android - 自定义视图 custom view

参考：https://stackoverflow.com/documentation/android/1446/creating-custom-views#t=201709020218142901513
关于3 个constructor的解释，见 : https://stackoverflow.com/questions/9195713/do-i-need-all-three-constructors-for-an-android-custom-view

1. 建立xml (非常简单）

    <com.yunbi.SmileyView
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        />

2. 建立对应的class . 这里注意：　3个constructor，三个参数的很少用（用于为所有的view统一设置style) , 二个参数的用于通过XML来　inflate , 一个参数的用于使用代码来创建， onMeasure,  onDraw必须都在。

package com.yunbi;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;

/**
 */

public class SmileyView extends View {

    Paint circle;
    float centerX;
    float centerY;
    float radius;


    public SmileyView(Context context) {
        this(context, null, 0);
    }

    public SmileyView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public SmileyView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initPaints();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec){
        int w = MeasureSpec.getSize(widthMeasureSpec);
        int h = MeasureSpec.getSize(heightMeasureSpec);
        int size = Math.min(w, h);
        setMeasuredDimension(size, size);
    }

    @Override
    protected void onDraw(Canvas canvas){
        canvas.drawCircle(centerX, centerY, radius, circle);

    }

    private void initPaints(){
        circle = new Paint(Paint.ANTI_ALIAS_FLAG);
        circle.setStyle(Paint.Style.FILL);
        circle.setColor(Color.YELLOW);
    }

    @Override
    protected void onSizeChanged(int w, int h ,int oldW, int oldH){
        centerX = w / 2f;
        centerY = h / 2f;
        radius = Math.min(w, h ) / 2f;
    }
}