/**
 * 自定义一个简单的 View
 * 
 * @author GAOYUAN
 *
 */
public class CustomView1 extends View {
    private Paint mPaint = new Paint();
    private Rect mRect = new Rect();
    // 一般要实现下面两个构造方法
    public CustomView1(Context context) {
        super(context);
    }
    // 第二个参数后面会讲到
    public CustomView1(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        mPaint.setStyle(Style.FILL);
        mPaint.setColor(Color.BLUE);
        mRect.set(0, 0, 100, 100);
        // Avoid object allocations during draw/layout operations (preallocate
        // and reuse instead)
        // 避免在 draw/layout 方法中进行对象分配, 提前分配, 或者重用.
        // canvas.drawRect(new Rect(0, 0, 100, 100), mPaint);
        canvas.drawRect(mRect, mPaint);
        // mPaint.reset();
        mPaint.setColor(Color.RED);
        mPaint.setTextSize(30.0f);
        // 注意, 画文字的时候, 是从左下角开始画的.
        canvas.drawText("Hello!", 0, 100, mPaint);
    }
}

    <TextView
        android:id="@+id/tv_hello"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />
    <com.gaoyuan4122.customui.views.CustomView1
        android:id="@+id/cv_custom1"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_below="@id/tv_hello" />
    
    <TextView
        android:id="@+id/tv_hello2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world"
        android:layout_below="@id/cv_custom1" />

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        widthMeasureSpec = MeasureSpec.makeMeasureSpec(100, MeasureSpec.EXACTLY);
        heightMeasureSpec = MeasureSpec.makeMeasureSpec(100, MeasureSpec.EXACTLY);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }