Scott Main | 569ed22 | 2011-12-02 13:49:44 -0800 | [diff] [blame] | 1 | page.title=Making ListView Scrolling Smooth |
Scott Main | 580f014 | 2011-12-15 16:47:26 -0800 | [diff] [blame] | 2 | parent.title=Improving Layout Performance |
Scott Main | 569ed22 | 2011-12-02 13:49:44 -0800 | [diff] [blame] | 3 | parent.link=index.html |
| 4 | |
| 5 | trainingnavtop=true |
| 6 | previous.title=Loading Views On Demand |
| 7 | previous.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 |
| 26 | Backgrounds: 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 |
| 33 | thread (the UI thread) free from heavy processing. Ensure you do any disk access, network access, or |
| 34 | SQL access in a separate thread. To test the status of your app, you can enable {@link |
| 35 | android.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 |
| 41 | on drawing the UI. In many cases, using {@link android.os.AsyncTask} provides a simple way to |
| 42 | perform your work outside the main thread. {@link android.os.AsyncTask} automatically queues up all |
| 43 | the {@link android.os.AsyncTask#execute execute()} requests and performs them serially. This |
| 44 | behavior is global to a particular process and means you don’t need to worry about creating your |
| 45 | own thread pool.</p> |
| 46 | |
| 47 | <p>In the sample code below, an {@link android.os.AsyncTask} is used to load |
| 48 | images in a background thread, then apply them to the UI once finished. It also shows a |
| 49 | progress 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 |
| 53 | new 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 |
| 77 | android.os.AsyncTask} so you can enable it to run across multiple processor cores. Instead of |
| 78 | calling {@link android.os.AsyncTask#execute execute()} you can specify {@link |
| 79 | android.os.AsyncTask#executeOnExecutor executeOnExecutor()} and multiple requests can be executed at |
| 80 | the 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 |
| 86 | during the scrolling of {@link android.widget.ListView}, which can slow down performance. Even when |
| 87 | the {@link |
| 88 | android.widget.Adapter} returns an inflated view for recycling, you still need to look up the |
| 89 | elements |
| 90 | and update them. A way around repeated use of {@link android.app.Activity#findViewById |
| 91 | findViewById()} 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 |
| 94 | Layout, so you can immediately access them without the need to look them up repeatedly. First, you |
| 95 | need to create a class to hold your exact set of views. For example:</p> |
| 96 | |
| 97 | <pre> |
| 98 | static 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> |
| 110 | ViewHolder holder = new ViewHolder(); |
| 111 | holder.icon = (ImageView) convertView.findViewById(R.id.listitem_image); |
| 112 | holder.text = (TextView) convertView.findViewById(R.id.listitem_text); |
| 113 | holder.timestamp = (TextView) convertView.findViewById(R.id.listitem_timestamp); |
| 114 | holder.progress = (ProgressBar) convertView.findViewById(R.id.progress_spinner); |
| 115 | convertView.setTag(holder); |
| 116 | </pre> |
| 117 | |
| 118 | <p>Now you can easily access each view without the need for the look-up, saving valuable processor |
| 119 | cycles.</p> |
| 120 | |
| 121 | |
| 122 | |
| 123 | |
| 124 | |