在自定義控制元件時，很多童鞋發現在XML佈局中寫上padding屬性卻不起作用，而且wrap_content和march_parent顯示效果一樣，這就需要我們在程式碼中對自定義View寬高做相應的改動，以自定義一個簡單的圓示例，如下圖（加了padding屬性）：

程式碼如下：

    public class CircleView extends View {
    private Paint mPaint;

    public CircleView(Context context, AttributeSet attrs, int defStyleAttr) {
        super
 
(context, attrs, defStyleAttr);
        initView();
    }

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

    public CircleView(Context context) {
        super(context);
        initView();
    }

    private void initView() {
        mPaint = new
 
 Paint(Paint.ANTI_ALIAS_FLAG);
        mPaint.setColor(Color.RED);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeWidth(8);
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // TODO Auto-generated method stub
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        //對寬高的測量
 

        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        if (widthMode==MeasureSpec.AT_MOST&&heightMode==MeasureSpec.AT_MOST) {
            setMeasuredDimension(300, 100);
        }else if (widthMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(300, heightSize);
        }else if (heightMode ==MeasureSpec.AT_MOST) {
            setMeasuredDimension(widthSize, 100);
        }
    }
    @Override
    protected void onDraw(Canvas canvas) {
        // TODO Auto-generated method stub
        super.onDraw(canvas);
        int paddingBottom = getPaddingBottom();
        int paddingLeft = getPaddingLeft();
        int paddingRight = getPaddingRight();
        int paddingTop = getPaddingTop();
        //主要是考慮到VIEW周圍的空白
        int width = getWidth()-paddingLeft-paddingRight;
        int height = getHeight()-paddingTop-paddingBottom;
        int radius = Math.min(width, height)/2;
        canvas.drawCircle(paddingLeft+width/2, paddingTop+height/2, radius, mPaint);
    }
}

circle.xml佈局程式碼如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#ffffff"
    android:orientation="vertical" >

    <com.example.customview.CircleView
        android:layout_width="wrap_content"
        android:layout_height="100dp"
        android:layout_margin="20dp"
        android:background="#000000"
        android:padding="20dp" />

</LinearLayout>

關於自定義view，可以參考《android開發藝術探索》《android群英傳》一書，