In this exercise, you will add a second Activity to your notepad application, to let the user create and edit notes. You will also allow the user to delete existing notes through a context menu. The new Activity assumes responsibility for creating new notes by collecting user input and packing it into a return Bundle provided by the intent. This exercise demonstrates:
- Constructing a new Activity and adding it to the Android manifest
- Invoking another Activity asynchronously using
startActivityForResult()
- Passing data between Activity in Bundle objects
- How to use a more advanced screen layout
- How to create a context menu
Step 1
Create a new Android project using the sources from Notepadv2
under the
NotepadCodeLab
folder, just like you did for the first exercise. If you see an error about
AndroidManifest.xml
, or some problems related to an
android.zip
file, right click on the project and select Android
Tools > Fix Project Properties.
Open the Notepadv2
project and take a look around:
-
Open and look at the
strings.xml
file underres/values
— there are several new strings which we will use for our new functionality -
Also, open and take a look at the top of the
Notepadv2
class, you will notice several new constants have been defined along with a newmNotesCursor
field used to hold the cursor we are using. -
Note also that the
fillData()
method has a few more comments and now uses the new field to store the notes Cursor. TheonCreate()
method is unchanged from the first exercise. Also notice that the member field used to store the notes Cursor is now calledmNotesCursor
. Them
denotes a member field and is part of the Android coding style standards. -
There are also a couple of new overridden methods
(
onCreateContextMenu()
,onContextItemSelected()
,onListItemClick()
andonActivityResult()
) which we will be filling in below.
Step 2
Context menus should always be used when performing actions upon specific elements in the UI. When you register a View to a context menu, the context menu is revealed by performing a "long-click" on the UI component (press and hold the touchscreen or highlight and hold down the selection key for about two seconds).
First, let's create the context menu that will allow users to delete individual notes. Open the Notepadv2 class.
- In order for each list item in the ListView to register for the context menu, we call
registerForContextMenu()
and pass it our ListView. So, at the very end of theonCreate()
method add this line:registerForContextMenu(getListView());
Because our Activity extends the ListActivity class,
getListView()
will return us the local ListView object for the Activity. Now, each list item in this ListView will activate the context menu. -
Now fill in the
onCreateContextMenu()
method. This callback is similar to the other menu callback used for the options menu. Here, we add just one line, which will add a menu item to delete a note. Callmenu.add()
like so:public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) { super.onCreateContextMenu(menu, v, menuInfo); menu.add(0, DELETE_ID, 0, R.string.menu_delete); }
The
onCreateContextMenu()
callback passes some other information in addition to the Menu object, such as the View that has been triggered for the menu and an extra object that may contain additional information about the object selected. However, we don't care about these here, because we only have one kind of object in the Activity that uses context menus. In the next step, we'll handle the menu item selection.
Step 3
Now that we've registered our ListView for a context menu and defined our context menu item, we need
to handle the callback when it is selected. For this, we need to identify the list ID of the
selected item, then delete it. So fill in the
onContextItemSelected()
method like this:
public boolean onContextItemSelected(MenuItem item) { switch(item.getItemId()) { case DELETE_ID: AdapterContextMenuInfo info = (AdapterContextMenuInfo) item.getMenuInfo(); mDbHelper.deleteNote(info.id); fillData(); return true; } return super.onContextItemSelected(item); }
Here, we retrieve the AdapterContextMenuInfo
with getMenuInfo()
. The id field of this object tells us
the position of the item in the ListView. We then pass this to the deleteNote()
method of our NotesDbAdapter and the note is deleted. That's it for the context menu — notes
can now be deleted.
Step 4
Starting Other Activities
In this example our Intent uses a class name specifically. As well as starting intents in classes we already know about, be they in our own application or another application, we can also create Intents without knowing exactly which application will handle it.
For example, we might want to open a page in a
browser, and for this we still use
an Intent. But instead of specifying a class to handle it, we use
a predefined Intent constant, and a content URI that describes what we
want to do. See android.content.Intent
for more information.
Fill in the body of the createNote()
method:
Create a new Intent
to create a note
(ACTIVITY_CREATE
) using the NoteEdit
class.
Then fire the Intent using the startActivityForResult()
method
call:
Intent i = new Intent(this, NoteEdit.class); startActivityForResult(i, ACTIVITY_CREATE);
This form of the Intent call targets a specific class in our Activity, in this case
NoteEdit
. Since the Intent class will need to communicate with the Android
operating system to route requests, we also have to provide a Context (this
).
The startActivityForResult()
method fires the Intent in a way that causes a method
in our Activity to be called when the new Activity is completed. The method in our Activity
that receives the callback is called
onActivityResult()
and we will implement it in a later step. The other way
to call an Activity is using startActivity()
but this is a "fire-and-forget" way
of calling it — in this manner, our Activity is not informed when the Activity is completed, and there is
no way to return result information from the called Activity with startActivity()
.
Don't worry about the fact that NoteEdit
doesn't exist yet,
we will fix that soon.
Step 5
Fill in the body of the onListItemClick()
override.
onListItemClick()
is a callback method that we'll override. It is called when
the user selects an item from the list. It is passed four parameters: the
ListView
object it was invoked from, the View
inside the ListView
that was clicked on, the
position
in the list that was clicked, and the
mRowId
of the item that was clicked. In this instance we can
ignore the first two parameters (we only have one ListView
it
could be), and we ignore the mRowId
as well. All we are
interested in is the position
that the user selected. We use
this to get the data from the correct row, and bundle it up to send to
the NoteEdit
Activity.
In our implementation of the callback, the method creates an
Intent
to edit the note using
the NoteEdit
class. It then adds data into the extras Bundle of
the Intent, which will be passed to the called Activity. We use it
to pass in the title and body text, and the mRowId
for the note we are
editing. Finally, it will fire the Intent using the
startActivityForResult()
method call. Here's the code that
belongs in onListItemClick()
:
super.onListItemClick(l, v, position, id); Cursor c = mNotesCursor; c.moveToPosition(position); Intent i = new Intent(this, NoteEdit.class); i.putExtra(NotesDbAdapter.KEY_ROWID, id); i.putExtra(NotesDbAdapter.KEY_TITLE, c.getString( c.getColumnIndexOrThrow(NotesDbAdapter.KEY_TITLE))); i.putExtra(NotesDbAdapter.KEY_BODY, c.getString( c.getColumnIndexOrThrow(NotesDbAdapter.KEY_BODY))); startActivityForResult(i, ACTIVITY_EDIT);
-
putExtra()
is the method to add items into the extras Bundle to pass in to intent invocations. Here, we are using the Bundle to pass in the title, body and mRowId of the note we want to edit. -
The details of the note are pulled out from our query Cursor, which we move to the
proper position for the element that was selected in the list, with
the
moveToPosition()
method. - With the extras added to the Intent, we invoke the Intent on the
NoteEdit
class by passingstartActivityForResult()
the Intent and the request code. (The request code will be returned toonActivityResult
as therequestCode
parameter.)
Note: We assign the mNotesCursor field to a local variable at the start of the method. This is done as an optimization of the Android code. Accessing a local variable is much more efficient than accessing a field in the Dalvik VM, so by doing this we make only one access to the field, and five accesses to the local variable, making the routine much more efficient. It is recommended that you use this optimization when possible.
Step 6
The above createNote()
and onListItemClick()
methods use an asynchronous Intent invocation. We need a handler for the callback, so here we fill
in the body of the onActivityResult()
.
onActivityResult()
is the overridden method
which will be called when an Activity returns with a result. (Remember, an Activity
will only return a result if launched with startActivityForResult
.) The parameters provided
to the callback are:
requestCode
— the original request code specified in the Intent invocation (eitherACTIVITY_CREATE
orACTIVITY_EDIT
for us).resultCode
— the result (or error code) of the call, this should be zero if everything was OK, but may have a non-zero code indicating that something failed. There are standard result codes available, and you can also create your own constants to indicate specific problems.intent
— this is an Intent created by the Activity returning results. It can be used to return data in the Intent "extras."
The combination of startActivityForResult()
and
onActivityResult()
can be thought of as an asynchronous RPC
(remote procedure call) and forms the recommended way for an Activity to invoke
another and share services.
Here's the code that belongs in your onActivityResult()
:
super.onActivityResult(requestCode, resultCode, intent); Bundle extras = intent.getExtras(); switch(requestCode) { case ACTIVITY_CREATE: String title = extras.getString(NotesDbAdapter.KEY_TITLE); String body = extras.getString(NotesDbAdapter.KEY_BODY); mDbHelper.createNote(title, body); fillData(); break; case ACTIVITY_EDIT: Long mRowId = extras.getLong(NotesDbAdapter.KEY_ROWID); if (mRowId != null) { String editTitle = extras.getString(NotesDbAdapter.KEY_TITLE); String editBody = extras.getString(NotesDbAdapter.KEY_BODY); mDbHelper.updateNote(mRowId, editTitle, editBody); } fillData(); break; }
-
We are handling both the
ACTIVITY_CREATE
andACTIVITY_EDIT
activity results in this method. - In the case of a create, we pull the title and body from the extras (retrieved from the returned Intent) and use them to create a new note.
- In the case of an edit, we pull the mRowId as well, and use that to update the note in the database.
-
fillData()
at the end ensures everything is up to date .
Step 7
The Art of Layout
The provided note_edit.xml layout file is the most sophisticated one in the application we will be building, but that doesn't mean it is even close to the kind of sophistication you will be likely to want in real Android applications.
Creating a good UI is part art and part science, and the rest is work. Mastery of XML Layouts is an essential part of creating a good looking Android application.
Take a look at the Hello Views for some example layouts and how to use them. The ApiDemos sample project is also a great resource from which to learn how to create different layouts.
Open the file note_edit.xml
that has been provided and take a
look at it. This is the UI code for the Note Editor.
This is the most sophisticated UI we have dealt with yet. The file is given to you to avoid problems that may sneak in when typing the code. (The XML is very strict about case sensitivity and structure, mistakes in these are the usual cause of problems with layout.)
There is a new parameter used
here that we haven't seen before: android:layout_weight
(in
this case set to use the value 1 in each case).
layout_weight
is used in LinearLayouts
to assign "importance" to Views within the layout. All Views have a default
layout_weight
of zero, meaning they take up only as much room
on the screen as they need to be displayed. Assigning a value higher than
zero will split up the rest of the available space in the parent View, according
to the value of each View's layout_weight
and its ratio to the
overall layout_weight
specified in the current layout for this
and other View elements.
To give an example: let's say we have a text label
and two text edit elements in a horizontal row. The label has no
layout_weight
specified, so it takes up the minimum space
required to render. If the layout_weight
of each of the two
text edit elements is set to 1, the remaining width in the parent layout will
be split equally between them (because we claim they are equally important).
If the first one has a layout_weight
of 1
and the second has a layout_weight
of 2, then one third of the
remaining space will be given to the first, and two thirds to the
second (because we claim the second one is more important).
This layout also demonstrates how to nest multiple layouts inside each other to achieve a more complex and pleasant layout. In this example, a horizontal linear layout is nested inside the vertical one to allow the title label and text field to be alongside each other, horizontally.
Step 8
Create a NoteEdit
class that extends
android.app.Activity
.
This is the first time we will have
created an Activity without the Android Eclipse plugin doing it for us. When
you do so, the onCreate()
method is not automatically
overridden for you. It is hard to imagine an Activity that doesn't override
the onCreate()
method, so this should be the first thing you do.
- Right click on the
com.android.demo.notepad2
package in the Package Explorer, and select New > Class from the popup menu. - Fill in
NoteEdit
for theName:
field in the dialog. - In the
Superclass:
field, enterandroid.app.Activity
(you can also just type Activity and hit Ctrl-Space on Windows and Linux or Cmd-Space on the Mac, to invoke code assist and find the right package and class). - Click Finish.
- In the resulting
NoteEdit
class, right click in the editor window and select Source > Override/Implement Methods... - Scroll down through the checklist in the dialog until you see
onCreate(Bundle)
— and check the box next to it. - Click OK.
The method should now appear in your class.
Step 9
Fill in the body of the onCreate()
method for NoteEdit
.
This will set the title of our new Activity to say "Edit Note" (one
of the strings defined in strings.xml
). It will also set the
content view to use our note_edit.xml
layout file. We can then
grab handles to the title and body text edit views, and the confirm button,
so that our class can use them to set and get the note title and body,
and attach an event to the confirm button for when it is pressed by the
user.
We can then unbundle the values that were passed in to the Activity
with the extras Bundle attached to the calling Intent. We'll use them to pre-populate
the title and body text edit views so that the user can edit them.
Then we will grab and store the mRowId
so we can keep
track of what note the user is editing.
-
Inside
onCreate()
, set up the layout:
setContentView(R.layout.note_edit);
-
Change the Activity title to the "Edit Note" string:
setTitle(R.string.edit_note);
-
Find the
EditText
andButton
components we need:These are found by the IDs associated to them in the R class, and need to be cast to the right type of
View
(EditText
for the two text views, andButton
for the confirm button):mTitleText = (EditText) findViewById(R.id.title); mBodyText = (EditText) findViewById(R.id.body); Button confirmButton = (Button) findViewById(R.id.confirm);
Note that
mTitleText
andmBodyText
are member fields (you need to declare them at the top of the class definition). - At the top of the class, declare a
Long mRowId
private field to store the currentmRowId
being edited (if any). - Continuing inside
onCreate()
, add code to initialize thetitle
,body
andmRowId
from the extras Bundle in the Intent (if it is present):
mRowId = null; Bundle extras = getIntent().getExtras(); if (extras != null) { String title = extras.getString(NotesDbAdapter.KEY_TITLE); String body = extras.getString(NotesDbAdapter.KEY_BODY); mRowId = extras.getLong(NotesDbAdapter.KEY_ROWID); if (title != null) { mTitleText.setText(title); } if (body != null) { mBodyText.setText(body); } }
-
We are pulling the
title
andbody
out of theextras
Bundle that was set from the Intent invocation. - We also null-protect the text field setting (i.e., we don't want to set the text fields to null accidentally).
-
We are pulling the
-
Create an
onClickListener()
for the button:Listeners can be one of the more confusing aspects of UI implementation, but what we are trying to achieve in this case is simple. We want an
onClick()
method to be called when the user presses the confirm button, and use that to do some work and return the values of the edited note to the Intent caller. We do this using something called an anonymous inner class. This is a bit confusing to look at unless you have seen them before, but all you really need to take away from this is that you can refer to this code in the future to see how to create a listener and attach it to a button. (Listeners are a common idiom in Java development, particularly for user interfaces.) Here's the empty listener:
confirmButton.setOnClickListener(new View.OnClickListener() { public void onClick(View view) { } });
Step 10
Fill in the body of the onClick()
method of the OnClickListener
created in the last step.
This is the code that will be run when the user clicks on the
confirm button. We want this to grab the title and body text from the edit
text fields, and put them into the return Bundle so that they can be passed
back to the Activity that invoked this NoteEdit
Activity. If the
operation is an edit rather than a create, we also want to put the
mRowId
into the Bundle so that the
Notepadv2
class can save the changes back to the correct
note.
-
Create a
Bundle
and put the title and body text into it using the constants defined in Notepadv2 as keys:
Bundle bundle = new Bundle(); bundle.putString(NotesDbAdapter.KEY_TITLE, mTitleText.getText().toString()); bundle.putString(NotesDbAdapter.KEY_BODY, mBodyText.getText().toString()); if (mRowId != null) { bundle.putLong(NotesDbAdapter.KEY_ROWID, mRowId); }
-
Set the result information (the Bundle) in a new Intent and finish the Activity:
Intent mIntent = new Intent(); mIntent.putExtras(bundle); setResult(RESULT_OK, mIntent); finish();
- The Intent is simply our data carrier that carries our Bundle (with the title, body and mRowId).
- The
setResult()
method is used to set the result code and return Intent to be passed back to the Intent caller. In this case everything worked, so we return RESULT_OK for the result code. - The
finish()
call is used to signal that the Activity is done (like a return call). Anything set in the Result will then be returned to the caller, along with execution control.
The full onCreate()
method (plus supporting class fields) should
now look like this:
private EditText mTitleText; private EditText mBodyText; private Long mRowId; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.note_edit); mTitleText = (EditText) findViewById(R.id.title); mBodyText = (EditText) findViewById(R.id.body); Button confirmButton = (Button) findViewById(R.id.confirm); mRowId = null; Bundle extras = getIntent().getExtras(); if (extras != null) { String title = extras.getString(NotesDbAdapter.KEY_TITLE); String body = extras.getString(NotesDbAdapter.KEY_BODY); mRowId = extras.getLong(NotesDbAdapter.KEY_ROWID); if (title != null) { mTitleText.setText(title); } if (body != null) { mBodyText.setText(body); } } confirmButton.setOnClickListener(new View.OnClickListener() { public void onClick(View view) { Bundle bundle = new Bundle(); bundle.putString(NotesDbAdapter.KEY_TITLE, mTitleText.getText().toString()); bundle.putString(NotesDbAdapter.KEY_BODY, mBodyText.getText().toString()); if (mRowId != null) { bundle.putLong(NotesDbAdapter.KEY_ROWID, mRowId); } Intent mIntent = new Intent(); mIntent.putExtras(bundle); setResult(RESULT_OK, mIntent); finish(); } }); }
Step 11
The All-Important Android Manifest File
The AndroidManifest.xml file is the way in which Android sees your application. This file defines the category of the application, where it shows up (or even if it shows up) in the launcher or settings, what activities, services, and content providers it defines, what intents it can receive, and more.
For more information, see the reference document The AndroidManifest.xml File
Finally, the new Activity has to be defined in the manifest file:
Before the new Activity can be seen by Android, it needs its own
Activity entry in the AndroidManifest.xml
file. This is to let
the system know that it is there and can be called. We could also specify
which IntentFilters the activity implements here, but we are going to skip
this for now and just let Android know that the Activity is
defined.
There is a Manifest editor included in the Eclipse plugin that makes it much easier to edit the AndroidManifest file, and we will use this. If you prefer to edit the file directly or are not using the Eclipse plugin, see the box at the end for information on how to do this without using the new Manifest editor.
- Double click on the
AndroidManifest.xml
file in the package explorer to open it. - Click the Application tab at the bottom of the Manifest editor.
- Click Add... in the Application Nodes section.
If you see a dialog with radiobuttons at the top, select the top radio button: "Create a new element at the top level, in Application".
- Make sure "(A) Activity" is selected in the selection pane of the dialog, and click OK.
- Click on the new "Activity" node, in the Application Nodes section, then
type
.NoteEdit
into the Name* field to the right. Press Return/Enter.
The Android Manifest editor helps you add more complex entries into the AndroidManifest.xml file, have a look around at some of the other options available (but be careful not to select them otherwise they will be added to your Manifest). This editor should help you understand and alter the AndroidManifest.xml file as you move on to more advanced Android applications.
If you prefer to edit this file directly, simply open the
AndroidManifest.xml
file and look at the source (use the
AndroidManifest.xml
tab in the eclipse editor to see the source code directly).
Then edit the file as follows:
<activity android:name=".NoteEdit" />
This should be placed just below the line that reads:
</activity>
for the .Notepadv2
activity.
Step 12
Now Run it!
You should now be able to add real notes from the menu, as well as delete an existing one. Notice that in order to delete, you must first use the directional controls on the device to highlight the note. Furthermore, selecting a note title from the list should bring up the note editor to let you edit it. Press confirm when finished to save the changes back to the database.
Solution and Next Steps
You can see the solution to this exercise in Notepadv2Solution
from the zip file to compare with your own.
Now try editing a note, and then hitting the back button on the emulator instead of the confirm button (the back button is below the menu button). You will see an error come up. Clearly our application still has some problems. Worse still, if you did make some changes and hit the back button, when you go back into the notepad to look at the note you changed, you will find that all your changes have been lost. In the next exercise we will fix these problems.
Once you are ready, move on to Tutorial Exercise 3 where you will fix the problems with the back button and lost edits by introducing a proper life cycle into the NoteEdit Activity.