.android.material.snackbar.Snackbar;

/**
 * Launcher Activity that demonstrates the use of runtime permissions for Android M.
 * This Activity requests permissions to access the camera
 * ({@link android.Manifest.permission#CAMERA})
 * when the 'Show Camera Preview' button is clicked to start  {@link CameraPreviewActivity} once
 * the permission has been granted.
 * <p>
 * First, the status of the Camera permission is checked using {@link
 * ActivityCompat#checkSelfPermission(Context, String)}
 * If it has not been granted ({@link PackageManager#PERMISSION_GRANTED}), it is requested by
 * calling
 * {@link ActivityCompat#requestPermissions(Activity, String[], int)}. The result of the request is
 * returned to the
 * {@link androidx.core.app.ActivityCompat.OnRequestPermissionsResultCallback}, which starts
 * {@link
 * CameraPreviewActivity} if the permission has been granted.
 * <p>
 * Note that there is no need to check the API level, the support library
 * already takes care of this. Similar helper methods for permissions are also available in
 * ({@link ActivityCompat},
 * {@link androidx.core.content.ContextCompat} and {@link androidx.fragment.app.Fragment}).
 */
public class MainPermissionActivity extends AppCompatActivity
        implements ActivityCompat.OnRequestPermissionsResultCallback {

    private static final int PERMISSION_REQUEST_CAMERA = 0;

    private View mLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main1);
        mLayout = findViewById(R.id.main_layout);

        // Register a listener for the 'Show Camera Preview' button.
        findViewById(R.id.button_open_camera).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                showCameraPreview();
            }
        });
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions,
                                           @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        // BEGIN_INCLUDE(onRequestPermissionsResult)
        if (requestCode == PERMISSION_REQUEST_CAMERA) {
            // Request for camera permission.
            if (grantResults.length == 1 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                // Permission has been granted. Start camera preview Activity.
                Snackbar.make(mLayout, R.string.camera_permission_granted,
                        Snackbar.LENGTH_LONG)
                        .show();
                startCamera();
            } else {
                // Permission request was denied.
                Snackbar.make(mLayout, R.string.camera_permission_denied,
                        Snackbar.LENGTH_LONG)
                        .show();
            }
        }
        // END_INCLUDE(onRequestPermissionsResult)
    }

    private void showCameraPreview() {
        // BEGIN_INCLUDE(startCamera)
        // Check if the Camera permission has been granted
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.CAMERA)
                == PackageManager.PERMISSION_GRANTED) {
            // Permission is already available, start camera preview
            Snackbar.make(mLayout,
                    R.string.camera_permission_available,
                    Snackbar.LENGTH_LONG).show();
            startCamera();
        } else {
            // Permission is missing and must be requested.
            requestCameraPermission();
        }
        // END_INCLUDE(startCamera)
    }

    /**
     * Requests the {@link android.Manifest.permission#CAMERA} permission.
     * If an additional rationale should be displayed, the user has to launch the request from
     * a SnackBar that includes additional information.
     */
    private void requestCameraPermission() {
        // Permission has not been granted and must be requested.
        if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                Manifest.permission.CAMERA)) {
            // Provide an additional rationale to the user if the permission was not granted
            // and the user would benefit from additional context for the use of the permission.
            // Display a SnackBar with cda button to request the missing permission.
            Snackbar.make(mLayout, R.string.camera_access_required,
                    Snackbar.LENGTH_INDEFINITE).setAction(R.string.ok, new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    // Request the permission
                    ActivityCompat.requestPermissions(MainPermissionActivity.this,
                            new String[]{Manifest.permission.CAMERA},
                            PERMISSION_REQUEST_CAMERA);
                }
            }).show();

        } else {
            Snackbar.make(mLayout, R.string.camera_unavailable, Snackbar.LENGTH_LONG).show();
            // Request the permission. The result will be received in onRequestPermissionResult().
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.CAMERA}, PERMISSION_REQUEST_CAMERA);
        }
    }

    private void startCamera() {
        Intent intent = new Intent(this, CameraPreviewActivity.class);
        startActivity(intent);
    }

}

2、shouldShowRequestPermissionRationale   

/**
 * Gets whether you should show UI with rationale before requesting a permission.
 *
 * @param permission A permission your app wants to request.
 * @return Whether you should show permission rationale UI.
 *
 * @see #checkSelfPermission(String)
 * @see #requestPermissions(String[], int)
 * @see #onRequestPermissionsResult(int, String[], int[])
 */
public boolean shouldShowRequestPermissionRationale(@NonNull String permission) {
    return getPackageManager().shouldShowRequestPermissionRationale(permission);
}

        1、应用第一次申请危险权限时，shouldShowRequestPermissionRationale(permission)=false，不会弹出请求权限原因说明弹窗，而是直接弹出请求权限弹窗让用户选择授予或拒绝权限；

         2、当且仅当用户之前有拒绝了危险权限，且没有勾选”拒绝且不再询问“复选框时shouldShowRequestPermissionRationale(permission)=true，则申请危险权限时，会先弹出请求权限原因说明弹窗，在弹窗允许后才会弹出请求权限弹窗让用户选择授予或拒绝权限；

        3、如果用户勾选了”拒绝且不再询问“复选框，则shouldShowRequestPermissionRationale(permission)=false,则申请危险权限时，不再弹出权限原因说明弹窗，并且调用requestPermissions()申请危险权限时也不再弹出请求权限弹窗，而是直接走到onRequestPermissionsResult()函数中权限请求失败的逻辑。

       疑问：为什么勾选了”拒绝且不再询问“复选框后再请求权限就不会再弹窗请求权限弹窗，我没有从源码中看出是如何实现的。

注意：上面的逻辑我是在android api 28上能够复现的，在api30上好像不行（api 30 没有”拒绝且不再询问“复选框），具体原因不清楚。

相关代码如下：

reqoneper.setOnClickListener {
    if (checkSelfPermission(Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
        //if (ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.CAMERA)) {
        if (shouldShowRequestPermissionRationale( Manifest.permission.CAMERA)) {
            var dialog = AlertDialog.Builder(this)
                .setTitle("权限申请原因")
                .setMessage("您必须申请相机权限才能正常使用我们的功能，请开启相机权限")
                .setPositiveButton(
                    "允许",
                    DialogInterface.OnClickListener() { dialog, which ->
                        requestPermissions(
                            onePermission,
                            reqonecode
                        )
                    })
                .setNegativeButton(
                    "取消",
                    DialogInterface.OnClickListener() { dialog, which ->
                        dialog.cancel()
                    }).create()
            dialog.setCanceledOnTouchOutside(false)
            dialog.show()
        } else {
            requestPermissions(onePermission, reqonecode)
        }
    } else {
        openCamera()
    }
}

override fun onRequestPermissionsResult(
    requestCode: Int,
    permissions: Array<out String>,
    grantResults: IntArray
) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults)
    when (requestCode) {
        reqonecode -> {
            if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                openCamera()
            } else {
                Toast.makeText(this, "申请相机失败", Toast.LENGTH_SHORT).show()
            }
        }
    }
}

参考文章：

Android内部分享[8]——Android系统的应用程序权限申请 - DP2PX.COM