Android 自定义View(二) onSizeChanged、onDraw、onMeasure顺序

上一篇《Android 自定义View(一) Paint、Rect、Canvas介绍》讲了最基础的如何自定义一个View，以及View用到的一些工具类。下面讲下View绘制的方法及过程

public class MyView extends View {
    private String TAG = "--------MyView";
    private int width, height;
    public MyView(Context context, AttributeSet attrs) {
        super(context, attrs);
        Log.i(TAG,"MyView()");
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        width = w;
        height = h;
        Log.i(TAG,"onSizeChanged()");
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Log.i(TAG,"onDraw()");

        Rect rect = new Rect(0, 0, width, height);
        Paint paint = new Paint();
        paint.setColor(Color.RED);
        canvas.drawRect(rect, paint);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        Log.i(TAG,"onMeasure()");
    }
    

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        Log.i(TAG,"onFinishInflate()");
    }
}

03-15 01:28:53.111 30961-30961/com.myView I/--------MyView: MyView()
03-15 01:28:53.111 30961-30961/com.myView I/--------MyView: onFinishInflate()
03-15 01:28:53.210 30961-30961/com.myView I/--------MyView: onMeasure()
03-15 01:28:53.210 30961-30961/com.myView I/--------MyView: onMeasure()
03-15 01:28:53.520 30961-30961/com.myView I/--------MyView: onMeasure()
03-15 01:28:53.520 30961-30961/com.myView I/--------MyView: onMeasure()
03-15 01:28:53.520 30961-30961/com.myView I/--------MyView: onSizeChanged()
03-15 01:28:53.543 30961-30961/com.myView I/--------MyView: onDraw()

---

1、MyView() 构造方法，这个不做解释，铁定第一个被调用。

作用：传入Context

2、onFinishInflate() 当View中所有的子控件均被映射成xml后触发

3、onMeasure() 在View放置到父容器时调用

作用：测量View的大小，也可以通过下面方式，修改View的大小

 @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        setMeasuredDimension(100,100);
    }

也可以通过以下方式获取控件的宽和高

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int width = MeasureSpec.getSize(widthMeasureSpec);
       int height = MeasureSpec.getSize(heightMeasureSpec));
    }

4、onSizeChanged() 在控件大小发生改变时调用。所以这里初始化会被调用一次

作用：获取控件的宽和高度

5、onDraw() 最后就是绘制图形了