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。咱们到Eclipse中按ctrl+shift+t打开ListView的源码(前提是要和源码关联才能看到),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:
Demo下载地址:http://download.csdn.net/detail/xyang81/6788411