一、 why ?
由于才疏学浅，在开发中requestCode的让我很困惑。困惑是因为什么呢，是因为弄混了。要想弄明白，不困惑，来想一想用到requestCode的地方：
① startActivityForResult开启另一个Activity的时候：
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
startActivityForResult(intent, requestCode, null);
}
② 请求权限的时候：
public final void requestPermissions(@NonNull String[] permissions, int requestCode) {
if (mHasCurrentPermissionsRequest) {
Log.w(TAG, "Can reqeust 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;
}
Intent intent = getPackageManager().buildRequestPermissionsIntent(permissions);
startActivityForResult(REQUEST_PERMISSIONS_WHO_PREFIX, intent, requestCode, null);
mHasCurrentPermissionsRequest = true;
}
（1）首先来看startActivityForResult()的requestCode:
通过查看源码发现：在Activity中startActivityForResult() 和Activity的子类FragmentActivity中的startActivityForResult()是不一样的，代码展示区别：
/** |
/** /** |
通过以上代码的比较我们知道，会造成下面的错误，是FragmentActivity添加的限制，而Activity中没有该限制，这里解决了一点点疑惑：
java.lang.IllegalArgumentException: Can only use lower bits for requestCode
所以这里总结一下：在startActivityForResult()中的resquestCode的限制是 0 <= requestCode <= 216，小于零会造成什么问题，你可以试试，我没有尝试。
（2）再来看ActivityCompat请求权限中的requestCode：
public static void requestPermissions(final @NonNull Activity activity,
final @NonNull String[] permissions, final int requestCode) {
if (Build.VERSION.SDK_INT >= 23) {
ActivityCompatApi23.requestPermissions(activity, permissions, requestCode);
} else if (activity instanceof OnRequestPermissionsResultCallback) {
Handler handler = new Handler(Looper.getMainLooper());
handler.post(new Runnable() {
@Override
public void run() {
final int[] grantResults = new int[permissions.length]; PackageManager packageManager = activity.getPackageManager();
String packageName = activity.getPackageName(); final int permissionCount = permissions.length;
for (int i = 0; i < permissionCount; i++) {
grantResults[i] = packageManager.checkPermission(
permissions[i], packageName);
} ((OnRequestPermissionsResultCallback) activity).onRequestPermissionsResult(
requestCode, permissions, grantResults);
}
});
}
}
由源码可以知道：当sdk >=23的时候，会走 ActivityCompatApi23.requestPermissions(activity, permissions, requestCode)，来看看它干了什么：
public static void requestPermissions(Activity activity, String[] permissions,
int requestCode) {
if (activity instanceof RequestPermissionsRequestCodeValidator) {
((RequestPermissionsRequestCodeValidator) activity)
.validateRequestPermissionsRequestCode(requestCode);
}
activity.requestPermissions(permissions, requestCode);
}
如果Activity实现了接口RequestPermissionsRequestCodeValidator 就会对requestCode进行判断，天啊，终于说到它了。validateRequestPermissionsRequestCode 见名之意，它对requestCode作了限制，看看FragmentActivity实现的该接口是怎么限制的：checkForValidRequestCode() 这个方法是不是很熟悉：上面就有看看吧，看来限制是一样的： 请求权限 requestCode <= 216
@Override
public final void validateRequestPermissionsRequestCode(int requestCode) {
// We use 16 bits of the request code to encode the fragment id when
// requesting permissions from a fragment. Hence, requestPermissions()
// should validate the code against that but we cannot override it as
// we can not then call super and also the ActivityCompat would call
// back to this override. To handle this we use dependency inversion
// where we are the validator of request codes when requesting
// permissions in ActivityCompat.
if (!mRequestedPermissionsFromFragment
&& requestCode != -1) {
checkForValidRequestCode(requestCode);
}
}
你会不会以为这个限制只有在sdk >=23以上才有，那就错了。我们来看看小于23时的分支会直接走到onRequestPermissionsResult()，来看看它：哇，同样的限制。
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions,
@NonNull int[] grantResults) {
int index = (requestCode >> 16) & 0xffff;
if (index != 0) {
index--; String who = mPendingFragmentActivityResults.get(index);
mPendingFragmentActivityResults.remove(index);
if (who == null) {
Log.w(TAG, "Activity result delivered for unknown Fragment.");
return;
}
Fragment frag = mFragments.findFragmentByWho(who);
if (frag == null) {
Log.w(TAG, "Activity result no fragment exists for who: " + who);
} else {
frag.onRequestPermissionsResult(requestCode & 0xffff, permissions, grantResults);
}
}
}
二、总结：
FragmentActivity是对Activtiy的一层包装，requestCode的限制是在它里面添加的，而Activity本身对requestCode是没有限制的。所以如果发现有的requestCode大于216仍然能通过，不要疑惑，你懂得！