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    <h2>Android <code>id</code> overview</h2> <p>An Android <code>id</code> is an integer commonly used to identify views; this <code>id</code> can be assigned via XML (when possible) and via code (programmatically.) The <code>id</code> is most useful for getting references for XML-defined <code>View</code>s generated by an <code>Inflater</code> (such as by using <code>setContentView</code>.)</p> <h3>Assign <code>id</code> via <code>XML</code></h3> <ul> <li>Add an attribute of <code>android:id="@+id/</code>somename<code>"</code> to your view.</li> <li>When your application is built, the <code>android:id</code> will be assigned a <strong>unique</strong> <code>int</code> for use in code.</li> <li>Reference your <code>android:id</code>'s <code>int</code> value in code using "<code>R.id.</code>somename" (effectively a constant.)</li> <li><em>this <code>int</code> can change from build to build</em> so <strong>never copy an id</strong> from <code>gen/</code>package.name/<code>R.java</code>, just use "<code>R.id.</code>somename".</li> <li>(Also, an <code>id</code> assigned to a <code>Preference</code> in XML is not used when the <code>Preference</code> generates its <code>View</code>.)</li> </ul> <h3>Assign <code>id</code> via code (programmatically)</h3> <ul> <li>Manually set <code>id</code>s using <code>someView.setId(</code>int<code>);</code></li> <li>The <code>int</code> must be positive, but is otherwise arbitrary- it can be whatever you want (keep reading if this is frightful.)</li> <li><em>For example, if creating and numbering several views representing items, you could use their item number.</em></li> </ul> <h3>Uniqueness of <code>id</code>s</h3> <ul> <li><code>XML</code>-assigned <code>id</code>s will be unique.</li> <li>Code-assigned <code>id</code>s do <em>not</em> have to be unique</li> <li>Code-assigned <code>id</code>s can (theoretically) conflict with <code>XML</code>-assigned <code>id</code>s.</li> <li>These conflicting <code>id</code>s won't matter if queried correctly <em>(keep reading)</em>.</li> </ul> <h3>When (and why) conflicting <code>id</code>s don't matter</h3> <ul> <li><code>findViewById(int)</code> will iterate depth-first recursively through the view hierarchy <em>from the View you specify</em> and return the first <code>View</code> it finds with a matching <code>id</code>.</li> <li>As long as there are no code-assigned <code>id</code>s assigned before an XML-defined <code>id</code> in the hierarchy, <code>findViewById(R.id.somename)</code> will always return the XML-defined View so <code>id</code>'d.</li> </ul> <h3>Dynamically Creating Views and Assigning <code>ID</code>s</h3> <ul> <li>In layout XML, define an empty <code>ViewGroup</code> with <code>id</code>.</li> <li><em>Such as a <code>LinearLayout</code> with <code>android:id="@+id/placeholder"</code>.</em></li> <li>Use code to populate the placeholder <code>ViewGroup</code> with <code>View</code>s.</li> <li>If you need or want, assign any <code>id</code>s that are convenient to each view.</li> <li><p>Query these child views using placeholder.findViewById(convenientInt);</p></li> <li><p>API 17 introduced <code>View.generateViewId()</code> which allows you to generate a unique ID. </p></li> </ul> <p><strong>If you choose to keep references to your views around</strong>, be sure to instantiate them with <code>getApplicationContext()</code> and be sure to set each reference to null in <code>onDestroy</code>. Apparently <em>leaking</em> the <code>Activity</code> (hanging onto it after is is destroyed) is wasteful.. :)</p> <h3>Reserve an XML <code>android:id</code> for use in code</h3> <p><em>API 17 introduced</em> <code>View.generateViewId()</code> <em>which generates a unique ID.</em> (Thanks to take-chances-make-changes for pointing this out.)*</p> <p>If your <code>ViewGroup</code> cannot be defined via XML (or you don't want it to be) you can reserve the id via XML to ensure it remains unique:</p> <p><strong>Here, <em>values/ids.xml</em> defines a custom <code>id</code>:</strong></p> <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt; &lt;resources&gt; &lt;item name="reservedNamedId" type="id"/&gt; &lt;/resources&gt; </code></pre> <p><strong>Then once the ViewGroup or View has been created, you can attach the custom id</strong></p> <pre><code>myViewGroup.setId(R.id.reservedNamedId); </code></pre> <h2>Conflicting <code>id</code> example</h2> <p>For clarity by way of obfuscating example, lets examine what happens when there is an <code>id</code> conflict behind the scenes.</p> <p><strong>layout/mylayout.xml</strong></p> <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt; &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" &gt; &lt;LinearLayout android:id="@+id/placeholder" android:layout_width="fill_parent" android:layout_height="wrap_content" android:orientation="horizontal" &gt; &lt;/LinearLayout&gt; </code></pre> <p><em>To simulate a conflict, lets say our latest build assigned <code>R.id.placeholder</code>(<code>@+id/placeholder</code>) an <code>int</code> value of</em> <code>12</code>..</p> <p>Next, <strong>MyActivity.java</strong> defines some adds views programmatically (via code):</p> <pre><code>int placeholderId = R.id.placeholder; // placeholderId==12 // returns *placeholder* which has id==12: ViewGroup placeholder = (ViewGroup)this.findViewById(placeholderId); for (int i=0; i&lt;20; i++){ TextView tv = new TextView(this.getApplicationContext()); // One new TextView will also be assigned an id==12: tv.setId(i); placeholder.addView(tv); } </code></pre> <p>So <code>placeholder</code> and one of our new <code>TextView</code>s both have an <code>id</code> of 12! But this isn't really a problem if we query placeholder's child views:</p> <pre><code>// Will return a generated TextView: placeholder.findViewById(12); // Whereas this will return the ViewGroup *placeholder*; // as long as its R.id remains 12: Activity.this.findViewById(12); </code></pre> <p>*Not so bad</p>
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