small changing to spring (#201)

Author: jafu888

constraintlayout/constraintlayout/src/main/java/androidx/constraintlayout/motion/widget/TouchResponse.java

@@ -104,7 +104,7 @@ class TouchResponse {
     static final int FLAG_DISABLE_POST_SCROLL = 1;
     static final int FLAG_DISABLE_SCROLL = 2;
     private float mDragThreshold = 10;
-    private float mSpringDamping = Float.NaN;
+    private float mSpringDamping = 10;
     private float mSpringMass = 1;
     private float mSpringStiffness = Float.NaN;
     private float mSpringStopThreshold = Float.NaN;

constraintlayout/constraintlayout/src/main/java/androidx/constraintlayout/utils/widget/MotionButton.java

@@ -36,7 +36,7 @@
 /**
  * An ImageButton that can display, combine  and filter images. <b>Added in 2.0</b>
  * <p>
- * Subclass of AppCompatImageButton to handle rounding edges dynamically.
+ * Subclass of AppCompatButton to handle rounding edges dynamically.
  * </p>
  * <h2>MotionButton attributes</h2>
  * <td>round</td>

constraintlayout/core/src/main/java/androidx/constraintlayout/core/motion/utils/SpringStopEngine.java

@@ -98,10 +98,17 @@ public boolean isStopped() {
 
     private void compute(double dt) {
         double x = (mPos - mTargetPos);
-        double a = getAcceleration();
-        double dv = a * dt;
-        double avgV = mV + dv / 2;
-        mV += dv / 2;
+        double k = mStiffness;
+        double c = mDamping;
+        double a = (-k * x - c * mV) / mMass;
+        // This refinement of a simple coding of the acceleration increases accuracy
+        double avgV = mV + a * dt / 2; // pass 1 calculate the average velocity
+        double avgX = x + dt * (avgV) / 2 - mTargetPos;// pass 1 calculate the average pos
+        a = (-avgX * k - avgV * c) / mMass; //  calculate acceleration over that average pos
+
+        double dv = a * dt; //  calculate change in velocity
+        avgV = mV + dv / 2; //  average  velocity is current + half change
+        mV += dv;
         mPos += avgV * dt;
         if (mBoundaryMode > 0) {
             if (mPos < 0 && ((mBoundaryMode & 1) == 1)) {
@@ -114,4 +121,5 @@ private void compute(double dt) {
             }
         }
     }
+    
 }

projects/MotionLayoutVerification/app/src/main/res/xml/verification_scene_501.xml

@@ -10,9 +10,7 @@
         <OnSwipe
             motion:touchAnchorId="@+id/clock"
             motion:dragDirection="dragLeft"
-            motion:springDamping="10"
             motion:onTouchUp="neverCompleteToEnd"
-            motion:springMass="1"
             motion:springBoundary="overshoot"
             motion:springStiffness="500" />
         <KeyFrameSet></KeyFrameSet>
@@ -27,7 +25,7 @@
         <OnSwipe
             motion:touchAnchorId="@+id/clock"
             motion:dragDirection="dragRight"
-            motion:springDamping="10"
+            motion:springDamping="0.2"
             motion:springMass="1"
             motion:springBoundary="overshoot"
             motion:onTouchUp="neverCompleteToEnd"