自定义控件之onMeasure

最近一直在接触自定义控件的知识，自己就尝试着写了一个小的demo，算是对自定义知识点进行下总结

今天先来看下自定义控件需要重写的三个重要方法

看代码

package com.example.testcode;

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

public class DrawView extends View {

    public DrawView(Context context) {
        super(context);
        Log.e("123", "drawview_1");
        // TODO Auto-generated constructor stub
    }

    public DrawView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        Log.e("123", "drawview_3");
        // TODO Auto-generated constructor stub
    }

    public DrawView(Context context, AttributeSet attrs) {
        super(context, attrs);
        Log.e("123", "drawview_2");
        // TODO Auto-generated constructor stub
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // TODO Auto-generated method stub
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        Log.e("123", "MeasureSpec.UNSPECIFIED==" + MeasureSpec.UNSPECIFIED);
        Log.e("123", "MeasureSpec.AT_MOST==" + MeasureSpec.AT_MOST);
        Log.e("123", "MeasureSpec.EXACTLY==" + MeasureSpec.EXACTLY);
        Log.e("123", "widthMeasureSpec===" + widthMeasureSpec);
        Log.e("123", "heightMeasureSpec===" + heightMeasureSpec);
        Log.e("123", "widthMode==" + widthMode + " widthSize===" + widthSize);
        Log.e("123", "heightMode==" + heightMode + " heightSize==="
                + heightSize);
        //这两个方法必须有一个，否则会报错
        //super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        setMeasuredDimension(75, 75);
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right,
            int bottom) {
        // TODO Auto-generated method stub
        Log.e("123", "change===" + changed + " left===" + left + " top==="
                + top + " right===" + right + " bottom===" + bottom);
        super.onLayout(changed, left, top, right, bottom);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        // TODO Auto-generated method stub
        Log.e("123", "onDraw");
        super.onDraw(canvas);
    }
}

xml中使用

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.testcode.MainActivity" >

    <com.example.testcode.DrawView
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:background="#ff0000" />

</RelativeLayout>

先看下我们的打印结果

   - ::26.901: E/(): drawview_2
   - ::26.959: E/(): MeasureSpec.UNSPECIFIED==
   - ::26.959: E/(): MeasureSpec.AT_MOST==-
   - ::26.959: E/(): MeasureSpec.EXACTLY==
   - ::26.959: E/(): widthMeasureSpec===
   - ::26.960: E/(): heightMeasureSpec===-
   - ::26.960: E/(): widthMode== widthSize===
   - ::26.960: E/(): heightMode==- heightSize===
   - ::26.960: E/(): MeasureSpec.UNSPECIFIED==
  - ::26.960: E/(): MeasureSpec.AT_MOST==-
  - ::26.961: E/(): MeasureSpec.EXACTLY==
  - ::26.961: E/(): widthMeasureSpec===
  - ::26.961: E/(): heightMeasureSpec===
  - ::26.961: E/(): widthMode== widthSize===
  - ::26.961: E/(): heightMode== heightSize===
  - ::27.001: E/(): change===true left=== top=== right=== bottom===
  - ::27.030: E/(): MeasureSpec.UNSPECIFIED==
  - ::27.031: E/(): MeasureSpec.AT_MOST==-
  - ::27.031: E/(): MeasureSpec.EXACTLY==
  - ::27.031: E/(): widthMeasureSpec===
  - ::27.031: E/(): heightMeasureSpec===-
  - ::27.031: E/(): widthMode== widthSize===
  - ::27.031: E/(): heightMode==- heightSize===
  - ::27.031: E/(): MeasureSpec.UNSPECIFIED==
  - ::27.031: E/(): MeasureSpec.AT_MOST==-
  - ::27.031: E/(): MeasureSpec.EXACTLY==
  - ::27.032: E/(): widthMeasureSpec===
  - ::27.032: E/(): heightMeasureSpec===
  - ::27.032: E/(): widthMode== widthSize===
  - ::27.032: E/(): heightMode== heightSize===
  - ::27.033: E/(): change===false left=== top=== right=== bottom===
  - ::27.045: E/(): onDraw

从上面的结果我们可以知道xml中控件加载过程

1.xml中使用的控件，加载的时候，调用的是控件两个参数的方法

public DrawView(Context context, AttributeSet attrs) {
}

一个是上下文，一个是属性

2.解析来会调用

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
}

方法，这个方法是用来确定控件的宽高的，这个值是从控件的width、height中读出来的。但是我们发现，这两个值的打印结果很奇怪，甚至还有负数。网上对此的解释是

onMeasure传入的widthMeasureSpec和heightMeasureSpec不是一般的尺寸数值，而是将模式和尺寸组合在一起的数值。

具体什么我也不清楚，不过，它其实是包含了很多信息在里面的。一个就是，从这个数值，我们可以获得这个控件宽跟高的形式

使用如下方法

int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);

这个mode的取值，可以有一下三种

MeasureSpec.EXACTLY-是精确尺寸，当我们将控件的layout_width或layout_height指定为具体数值时如andorid:layout_width="50dip"，或者为FILL_PARENT是，都是控件大小已经确定的情况，都是精确尺寸。

MeasureSpec.AT_MOST 是最大尺寸，当控件的layout_width或layout_height指定为WRAP_CONTENT时，控件大小一般随着控件的子空间或内容进行 变化，此时控件尺寸只要不超过父控件允许的最大尺寸即可。因此，此时的mode是AT_MOST，size给出了父控件允许的最大尺寸。

MeasureSpec.UNSPECIFIED是未指定尺寸，这种情况不多，一般都是父控件是AdapterView，通过measure方法传入的模式。