Android onActivityResult() resul

问题

当 Activity调用startActivityForResult()方法后 ，回调函数 onActivityResult()中参数 resultCode 等于0？

原因

主要原因是跟Activity的 launchMode启动方式有关：

 /**
     * Launch an activity for which you would like a result when it finished.
     * When this activity exits, your
     * onActivityResult() method will be called with the given requestCode.
     * Using a negative requestCode is the same as calling
     * {@link #startActivity} (the activity is not launched as a sub-activity).
     *
     * <p>Note that this method should only be used with Intent protocols
     * that are defined to return a result.  In other protocols (such as
     * {@link Intent#ACTION_MAIN} or {@link Intent#ACTION_VIEW}), you may
     * not get the result when you expect.  For example, if the activity you
     * are launching uses {@link Intent#FLAG_ACTIVITY_NEW_TASK}, it will not
     * run in your task and thus you will immediately receive a cancel result.
     *
     * <p>As a special case, if you call startActivityForResult() with a requestCode
     * >= 0 during the initial onCreate(Bundle savedInstanceState)/onResume() of your
     * activity, then your window will not be displayed until a result is
     * returned back from the started activity.  This is to avoid visible
     * flickering when redirecting to another activity.
     *
     * <p>This method throws {@link android.content.ActivityNotFoundException}
     * if there was no Activity found to run the given Intent.
     *
     * @param intent The intent to start.
     * @param requestCode If >= 0, this code will be returned in
     *                    onActivityResult() when the activity exits.
     * @param options Additional options for how the Activity should be started.
     * See {@link android.content.Context#startActivity(Intent, Bundle)}
     * Context.startActivity(Intent, Bundle)} for more details.
     *
     * @throws android.content.ActivityNotFoundException
     *
     * @see #startActivity
     */
    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {}

在上面注释中有这么一句话：
For example, if the activity you are launching uses {@link Intent#FLAG_ACTIVITY_NEW_TASK}, it will not run in your task and thus you will immediately receive a cancel result.
意思是：如果你的Activity为FLAG_ACTIVITY_NEW_TASK 模式， 即为singleTask或 singleInstance模式时，将立即接口一个cancel result ，即resultCode为0

• launchMode 为standard或singleTop
  调用 startActivityForResult() 的Activity launchMode 为standard、singleTop两个模式其中之一， onActivityResult() 会在Activity中回调，此时 resultCode = -1，也是RESULT_OK;

• launchMode 为singleTask或 singleInstance
  调用 startActivityForResult() 的Activity launchMode 为singleTask、singleInstance两个模式其中之一， onActivityResult() 会在Activity中回调，此时 resultCode = 0 ;