1 page.title=Input Events 2 parent.title=User Interface 3 parent.link=index.html 4 @jd:body 5 6 <div id="qv-wrapper"> 7 <div id="qv"> 8 <h2>In this document</h2> 9 <ol> 10 <li><a href="#EventListeners">Event Listeners</a></li> 11 <li><a href="#EventHandlers">Event Handlers</a></li> 12 <li><a href="#TouchMode">Touch Mode</a></li> 13 <li><a href="#HandlingFocus">Handling Focus</a></li> 14 </ol> 15 16 </div> 17 </div> 18 19 <p>On Android, there's more than one way to intercept the events from a user's interaction with your application. 20 When considering events within your user interface, the approach is to capture the events from 21 the specific View object that the user interacts with. The View class provides the means to do so.</p> 22 23 <p>Within the various View classes that you'll use to compose your layout, you may notice several public callback 24 methods that look useful for UI events. These methods are called by the Android framework when the 25 respective action occurs on that object. For instance, when a View (such as a Button) is touched, 26 the <code>onTouchEvent()</code> method is called on that object. However, in order to intercept this, you must extend 27 the class and override the method. However, extending every View object 28 in order to handle such an event would not be practical. This is why the View class also contains 29 a collection of nested interfaces with callbacks that you can much more easily define. These interfaces, 30 called <a href="#EventListeners">event listeners</a>, are your ticket to capturing the user interaction with your UI.</p> 31 32 <p>While you will more commonly use the event listeners to listen for user interaction, there may 33 come a time when you do want to extend a View class, in order to build a custom component. 34 Perhaps you want to extend the {@link android.widget.Button} 35 class to make something more fancy. In this case, you'll be able to define the default event behaviors for your 36 class using the class <a href="#EventHandlers">event handlers</a>.</p> 37 38 39 <h2 id="EventListeners">Event Listeners</h2> 40 41 <p>An event listener is an interface in the {@link android.view.View} class that contains a single 42 callback method. These methods will be called by the Android framework when the View to which the listener has 43 been registered is triggered by user interaction with the item in the UI.</p> 44 45 <p>Included in the event listener interfaces are the following callback methods:</p> 46 47 <dl> 48 <dt><code>onClick()</code></dt> 49 <dd>From {@link android.view.View.OnClickListener}. 50 This is called when the user either touches the item 51 (when in touch mode), or focuses upon the item with the navigation-keys or trackball and 52 presses the suitable "enter" key or presses down on the trackball.</dd> 53 <dt><code>onLongClick()</code></dt> 54 <dd>From {@link android.view.View.OnLongClickListener}. 55 This is called when the user either touches and holds the item (when in touch mode), or 56 focuses upon the item with the navigation-keys or trackball and 57 presses and holds the suitable "enter" key or presses and holds down on the trackball (for one second).</dd> 58 <dt><code>onFocusChange()</code></dt> 59 <dd>From {@link android.view.View.OnFocusChangeListener}. 60 This is called when the user navigates onto or away from the item, using the navigation-keys or trackball.</dd> 61 <dt><code>onKey()</code></dt> 62 <dd>From {@link android.view.View.OnKeyListener}. 63 This is called when the user is focused on the item and presses or releases a hardware key on the device.</dd> 64 <dt><code>onTouch()</code></dt> 65 <dd>From {@link android.view.View.OnTouchListener}. 66 This is called when the user performs an action qualified as a touch event, including a press, a release, 67 or any movement gesture on the screen (within the bounds of the item).</dd> 68 <dt><code>onCreateContextMenu()</code></dt> 69 <dd>From {@link android.view.View.OnCreateContextMenuListener}. 70 This is called when a Context Menu is being built (as the result of a sustained "long click"). See the discussion 71 on context menus in the <a href="{@docRoot}guide/topics/ui/menus.html#context-menu">Menus</a> 72 developer guide.</dd> 73 </dl> 74 75 <p>These methods are the sole inhabitants of their respective interface. To define one of these methods 76 and handle your events, implement the nested interface in your Activity or define it as an anonymous class. 77 Then, pass an instance of your implementation 78 to the respective <code>View.set...Listener()</code> method. (E.g., call 79 <code>{@link android.view.View#setOnClickListener(View.OnClickListener) setOnClickListener()}</code> 80 and pass it your implementation of the {@link android.view.View.OnClickListener OnClickListener}.)</p> 81 82 <p>The example below shows how to register an on-click listener for a Button. </p> 83 84 <pre> 85 // Create an anonymous implementation of OnClickListener 86 private OnClickListener mCorkyListener = new OnClickListener() { 87 public void onClick(View v) { 88 // do something when the button is clicked 89 } 90 }; 91 92 protected void onCreate(Bundle savedValues) { 93 ... 94 // Capture our button from layout 95 Button button = (Button)findViewById(R.id.corky); 96 // Register the onClick listener with the implementation above 97 button.setOnClickListener(mCorkyListener); 98 ... 99 } 100 </pre> 101 102 <p>You may also find it more convenient to implement OnClickListener as a part of your Activity. 103 This will avoid the extra class load and object allocation. For example:</p> 104 <pre> 105 public class ExampleActivity extends Activity implements OnClickListener { 106 protected void onCreate(Bundle savedValues) { 107 ... 108 Button button = (Button)findViewById(R.id.corky); 109 button.setOnClickListener(this); 110 } 111 112 // Implement the OnClickListener callback 113 public void onClick(View v) { 114 // do something when the button is clicked 115 } 116 ... 117 } 118 </pre> 119 120 <p>Notice that the <code>onClick()</code> callback in the above example has 121 no return value, but some other event listener methods must return a boolean. The reason 122 depends on the event. For the few that do, here's why:</p> 123 <ul> 124 <li><code>{@link android.view.View.OnLongClickListener#onLongClick(View) onLongClick()}</code> - 125 This returns a boolean to indicate whether you have consumed the event and it should not be carried further. 126 That is, return <em>true</em> to indicate that you have handled the event and it should stop here; 127 return <em>false</em> if you have not handled it and/or the event should continue to any other 128 on-click listeners.</li> 129 <li><code>{@link android.view.View.OnKeyListener#onKey(View,int,KeyEvent) onKey()}</code> - 130 This returns a boolean to indicate whether you have consumed the event and it should not be carried further. 131 That is, return <em>true</em> to indicate that you have handled the event and it should stop here; 132 return <em>false</em> if you have not handled it and/or the event should continue to any other 133 on-key listeners.</li> 134 <li><code>{@link android.view.View.OnTouchListener#onTouch(View,MotionEvent) onTouch()}</code> - 135 This returns a boolean to indicate whether your listener consumes this event. The important thing is that 136 this event can have multiple actions that follow each other. So, if you return <em>false</em> when the 137 down action event is received, you indicate that you have not consumed the event and are also 138 not interested in subsequent actions from this event. Thus, you will not be called for any other actions 139 within the event, such as a finger gesture, or the eventual up action event.</li> 140 </ul> 141 142 <p>Remember that hardware key events are always delivered to the View currently in focus. They are dispatched starting from the top 143 of the View hierarchy, and then down, until they reach the appropriate destination. If your View (or a child of your View) 144 currently has focus, then you can see the event travel through the <code>{@link android.view.View#dispatchKeyEvent(KeyEvent) 145 dispatchKeyEvent()}</code> method. As an alternative to capturing key events through your View, you can also receive 146 all of the events inside your Activity with <code>{@link android.app.Activity#onKeyDown(int,KeyEvent) onKeyDown()}</code> 147 and <code>{@link android.app.Activity#onKeyUp(int,KeyEvent) onKeyUp()}</code>.</p> 148 149 <p>Also, when thinking about text input for your application, remember that many devices only have software input 150 methods. Such methods are not required to be key-based; some may use voice input, handwriting, and so on. Even if 151 an input method presents a keyboard-like interface, it will generally <strong>not</strong> trigger the 152 <code>{@link android.app.Activity#onKeyDown(int,KeyEvent) onKeyDown()}</code> family of events. You should never 153 build a UI that requires specific key presses to be controlled unless you want to limit your application to devices 154 with a hardware keyboard. In particular, do not rely on these methods to validate input when the user presses the 155 return key; instead, use actions like {@link android.view.inputmethod.EditorInfo#IME_ACTION_DONE} to signal the 156 input method how your application expects to react, so it may change its UI in a meaningful way. Avoid assumptions 157 about how a software input method should work and just trust it to supply already formatted text to your application.</p> 158 159 <p class="note"><strong>Note:</strong> Android will call event handlers first and then the appropriate default 160 handlers from the class definition second. As such, returning <em>true</em> from these event listeners will stop 161 the propagation of the event to other event listeners and will also block the callback to the 162 default event handler in the View. So be certain that you want to terminate the event when you return <em>true</em>.</p> 163 164 165 <h2 id="EventHandlers">Event Handlers</h2> 166 167 <p>If you're building a custom component from View, then you'll be able to define several callback methods 168 used as default event handlers. 169 In the document about <a href="{@docRoot}guide/topics/ui/custom-components.html">Custom 170 Components</a>, you'll learn see some of the common callbacks used for event handling, 171 including:</p> 172 <ul> 173 <li><code>{@link android.view.View#onKeyDown}</code> - Called when a new key event occurs.</li> 174 <li><code>{@link android.view.View#onKeyUp}</code> - Called when a key up event occurs.</li> 175 <li><code>{@link android.view.View#onTrackballEvent}</code> - Called when a trackball motion event occurs.</li> 176 <li><code>{@link android.view.View#onTouchEvent}</code> - Called when a touch screen motion event occurs.</li> 177 <li><code>{@link android.view.View#onFocusChanged}</code> - Called when the view gains or loses focus.</li> 178 </ul> 179 <p>There are some other methods that you should be aware of, which are not part of the View class, 180 but can directly impact the way you're able to handle events. So, when managing more complex events inside 181 a layout, consider these other methods:</p> 182 <ul> 183 <li><code>{@link android.app.Activity#dispatchTouchEvent(MotionEvent) 184 Activity.dispatchTouchEvent(MotionEvent)}</code> - This allows your {@link 185 android.app.Activity} to intercept all touch events before they are dispatched to the window.</li> 186 <li><code>{@link android.view.ViewGroup#onInterceptTouchEvent(MotionEvent) 187 ViewGroup.onInterceptTouchEvent(MotionEvent)}</code> - This allows a {@link 188 android.view.ViewGroup} to watch events as they are dispatched to child Views.</li> 189 <li><code>{@link android.view.ViewParent#requestDisallowInterceptTouchEvent(boolean) 190 ViewParent.requestDisallowInterceptTouchEvent(boolean)}</code> - Call this 191 upon a parent View to indicate that it should not intercept touch events with <code>{@link 192 android.view.ViewGroup#onInterceptTouchEvent(MotionEvent)}</code>.</li> 193 </ul> 194 195 <h2 id="TouchMode">Touch Mode</h2> 196 <p> 197 When a user is navigating a user interface with directional keys or a trackball, it is 198 necessary to give focus to actionable items (like buttons) so the user can see 199 what will accept input. If the device has touch capabilities, however, and the user 200 begins interacting with the interface by touching it, then it is no longer necessary to 201 highlight items, or give focus to a particular View. Thus, there is a mode 202 for interaction named "touch mode." 203 </p> 204 <p> 205 For a touch-capable device, once the user touches the screen, the device 206 will enter touch mode. From this point onward, only Views for which 207 {@link android.view.View#isFocusableInTouchMode} is true will be focusable, such as text editing widgets. 208 Other Views that are touchable, like buttons, will not take focus when touched; they will 209 simply fire their on-click listeners when pressed. 210 </p> 211 <p> 212 Any time a user hits a directional key or scrolls with a trackball, the device will 213 exit touch mode, and find a view to take focus. Now, the user may resume interacting 214 with the user interface without touching the screen. 215 </p> 216 <p> 217 The touch mode state is maintained throughout the entire system (all windows and activities). 218 To query the current state, you can call 219 {@link android.view.View#isInTouchMode} to see whether the device is currently in touch mode. 220 </p> 221 222 223 <h2 id="HandlingFocus">Handling Focus</h2> 224 225 <p>The framework will handle routine focus movement in response to user input. 226 This includes changing the focus as Views are removed or hidden, or as new 227 Views become available. Views indicate their willingness to take focus 228 through the <code>{@link android.view.View#isFocusable()}</code> method. To change whether a View can take 229 focus, call <code>{@link android.view.View#setFocusable(boolean) setFocusable()}</code>. When in touch mode, 230 you may query whether a View allows focus with <code>{@link android.view.View#isFocusableInTouchMode()}</code>. 231 You can change this with <code>{@link android.view.View#setFocusableInTouchMode(boolean) setFocusableInTouchMode()}</code>. 232 </p> 233 234 <p>Focus movement is based on an algorithm which finds the nearest neighbor in a 235 given direction. In rare cases, the default algorithm may not match the 236 intended behavior of the developer. In these situations, you can provide 237 explicit overrides with the following XML attributes in the layout file: 238 <var>nextFocusDown</var>, <var>nextFocusLeft</var>, <var>nextFocusRight</var>, and 239 <var>nextFocusUp</var>. Add one of these attributes to the View <em>from</em> which 240 the focus is leaving. Define the value of the attribute to be the id of the View 241 <em>to</em> which focus should be given. For example:</p> 242 <pre> 243 <LinearLayout 244 android:orientation="vertical" 245 ... > 246 <Button android:id="@+id/top" 247 android:nextFocusUp="@+id/bottom" 248 ... /> 249 <Button android:id="@+id/bottom" 250 android:nextFocusDown="@+id/top" 251 ... /> 252 </LinearLayout> 253 </pre> 254 255 <p>Ordinarily, in this vertical layout, navigating up from the first Button would not go 256 anywhere, nor would navigating down from the second Button. Now that the top Button has 257 defined the bottom one as the <var>nextFocusUp</var> (and vice versa), the navigation focus will 258 cycle from top-to-bottom and bottom-to-top.</p> 259 260 <p>If you'd like to declare a View as focusable in your UI (when it is traditionally not), 261 add the <code>android:focusable</code> XML attribute to the View, in your layout declaration. 262 Set the value <var>true</var>. You can also declare a View 263 as focusable while in Touch Mode with <code>android:focusableInTouchMode</code>.</p> 264 <p>To request a particular View to take focus, call <code>{@link android.view.View#requestFocus()}</code>.</p> 265 <p>To listen for focus events (be notified when a View receives or looses focus), use 266 <code>{@link android.view.View.OnFocusChangeListener#onFocusChange(View,boolean) onFocusChange()}</code>, 267 as discussed in the <a href="#EventListeners">Event Listeners</a> section, above.</p> 268 269 270 271 <!-- 272 <h2 is="EventCycle">Event Cycle</h2> 273 <p>The basic cycle of a View is as follows:</p> 274 <ol> 275 <li>An event comes in and is dispatched to the appropriate View. The View 276 handles the event and notifies any listeners.</li> 277 <li>If, in the course of processing the event, the View's bounds may need 278 to be changed, the View will call {@link android.view.View#requestLayout()}.</li> 279 <li>Similarly, if in the course of processing the event the View's appearance 280 may need to be changed, the View will call {@link android.view.View#invalidate()}.</li> 281 <li>If either {@link android.view.View#requestLayout()} or {@link android.view.View#invalidate()} were called, 282 the framework will take care of measuring, laying out, and drawing the tree 283 as appropriate.</li> 284 </ol> 285 286 <p class="note"><strong>Note:</strong> The entire View tree is single threaded. You must always be on 287 the UI thread when calling any method on any View. 288 If you are doing work on other threads and want to update the state of a View 289 from that thread, you should use a {@link android.os.Handler}. 290 </p> 291 --> 292
