Android中Fab(FloatingActionButton)实现上下滑动的渐变效果

前言

Promoted Actions是指一种操作按钮，它不是放在actionbar中，而是直接在可见的UI布局中（当然这里的UI指的是setContentView所管辖的范围）。因此它更容易在代码中被获取到（试想如果你要在actionbar中获取一个菜单按钮是不是很难？），Promoted Actions往往主要用于一个界面的主要操作，比如在email的邮件列表界面，promoted action可以用于接受一个新邮件。promoted action在外观上其实就是一个悬浮按钮，更常见的是漂浮在界面上的圆形按钮，一般我直接将promoted action称作悬浮按钮，英文名称Float Action Button 简称（FAB，不是FBI哈）。

系统自带的 Fab 也会随着页面上下滚动,但是淡出或者进入的效果太不自然。

这里记录一个小知识点,Fab 随着页面的 RecyclerView 上下滚动而渐变的动画效果。

包含 Fab 控件的布局如下:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".view.activity.MainActivity">

<android.support.design.widget.AppBarLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="@style/AppTheme.AppBarOverlay">

<android.support.v7.widget.Toolbar
 android:id="@+id/toolbar"
 android:layout_width="match_parent"
 android:layout_height="?attr/actionBarSize"
 android:background="?attr/colorPrimary"
 app:layout_scrollFlags="scroll|enterAlways"
 app:popupTheme="@style/AppTheme.PopupOverlay" />

<android.support.design.widget.TabLayout
 android:id="@+id/tab_layout"
 app:tabIndicatorColor="#FFFFFF"
 android:layout_width="match_parent"
 android:layout_height="wrap_content">
</android.support.design.widget.TabLayout>
</android.support.design.widget.AppBarLayout>

<include layout="@layout/content_main" />

<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="@dimen/fab_margin"
android:src="@android:drawable/ic_dialog_email"
app:layout_behavior="com.wu.allen.zhuanlan.util.ScrollAwareFABBehavior"/>

</android.support.design.widget.CoordinatorLayout>

实现的 Java 代码如下:

public class ScrollAwareFABBehavior extends FloatingActionButton.Behavior {
private static final Interpolator INTERPOLATOR = new FastOutSlowInInterpolator();
private boolean mIsAnimatingOut = false;

public ScrollAwareFABBehavior(Context context, AttributeSet attrs) {
super();
}

@Override
public boolean onStartNestedScroll(final CoordinatorLayout coordinatorLayout, final FloatingActionButton child,
    final View directTargetChild, final View target, final int nestedScrollAxes) {
// Ensure we react to vertical scrolling
return nestedScrollAxes == ViewCompat.SCROLL_AXIS_VERTICAL
 || super.onStartNestedScroll(coordinatorLayout, child, directTargetChild, target, nestedScrollAxes);
}

@Override
public void onNestedScroll(final CoordinatorLayout coordinatorLayout, final FloatingActionButton child,
   final View target, final int dxConsumed, final int dyConsumed,
   final int dxUnconsumed, final int dyUnconsumed) {
super.onNestedScroll(coordinatorLayout, child, target, dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed);
if (dyConsumed > 0 && !this.mIsAnimatingOut && child.getVisibility() == View.VISIBLE) {
 // User scrolled down and the FAB is currently visible -> hide the FAB
 animateOut(child);
} else if (dyConsumed < 0 && child.getVisibility() != View.VISIBLE) {
 // User scrolled up and the FAB is currently not visible -> show the FAB
 animateIn(child);
}
}

// Same animation that FloatingActionButton.Behavior uses to hide the FAB when the AppBarLayout exits
private void animateOut(final FloatingActionButton button) {
if (Build.VERSION.SDK_INT >= 14) {
 ViewCompat.animate(button).scaleX(0.0F).scaleY(0.0F).alpha(0.0F).setInterpolator(INTERPOLATOR).withLayer()
  .setListener(new ViewPropertyAnimatorListener() {
  public void onAnimationStart(View view) {
   ScrollAwareFABBehavior.this.mIsAnimatingOut = true;
  }

public void onAnimationCancel(View view) {
   ScrollAwareFABBehavior.this.mIsAnimatingOut = false;
  }

public void onAnimationEnd(View view) {
   ScrollAwareFABBehavior.this.mIsAnimatingOut = false;
   view.setVisibility(View.GONE);
  }
  }).start();
} else {
 Animation anim = AnimationUtils.loadAnimation(button.getContext(), R.anim.fab_out);
 anim.setInterpolator(INTERPOLATOR);
 anim.setDuration(200L);
 anim.setAnimationListener(new Animation.AnimationListener() {
 public void onAnimationStart(Animation animation) {
  ScrollAwareFABBehavior.this.mIsAnimatingOut = true;
 }

public void onAnimationEnd(Animation animation) {
  ScrollAwareFABBehavior.this.mIsAnimatingOut = false;
  button.setVisibility(View.GONE);
 }

@Override
 public void onAnimationRepeat(final Animation animation) {
 }
 });
 button.startAnimation(anim);
}
}

// Same animation that FloatingActionButton.Behavior uses to show the FAB when the AppBarLayout enters
private void animateIn(FloatingActionButton button) {
button.setVisibility(View.VISIBLE);
if (Build.VERSION.SDK_INT >= 14) {
 ViewCompat.animate(button).scaleX(1.0F).scaleY(1.0F).alpha(1.0F)
  .setInterpolator(INTERPOLATOR).withLayer().setListener(null)
  .start();
} else {
 Animation anim = AnimationUtils.loadAnimation(button.getContext(), R.anim.fab_in);
 anim.setDuration(200L);
 anim.setInterpolator(INTERPOLATOR);
 button.startAnimation(anim);
}
}
}

fab_in.xml 文件如下(fab_out.xml 同理),当然要改变淡出或者进入的样式,一般修改这里的 XML 文件就可以了 :

<?xml version="1.0" encoding="utf-8"?>
<set xmlns:android="http://schemas.android.com/apk/res/android">

<alpha android:fromAlpha="0.0"
android:toAlpha="1.0"/>

<scale android:fromXScale="0.0"
android:fromYScale="0.0"
android:toXScale="1.0"
android:toYScale="1.0"
android:pivotX="50％"
android:pivotY="50％"/>
</set>

<?xml version="1.0" encoding="utf-8"?>
<set xmlns:android="http://schemas.android.com/apk/res/android">

<alpha android:fromAlpha="1.0"
android:toAlpha="0.0"/>

<scale android:fromXScale="1.0"
android:fromYScale="1.0"
android:toXScale="0.0"
android:toYScale="0.0"
android:pivotX="50％"
android:pivotY="50％"/>

</set>

大致效果就像上面。

总结

好了，以上就是这篇文章的全部内容了，希望本文的内容对各位Android开发者们能带来一定的帮助，如果有疑问大家可以留言交流。