Add support for start delay in shimmer library. (#92)

Summary:
Shimmer library right now only supports repeat delay but not start delay while ValueAnimator supports both. This is useful to stagger the shimmer animations in case there are multiple ShimmerFrameLayouts in a view.
Pull Request resolved: #92

Differential Revision: D18642161

Pulled By: xiphirx

fbshipit-source-id: d30e13194d27d7a12e6a01dc79b9f876679de8a4

shimmer/src/main/java/com/facebook/shimmer/Shimmer.java

@@ -78,6 +78,7 @@ public class Shimmer {
   int repeatMode = ValueAnimator.RESTART;
   long animationDuration = 1000L;
   long repeatDelay;
+  long startDelay;
 
   Shimmer() {}
 
@@ -179,7 +180,11 @@ T consumeAttributes(TypedArray a) {
         setRepeatMode(
             a.getInt(R.styleable.ShimmerFrameLayout_shimmer_repeat_mode, mShimmer.repeatMode));
       }
-
+      if (a.hasValue(R.styleable.ShimmerFrameLayout_shimmer_start_delay)) {
+        setStartDelay(
+            a.getInt(
+                R.styleable.ShimmerFrameLayout_shimmer_start_delay, (int) mShimmer.startDelay));
+      }
       if (a.hasValue(R.styleable.ShimmerFrameLayout_shimmer_direction)) {
         int direction =
             a.getInt(R.styleable.ShimmerFrameLayout_shimmer_direction, mShimmer.direction);
@@ -260,6 +265,7 @@ public T copyFrom(Shimmer other) {
       setRepeatCount(other.repeatCount);
       setRepeatMode(other.repeatMode);
       setRepeatDelay(other.repeatDelay);
+      setStartDelay(other.startDelay);
       setDuration(other.animationDuration);
       mShimmer.baseColor = other.baseColor;
       mShimmer.highlightColor = other.highlightColor;
@@ -399,6 +405,15 @@ public T setRepeatDelay(long millis) {
       return getThis();
     }
 
+    /** Sets how long to wait for starting the shimmering animation. */
+    public T setStartDelay(long millis) {
+      if (millis < 0) {
+        throw new IllegalArgumentException("Given a negative start delay: " + millis);
+      }
+      mShimmer.startDelay = millis;
+      return getThis();
+    }
+
     /** Sets how long the shimmering animation takes to do one full sweep. */
     public T setDuration(long millis) {
       if (millis < 0) {

shimmer/src/main/java/com/facebook/shimmer/ShimmerDrawable.java

@@ -163,6 +163,7 @@ private void updateValueAnimator() {
     mValueAnimator =
         ValueAnimator.ofFloat(0f, 1f + (float) (mShimmer.repeatDelay / mShimmer.animationDuration));
     mValueAnimator.setRepeatMode(mShimmer.repeatMode);
+    mValueAnimator.setStartDelay(mShimmer.startDelay);
     mValueAnimator.setRepeatCount(mShimmer.repeatCount);
     mValueAnimator.setDuration(mShimmer.animationDuration + mShimmer.repeatDelay);
     mValueAnimator.addUpdateListener(mUpdateListener);

shimmer/src/main/res/values/attrs.xml

@@ -15,6 +15,7 @@
       <enum name="restart" value="1"/>
       <enum name="reverse" value="2"/>
     </attr>
+    <attr name="shimmer_start_delay" format="integer"/>
     <attr name="shimmer_direction" format="enum">
       <enum name="left_to_right" value="0"/>
       <enum name="top_to_bottom" value="1"/>