View工作原理 -- 自定义View（2）

继承View，例子：CircleView

//自定义属性XML，attrs_circleview.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="CircleView">
        <attr name="circle_color" format="color" />
    </declare-styleable>
</resources>

//自定义控件CircleView，继承View
public class CircleView extends View {
    private int mColor = Color.RED;
    private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);

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

    public CircleView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0); //调用重写的构造方法
    }

    public CircleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.CircleView);
        mColor = a.getColor(R.styleable.CircleView_circle_color, Color.RED);
        a.recycle();
        init();
    }

    private void init() {
        mPaint.setColor(mColor);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
        //支持wrap_content
        if(widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(200, 200);
        } else if(widthSpecMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(200, heightSpecSize);
        } else if(heightSpecMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(widthSpecSize, 200);
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //处理padding情况
        final int paddingLeft = getPaddingLeft();
        final int paddingRight = getPaddingRight();
        final int paddingTop = getPaddingTop();
        final int paddingBottom = getPaddingBottom();
        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);
    }
}

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

    <com.tomorrow.androidtest3.CircleView
        android:layout_width="wrap_content"
        android:layout_height="100dp"
        android:layout_margin="20dp"
        android:padding="20dp"
        app:circle_color="#0000FF"
        android:background="#000000"/>

</LinearLayout>

//MainActivity
setContentView(R.layout.activity_main);