How can I retrieve an instance of my fragment class so I can call a method on it?

Question

I am learning how to use fragments, and in my MainActivity.java I have this:

@Override

protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);



    A a = new A();



    B b = new B(a);



    if (savedInstanceState == null) {

        getSupportFragmentManager().beginTransaction()

                .add(R.id.container, new ContentFragment())

                .commit();

        ContentFragment frag = (ContentFragment) getSupportFragmentManager()

                .findFragmentById(R.id.container);

        frag.doInject(b);

    }

}

If I try to inject the dependency directly into the constructor of the fragment, Android Studio barks at me. So I'm trying this approach, but when debugging I get to where I am setting frag, and it's just null and is always null. So, I guess what I need to know is, in the simplest way possible, how do I call my doInject(b) and inject b into my fragment?

Another thing too, which I'm not sure about, is that later if I have a listener in the same MainActivity.java file, and I want to update the fragment with new content, I was hoping to do something like this:

@Override

public void handleResponseData(String s) {

    frag.updateTextbox(s);

}

But I have a feeling that I'm going to run into problems with that too. So I need some help understanding what it is that I'm trying to do. I've spent at least 5 hours trying to figure this out, and need some help, please!

Edit to show layout files:

activity_main.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    android:id="@+id/container"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context=".MainActivity"

    tools:ignore="MergeRootFrame" />

fragment_main.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:paddingLeft="@dimen/activity_horizontal_margin"



    android:paddingRight="@dimen/activity_horizontal_margin"

    android:paddingTop="@dimen/activity_vertical_margin"

    android:paddingBottom="@dimen/activity_vertical_margin"

    tools:context=".MainActivity">



    <TextView

        android:id="@+id/textbox"

        android:layout_width="match_parent"

        android:layout_height="match_parent"

        android:text="Hello World" />



</FrameLayout>

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Answer 1

If I try to inject the dependency directly into the constructor of the fragment

I'm not sure about that doInject method, but you should read Best practice for instantiating a new Android Fragment

For example,

ContentFragment frag = ContentFragment.newInstance(b);
getSupportFragmentManager().beginTransaction()
            .add(R.id.container, frag, "TAG")
            .commit();
// frag.doInject(b); // Can do, but not necessary

later if I have a listener in the same MainActivity.java file, and I want to update the fragment with new content, I was hoping to do something like this

Generally, this is not how you do that.

frag.updateTextbox(s);

I assume you have the Activity attached to the Fragment via a listener with a handleResponseData(String s), and you'd have to do

String s = "foo";
if (listener != null) {
    listener.handleResponseData(s);
}

therefore, instead of that, you only need do this

String s = "foo";
updateTextbox(s);
/*
if (listener != null) {
    listener.handleResponseData(s);
}
*/

How can I retrieve an instance of my fragment class so I can call a method on it?

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