1page.title=Custom Components 2page.tags=view,widget 3@jd:body 4 5<div id="qv-wrapper"> 6<div id="qv"> 7 <h2>In this document</h2> 8 <ol> 9 <li><a href="#basic">The Basic Approach</a></li> 10 <li><a href="#custom">Fully Customized Components</a></li> 11 <li><a href="#compound">Compound Controls</a></li> 12 <li><a href="#modifying">Modifying an Existing View Type</a></li> 13 </ol> 14</div> 15</div> 16 17<p>Android offers a sophisticated and powerful componentized model for building your UI, 18based on the fundamental layout classes: {@link android.view.View} and 19{@link android.view.ViewGroup}. To start with, the platform includes a variety of prebuilt 20View and ViewGroup subclasses — called widgets and layouts, respectively — 21that you can use to construct your UI.</p> 22 23<p>A partial list of available widgets includes {@link android.widget.Button Button}, 24{@link android.widget.TextView TextView}, 25{@link android.widget.EditText EditText}, 26{@link android.widget.ListView ListView}, 27{@link android.widget.CheckBox CheckBox}, 28{@link android.widget.RadioButton RadioButton}, 29{@link android.widget.Gallery Gallery}, 30{@link android.widget.Spinner Spinner}, and the more special-purpose 31{@link android.widget.AutoCompleteTextView AutoCompleteTextView}, 32{@link android.widget.ImageSwitcher ImageSwitcher}, and 33{@link android.widget.TextSwitcher TextSwitcher}. </p> 34 35<p>Among the layouts available are {@link android.widget.LinearLayout LinearLayout}, 36{@link android.widget.FrameLayout FrameLayout}, {@link android.widget.RelativeLayout RelativeLayout}, 37and others. For more examples, see <a href="layout-objects.html">Common Layout Objects</a>.</p> 38 39<p>If none of the prebuilt widgets or layouts meets your needs, you can create your own View subclass. 40If you only need to make small adjustments to an existing widget or layout, you can simply subclass 41the widget or layout and override its methods. 42</p> 43 44<p>Creating your own View subclasses gives you precise control over the appearance and function 45of a screen element. To give an idea of the control you get with custom views, here are some 46examples of what you could do with them:</p> 47 48<ul> 49 <li> 50 You could create a completely custom-rendered View type, for example a "volume 51 control" knob rendered using 2D graphics, and which resembles an 52 analog electronic control. 53 </li> 54 <li> 55 You could combine a group of View components into a new single component, perhaps 56 to make something like a ComboBox (a combination of popup list and free 57 entry text field), a dual-pane selector control (a left and right pane 58 with a list in each where you can re-assign which item is in which 59 list), and so on. 60 </li> 61 <li> 62 You could override the way that an EditText component is rendered on the screen 63 (the <a href="{@docRoot}resources/samples/NotePad/index.html">Notepad Tutorial</a> uses this to good effect, 64 to create a lined-notepad page). 65 </li> 66 <li> 67 You could capture other events like key presses and handle them in some custom 68 way (such as for a game). 69 </li> 70</ul> 71<p> 72The sections below explain how to create custom Views and use them in your application. 73For detailed reference information, see the {@link android.view.View} class. </p> 74 75 76<h2 id="basic">The Basic Approach</h2> 77 78<p>Here is a high level overview of what you need to know to get started in creating your own 79View components:</p> 80 81<ol> 82 <li> 83 Extend an existing {@link android.view.View View} class or subclass 84 with your own class. 85 </li> 86 <li> 87 Override some of the methods from the superclass. The superclass methods 88 to override start with '<code>on</code>', for 89 example, {@link android.view.View#onDraw onDraw()}, 90 {@link android.view.View#onMeasure onMeasure()}, and 91 {@link android.view.View#onKeyDown onKeyDown()}. 92 This is similar to the <code>on...</code> events in {@link android.app.Activity Activity} 93 or {@link android.app.ListActivity ListActivity} 94 that you override for lifecycle and other functionality hooks. 95 <li> 96 Use your new extension class. Once completed, your new extension class 97 can be used in place of the view upon which it was based. 98 </li> 99</ol> 100<p class="note"><strong>Tip:</strong> 101 Extension classes can be defined as inner classes inside the activities 102 that use them. This is useful because it controls access to them but 103 isn't necessary (perhaps you want to create a new public View for 104 wider use in your application). 105</p> 106 107 108 109<h2 id="custom">Fully Customized Components</h2> 110<p> 111Fully customized components can be used to create graphical components that 112appear however you wish. Perhaps a graphical VU 113meter that looks like an old analog gauge, or a sing-a-long text view where 114a bouncing ball moves along the words so you can sing along with a karaoke 115machine. Either way, you want something that the built-in components just 116won't do, no matter how you combine them.</p> 117<p>Fortunately, you can easily create components that look and behave in any 118way you like, limited perhaps only by your imagination, the size of the 119screen, and the available processing power (remember that ultimately your 120application might have to run on something with significantly less power 121than your desktop workstation).</p> 122<p>To create a fully customized component:</p> 123<ol> 124 <li> 125 The most generic view you can extend is, unsurprisingly, {@link 126 android.view.View View}, so you will usually start by extending this to 127 create your new super component. 128 </li> 129 <li> 130 You can supply a constructor which can 131 take attributes and parameters from the XML, and you can also consume 132 your own such attributes and parameters (perhaps the color and range of 133 the VU meter, or the width and damping of the needle, etc.) 134 </li> 135 <li> 136 You will probably want to create your own event listeners, 137 property accessors and modifiers, and possibly more sophisticated 138 behavior in your component class as well. 139 </li> 140 <li> 141 You will almost certainly want to override <code>onMeasure()</code> and 142 are also likely to need to override <code>onDraw()</code> if you want 143 the component to show something. While both have default behavior, 144 the default <code>onDraw()</code> will do nothing, and the default 145 <code>onMeasure()</code> will always set a size of 100x100 — which is 146 probably not what you want. 147 </li> 148 <li> 149 Other <code>on...</code> methods may also be overridden as required. 150 </li> 151</ol> 152 153<h3>Extend <code>onDraw()</code> and <code>onMeasure()</code></h3> 154<p>The <code>onDraw()</code> method delivers you a {@link android.graphics.Canvas Canvas} 155upon which you can implement anything you want: 2D graphics, other standard or 156custom components, styled text, or anything else you can think of.</p> 157 158<p class="note"><strong>Note:</strong> 159This does not apply to 3D graphics. If you want to 160use 3D graphics, you must extend {@link android.view.SurfaceView SurfaceView} 161instead of View, and draw from a separate thread. See the 162GLSurfaceViewActivity sample 163for details.</p> 164 165<p><code>onMeasure()</code> is a little more involved. <code>onMeasure()</code> 166is a critical piece of the rendering contract between your component and its 167container. <code>onMeasure()</code> should be overridden to efficiently and 168accurately report the measurements of its contained parts. This is made 169slightly more complex by the requirements of limits from the parent 170(which are passed in to the <code>onMeasure()</code> method) and by the 171requirement to call the <code>setMeasuredDimension()</code> method with the 172measured width and height once they have been calculated. If you fail to 173call this method from an overridden <code>onMeasure()</code> method, the 174result will be an exception at measurement time.</p> 175<p>At a high level, implementing <code>onMeasure()</code> looks something 176 like this:</p> 177 178<ol> 179 <li> 180 The overridden <code>onMeasure()</code> method is called with width and 181 height measure specifications (<code>widthMeasureSpec</code> and 182 <code>heightMeasureSpec</code> parameters, both are integer codes 183 representing dimensions) which should be treated as requirements for 184 the restrictions on the width and height measurements you should produce. A 185 full reference to the kind of restrictions these specifications can require 186 can be found in the reference documentation under {@link 187 android.view.View#onMeasure View.onMeasure(int, int)} (this reference 188 documentation does a pretty good job of explaining the whole measurement 189 operation as well). 190 </li> 191 <li> 192 Your component's <code>onMeasure()</code> method should calculate a 193 measurement width and height which will be required to render the 194 component. It should try to stay within the specifications passed in, 195 although it can choose to exceed them (in this case, the parent can 196 choose what to do, including clipping, scrolling, throwing an exception, 197 or asking the <code>onMeasure()</code> to try again, perhaps with 198 different measurement specifications). 199 </li> 200 <li> 201 Once the width and height are calculated, the <code>setMeasuredDimension(int 202 width, int height)</code> method must be called with the calculated 203 measurements. Failure to do this will result in an exception being 204 thrown. 205 </li> 206</ol> 207 208<p> 209Here's a summary of some of the other standard methods that the framework calls on views: 210</p> 211<table border="2" width="85%" align="center" cellpadding="5"> 212 <thead> 213 <tr><th>Category</th> <th>Methods</th> <th>Description</th></tr> 214 </thead> 215 216 <tbody> 217 <tr> 218 <td rowspan="2">Creation</td> 219 <td>Constructors</td> 220 <td>There is a form of the constructor that are called when the view 221 is created from code and a form that is called when the view is 222 inflated from a layout file. The second form should parse and apply 223 any attributes defined in the layout file. 224 </td> 225 </tr> 226 <tr> 227 <td><code>{@link android.view.View#onFinishInflate()}</code></td> 228 <td>Called after a view and all of its children has been inflated 229 from XML.</td> 230 </tr> 231 232 <tr> 233 <td rowspan="3">Layout</td> 234 <td><code>{@link android.view.View#onMeasure}</code></td> 235 <td>Called to determine the size requirements for this view and all 236 of its children. 237 </td> 238 </tr> 239 <tr> 240 <td><code>{@link android.view.View#onLayout}</code></td> 241 <td>Called when this view should assign a size and position to all 242 of its children. 243 </td> 244 </tr> 245 <tr> 246 <td><code>{@link android.view.View#onSizeChanged}</code></td> 247 <td>Called when the size of this view has changed. 248 </td> 249 </tr> 250 251 <tr> 252 <td>Drawing</td> 253 <td><code>{@link android.view.View#onDraw}</code></td> 254 <td>Called when the view should render its content. 255 </td> 256 </tr> 257 258 <tr> 259 <td rowspan="4">Event processing</td> 260 <td><code>{@link android.view.View#onKeyDown}</code></td> 261 <td>Called when a new key event occurs. 262 </td> 263 </tr> 264 <tr> 265 <td><code>{@link android.view.View#onKeyUp}</code></td> 266 <td>Called when a key up event occurs. 267 </td> 268 </tr> 269 <tr> 270 <td><code>{@link android.view.View#onTrackballEvent}</code></td> 271 <td>Called when a trackball motion event occurs. 272 </td> 273 </tr> 274 <tr> 275 <td><code>{@link android.view.View#onTouchEvent}</code></td> 276 <td>Called when a touch screen motion event occurs. 277 </td> 278 </tr> 279 280 <tr> 281 <td rowspan="2">Focus</td> 282 <td><code>{@link android.view.View#onFocusChanged}</code></td> 283 <td>Called when the view gains or loses focus. 284 </td> 285 </tr> 286 287 <tr> 288 <td><code>{@link android.view.View#onWindowFocusChanged}</code></td> 289 <td>Called when the window containing the view gains or loses focus. 290 </td> 291 </tr> 292 293 <tr> 294 <td rowspan="3">Attaching</td> 295 <td><code>{@link android.view.View#onAttachedToWindow()}</code></td> 296 <td>Called when the view is attached to a window. 297 </td> 298 </tr> 299 300 <tr> 301 <td><code>{@link android.view.View#onDetachedFromWindow}</code></td> 302 <td>Called when the view is detached from its window. 303 </td> 304 </tr> 305 306 <tr> 307 <td><code>{@link android.view.View#onWindowVisibilityChanged}</code></td> 308 <td>Called when the visibility of the window containing the view 309 has changed. 310 </td> 311 </tr> 312 </tbody> 313 314 </table> 315 316 317 318<h3 id="customexample">A Custom View Example</h3> 319<p>The CustomView sample in the 320<a href="{@docRoot}resources/samples/ApiDemos/index.html">API Demos</a> provides an example 321of a customized View. The custom View is defined in the 322<a href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/view/LabelView.html">LabelView</a> 323class.</p> 324<p>The LabelView sample demonstrates a number of different aspects of custom components:</p> 325<ul> 326 <li>Extending the View class for a completely custom component.</li> 327 <li>Parameterized constructor that takes the view inflation parameters 328 (parameters defined in the XML). Some of these are passed through to the 329 View superclass, but more importantly, there are some custom attributes defined 330 and used for LabelView.</li> 331 <li>Standard public methods of the type you would expect to see for a label 332 component, for example <code>setText()</code>, <code>setTextSize()</code>, 333 <code>setTextColor()</code> and so on.</li> 334 <li>An overridden <code>onMeasure</code> method to determine and set the 335 rendering size of the component. (Note that in LabelView, the real work is done 336 by a private <code>measureWidth()</code> method.)</li> 337 <li>An overridden <code>onDraw()</code> method to draw the label onto the 338 provided canvas.</li> 339</ul> 340<p>You can see some sample usages of the LabelView custom View in 341<a href="{@docRoot}resources/samples/ApiDemos/res/layout/custom_view_1.html">custom_view_1.xml</a> 342from the samples. In particular, you can see a mix of both <code>android:</code> 343namespace parameters and custom <code>app:</code> namespace parameters. These 344<code>app:</code> parameters are the custom ones that the LabelView recognizes 345and works with, and are defined in a styleable inner class inside of the 346samples R resources definition class.</p> 347 348 349<h2 id="compound">Compound Controls 350</h2> 351<p>If you don't want to create a completely customized component, but instead 352are looking to put together a reusable component that consists of a group of 353existing controls, then creating a Compound Component (or Compound Control) might 354fit the bill. In a nutshell, this brings together a number of more atomic 355controls (or views) into a logical group of items that can be treated as a 356single thing. For example, a Combo Box can be thought of as a 357combination of a single line EditText field and an adjacent button with an attached 358 PopupList. If you press the button and select 359something from the list, it populates the EditText field, but the user can 360also type something directly into the EditText if they prefer.</p> 361<p>In Android, there are actually two other Views readily available to do 362this: {@link android.widget.Spinner Spinner} and 363{@link android.widget.AutoCompleteTextView AutoCompleteTextView}, but 364regardless, the concept of a Combo Box makes an easy-to-understand 365example.</p> 366<p>To create a compound component:</p> 367<ol> 368 <li> 369 The usual starting point is a Layout of some kind, so create a class 370 that extends a Layout. Perhaps in the case of a Combo box we might use 371 a LinearLayout with horizontal orientation. Remember that other layouts 372 can be nested inside, so the compound component can be arbitrarily 373 complex and structured. Note that just like with an Activity, you can 374 use either the declarative (XML-based) approach to creating the 375 contained components, or you can nest them programmatically from your 376 code. 377 </li> 378 <li> 379 In the constructor for the new class, take whatever parameters the 380 superclass expects, and pass them through to the superclass constructor 381 first. Then you can set up the other views to use within your new 382 component; this is where you would create the EditText field and the 383 PopupList. Note that you also might introduce your own attributes and 384 parameters into the XML that can be pulled out and used by your 385 constructor. 386 </li> 387 <li> 388 You can also create listeners for events that your contained views might 389 generate, for example, a listener method for the List Item Click Listener 390 to update the contents of the EditText if a list selection is made. 391 </li> 392 <li> 393 You might also create your own properties with accessors and modifiers, 394 for example, allow the EditText value to be set initially in the 395 component and query for its contents when needed. 396 </li> 397 <li> 398 In the case of extending a Layout, you don't need to override the 399 <code>onDraw()</code> and <code>onMeasure()</code> methods since the 400 layout will have default behavior that will likely work just fine. However, 401 you can still override them if you need to. 402 </li> 403 <li> 404 You might override other <code>on...</code> methods, like 405 <code>onKeyDown()</code>, to perhaps choose certain default values from 406 the popup list of a combo box when a certain key is pressed. 407 </li> 408</ol> 409<p> 410 To summarize, the use of a Layout as the basis for a Custom Control has a 411number of advantages, including:</p> 412 413<ul> 414 <li> 415 You can specify the layout using the declarative XML files just like 416 with an activity screen, or you can create views programmatically and 417 nest them into the layout from your code. 418 </li> 419 <li> 420 The <code>onDraw()</code> and <code>onMeasure()</code> methods (plus 421 most of the other <code>on...</code> methods) will likely have suitable behavior so 422 you don't have to override them. 423 </li> 424 <li> 425 In the end, you can very quickly construct arbitrarily complex compound 426 views and re-use them as if they were a single component. 427 </li> 428</ul> 429<h4>Examples of Compound Controls</h4> 430<p>In the API Demos project 431 that comes with the SDK, there are two List 432 examples — Example 4 and Example 6 under Views/Lists demonstrate a 433 SpeechView which extends LinearLayout to make a component for displaying 434 Speech quotes. The corresponding classes in the sample code are 435 <code>List4.java</code> and <code>List6.java</code>.</p> 436 437 438 439<h2 id="modifying">Modifying an Existing View Type</h2> 440<p>There is an even easier option for creating a custom View which is 441useful in certain circumstances. If there is a component that is already very 442similar to what you want, you can simply extend that component and just 443override the behavior that you want to change. You can do all of the things 444you would do with a fully customized component, but by starting with a more 445specialized class in the View hierarchy, you can also get a lot of behavior for 446free that probably does exactly what you want.</p> 447<p>For example, the SDK includes a <a 448href="{@docRoot}resources/samples/NotePad/index.html">NotePad application</a> in the 449samples. This demonstrates many aspects of using the Android platform, among 450them is extending an EditText View to make a lined notepad. This is not a 451perfect example, and the APIs for doing this might change from this early 452preview, but it does demonstrate the principles.</p> 453<p>If you haven't done so already, import the 454NotePad sample into Android Studio (or 455just look at the source using the link provided). In particular look at the definition of 456<code>MyEditText</code> in the <a 457href="{@docRoot}resources/samples/NotePad/src/com/example/android/notepad/NoteEditor.html">NoteEditor.java</a> 458file.</p> 459<p>Some points to note here</p> 460<ol> 461 <li> 462 <strong>The Definition</strong> 463 <p>The class is defined with the following line:<br/> 464 <code>public static class MyEditText extends EditText</code></p> 465 466 <ul> 467 <li> 468 It is defined as an inner class within the <code>NoteEditor</code> 469 activity, but it is public so that it could be accessed as 470 <code>NoteEditor.MyEditText</code> from outside of the <code>NoteEditor</code> 471 class if desired. 472 </li> 473 <li> 474 It is <code>static</code>, meaning it does not generate the so-called 475 "synthetic methods" that allow it to access data from the parent 476 class, which in turn means that it really behaves as a separate 477 class rather than something strongly related to <code>NoteEditor</code>. 478 This is a cleaner way to create inner classes if they do not need 479 access to state from the outer class, keeps the generated class 480 small, and allows it to be used easily from other classes. 481 </li> 482 <li> 483 It extends <code>EditText</code>, which is the View we have chosen to 484 customize in this case. When we are finished, the new class will be 485 able to substitute for a normal <code>EditText</code> view. 486 </li> 487 </ul> 488 </li> 489 <li> 490 <strong>Class Initialization</strong> 491 <p>As always, the super is called first. Furthermore, 492 this is not a default constructor, but a parameterized one. The 493 EditText is created with these parameters when it is inflated from an 494 XML layout file, thus, our constructor needs to both take them and pass them 495 to the superclass constructor as well.</p> 496 </li> 497 <li> 498 <strong>Overridden Methods</strong> 499 <p>In this example, there is only one method to be overridden: 500 <code>onDraw()</code> — but there could easily be others needed when you 501 create your own custom components.</p> 502 <p>For the NotePad sample, overriding the <code>onDraw()</code> method allows 503 us to paint the blue lines on the <code>EditText</code> view canvas (the 504 canvas is passed into the overridden <code>onDraw()</code> method). The 505 super.onDraw() method is called before the method ends. The 506 superclass method should be invoked, but in this case, we do it at the 507 end after we have painted the lines we want to include.</p> 508 <li> 509 <strong>Use the Custom Component</strong> 510 <p>We now have our custom component, but how can we use it? In the 511 NotePad example, the custom component is used directly from the 512 declarative layout, so take a look at <code>note_editor.xml</code> in the 513 <code>res/layout</code> folder.</p> 514<pre> 515<view 516 class="com.android.notepad.NoteEditor$MyEditText" 517 id="@+id/note" 518 android:layout_width="fill_parent" 519 android:layout_height="fill_parent" 520 android:background="@android:drawable/empty" 521 android:padding="10dip" 522 android:scrollbars="vertical" 523 android:fadingEdge="vertical" /> 524</pre> 525 526 <ul> 527 <li> 528 The custom component is created as a generic view in the XML, and 529 the class is specified using the full package. Note also that the 530 inner class we defined is referenced using the 531 <code>NoteEditor$MyEditText</code> notation which is a standard way to 532 refer to inner classes in the Java programming language. 533 <p>If your custom View component is not defined as an inner class, then you can, 534 alternatively, declare the View component 535 with the XML element name, and exclude the <code>class</code> attribute. For example:</p> 536<pre> 537<com.android.notepad.MyEditText 538 id="@+id/note" 539 ... /> 540</pre> 541 <p>Notice that the <code>MyEditText</code> class is now a separate class file. When the class 542 is nested in the <code>NoteEditor</code> class, this technique will not work.</p> 543 </li> 544 <li> 545 The other attributes and parameters in the definition are the ones 546 passed into the custom component constructor, and then passed 547 through to the EditText constructor, so they are the same 548 parameters that you would use for an EditText view. Note that it is 549 possible to add your own parameters as well, and we will touch on 550 this again below. 551 </li> 552 </ul> 553 </li> 554</ol> 555<p>And that's all there is to it. Admittedly this is a simple case, but 556that's the point — creating custom components is only as complicated as you 557need it to be.</p> 558<p>A more sophisticated component may override even more <code>on...</code> methods and 559introduce some of its own helper methods, substantially customizing its properties and 560behavior. The only limit is your imagination and what you need the component to 561do.</p> 562 563