1page.title=Making ListView Scrolling Smooth 2parent.title=Improving Layout Performance 3parent.link=index.html 4 5trainingnavtop=true 6previous.title=Loading Views On Demand 7previous.link=loading-ondemand.html 8 9@jd:body 10 11 12<div id="tb-wrapper"> 13<div id="tb"> 14 15<!-- table of contents --> 16<h2>This lesson teaches you to</h2> 17<ol> 18 <li><a href="#AsyncTask">Use a Background Thread</a></li> 19 <li><a href="#ViewHolder">Hold View Objects in a View Holder</a></li> 20</ol> 21 22<!-- other docs (NOT javadocs) --> 23<h2>You should also read</h2> 24<ul> 25 <li><a href="{@docRoot}resources/articles/listview-backgrounds.html">ListView 26Backgrounds: An Optimization</a></li> 27</ul> 28 29</div> 30</div> 31 32<p>The key to a smoothly scrolling {@link android.widget.ListView} is to keep the application’s main 33thread (the UI thread) free from heavy processing. Ensure you do any disk access, network access, or 34SQL access in a separate thread. To test the status of your app, you can enable {@link 35android.os.StrictMode}.</p> 36 37 38<h2 id="AsyncTask">Use a Background Thread</h2> 39 40<p>Using a background thread ("worker thread") removes strain from the main thread so it can focus 41on drawing the UI. In many cases, using {@link android.os.AsyncTask} provides a simple way to 42perform your work outside the main thread. {@link android.os.AsyncTask} automatically queues up all 43the {@link android.os.AsyncTask#execute execute()} requests and performs them serially. This 44behavior is global to a particular process and means you don’t need to worry about creating your 45own thread pool.</p> 46 47<p>In the sample code below, an {@link android.os.AsyncTask} is used to load 48images in a background thread, then apply them to the UI once finished. It also shows a 49progress spinner in place of the images while they are loading.</p> 50 51<pre> 52// Using an AsyncTask to load the slow images in a background thread 53new AsyncTask<ViewHolder, Void, Bitmap>() { 54 private ViewHolder v; 55 56 @Override 57 protected Bitmap doInBackground(ViewHolder... params) { 58 v = params[0]; 59 return mFakeImageLoader.getImage(); 60 } 61 62 @Override 63 protected void onPostExecute(Bitmap result) { 64 super.onPostExecute(result); 65 if (v.position == position) { 66 // If this item hasn't been recycled already, hide the 67 // progress and set and show the image 68 v.progress.setVisibility(View.GONE); 69 v.icon.setVisibility(View.VISIBLE); 70 v.icon.setImageBitmap(result); 71 } 72 } 73}.execute(holder); 74</pre> 75 76<p>Beginning with Android 3.0 (API level 11), an extra feature is available in {@link 77android.os.AsyncTask} so you can enable it to run across multiple processor cores. Instead of 78calling {@link android.os.AsyncTask#execute execute()} you can specify {@link 79android.os.AsyncTask#executeOnExecutor executeOnExecutor()} and multiple requests can be executed at 80the same time depending on the number of cores available.</p> 81 82 83<h2 id="ViewHolder">Hold View Objects in a View Holder</h2> 84 85<p>Your code might call {@link android.app.Activity#findViewById findViewById()} frequently 86during the scrolling of {@link android.widget.ListView}, which can slow down performance. Even when 87the {@link 88android.widget.Adapter} returns an inflated view for recycling, you still need to look up the 89elements 90and update them. A way around repeated use of {@link android.app.Activity#findViewById 91findViewById()} is to use the "view holder" design pattern.</p> 92 93<p>A {@code ViewHolder} object stores each of the component views inside the tag field of the 94Layout, so you can immediately access them without the need to look them up repeatedly. First, you 95need to create a class to hold your exact set of views. For example:</p> 96 97<pre> 98static class ViewHolder { 99 TextView text; 100 TextView timestamp; 101 ImageView icon; 102 ProgressBar progress; 103 int position; 104} 105</pre> 106 107<p>Then populate the {@code ViewHolder} and store it inside the layout.</p> 108 109<pre> 110ViewHolder holder = new ViewHolder(); 111holder.icon = (ImageView) convertView.findViewById(R.id.listitem_image); 112holder.text = (TextView) convertView.findViewById(R.id.listitem_text); 113holder.timestamp = (TextView) convertView.findViewById(R.id.listitem_timestamp); 114holder.progress = (ProgressBar) convertView.findViewById(R.id.progress_spinner); 115convertView.setTag(holder); 116</pre> 117 118<p>Now you can easily access each view without the need for the look-up, saving valuable processor 119cycles.</p> 120 121 122 123 124 125