The Android notification subsystem was significantly revamped in Jelly Bean. Among the new features, notifications can now be presented in two modes—collapsed and expanded. The user can toggle between the two modes by using a 2-finger swipe gesture, dragging up or down on the item in the notification panel. The GMail notification uses this new feature to present a summary of the new emails received in expanded mode while only giving a count of new mails in collapsed mode:
I wanted to provide a similar effect in one of my own applications, allowing items in a particular ListView to similarly expand and contract. In my application, there is a list that displays a map image generated with the Google Static Map API in each row. Downloading and instantiating the map image for each row turned out to be very costly and prevented fluid scrolling. That problem led me to the idea of having two modes for the list item—the map image is only instantiated when needed, after the item is expanded.
The code to support the expanding item is not available as a public API in the framework, so I ported the underlying Java code over to C# and integrated it with ListView to replicate the behavior. I published the three-part code implementation in this Gist on GitHub:
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 | /* * Copyright (C) 2010 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ using System; using Android.Runtime; using Android.Content; using Android.Util; using Android.Views; namespace FriendTab { public class DoubleSwipeDetector { public interface IOnScaleGestureListener { bool OnScale(DoubleSwipeDetector detector); bool OnScaleBegin(DoubleSwipeDetector detector); void OnScaleEnd(DoubleSwipeDetector detector); } public class SimpleOnScaleGestureListener : IOnScaleGestureListener { public bool OnScale(DoubleSwipeDetector detector) { return false; } public bool OnScaleBegin(DoubleSwipeDetector detector) { return true; } public void OnScaleEnd(DoubleSwipeDetector detector) { // Intentionally empty } } private static float PRESSURE_THRESHOLD = 0.67f; private Context mContext; private IOnScaleGestureListener mListener; private bool mGestureInProgress; private MotionEvent mPrevEvent; private MotionEvent mCurrEvent; private float mFocusX; private float mFocusY; private float mPrevFingerDiffX; private float mPrevFingerDiffY; private float mCurrFingerDiffX; private float mCurrFingerDiffY; private float mCurrLen; private float mPrevLen; private float mScaleFactor; private float mCurrPressure; private float mPrevPressure; private long mTimeDelta; private bool mInvalidGesture; // Pointer IDs currently responsible for the two fingers controlling the gesture private int mActiveId0; private int mActiveId1; private bool mActive0MostRecent; public DoubleSwipeDetector (Context context, IOnScaleGestureListener listener) { mContext = context; mListener = listener; } public bool OnTouchEvent (MotionEvent evt) { var action = evt.ActionMasked; if (action == MotionEventActions.Down) { reset(); // Start fresh } bool handled = true; if (mInvalidGesture) { handled = false; } else if (!mGestureInProgress) { switch (action) { case MotionEventActions.Down: { mActiveId0 = evt.GetPointerId(0); mActive0MostRecent = true; } break; case MotionEventActions.Up: reset(); break; case MotionEventActions.PointerDown: { // We have a new multi-finger gesture if (mPrevEvent != null) mPrevEvent.Recycle(); mPrevEvent = MotionEvent.Obtain(evt); mTimeDelta = 0; int index1 = evt.ActionIndex; int index0 = evt.FindPointerIndex(mActiveId0); mActiveId1 = evt.GetPointerId(index1); if (index0 < 0 || index0 == index1) { // Probably someone sending us a broken evt stream. index0 = findNewActiveIndex(evt, mActiveId1, -1); mActiveId0 = evt.GetPointerId(index0); } mActive0MostRecent = false; setContext(evt); mGestureInProgress = mListener.OnScaleBegin(this); break; } } } else { // Transform gesture in progress - attempt to handle it switch (action) { case MotionEventActions.Down: { // End the old gesture and begin a new one with the most recent two fingers. mListener.OnScaleEnd(this); int oldActive0 = mActiveId0; int oldActive1 = mActiveId1; reset(); mPrevEvent = MotionEvent.Obtain(evt); mActiveId0 = mActive0MostRecent ? oldActive0 : oldActive1; mActiveId1 = evt.GetPointerId(evt.ActionIndex); mActive0MostRecent = false; int index0 = evt.FindPointerIndex (mActiveId0); if (index0 < 0 || mActiveId0 == mActiveId1) { index0 = findNewActiveIndex(evt, mActiveId1, -1); mActiveId0 = evt.GetPointerId(index0); } setContext(evt); mGestureInProgress = mListener.OnScaleBegin(this); } break; case MotionEventActions.PointerUp: { int pointerCount = evt.PointerCount; int actionIndex = evt.ActionIndex; int actionId = evt.GetPointerId(actionIndex); bool gestureEnded = false; if (pointerCount > 2) { if (actionId == mActiveId0) { int newIndex = findNewActiveIndex(evt, mActiveId1, actionIndex); if (newIndex >= 0) { mListener.OnScaleEnd(this); mActiveId0 = evt.GetPointerId(newIndex); mActive0MostRecent = true; mPrevEvent = MotionEvent.Obtain(evt); setContext(evt); mGestureInProgress = mListener.OnScaleBegin(this); } else { gestureEnded = true; } } else if (actionId == mActiveId1) { int newIndex = findNewActiveIndex(evt, mActiveId0, actionIndex); if (newIndex >= 0) { mListener.OnScaleEnd(this); mActiveId1 = evt.GetPointerId(newIndex); mActive0MostRecent = false; mPrevEvent = MotionEvent.Obtain(evt); setContext(evt); mGestureInProgress = mListener.OnScaleBegin(this); } else { gestureEnded = true; } } mPrevEvent.Recycle(); mPrevEvent = MotionEvent.Obtain(evt); setContext(evt); } else { gestureEnded = true; } if (gestureEnded) { // Gesture ended setContext(evt); // Set focus point to the remaining finger int activeId = actionId == mActiveId0 ? mActiveId1 : mActiveId0; int index = evt.FindPointerIndex(activeId); mFocusX = evt.GetX(index); mFocusY = evt.GetY(index); mListener.OnScaleEnd(this); reset(); mActiveId0 = activeId; mActive0MostRecent = true; } } break; case MotionEventActions.Cancel: mListener.OnScaleEnd(this); reset(); break; case MotionEventActions.Up: reset(); break; case MotionEventActions.Move: { setContext(evt); // Only accept the evt if our relative pressure is within // a certain limit - this can help filter shaky data as a // finger is lifted. if (mCurrPressure / mPrevPressure > PRESSURE_THRESHOLD) { bool updatePrevious = mListener.OnScale(this); if (updatePrevious) { mPrevEvent.Recycle(); mPrevEvent = MotionEvent.Obtain(evt); } } } break; } } return handled; } private int findNewActiveIndex(MotionEvent ev, int otherActiveId, int removedPointerIndex) { int pointerCount = ev.PointerCount; // It's ok if this isn't found and returns -1, it simply won't match. int otherActiveIndex = ev.FindPointerIndex(otherActiveId); // Pick a new id and update tracking state. for (int i = 0; i < pointerCount; i++) { if (i != removedPointerIndex && i != otherActiveIndex) { return i; } } return -1; } private void setContext(MotionEvent curr) { if (mCurrEvent != null) { mCurrEvent.Recycle(); } mCurrEvent = MotionEvent.Obtain(curr); mCurrLen = -1; mPrevLen = -1; mScaleFactor = -1; MotionEvent prev = mPrevEvent; int prevIndex0 = prev.FindPointerIndex(mActiveId0); int prevIndex1 = prev.FindPointerIndex(mActiveId1); int currIndex0 = curr.FindPointerIndex(mActiveId0); int currIndex1 = curr.FindPointerIndex(mActiveId1); if (prevIndex0 < 0 || prevIndex1 < 0 || currIndex0 < 0 || currIndex1 < 0) { mInvalidGesture = true; if (mGestureInProgress) { mListener.OnScaleEnd(this); } return; } float px0 = prev.GetX(prevIndex0); float py0 = prev.GetY(prevIndex0); float px1 = prev.GetX(prevIndex1); float py1 = prev.GetY(prevIndex1); float cx0 = curr.GetX(currIndex0); float cy0 = curr.GetY(currIndex0); float cx1 = curr.GetX(currIndex1); float cy1 = curr.GetY(currIndex1); float pvx = px1 - px0; float pvy = py1 - py0; float cvx = cx1 - cx0; float cvy = cy1 - cy0; mPrevFingerDiffX = pvx; mPrevFingerDiffY = pvy; mCurrFingerDiffX = cvx; mCurrFingerDiffY = cvy; mFocusX = cx0 + cvx * 0.5f; mFocusY = cy0 + cvy * 0.5f; mTimeDelta = curr.EventTime - prev.EventTime; mCurrPressure = curr.GetPressure(currIndex0) + curr.GetPressure(currIndex1); mPrevPressure = prev.GetPressure(prevIndex0) + prev.GetPressure(prevIndex1); } private void reset() { if (mPrevEvent != null) { mPrevEvent.Recycle(); mPrevEvent = null; } if (mCurrEvent != null) { mCurrEvent.Recycle(); mCurrEvent = null; } mGestureInProgress = false; mActiveId0 = -1; mActiveId1 = -1; mInvalidGesture = false; } public bool IsInProgress { get { return mGestureInProgress; } } public float FocusX { get { return mFocusX; } } public float FocusY { get { return mFocusY; } } public float CurrentSpan { get { if (mCurrLen == -1) { float cvx = mCurrFingerDiffX; float cvy = mCurrFingerDiffY; mCurrLen = (float)Math.Sqrt (cvx * cvx + cvy * cvy); } return mCurrLen; } } public float CurrentSpanX { get { return mCurrFingerDiffX; } } public float CurrentSpanY { get { return mCurrFingerDiffY; } } public float PreviousSpan { get { if (mPrevLen == -1) { float pvx = mPrevFingerDiffX; float pvy = mPrevFingerDiffY; mPrevLen = (float)Math.Sqrt (pvx * pvx + pvy * pvy); } return mPrevLen; } } public float PreviousSpanX { get { return mPrevFingerDiffX; } } public float PreviousSpanY { get { return mPrevFingerDiffY; } } public float ScaleFactor { get { if (mScaleFactor == -1) { mScaleFactor = CurrentSpan / PreviousSpan; } return mScaleFactor; } } public long TimeDelta { get { return mTimeDelta; } } public long EventTime { get { return mCurrEvent.EventTime; } } } } |
ExpandableListView is the type that you use to incorporate the control into your own application. You also need to download and put in your resource directory the following two images from the Android framework to provide the glow.
I use a custom ActivityItem view as my ListView row, which is instantiated from an XML layout containing the following item. When used, the item can be collapsed and expanded. It will start in the collapsed state, as the layout_height property indicates.
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