Android Execute Code On Variable Change

I alter a variable on different button clicks, (i.e. inside the onclick listener for multiple buttons) and I have a textview that is suppose to show this variable, but remains out

Solution 1:

There is no way in Java to be notified when a variable or class field changes. What you need to do is to implement a simple wrapper class for your int so that clients can register for callbacks whenever the value changes. This class might look something like this:

package com.example.android;

/**
 * A store of an int value. You can register a listener that will be notified
 * when the value changes.
 */publicclassIntValueStore{

    /**
     * The current value.
     */int mValue;

    /**
     * The listener (you might want turn this into an array to support many
     * listeners)
     */private IntValueStoreListener mListener;

    /**
     * Construct a the int store.
     *
     * @param initialValue The initial value.
     */public IntValueStore(int initialValue) {
        mValue = initialValue;
    }

    /**
     * Sets a listener on the store. The listener will be modified when the
     * value changes.
     *
     * @param listener The {@link IntValueStoreListener}.
     */publicvoid setListener(IntValueStoreListener listener) {
        mListener = listener;
    }

    /**
     * Set a new int value.
     *
     * @param newValue The new value.
     */publicvoid setValue(int newValue) {
        mValue = newValue;
        if (mListener != null) {
            mListener.onValueChanged(mValue);
        }
    }

    /**
     * Get the current value.
     *
     * @return The current int value.
     */publicint getValue() {
        return mValue;
    }

    /**
     * Callbacks by {@link IntValueModel}.
     */publicstaticinterfaceIntValueStoreListener{
        /**
         * Called when the value of the int changes.
         *
         * @param newValue The new value.
         */void onValueChanged(int newValue);
    }
}

Now you need to have some class that implements the IntValueStoreListener interface. You could let that be the Activity, which would then keep track of a TextView to update. I would implement a trivial custom TextView instead, like this:

package com.example.android;

import com.example.android.IntValueStore.IntValueStoreListener;

import android.content.Context;
import android.util.AttributeSet;
import android.widget.TextView;

publicclassIntTextViewextendsTextViewimplementsIntValueStoreListener{

    publicIntTextView(Context context) {
        super(context);
    }

    publicIntTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    publicvoidonValueChanged(int newValue) {
        // The int got a new value! Update the text
        setText(String.valueOf(newValue));
    }
}

You can now setup your layout XML. For example:

<?xml version="1.0" encoding="utf-8"?><LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="match_parent"android:layout_height="match_parent"android:orientation="vertical" ><Buttonandroid:id="@+id/btn_increment"android:layout_width="match_parent"android:layout_height="wrap_content"android:text="Increment" /><Buttonandroid:id="@+id/btn_double"android:layout_width="match_parent"android:layout_height="wrap_content"android:text="Double" /><com.example.android.IntTextViewandroid:id="@+id/text_current_value"android:layout_width="match_parent"android:layout_height="wrap_content" /></LinearLayout>

An activity that does the necessary setup would look like this:

package com.example.android;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;

publicclassIntValueStoreActivityextendsActivity {

    privateIntValueStoremIntValueModel=newIntValueStore(1);

    @OverridepublicvoidonCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        // Initialize the text view with the initial valueIntTextViewintTextView= (IntTextView)findViewById(R.id.text_current_value);
        intTextView.setText(String.valueOf(mIntValueModel.getValue()));

        // Setup the text view as a listener so it gets updated whenever the int// value changes
        mIntValueModel.setListener(intTextView);

        // Setup an OnClickListener that will increment the int value by 1. The// TextView will be automatically updated since it is setup as a// listener to the int value store
        findViewById(R.id.btn_increment).setOnClickListener(newView.OnClickListener() {
            publicvoidonClick(View view) {
                mIntValueModel.setValue(mIntValueModel.getValue() + 1);
            }
        });

        // Setup an OnClickListener that will double the int value. The TextView// will be automatically updated since it is setup as a listener to the// int value store
        findViewById(R.id.btn_double).setOnClickListener(newView.OnClickListener() {
            publicvoidonClick(View view) {
                mIntValueModel.setValue(mIntValueModel.getValue() * 2);
            }
        });
    }
}

This will give you the following UI:

Whenever you click any of the buttons, the TextView will almost magically update the value it displays, even though none of the OnClickListeners have any code to touch the TextView!

This is actually a common pattern in programming called Model-view-controller. In the sample code above, the Model is IntValueStore and the View is IntTextView and there is no Controller.

Solution 2:

In C# there is an easy way to trigger some action once a variable changes so it is in Java. Java afaik do not support operators overloading, but this feature is not necessary. When it comes to classes (object) oriented programming, just create your special type out of any standard type. A new class will extend the features od any type (int, long, string, etc.). Define, for example your own "SET" and/or "GET" methods and just use these instead of standard "=" or get value/ponter as left operand.

Example:

publicclassmyINT
{
 publicint Value;

 publicvoidSET(int v){Value = v;  // PLUS additional ACTION }publicintGET(int v){return Value ;}
 publicvoidADD(int v){Value += v; // PLUS additional ACTION }publicvoidSUB(int v){Value -= v; // PLUS additional ACTION }
}