fastScrollEnabled:ListView的快速滑动模式,以及滑块的自定义
一、显示ListView快速滑动块图标
设想这样一个场景,当ListView的内容有大于100页的情况下,如果想滑动到第80页,用手指滑动到指定位置,无疑是一件很费时的事情,如果想快速滑动到指定的位置,只需加上ListView的fastScrollEnabled属性等于true,启用快速滑动功能即可。
<ListView
android:id="@android:id/list"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fastScrollEnabled="true" />
注意:默认只有当ListView的内容大于4页时,才会显示快速滑动块。读者可能会问,我是怎么知道要大于4页时,ListView才会显示快速滑动块图标。想知道原因,查一下ListView源码中fastScrollEnabled这个属性是怎么被初始化的吧。打开ListView的源码,你发现在ListView中并没有fastScrollEnabled这个属性,那就去父类AbsListView中找找吧,发现在AbsListView的构造方法中发现了初始化快速滑动块的代码(4.1.2源码在788-789行):
boolean enableFastScroll = a.getBoolean(R.styleable.AbsListView_fastScrollEnabled, false);
setFastScrollEnabled(enableFastScroll);
下面定位到setFastScrollEnabled方法:
/**
* Enables fast scrolling by letting the user quickly scroll through lists by
* dragging the fast scroll thumb. The adapter attached to the list may want
* to implement {@link SectionIndexer} if it wishes to display alphabet preview and
* jump between sections of the list.
* @see SectionIndexer
* @see #isFastScrollEnabled()
* @param enabled whether or not to enable fast scrolling
*/
public void setFastScrollEnabled(boolean enabled) {
mFastScrollEnabled = enabled;
if (enabled) {
if (mFastScroller == null) {
mFastScroller = new FastScroller(getContext(), this);
}
} else {
if (mFastScroller != null) {
mFastScroller.stop();
mFastScroller = null;
}
}
}
从setFastScrollEnabled方法得知,ListView的快速滑动块是通过FastScroller这个类来创建的,接下来打开FastScroller的构造方法,来看下它创建滑动块的流程。(FastScroller这个类在Eclipse关联的源码中看不到,要到Android Framework源码中才能找到,Demo下载地址中包含了这个文件),下面我们来看看FastScroller这个类的关键代码:
package android.widget;
class FastScroller {
//....省略部分源码
// Minimum number of pages to justify showing a fast scroll thumb
private static int MIN_PAGES = 4;
private static final int THUMB_DRAWABLE = 1;
private static final int\[\] ATTRS = new int\[\] {
android.R.attr.fastScrollTextColor,
android.R.attr.fastScrollThumbDrawable, // 定义快速滑动块图标的属性
android.R.attr.fastScrollTrackDrawable,
android.R.attr.fastScrollPreviewBackgroundLeft,
android.R.attr.fastScrollPreviewBackgroundRight,
android.R.attr.fastScrollOverlayPosition
};
//....省略部分源码
private Drawable mThumbDrawable;// 快速滑动块图标
public FastScroller(Context context, AbsListView listView) {
mList = listView;
init(context);
}
private void init(Context context) {
// Get both the scrollbar states drawables
TypedArray ta = context.getTheme().obtainStyledAttributes(ATTRS);
useThumbDrawable(context, ta.getDrawable(THUMB_DRAWABLE)); // 获取当前主题ListView的快速滑动块图标
//....省略部分源码
}
private void useThumbDrawable(Context context, Drawable drawable) {
mThumbDrawable = drawable;
if (drawable instanceof NinePatchDrawable) {
mThumbW = context.getResources().getDimensionPixelSize(
com.android.internal.R.dimen.fastscroll_thumb_width);
mThumbH = context.getResources().getDimensionPixelSize(
com.android.internal.R.dimen.fastscroll_thumb_height);
} else {
mThumbW = drawable.getIntrinsicWidth();
mThumbH = drawable.getIntrinsicHeight();
}
mChangedBounds = true;
}
void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount,
int totalItemCount) {
// Are there enough pages to require fast scroll? Recompute only if total count changes
if (mItemCount != totalItemCount && visibleItemCount > 0) {
mItemCount = totalItemCount;
mLongList = mItemCount / visibleItemCount >= MIN_PAGES; // 至少4页才显示滑动块
}
if (mAlwaysShow) { // android:fastScrollAlwaysVisible="true"
mLongList = true;
}
//....省略部分源码
}
//....省略部分源码
}
由上述代码得知,快速滑动块图标是由fastScrollThumbDrawable定义的,第8行定义了显示快速滑动块的最少页数,第51行onScroll方法负责处理显示快速滑动块的逻辑。
二、自定义快速滑动块图标
在eoe上看到有个贴子通过反射,动态修改FastScroller对象的mThumbDrawable属性来改变快速滑动块的图标,这也不为于一种实现方式,但反射的效率较低。下面将介绍使用Style的方式来自定义图标。
从FastScroller类的init方法中可以得知,mThumbDrawable是通过获取当前Activity主题的android.R.attr.fastScrollThumbDrawable属性赋值,既然是这样的话,我们完全可以自定义一个主题,覆盖android.R.attr.fastScrollThumbDrawable属性对应的Drawable不就搞定了!
1、定义一个主题
<style name="ListViewFastScrollThumb" parent="@android:style/Theme.Light.NoTitleBar.Fullscreen">
<item name="android:fastScrollThumbDrawable">@drawable/ic_launcher</item>
</style>
2、当前ListView所在Activity应用自定义的主题
<activity
android:name="com.example.actionbardemo.MainActivity"
android:label="@string/app_name"
android:theme="@style/ListViewFastScrollThumb" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
3、验证
public class MainActivity extends ListActivity {
private static final int\[\] ATTRS = new int\[\] {
android.R.attr.fastScrollThumbDrawable,
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
setListAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, Cheeses.sCheeseStrings));
ImageView imageView = (ImageView) findViewById(R.id.fastScrollDrawable);
Theme theme = getTheme();
TypedArray a = theme.obtainStyledAttributes(ATTRS);
Drawable drawable = a.getDrawable(0);
imageView.setBackgroundDrawable(drawable);
}
}
布局:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<ImageView
android:id="@+id/fastScrollDrawable"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<ListView
android:id="@android:id/list"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fastScrollEnabled="true"
/>
</LinearLayout>
4、 原生的和自定义的快速滑动块效果图对比: