44. Events in ListView

In this lesson:

- we consider ListView events: pressing - onItemClick, selection - onItemSelect, scrolling - onScroll

When interacting with the list, it may be necessary to handle events - clicking on an item and scrolling. Let's try to do it.

Let's create   project :

Project name : P0441_SimpleListEvents
Build Target : Android 2.3.3
Application name : SimpleListEvents
Package name : ru.startandroid.develop.p0441simplelistevents
Create Activity : MainActivity

Let's draw the main.xml screen:

  <? xml version = "1.0" encoding = "utf-8"?> 
  <LinearLayout 
  xmlns: android = "http://schemas.android.com/apk/res/android" 
  android: layout_width = "fill_parent" 
  android: layout_height = "fill_parent" 
  android: orientation = "vertical"> 
  <ListView 
  android: id = "@ + id / lvMain" 
  android: layout_width = "match_parent" 
  android: layout_height = "wrap_content"> 
  </ Listview> 
  </ LinearLayout> 

On the screen only ListView .

Just like in the last lesson, add a list of names to the resource res / values ​​/ strings.xml :

  <? xml version = "1.0" encoding = "utf-8"?> 
  <resources> 
  <string name = "hello"> Hello World, MainActivity! </ string> 
  <string name = "app_name"> SimpleListEvents </ string> 
  <string-array name = "names"> 
  <item> Ivan </ item> 
  <item> Marya </ item> 
  <item> Peter </ item> 
  <item> Anton </ item> 
  <item> Dasha </ item> 
  <item> Boris </ item> 
  <item> Kostya </ item> 
  <item> Igor </ item> 
  <item> Anna </ item> 
  <item> Denis </ item> 
  <item> Vadim </ item> 
  <item> Olga </ item> 
  <item> Sergey </ item> 
  </ string-array> 
  </ resources> 

We write the code MainActivity.java :

We look code. We find the screen elements, create and assign an adapter to the list. Next, we assign two event handlers to the list:

1) OnItemClickListener - handles clicking on a list item

Provides us with the onItemClick method (AdapterView <?> Parent, View view, int position, long id), where

parent - View-parent for the clicked item, in our case - ListView
view is the clicked item, in our case the TextView of android.R.layout.simple_list_item_1
position - the ordinal number of the item in the list
id - the identifier of the element

We will log the id and position for the element we clicked on.

2) OnItemSelectedListener - handles the selection of list items (not check, as in the previous lesson)

Provides us with the onItemSelected method is completely similar in parameters to the onItemClick method described above. I will not repeat.

There is also a method onNothingSelected - when the list loses the selection of an item and no item is selected.

All save and run the application.

We tap some element, for example - Peter. We look at the log:

itemClick: position = 2, id = 2

That's right. Because position is not counted from one, but from zero - Peter has position 2. (In our case, id is equal to position. I haven’t seen any id! = position, but surely they are)

Now spin the mouse wheel or press the keys up and down on the keyboard. It is visible that there is a visual selection of elements of the list.

And in the logs we see such records:

itemSelect: position = 2, id = 2
itemSelect: position = 3, id = 3
itemSelect: position = 4, id = 4
itemSelect: position = 5, id = 5
itemSelect: position = 4, id = 4
itemSelect: position = 3, id = 3
itemSelect: position = 2, id = 2

Those. handler fixes which item is selected . Frankly, I do not really understand how you can use this selection. But there is a handler for it and I decided to tell about it. Let it be.

Now click again on any item in the list, we see that the selection is gone. Logs:

itemSelect: nothing
itemClick: position = 3, id = 3

Nothing is selected and the item with position 3 is pressed.

Let's add another handler to the list:

OnScrollListener - handles list scrolling.

Methods:

1) onScrollStateChanged (AbsListView view, int scrollState) - processing scroll states

view is a scrollable item, i.e. Listview
scrollState - list state. It can take three values:

SCROLL_STATE_IDLE = 0, the list has finished scrolling
SCROLL_STATE_TOUCH_SCROLL = 1, the list started scrolling
SCROLL_STATE_FLING = 2, the list is “rolled”, i.e. when scrolling released a finger and scrolling goes further "by inertia"

I logged the log output so that it does not interfere. A little later repent.

2) onScroll (AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) - scroll processing

view - scrollable item
firstVisibleItem - the first visible item on the screen
visibleItemCount - how many points are visible on the screen
totalItemCount - how many items in the list

Moreover, for the firstVisibleItem and visibleItemCount parameters, the item is considered to be visible on the screen even if it is not fully visible.

All save and run.

Now drag the list back and forth with the cursor (as if with a finger) and see the logs. There are too many things displayed. I will not lay out here. But the principle is clear - the first visible item changes (firstVisibleItem) and the number of visible items (visibleItemCount) can change by one.

Now we comment out the output to the log in the onScroll method (so as not to spam us the log) and replicate in onScrollStateChanged .

Save, run.

We grab the list, pull it back a little and let it go. We look at the log:

scrollState = 1
scrollState = 0

Two events worked out - the list started scrolling, the list finished scrolling.

Let's try to take a list, “roll” it and release.

scrollState = 1
scrollState = 2
scrollState = 0

We see three events - the scrolling has begun , the list is “ knocked out ”, the scrolling is over .

Full lesson code:

In the next lesson:

- build an ExpandableListView tree list