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diff --git a/src/com/android/gallery3d/anim/Animation.java b/src/com/android/gallery3d/anim/Animation.java
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+/*
+ * 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.
+ */
+
+package com.android.gallery3d.anim;
+
+import android.view.animation.Interpolator;
+
+import com.android.gallery3d.common.Utils;
+
+// Animation calculates a value according to the current input time.
+//
+// 1. First we need to use setDuration(int) to set the duration of the
+// animation. The duration is in milliseconds.
+// 2. Then we should call start(). The actual start time is the first value
+// passed to calculate(long).
+// 3. Each time we want to get an animation value, we call
+// calculate(long currentTimeMillis) to ask the Animation to calculate it.
+// The parameter passed to calculate(long) should be nonnegative.
+// 4. Use get() to get that value.
+//
+// In step 3, onCalculate(float progress) is called so subclasses can calculate
+// the value according to progress (progress is a value in [0,1]).
+//
+// Before onCalculate(float) is called, There is an optional interpolator which
+// can change the progress value. The interpolator can be set by
+// setInterpolator(Interpolator). If the interpolator is used, the value passed
+// to onCalculate may be (for example, the overshoot effect).
+//
+// The isActive() method returns true after the animation start() is called and
+// before calculate is passed a value which reaches the duration of the
+// animation.
+//
+// The start() method can be called again to restart the Animation.
+//
+abstract public class Animation {
+ private static final long ANIMATION_START = -1;
+ private static final long NO_ANIMATION = -2;
+
+ private long mStartTime = NO_ANIMATION;
+ private int mDuration;
+ private Interpolator mInterpolator;
+
+ public void setInterpolator(Interpolator interpolator) {
+ mInterpolator = interpolator;
+ }
+
+ public void setDuration(int duration) {
+ mDuration = duration;
+ }
+
+ public void start() {
+ mStartTime = ANIMATION_START;
+ }
+
+ public void setStartTime(long time) {
+ mStartTime = time;
+ }
+
+ public boolean isActive() {
+ return mStartTime != NO_ANIMATION;
+ }
+
+ public void forceStop() {
+ mStartTime = NO_ANIMATION;
+ }
+
+ public boolean calculate(long currentTimeMillis) {
+ if (mStartTime == NO_ANIMATION) return false;
+ if (mStartTime == ANIMATION_START) mStartTime = currentTimeMillis;
+ int elapse = (int) (currentTimeMillis - mStartTime);
+ float x = Utils.clamp((float) elapse / mDuration, 0f, 1f);
+ Interpolator i = mInterpolator;
+ onCalculate(i != null ? i.getInterpolation(x) : x);
+ if (elapse >= mDuration) mStartTime = NO_ANIMATION;
+ return mStartTime != NO_ANIMATION;
+ }
+
+ abstract protected void onCalculate(float progress);
+}