core/java/android/widget/EdgeGlow.java

/*
 * 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 android.widget;

import android.graphics.Canvas;
import android.graphics.drawable.Drawable;
import android.view.animation.AnimationUtils;
import android.view.animation.DecelerateInterpolator;
import android.view.animation.Interpolator;

/**
 * This class performs the glow effect used at the edges of scrollable widgets.
 * @hide
 */
public class EdgeGlow {
    private static final String TAG = "EdgeGlow";

    // Time it will take the effect to fully recede in ms
    private static final int RECEDE_TIME = 1000;

    // Time it will take before a pulled glow begins receding
    private static final int PULL_TIME = 250;

    // Time it will take for a pulled glow to decay to partial strength before release
    private static final int PULL_DECAY_TIME = 10000;

    private static final float MAX_ALPHA = 1.f;
    private static final float HELD_EDGE_ALPHA = 0.7f;
    private static final float HELD_EDGE_SCALE_Y = 0.5f;
    private static final float HELD_GLOW_ALPHA = 0.5f;
    private static final float HELD_GLOW_SCALE_Y = 0.5f;

    private static final float MAX_GLOW_HEIGHT = 3.f;

    private static final float PULL_GLOW_BEGIN = 1.f;
    private static final float PULL_EDGE_BEGIN = 0.6f;

    // Minimum velocity that will be absorbed
    private static final int MIN_VELOCITY = 100;

    private static final float EPSILON = 0.001f;

    private final Drawable mEdge;
    private final Drawable mGlow;
    private int mWidth;
    private int mHeight;

    private float mEdgeAlpha;
    private float mEdgeScaleY;
    private float mGlowAlpha;
    private float mGlowScaleY;

    private float mEdgeAlphaStart;
    private float mEdgeAlphaFinish;
    private float mEdgeScaleYStart;
    private float mEdgeScaleYFinish;
    private float mGlowAlphaStart;
    private float mGlowAlphaFinish;
    private float mGlowScaleYStart;
    private float mGlowScaleYFinish;

    private long mStartTime;
    private float mDuration;

    private final Interpolator mInterpolator;

    private static final int STATE_IDLE = 0;
    private static final int STATE_PULL = 1;
    private static final int STATE_ABSORB = 2;
    private static final int STATE_RECEDE = 3;
    private static final int STATE_PULL_DECAY = 4;

    // How much dragging should effect the height of the edge image.
    // Number determined by user testing.
    private static final int PULL_DISTANCE_EDGE_FACTOR = 5;

    // How much dragging should effect the height of the glow image.
    // Number determined by user testing.
    private static final int PULL_DISTANCE_GLOW_FACTOR = 5;

    private static final int VELOCITY_EDGE_FACTOR = 8;
    private static final int VELOCITY_GLOW_FACTOR = 16;

    private int mState = STATE_IDLE;

    private float mPullDistance;

    public EdgeGlow(Drawable edge, Drawable glow) {
        mEdge = edge;
        mGlow = glow;

        mInterpolator = new DecelerateInterpolator();
    }

    public void setSize(int width, int height) {
        mWidth = width;
        mHeight = height;
    }

    public boolean isFinished() {
        return mState == STATE_IDLE;
    }

    public void finish() {
        mState = STATE_IDLE;
    }

    /**
     * Call when the object is pulled by the user.
     *
     * @param deltaDistance Change in distance since the last call
     */
    public void onPull(float deltaDistance) {
        final long now = AnimationUtils.currentAnimationTimeMillis();
        if (mState == STATE_PULL_DECAY && now - mStartTime < mDuration) {
            return;
        }
        if (mState != STATE_PULL) {
            mGlowScaleY = PULL_GLOW_BEGIN;
        }
        mState = STATE_PULL;

        mStartTime = now;
        mDuration = PULL_TIME;

        mPullDistance += deltaDistance;
        float distance = Math.abs(mPullDistance);

        mEdgeAlpha = mEdgeAlphaStart = Math.max(PULL_EDGE_BEGIN, Math.min(distance, MAX_ALPHA));
        mEdgeScaleY = mEdgeScaleYStart = Math.max(
                HELD_EDGE_SCALE_Y, Math.min(distance * PULL_DISTANCE_EDGE_FACTOR, 1.f));

        mGlowAlpha = mGlowAlphaStart = Math.max(
                0.5f, Math.min(mGlowAlpha + Math.abs(deltaDistance), MAX_ALPHA));

        float glowChange = Math.abs(deltaDistance);
        if (deltaDistance > 0 && mPullDistance < 0) {
            glowChange = -glowChange;
        }
        if (mPullDistance == 0) {
            mGlowScaleY = 0;
        }
        mGlowScaleY = mGlowScaleYStart = Math.max(
                0, mGlowScaleY + glowChange * PULL_DISTANCE_GLOW_FACTOR);

        mEdgeAlphaFinish = mEdgeAlpha;
        mEdgeScaleYFinish = mEdgeScaleY;
        mGlowAlphaFinish = mGlowAlpha;
        mGlowScaleYFinish = mGlowScaleY;
    }

    /**
     * Call when the object is released after being pulled.
     */
    public void onRelease() {
        mPullDistance = 0;

        if (mState != STATE_PULL && mState != STATE_PULL_DECAY) {
            return;
        }

        mState = STATE_RECEDE;
        mEdgeAlphaStart = mEdgeAlpha;
        mEdgeScaleYStart = mEdgeScaleY;
        mGlowAlphaStart = mGlowAlpha;
        mGlowScaleYStart = mGlowScaleY;

        mEdgeAlphaFinish = 0.f;
        mEdgeScaleYFinish = 0.f;
        mGlowAlphaFinish = 0.f;
        mGlowScaleYFinish = 0.f;

        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mDuration = RECEDE_TIME;
    }

    /**
     * Call when the effect absorbs an impact at the given velocity.
     *
     * @param velocity Velocity at impact in pixels per second.
     */
    public void onAbsorb(int velocity) {
        mState = STATE_ABSORB;
        velocity = Math.max(MIN_VELOCITY, Math.abs(velocity));

        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mDuration = 0.1f + (velocity * 0.03f);

        // The edge should always be at least partially visible, regardless
        // of velocity.
        mEdgeAlphaStart = 0.5f;
        mEdgeScaleYStart = 0.2f;
        // The glow depends more on the velocity, and therefore starts out
        // nearly invisible.
        mGlowAlphaStart = 0.5f;
        mGlowScaleYStart = 0.f;

        // Factor the velocity by 8. Testing on device shows this works best to
        // reflect the strength of the user's scrolling.
        mEdgeAlphaFinish = Math.max(0, Math.min(velocity * VELOCITY_EDGE_FACTOR, 1));
        // Edge should never get larger than the size of its asset.
        mEdgeScaleYFinish = 1.f;

        // Growth for the size of the glow should be quadratic to properly
        // respond
        // to a user's scrolling speed. The faster the scrolling speed, the more
        // intense the effect should be for both the size and the saturation.
        mGlowScaleYFinish = Math.min(0.025f + (velocity * (velocity / 100) * 0.00015f), 1.75f);
        // Alpha should change for the glow as well as size.
        mGlowAlphaFinish = Math.max(
                mGlowAlphaStart, Math.min(velocity * VELOCITY_GLOW_FACTOR * .00001f, MAX_ALPHA));
    }


    /**
     * Draw into the provided canvas. Assumes that the canvas has been rotated
     * accordingly and the size has been set. The effect will be drawn the full
     * width of X=0 to X=width, emitting from Y=0 and extending to some factor <
     * 1.f of height.
     *
     * @param canvas Canvas to draw into
     * @return true if drawing should continue beyond this frame to continue the
     *         animation
     */
    public boolean draw(Canvas canvas) {
        update();

        final int edgeHeight = mEdge.getIntrinsicHeight();
        final int glowHeight = mGlow.getIntrinsicHeight();

        final float distScale = (float) mHeight / mWidth;

        mGlow.setAlpha((int) (Math.max(0, Math.min(mGlowAlpha, 1)) * 255));
        // Width of the image should be 3 * the width of the screen.
        // Should start off screen to the left.
        mGlow.setBounds(-mWidth, 0, mWidth * 2, (int) Math.min(
                glowHeight * mGlowScaleY * distScale * 0.6f, mHeight * MAX_GLOW_HEIGHT));
        mGlow.draw(canvas);

        mEdge.setAlpha((int) (Math.max(0, Math.min(mEdgeAlpha, 1)) * 255));
        mEdge.setBounds(0, 0, mWidth, (int) (edgeHeight * mEdgeScaleY));
        mEdge.draw(canvas);

        return mState != STATE_IDLE;
    }

    private void update() {
        final long time = AnimationUtils.currentAnimationTimeMillis();
        final float t = Math.min((time - mStartTime) / mDuration, 1.f);

        final float interp = mInterpolator.getInterpolation(t);

        mEdgeAlpha = mEdgeAlphaStart + (mEdgeAlphaFinish - mEdgeAlphaStart) * interp;
        mEdgeScaleY = mEdgeScaleYStart + (mEdgeScaleYFinish - mEdgeScaleYStart) * interp;
        mGlowAlpha = mGlowAlphaStart + (mGlowAlphaFinish - mGlowAlphaStart) * interp;
        mGlowScaleY = mGlowScaleYStart + (mGlowScaleYFinish - mGlowScaleYStart) * interp;

        if (t >= 1.f - EPSILON) {
            switch (mState) {
                case STATE_ABSORB:
                    mState = STATE_RECEDE;
                    mStartTime = AnimationUtils.currentAnimationTimeMillis();
                    mDuration = RECEDE_TIME;

                    mEdgeAlphaStart = mEdgeAlpha;
                    mEdgeScaleYStart = mEdgeScaleY;
                    mGlowAlphaStart = mGlowAlpha;
                    mGlowScaleYStart = mGlowScaleY;

                    mEdgeAlphaFinish = 0.f;
                    mEdgeScaleYFinish = mEdgeScaleY;
                    mGlowAlphaFinish = 0.f;
                    mGlowScaleYFinish = mGlowScaleY;
                    break;
                case STATE_PULL:
                    mState = STATE_PULL_DECAY;
                    mStartTime = AnimationUtils.currentAnimationTimeMillis();
                    mDuration = PULL_DECAY_TIME;

                    mEdgeAlphaStart = mEdgeAlpha;
                    mEdgeScaleYStart = mEdgeScaleY;
                    mGlowAlphaStart = mGlowAlpha;
                    mGlowScaleYStart = mGlowScaleY;

                    // After a pull, the glow should fade to nothing.
                    mEdgeAlphaFinish = 0.f;
                    mEdgeScaleYFinish = 0.f;
                    mGlowAlphaFinish = 0.f;
                    mGlowScaleYFinish = 0.f;
                    break;
                case STATE_PULL_DECAY:
                    // Do nothing; wait for release
                    break;
                case STATE_RECEDE:
                    mState = STATE_IDLE;
                    break;
            }
        }
    }
}