Don't show gap in SlidingRelativeLayout (Fix #841) (#881)

* Don't show gap in SlidingRelativeLayout (Fix #841)

* minor improvemenents / bugfixes

- add getters (again)
- fix: check if view tree observer is alive before adding/removing listener to avoid possible crash
- fix: calling both setXFraction and setYFraction before view is measured did not correctly set the former onPreDraw

app/src/main/java/de/westnordost/streetcomplete/view/SlidingRelativeLayout.java

@@ -2,11 +2,14 @@
 
 import android.content.Context;
 import android.util.AttributeSet;
-import android.view.View;
+import android.view.ViewTreeObserver;
 import android.widget.RelativeLayout;
 
 public class SlidingRelativeLayout extends RelativeLayout
 {
+	private float yFraction;
+	private float xFraction;
+
 	public SlidingRelativeLayout(Context context)
 	{
 		super(context);
@@ -24,36 +27,49 @@ public SlidingRelativeLayout(Context context, AttributeSet attrs, int defStyleAt
 
 	public void setYFraction(float fraction)
 	{
-		/* Here we have to tackle two problems:
-		   - the layout may not be measured yet. If it isn't hidden, this will result in an ugly
-		     flicker in the first frame of the animation
-		   - users can turn animations off (i.e. battery saver mode), which results in that all
-		     animations "start" in their final frame immediately. If the view is hidden in the first
-		     frame, it will never appear
-		 solution: hide it only if it was not measured yet AND it should be translated */
-		setVisibility(getHeight() == 0 && fraction != 0 ? View.INVISIBLE : View.VISIBLE);
-
-		float translationY = getHeight() * fraction;
-		setTranslationY(translationY);
+		this.yFraction = fraction;
+		postAfterViewMeasured(() -> setTranslationY(getHeight() * yFraction));
 	}
 
 	public float getYFraction()
 	{
-		if(getHeight() == 0) return 0;
-		return getTranslationY() / getHeight();
+		return yFraction;
 	}
 
 	public void setXFraction(float fraction)
 	{
-		setVisibility(getWidth() == 0 && fraction != 0 ? View.INVISIBLE : View.VISIBLE);
-
-		float translationX = getWidth() * fraction;
-		setTranslationX(translationX);
+		this.xFraction = fraction;
+		postAfterViewMeasured(() -> setTranslationX(getWidth() * xFraction));
 	}
 
 	public float getXFraction()
 	{
-		if(getWidth() == 0) return 0;
-		return getTranslationX() / getWidth();
+		return xFraction;
+	}
+
+	private void postAfterViewMeasured(Runnable runnable)
+	{
+		if (getWidth() != 0 || getHeight() != 0)
+		{
+			runnable.run();
+		}
+		else
+		{
+			if(getViewTreeObserver().isAlive())
+			{
+				getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener()
+				{
+					@Override public boolean onPreDraw()
+					{
+						runnable.run();
+						if(getViewTreeObserver().isAlive())
+						{
+							getViewTreeObserver().removeOnPreDrawListener(this);
+						}
+						return true;
+					}
+				});
+			}
+		}
 	}
 }