AndroidGUI21:ImageSwitcher常用技巧

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ImageSwitcher ViewGroup 的派生类, ViewGroup View 的派生类。

ImageSwitcher 很简单见,但在使用它的时候,有一个地方必须要注意,否则就会出现 NullPointerException 。具体情况,请见下面的例子。

在这个例子中,我们将大致要重复做一遍 AndroidGUI19 Gallery 常用技巧中的提到的事情。也就是说,这次我们还要用到 Gallery ,不过这次不再用 ImageView 来显示大图了,而是改用 ImageSwitcher 来代替它。

1. 创建一个 Android Project ,修改 main.xml 使之如下:

<? xml version = "1.0" encoding = "utf-8" ?>

< RelativeLayout xmlns:android = "http://schemas.android.com/apk/res/android"

android:orientation = "vertical"

android:layout_width = "fill_parent"

android:layout_height = "fill_parent"

>

< ImageSwitcher

android:id = "@+id/imageswitcher"

android:layout_width = "match_parent"

android:layout_height = "wrap_content"

android:layout_alignParentTop = "true"

/>

< Gallery

android:id = "@+id/gallery"

android:layout_width = "fill_parent"

android:layout_height = "wrap_content"

android:gravity = "center_vertical"

android:spacing = "10px"

android:layout_marginTop = "10px"

android:layout_alignParentBottom = "true"

/>

<!--

android:layout_alignParentBottom="true"

就是让 Gallery 位于屏幕底端, LinearLayout 不存在这个属性

-->

</ RelativeLayout >

2. 把项目需要用到的图片文件拖入 res/drawable-mdpi 文件夹下,如下图所示

AndroidGUI21:ImageSwitcher常用技巧

3. 实现一个自定义的 Adapter: ImageAdapter ,代码如下:

package com.pat.gui;

import android.content.Context;

import android.graphics.Color;

import android.view.View;

import android.view.ViewGroup;

import android.widget.BaseAdapter;

import android.widget.Gallery;

import android.widget.ImageView;

public class ImageAdapter extends BaseAdapter

{

private Context ctx;

private int images[] =

{

R.drawable.lrt01,

R.drawable.lrt02,

R.drawable.lrt03,

R.drawable.lrt04,

R.drawable.lrt05,

R.drawable.lrt06,

R.drawable.lrt07,

R.drawable.lrt08,

R.drawable.lrt09

};

public ImageAdapter(Context ctx)

{

this.ctx = ctx;

}

//@Override

public int getCount()

{

return images.length;

}

//@Override

public Object getItem(int position)

{

return images[position];

}

//@Override

public long getItemId(int position)

{

return images[position];

}

//@Override

public View getView(int position, View convertView, ViewGroup parent)

{

ImageView view;

view = (ImageView)convertView;

if(view == null)

{

view = new ImageView(ctx);

}

view.setImageResource(images[position]);

view.setScaleType(ImageView.ScaleType.CENTER_INSIDE);

view.setLayoutParams(new Gallery.LayoutParams(60, 60));

view.setBackgroundColor(Color.LTGRAY);

return view;

}

}

4. 实现 Activity 所对应的代码,使之如下 ( 注意其中的粗体字部分 )

package com.pat.gui;

import android.app.Activity;

import android.os.Bundle;

import android.util.DisplayMetrics;

import android.view.View;

import android.view.Window;

import android.view.WindowManager;

import android.widget.AdapterView;

import android.widget.Gallery;

import android.widget.ImageSwitcher;

import android.widget.ImageView;

import android.widget.AdapterView.OnItemSelectedListener;

import android.widget.FrameLayout.LayoutParams;

import android.widget.ViewSwitcher.ViewFactory;

public class ControlImageSwitcher extends Activity

implements

OnItemSelectedListener, ViewFactory

{

private Gallery gallery;

private ImageSwitcher imageswitcher;

private ImageAdapter adapter;

private DisplayMetrics dm;

//@Override

public void onCreate(Bundle savedInstanceState)

{

super.onCreate(savedInstanceState);

requestWindowFeature(Window.FEATURE_NO_TITLE);// 隐藏标题

int flag = WindowManager.LayoutParams.FLAG_FULLSCREEN; // 全屏

this.getWindow().setFlags(flag, flag);

setContentView(R.layout.main);

// 得到屏幕尺寸

dm = new DisplayMetrics();

getWindowManager().getDefaultDisplay().getMetrics(dm);

gallery = (Gallery)this.findViewById(R.id.gallery);

imageswitcher = (ImageSwitcher)this.findViewById(R.id.imageswitcher);

// 下面这句必须要,不然会使 setImageSource 这个方法指向空指针

imageswitcher.setFactory(this);

adapter = new ImageAdapter(this);

gallery.setAdapter(adapter);

gallery.setOnItemSelectedListener(this);

}

//@Override

public void onItemSelected(AdapterView<?> parent, View view, int position, long id)

{

int imageID = (int) adapter.getItemId(position);

imageswitcher.setImageResource(imageID);

}

//@Override

public void onNothingSelected(AdapterView<?> parent)

{

imageswitcher.setImageResource((int) adapter.getItemId(0));

}

//@Override

// 接口 ViewFactory 中规定必须实现的方法,返回的 View 就是 ImageSwitcher 用于显示图片的 vie w

public View makeView()

{

ImageView iv = new ImageView(this);

// 设置背景颜色

iv.setBackgroundColor(0xFF646464);

iv.setScaleType(ImageView.ScaleType.FIT_CENTER);

iv.setLayoutParams(new ImageSwitcher.LayoutParams(LayoutParams.FILL_PARENT, dm.heightPixels - 70));

// 上面语句中 -70 的目的是应为, ImageSwitcher 下面要显示 Gallery ,我们在 ImageAdapter 中设定了

// view.setLayoutParams(new Gallery.LayoutParams(60, 60));

// 即高度是 60pixels ,在加在 main.xml Gallery 有下面这样的属性:

// android:layout_marginTop="10px"

// 因此加起来一共刚好是 70px

return iv;

}

}

运行结果:

AndroidGUI21:ImageSwitcher常用技巧

让下面的 Gallery 中的图片滚动,上面的 ImageSwitcher 会显示 Gallery 在屏幕上处于中间 ( 水平方向 ) 位置的那副图片:

AndroidGUI21:ImageSwitcher常用技巧

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