Android从入门到粗通（八）——List View

还记得那个BeautyActivity吗?它对应的layout的xml里定义了一个ListView。要想在ListView里面加入item，只能通过代码（不能直接在xml里）：

String[] items = {"Item1", "Item2", "Item3"};
        int itemResId = android.R.layout.simple_list_item_1; // TextView

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(
                this, itemResId, items);
        ListView lv = (ListView)findViewById(R.id.my_list_view);
        lv.setAdapter(adapter);

android.R.layout.simple_list_item_1就是TextView的id。通过一个ArrayAdapter，ListView设置了自己的item列表。

如果我们不想使用默认的TextView显示ListView的一个item，而是要自己定义item的外观，怎么办？首先定义一个View的子类（SadItemView.java）：

package tommy.myandroid;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.widget.TextView;

public class SadItemView extends TextView {
    public SadItemView(Context context) {
        super(context);
    }

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

    public SadItemView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas); // draw text
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(0xffffffff);
        canvas.drawLine(0, getMeasuredHeight() / 2,
                getMeasuredWidth(), getMeasuredHeight() / 2,
                paint);
    }
}

为了重用绘制文字的功能，SadItemView继承了TextView。类View有三个构造器，最好都定义好并调用父类的构造器。覆写onDraw可以自定义view的外观。

除了类文件，要在ListView里加入自己的item，还需要定义一个layout的xml（saditemview.xml）：

<?xml version="1.0" encoding="utf-8"?>
<tommy.myandroid.SadItemView
  xmlns:android=""
  android:orientation="vertical"
  android:layout_width="match_parent"
  android:layout_height="match_parent" />

注意saditemview.xml里只有一个结点，而且结点名与刚刚定义的类名一样。定义好java类和layou后，就可以在ListView里添加我们自定义的item了：

String[] items = {"Item1", "Item2", "Item3"};
//int itemResId = android.R.layout.simple_list_item_1;
int itemResId = R.layout.saditemview;

ArrayAdapter<String> adapter = new ArrayAdapter<String>(
    this, itemResId, items);
ListView lv = (ListView)findViewById(R.id.my_list_view);
lv.setAdapter(adapter);

上面的代码使用了我们刚刚定义好的resource id，运行结果：

自定义一个全新的View

class SimpleView extends View {
    public SimpleView(Context context) {
        super(context);
    }
    public SimpleView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    public SimpleView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int measuredWidth = radius * 2;
        if (widthSpecMode == MeasureSpec.AT_MOST) {
            if (measuredWidth > widthSpecSize)
                measuredWidth = widthSpecSize;
        } else if (widthSpecMode == MeasureSpec.EXACTLY){
            measuredWidth = widthSpecSize;
        }

        int measuredHeight = radius * 2;
        switch (MeasureSpec.getMode(heightMeasureSpec)) {
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
        case MeasureSpec.UNSPECIFIED:
            measuredHeight = MeasureSpec.getSize(heightMeasureSpec);
            break;
        }

        setMeasuredDimension(measuredWidth, measuredHeight);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        int width = getMeasuredWidth();
        int height = getMeasuredHeight();
        int r = Math.min(width, height) / 2;
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(color);
        canvas.drawCircle(width/2, height/2, r, paint);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        color &= 0x00ffffff;
        if (color == 0x000000ff)
            color = 0x00ff0000;
        else
            color >>= 8;
        color |= 0xff000000; // alpha
        invalidate();
        return true;
    }

    private int radius = 5;
    private int color = 0xffff0000;
}

自定义的View主要需要覆写OnMeaure方法和OnDraw方法（画一个圈）。上例的类还通过响应了onTouchEvent，并改变圆的颜色。可以使用setContentView来设置这个View。运行结果：