1 page.title=Loading Views On Demand 2 parent.title=Improving Layout Performance 3 parent.link=index.html 4 5 trainingnavtop=true 6 previous.title=Re-using Layouts with <include/> 7 previous.link=reusing-layouts.html 8 next.title=Making ListView Scrolling Smooth 9 next.link=smooth-scrolling.html 10 11 @jd:body 12 13 14 <div id="tb-wrapper"> 15 <div id="tb"> 16 17 <!-- table of contents --> 18 <h2>This lesson teaches you to</h2> 19 <ol> 20 <li><a href="#ViewStub">Define a ViewStub</a></li> 21 <li><a href="#Load">Load the ViewStub Layout</a></li> 22 </ol> 23 24 <!-- other docs (NOT javadocs) --> 25 <h2>You should also read</h2> 26 <ul> 27 <li><a href="http://android-developers.blogspot.com/2009/03/android-layout-tricks-3-optimize-with.html" 28 >Optimize with stubs (blog post)</a></li> 29 </ul> 30 31 </div> 32 </div> 33 34 35 <p>Sometimes your layout might require complex views that are rarely used. Whether 36 they are item details, progress indicators, or undo messages, you can reduce memory usage and speed 37 up rendering by loading the views only when they are needed.</p> 38 39 40 <h2 id="ViewStub">Define a ViewStub</h2> 41 42 <p>{@link android.view.ViewStub} is a lightweight view with no dimension and doesnt draw anything 43 or participate in the layout. As such, it's cheap to inflate and cheap to leave in a view hierarchy. 44 Each {@link android.view.ViewStub} simply needs to include the {@code android:layout} attribute to 45 specify the layout to inflate.</p> 46 47 <p>The following {@link android.view.ViewStub} is for a translucent progress bar overlay. It should 48 be visible only when new items are being imported into the application.</p> 49 50 <pre> 51 <ViewStub 52 android:id="@+id/stub_import" 53 android:inflatedId="@+id/panel_import" 54 android:layout="@layout/progress_overlay" 55 android:layout_width="fill_parent" 56 android:layout_height="wrap_content" 57 android:layout_gravity="bottom" /> 58 </pre> 59 60 61 <h2 id="Load">Load the ViewStub Layout</h2> 62 63 <p>When you want to load the layout specified by the {@link android.view.ViewStub}, either set it 64 visible by calling {@link android.view.View#setVisibility setVisibility(View.VISIBLE)} or call 65 {@link android.view.ViewStub#inflate()}.</p> 66 67 <pre> 68 ((ViewStub) findViewById(R.id.stub_import)).setVisibility(View.VISIBLE); 69 // or 70 View importPanel = ((ViewStub) findViewById(R.id.stub_import)).inflate(); 71 </pre> 72 73 <p class="note"><strong>Note:</strong> The {@link android.view.ViewStub#inflate()} method returns 74 the inflated {@link android.view.View} once complete. so you don't need to call {@link 75 android.app.Activity#findViewById findViewById()} if you need to interact with the layout.</p> 76 77 <p>Once visible/inflated, the {@link android.view.ViewStub} element is no longer part of the view 78 hierarchy. It is replaced by the inflated layout and the ID for the root view of that layout is 79 the one specified by the {@code android:inflatedId} attribute of the ViewStub. (The ID {@code 80 android:id} specified for the {@link android.view.ViewStub} is valid only until the {@link 81 android.view.ViewStub} layout is visible/inflated.)</p> 82 83 <p class="note"><strong>Note:</strong> One drawback of {@link android.view.ViewStub} is that it 84 doesnt currently support the {@code <merge/>} tag in the layouts to be inflated.</p> 85 86 87 88
