Weird behaviour of AndroidJavaObject in Unity, when implemented through C#

Question

Rather long post follows. It's fairly complex, needs someone who has worked with Unity Plugins before (creating them, not using them) and I'm not looking to get downvoted because a few common search terms show up. So, please read the entire post.

So, I've been having a rather weird issue happening when I try to call native functions through Unity's AndroidJavaObject

I've built native plugins in the past before, I'm trying to remove my dependancies on having to update multiple JARs each time I need to update my plugins. So, I'm moving a lot of my native Java code from plugins, into Unity itself.

Specifically, there's a way to call native Android functions (i.e. system functions) straight from C#, rather than creating a plugin and calling a function in the plugin that invokes the native function.

For example, earlier, if I had to create a Toast, I'd have to write some native code in Java, export the plugin and then call the function. These days, one can just call the Toast.makeText straight from C# as follows.

Here's how I make the call for a Toast.

    public static void Toast (string text, int duration) {
        #if UNITY_ANDROID && !UNITY_EDITOR
        CreateActivity();

        activity.Call("runOnUiThread", new AndroidJavaRunnable ( () => {
            new AndroidJavaObject("android.widget.Toast", activity)
                .CallStatic<AndroidJavaObject>("makeText", activity, text, duration)
                    .Call("show");
        }) );
        #endif
    }

and here's the CreateActivity method

private static void CreateActivity () {
    #if UNITY_ANDROID && !UNITY_EDITOR
    if(activity == null)
        activity = new AndroidJavaClass("com.unity3d.player.UnityPlayer").
            GetStatic<AndroidJavaObject>("currentActivity");
    #endif

}

This works perfectly. However, when I try the same approach for creating an AlertDialog, I get an androidjavaexception java.lang.classnotfoundexception

Here's the relevant code

public static void ShowAlert (string title, string message, string buttonText) {

    #if UNITY_ANDROID && !UNITY_EDITOR

    CreateActivity();

    activity.Call("runOnUiThread", new AndroidJavaRunnable ( () => {
        var builder = new AndroidJavaObject("android.app.AlertDialog.Builder", activity);
        ***//Exception happens in the above line***

        builder.Call<AndroidJavaObject>("setMessage", message)  
            .Call<AndroidJavaObject>("setTitle", title)
                .Call<AndroidJavaObject>("setCancelable", false)
                .Call<AndroidJavaObject>("setNeutralButton", buttonText)
                .Call("show");
    }) );
    #endif
}

What I've tried

Used android.support.v7.app.AlertDialog.Builder instead of android.app.AlertDialog.Builder
Added the support JARs for v7 & v4 (Specifically android-support-v4 & android-support-v7-appcompat)
Tried creating my own implementation for Unity's JNI and invoking that

All the above failed. (The third one, rather spectacularly)

Now, I tried other classes as well, and most of them work (such as ProgressDialog, which is a subclass of AlertDialog), and those work!

At this point, I'm rather stumped and I can't think of a VALID reason for certain functions to work, and the others to fail.

Any thoughts are appreciated.

EDIT 1: Added LogCat

I/Unity: AndroidJavaException: java.lang.ClassNotFoundException: android.app.AlertDialog.Builder
                                            at UnityEngine.AndroidJNISafe.CheckException () [0x00000] in <filename unknown>:0 
                                            at UnityEngine.AndroidJNISafe.CallStaticObjectMethod (IntPtr clazz, IntPtr methodID, UnityEngine.jvalue[] args) [0x00000] in <filename unknown>:0 
                                            at UnityEngine.AndroidJavaObject._CallStatic[AndroidJavaObject] (System.String methodName, System.Object[] args) [0x00000] in <filename unknown>:0 
                                            at UnityEngine.AndroidJavaObject.CallStatic[AndroidJavaObject] (System.String methodName, System.Object[] args) [0x00000] in <filename unknown>:0 
                                            at UnityEngine.AndroidJavaObject.FindClass (System.String name) [0x00000] in <filename unknown>:0 
                                            at UnityEngine.AndroidJavaObject._AndroidJavaObject (System.String className, System.Object[] args) [0x00000] in <filename unknown>:0 
                                            at UnityEngine.AndroidJavaObject..ctor (System.String className, System.Object[] args) [0x00000] in <filename unknown>:0 
                                            at AndroidNative+<ShowAlert>c__AnonSto

Interesting issue! Is it the actual android.app.AlertDialog.Builder class that is triggering the ClassNotFoundException (i.e. and not one of the implicit classes used in the applied method calls)? A logcat excerpt might provide useful extra information. — Paul-Jan
– Paul-Jan, Commented Mar 5, 2016 at 8:12

Paul-Jan · Accepted Answer · 2016-03-05 22:00:59Z

3

The problem you are running into, is that AlertDialog.Builder is a nested class of AlertDialog, and creating nested classes through reflection (which is what AndroidJavaObject is trying to do) requires a slightly different syntax in Java:

android.app.AlertDialog$Builder

There are some other tiny mistakes in your code (that you couldn't know about because your constructor wasn't working):

setNeutralButton requires a callback argument (which can be null)
show has to be called on the AlertDialog itself, not on the builder.

Taking these tiny corrections into consideration, a working code sample looks like this:

public static void ShowAlert(string title, string message, string buttonText)
{
    CreateActivity();

    activity.Call("runOnUiThread", new AndroidJavaRunnable(() =>
    {
        var builder = new AndroidJavaObject("android.app.AlertDialog$Builder", activity);

        builder.Call<AndroidJavaObject>("setMessage", message)
            .Call<AndroidJavaObject>("setTitle", title)
                .Call<AndroidJavaObject>("setCancelable", false)
                .Call<AndroidJavaObject>("setNeutralButton", buttonText, null)
                .Call<AndroidJavaObject>("create")
                .Call("show");
    }));
}

answered Mar 5, 2016 at 22:00

Paul-Jan

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2 Comments

Paul-Jan Over a year ago

If any of the other classes you couldn't access still can't be instantiated using this approach, please add them to your question (or start a new question) and I'll see if I can figure it out. It's good to get some more documentation out there on the whole Unity-Android interop process!

Venkat at Axiom Studios Over a year ago

Wow. I have no idea how I missed that. It's not the first time I'm working with nested classes, and now I'm laughing at myself going, "Oh how did you miss that?" Yeah, I know there's some inconsistencies on the rest of it that I'd have to fix. I took some code out (the OnClickListeners) for the purpose of this question. Thanks a bunch.

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