Develop Reference

Callbacks in Android

I am new to Android and programming as a whole and I need a little help with callbacks. I understand the gist of callbacks but I am unsure of how to go about implementing.
Context: I am writing a simple notetaking app that allows the user to write text and saving it to the app. The user can then request to read the file with a button. The text is then displayed on a textview in the main activity. There is an option to wipe this file and this is done with a confirmation pop up, which is another activity. This pop up contains 2 buttons, one to cancel and one to wipe. If the file is not empty it will wipe and does nothing if empty. I am not sure if this is the best way to implement it but I want to use the wipe button to callback to the main activity to clear the textview. The way I was thinking of was by using the callback to send a boolean value back. The main activity will check if the boolean is true and clear the textview if it is. I am unsure of how to implement the callback in my popup display to send this boolean value back to the main activity.
Code for main activity
public class MainActivity extends AppCompatActivity implements Popout.ClearTextView {
Button bnRead,bnWrite,bnClear;
TextView tvFileOP;
EditText etInput;
// private static final String INPUT_CONTENT = "inputContent";
public static final String TV_CONTENT = "textViewContent";
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
bnRead = (Button) findViewById(R.id.bnRead);
bnWrite = (Button) findViewById(R.id.bnWrite);
bnClear = (Button) findViewById(R.id.bnClear);
tvFileOP = (TextView) findViewById(R.id.tvFileOP);
etInput = (EditText) findViewById(R.id.etInput);
tvFileOP.setMovementMethod(new ScrollingMovementMethod());
final String fileName = "test_file";
String data;
bnRead.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
try {
FileInputStream fIn = openFileInput(fileName);
int c;
String temp = "";
while ( (c=fIn.read()) != -1){
temp = temp + Character.toString((char) c);
}
tvFileOP.setText(temp);
Toast.makeText(getBaseContext(),"file successfully read", Toast.LENGTH_LONG).show();
} catch (Exception e) {
e.printStackTrace();
}
}
});
bnWrite.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
String data = etInput.getText().toString();
try {
FileOutputStream fOut = openFileOutput(fileName,MODE_APPEND);
fOut.write(data.getBytes());
fOut.close();
etInput.setText("");
Toast.makeText(getBaseContext(),"file successfully written", Toast.LENGTH_LONG).show();
} catch (Exception e) {
e.printStackTrace();
}
}
});
bnClear.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
startActivity(new Intent(MainActivity.this,Popout.class));
}
});
}
#Override
protected void onSaveInstanceState(#NonNull Bundle outState) {
outState.putString(TV_CONTENT,tvFileOP.getText().toString());
super.onSaveInstanceState(outState);
}
#Override
protected void onRestoreInstanceState(#NonNull Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
tvFileOP.setText(savedInstanceState.getString(TV_CONTENT));
}
#Override
public void clearTextView(Boolean clear) {
if (clear){
tvFileOP.setText("");
}
}
}
Code for popup confirmation menu
public class Popout extends AppCompatActivity {
Button bnClosepopup,bnWipe;
TextView tvConfirmation;
String fileName = "test_file";
TextView tvFileOP;
public interface ClearTextView {
public void clearTextView(Boolean clear);
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.popupwindow);
bnClosepopup = (Button) findViewById(R.id.bnClosepopup);
bnWipe = (Button) findViewById(R.id.bnWipe);
tvConfirmation = (TextView) findViewById(R.id.tvConfirmation);
//HIDING THE TOOL BAR AT THE TOP OF THE SCREEN
this.getSupportActionBar().hide();
//GETTING THE SIZE OF THE SCREEN
DisplayMetrics displayMetrics = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);
int height = displayMetrics.heightPixels;
int width = displayMetrics.widthPixels;
getWindow().setLayout((int) (width*0.8) , (int) (0.8*height));
bnClosepopup.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
finish();
}
});
bnWipe.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
try {
File dir = getFilesDir();
File file = new File(dir, fileName);
boolean deleted = file.delete();
Toast.makeText(getBaseContext(),"file has been deleted",Toast.LENGTH_SHORT).show();
} catch (Exception e) {
Toast.makeText(getBaseContext(), e.getMessage(), Toast.LENGTH_LONG).show();
}
finish();
}
});
}
}
I am very new to android development and any tips on how to improve my code would be greatly appreciated :)

In this case there is no way to pass the interface to the other activity, because this is an activity to activity communication.
You have to use some other method, there is multiple ways to approach, the best way I can think of is to use startActivityForResult() to start the activity and then wait for a response to come back, and then query this response in the MainActivity by overriding the onActivityResult() method:
Example
In the MainActivity:
//on click of this button
bnClear.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this,Popout.class);
int requestCode = 12; //it could be whatever you want
startActivityForResult(intent , requestCode);
}
});
//override this method
#Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
//this is triggered when you finish the Popout Activity
if(requestCode == 12 && resultCode == Activity.RESULT_OK){
// get the boolean data returned from the Popout Activity
boolean deleted = data.getBooleanExtra("deleted_state" , false); //false is default if no value exists
}
}
In the Popout activity:
bnWipe.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
try {
File dir = getFilesDir();
File file = new File(dir, fileName);
boolean deleted = file.delete();
//send the result to onActivtyResult() in MainActivity
Intent result = new Intent();
result.putExtra("deleted_state", deleted );
setResult(Activity.RESULT_OK, result);
Toast.makeText(getBaseContext(),"file has been deleted",Toast.LENGTH_SHORT).show();
} catch (Exception e) {
Toast.makeText(getBaseContext(), e.getMessage(), Toast.LENGTH_LONG).show();
}
finish();
}
});
UPDATE:
It will be like this:
// get the boolean data returned from the Popout Activity
boolean deleted = data.getBooleanExtra("deleted_state" , false);
if (deleted){
tvFileOP.setText("");
}
..........

As far as what if Understood your problem correctly: You want to control your 'Wipe' button click event from your activity. Here is the solution which may help you.
1: Make an overridden constructor of your dialog class.
2: Create one abstract method in the dialog class. (say - onWipeButtonClick)
You need to make your dialog class abstract as well.
3: Inside on Click Listener of 'Wipe' button, call onWipeButtonClick abstract method.
4: Create the instance of dialog in the main activity where ever you want. The compiler will give you an error because you haven't implemented the call back method.
do implement your onWipeButtonClick method and do needful for wipe data inside the method.
public abstract class WipeDialog extends Dialog{
private Context context;
public WipeDialog(Context context){
this.context = context;
}
public abstract void onWipeButtonClick(boolean isTextEmpty);
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.<XML_FILE>);
<initialization>
btnWipe.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
onWipeButtonClick(<YOUR_BOOLEAN_CHECK>);
}
});
}
}
And now in Activity:
WipeDialog dialog = new WipeDialog(MainActivity.this) {
#Override
public void onWipeButtonClick(boolean isTextEmpty) {
//Do Need full with respected to your requirement on click of button 'WIPE'
}
};
Hope this will help.
Thanks!

Custom Listener for Compound component

I wrote a compound component and was adding a custom listener to react.
Inside the class for the compound component which uses an xml file.
public class VerticalCounterBlock extends LinearLayout {
public interface VerticalCounterBlockListener {
public void onCountChanged(int newCount);
}
private VerticalCounterBlockListener mVerticalCounterBlockListener = null;
public void setVerticalCounterBlockListener(VerticalCounterBlockListener listener){
mVerticalCounterBlockListener = listener;
}
// ... Other functions
}
I got my interface, I got the listener and I got the setter and I engage the listener like this in the button I have in the compound component. I can see that toast that is showing there when I test
addBtn = (Button)findViewById(R.id.btn_addcount);
addBtn.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View view) {
count++;
counttv.setText(String.format("%1$d", count));
Toast.makeText(getContext(), "VCB", Toast.LENGTH_SHORT).show();
if(mVerticalCounterBlockListener != null) {
mVerticalCounterBlockListener.onCountChanged(count);
}
}
});
In my main activity
m20_vcb = (VerticalCounterBlock) findViewById(R.id.vcb_m20);
m20_vcb.setVerticalCounterBlockListener(new VerticalCounterBlock.VerticalCounterBlockListener() {
#Override
public void onCountChanged(int newCount) {
increasePreachCountTotal();
Toast.makeText(CounterActivity.this, String.format("%1$d", newCount), Toast.LENGTH_SHORT).show();
}
});
I do not see that toast nor does it engage the function call. What am I missing?

I can suggest you several improvement scope here mainly restructuring the current format.
Lets not keep the interface as a inner class. So here's your VerticalCounterBlockListener.java
public interface VerticalCounterBlockListener {
public void onCountChanged(int newCount);
}
Now implement this interface in your MainActivity
public class MainActivity implements VerticalCounterBlockListener {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
m20_vcb = (VerticalCounterBlock) findViewById(R.id.vcb_m20);
m20_vcb.setVerticalCounterBlockListener(this);
}
// ... Other methods
// Override the onCountChanged function.
#Override
public void onCountChanged(int newCount) {
increasePreachCountTotal();
Toast.makeText(CounterActivity.this, String.format("%1$d", newCount), Toast.LENGTH_SHORT).show();
}
}
You might consider removing the Toast from the addBtn click listener which might create exception.
addBtn = (Button)findViewById(R.id.btn_addcount);
addBtn.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View view) {
count++;
counttv.setText(String.format("%1$d", count));
if(mVerticalCounterBlockListener != null) {
mVerticalCounterBlockListener.onCountChanged(count);
}
}
});

This was good there was something wrong with my system. i uninstaklled app and restarted computer and it worked as expected.

A custom function structure for my customView.onclick

I have searched but could not find answer of my question.
This is what I have:
private class BoxView extends View {
private String caption;
private OnClickListener bvClickListener = null
public BoxView(Context context) {
super(context);
this.bvClickListener = new this.OnClickListener(){
public void onClick (View v){
/*v.setCaption("X"); view don't have this method */
}}
}
public void setCaption(String s){
this.caption=s;
invalidate();
}
}
This is what I want to have:
private class BoxView extends View {
private String caption;
private OnClickListener bvClickListener = null
public BoxView(Context context) {
super(context);
this.bvClickListener = new this.OnClickListener(){
public void onClick (BoxView bv){
bv.setCaption("X");
}}
}
public void setCaption(String s){
this.caption=s;
invalidate();
}
}
I may need custom methods for my custom views. And I want to be able to pass my custom view instead of view version of it when onclick is triggered so I can access to it directly.
Updated
And I want to have access to real object not a converted one. So I want to avoid this:
public void onClick (View v){
((BoxView)v).setCaption("X");
}

Call setCaption method as in onClick :
public void onClick (View v){
((BoxView)v).setCaption("X");
}

Try this
class Main extents Activity
{
BoxView boxView;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// if view is used using layout then
boxView = (BoxView)findViewByID(id);
//else if directly used
boxView = new BoxView(this);
box.setOnClickListener(new onClickListener()
{
#Override
public void onClick(View view) {
boxView.setCaption("X");
boxView.invalidate();
}
});
}
}

Array of subclasses and onClick()

I want to create by code an array of objects that are subclasses of Button.
public class MyButton extends Button {
private Context ctx;
private int status;
public MyButton(Context context) {
super(context);
ctx = context;
status = 0;
}
private click() {
status = 1;
// OTHER CODE THAT NEEDS TO STAY HERE
}
}
In the main activity I do this:
public class myActivity extends Activity {
private MyButton[] myButtons = new MyButton[100];
#Override
public onCreate(Bundle si) {
super.onCreate(si);
createButtons();
}
private void createButtons() {
for (int w=0; w<100; w++) {
myButtons[w] = new MyButton(myActivity.this);
myButtons[w].setOnClickListener(new View.onClickListener() {
public void onClick(View v) {
// ... (A)
}
});
}
}
}
Now I want the click() method inside MyButton to be run each time the button is clicked.
Seems obvious but it is not at my eyes.
If I make the click() method public and run it directly from (A), I get an error because myButtons[w].click() is not static and cannot be run from there.
In the meantime, I an not able to understand where to put the code in the MyButton class to intercept a click and run click() from there. Should I override onClick? Or should I override onClickListener? Or what else should I do?
How can I run click() whenever one of myButtons[] object is clicked?
Thanks for the help.

You can cast View v you got in listener to MyButton and call click on it:
private void createButtons() {
View.OnClickListener listener = new View.onClickListener() {
public void onClick(View v) {
((MyButton) v).click();
}
};
for (int w=0; w<100; w++) {
myButtons[w] = new MyButton(myActivity.this);
myButtons[w].setOnClickListener(listener);
}
}

you can add:
View.onClickListener onclick = new View.onClickListener() {
public void onClick(View v) {
((MyButton)v).click();
//since v should be instance of MyButton
}
};
to your Activity
then use:
myButtons[w].setOnClickListener(onclick);
//one instance of onclick is enough, there is no need to create it for every button
in createButtons()
but ... why, oh why array of buttons we have ListView in android ...

Passing a reference of an Activity

Hi guys I can't figure out why I cant get it to work, to check the CheckBox of an activity from within another class.
In the onCreate method of the activity I'm passing a reference of itself to another class
public MainActivity()
...
dbi = new DBPrefsInterface(this);
...
}
public DBPrefsInterface(Context ctx)
{
MainActivity pma = (MainActivity)ctx;
this.ma = pma;
}
Now I try to Check a checkbox which is placed on the activity
this.ma.cbx.setChecked(true);
but it isn't working.
It seems I didn't pass a reference, only a Copy of MainActivity.
Thanks in adcance

Try passing a handler and a reference to the checkbox in the constructor of the other class
and make the handler send a message to the checkbox's original context
hanlder.post(new Runnable() {
#override
void run {
checkbox.setChecked(true);
}
});

Why don't you pass in the CheckBox itself? So your constructor becomes public DBPrefsInterface(CheckBox cbx) and you manipulate that reference.
Here is an example of a simple activity to demonstrate:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.checkbox);
CheckBox cbx = (CheckBox) findViewById(R.id.box);
Button button = (Button) findViewById(R.id.button);
final DBPrefsInterface iface = new DBPrefsInterface(cbx);
button.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View v) {
iface.toggle();
}
});
}
private class DBPrefsInterface {
CheckBox cbx = null;
public DBPrefsInterface(CheckBox cbx) {
this.cbx = cbx;
}
public void toggle() {
cbx.setChecked(cbx.isChecked());
}
}

I don't see why you would want to do this, but I think using the following constructor would do the work:
private MainActivity ma;
public DBPrefsInterface(MainActivity a){
ma = a;
}
Then to set the checkbox (but checkbox needs to be public in order to work):
ma.cbx.setChecked(true);