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Editor to share with you about the Android location permissions and the array to find the index of the pit, I believe that most people do not know, so share this article for your reference, I hope you can learn a lot after reading this article, let's go to know it!

I. Android dangerous permissions, holes from official documents

As Android developers all know, prior to Android 6. 0, permission requests only need to be declared in the AndroidManifest.xml file. Since the beginning of Android 6.0, the permission application has changed, and the dangerous permission needs to be applied dynamically in the application. Before, I wrote a note on the Android dynamic application for dangerous permission. For more information, please refer to: Android 6.0dynamic application for dangerous permission.

Take a screenshot and take a look at the official instructions of Android:

Let's take a look at the grouping of dangerous permissions:

This means that dangerous permissions are grouped, and as long as one permission is granted in the same group, other permissions in the same group are granted by default. For example, after I authorize the application to read the memory card, the application has the permission to write to the memory card, which is indeed the case.

The problem, however, is the code that uses GPS to get location information:

LocationManager locationManager = (LocationManager) getSystemService (Context.LOCATION_SERVICE); if (locationManager.isProviderEnabled (LocationManager.GPS_PROVIDER)) {if (ActivityCompat.checkSelfPermission (this, Manifest.permission.ACCESS_FINE_LOCATION)! = PackageManager.PERMISSION_GRANTED & & ActivityCompat.checkSelfPermission (MainActivity.this, Manifest.permission.ACCESS_COARSE_LOCATION)! = PackageManager.PERMISSION_GRANTED) {/ / request permissions /. /.} else {Location location = locationManager.getLastKnownLocation (LocationManager.GPS_PROVIDER If (location! = null) {double latitude = location.getLatitude (); double longitude = location.getLongitude ();}}

Through the official dangerous permission group, we can also see that the Location permission group contains two permissions: ACCESS_FINE_LOCATION and ACCESS_COARSE_LOCATION. According to the above instructions, as long as one of the two permissions is successfully applied for authorization, you can successfully obtain longitude and latitude. However, when you have successfully applied for ACCESS_COARSE_LOCATION permission, the program will still collapse with an error message and you need to obtain ACCESS_FINE_LOCATION permission.

Second, Spinner's setSelection () method comes from the trap that one takes for granted.

The drop-down list control spinner in Android has a method setSelection (int position) that displays the number of items. This method may not be effective. The first item is always displayed.

When doing two spinner linkage, spinner2 populates the data according to the choice of spinner1, and then uses setSeletion (2) to set the default. It turns out that spinner2 always shows the first item, but the actual selection is already the location of position 2.

Solution:

Old code:

Spinner.setAdapter (adapter); spinner.setSelection (2)

There are two solutions:

(1)

Spinner.setAdapter (adapter); spinner.setSelection (2jre true); / / spinner will re-layout

(2) recommendation

Spinner.setAdapter (adapter);! [] (https://images2018.cnblogs.com/blog/758949/201807/758949-20180726224508174-131546620.jpg) adapter.notifyDataSetChanged (); / / notify spinner to refresh data spinner.setSelection (2)

So, what is the cause of this? I think this is a bug. This usually happens after the data is repopulated, but otherwise, it can be displayed correctly using setSelection (int position).

The implementation mechanism of setSelection (int position, boolean animate) is quite different from that of setSelection (int position). When the former is called, re-layout will trigger the onItemSelected function immediately, which is equivalent to clicking directly with your hand. The latter sets the next selection location: setNextSelectedPositionInt (position); then requests the Layout;, and the requestLayout does not execute immediately, just a schedule. But the latter may lose some state when reloading the data and then Layout.

But goose, the pit I want to say is not like this. I don't have two Spinner linkage. The problem I have is like this. Go to the code.

Layout file:

Java Code:

Package com.example.sharpcj.helloworld;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.widget.ArrayAdapter;import android.widget.Spinner;import java.util.Arrays;public class MainActivity extends AppCompatActivity {private Spinner mSpTest; @ Override protected void onCreate (Bundle savedInstanceState) {super.onCreate (savedInstanceState); setContentView (R.layout.activity_main); String [] strs = new String [20]; for (int I = 0; I

< strs.length; i++) { strs[i] = "第" + i + "项"; } mSpTest = findViewById(R.id.sp_test); mSpTest.setAdapter(new ArrayAdapter(this, android.R.layout.simple_list_item_1, strs)); int index = Arrays.binarySearch(strs, "第11项"); mSpTest.setSelection(index); }} 运行结果：

What???

What causes it? Spinner says it doesn't carry this pot, but I take it for granted because I mistakenly use the method Arrays.binarySearch (Object [] a, Object key) and take it for granted that the return value is the index that found the array. In the code, the actual value of index is-2.

I looked for some information on the Internet:

BinarySearch (int [] a, int key) the rules for this method are as follows:

1. If the keyword is found, the return value is the index of the keyword's position in the array, and the index starts at 0

2. If the keyword is not found, a negative insertion point value is returned. The so-called insertion point value is the position index of the first element larger than the keyword in the array, and the location index starts at 1.

The source code of the final calling method of binarySearch (Object [] a, Object key) is as follows:

/ / Like public version, but without range checks. Private static int binarySearch0 (Object [] a, int fromIndex, int toIndex, Object key) {int low = fromIndex; int high = toIndex-1; while (low > > 1; @ SuppressWarnings ("rawtypes") Comparable midVal = (Comparable) a [mid]; @ SuppressWarnings ("unchecked") int cmp = midVal.compareTo (key); if (cmp)

< 0) low = mid + 1; else if (cmp >

0) high = mid-1; else return mid; / / key found} return-(low + 1); / / key not found. } these are all the contents of this article entitled "Android location permissions and what are the pits for array search indexes?" Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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