android中onMeasure初看,深刻理解布局之一!

今天学习android自定义组件:docs/guide/topics/ui/custom-components.html

其中有两个对布局界面影响很的方法,onDraw(),和onMeasure().

onDraw()比较好理解.onMeasure()就比较难理解一些,也更复杂些 ,引用文档中的说法就是:

onMeasure() is a little more involved.

其实还有另外一个方面的缘由就是我对这个单词measure不是很知道,而后果了下词典,就放了下心,确实是测量的意思.

实现onMeasure()方法基本须要完成下面三个方面的事情(最终结果是你本身写相应代码得出测量值并调用view的一个方法进行设置,告诉给你的view安排位置大小的父容器你要多大的空间.):

1.传递进来的参数,widthMeasureSpec,和heightMeasureSpec是你对你应该得出来的测量值的限制.

 

The overidden onMeasure() method is called with width and height measure specifications(widthMeasureSpec and heightMeasureSpec parameters,both are integer codes representing dimensions) which should be treated as requirements for the restrictions on the width and height measurements you should produce.

2. 你在onMeasure计算出来设置的width和height将被用来渲染组件.应当尽可能在传递进来的width和height 声明之间.

虽然你也能够选择你设置的尺寸超过传递进来的声明.可是这样的话,父容器能够选择,如clipping,scrolling,或者抛出异常,或者(也许是用新的声明参数)再次调用onMeasure()

Your component's onMeasure() method should calculate a measurement width and height which will be required to render the component.it should try to stay within the specified passed in.although it can choose to exceed them(in this case,the parent can choose what to do,including clipping,scrolling,throwing an excption,or asking the onMeasure to try again,perhaps with different measurement specifications).

3.一但width和height计算好了,就应该调用View.setMeasuredDimension(int width,int height)方法,不然将致使抛出异常.

Once the width and height are calculated,the setMeasureDimension(int width,int height) method must be called with the calculated measurements.Failure to do this will result in an exceptiion being thrown

   

在Android提提供的一个自定义View示例中(在API demos 中的 view/LabelView)能够看到一个重写onMeasure()方法的

实例,也比较好理解.

/**
     * @see android.view.View#measure(int, int)
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(measureWidth(widthMeasureSpec),
                measureHeight(heightMeasureSpec));
    }

    /**
     * Determines the width of this view
     * @param measureSpec A measureSpec packed into an int
     * @return The width of the view, honoring constraints from measureSpec
     */
    private int measureWidth(int measureSpec) {
        int result = 0;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        if (specMode == MeasureSpec.EXACTLY) {
            // We were told how big to be
            result = specSize;
        } else {
            // Measure the text
            result = (int) mTextPaint.measureText(mText) + getPaddingLeft()
                    + getPaddingRight();
            if (specMode == MeasureSpec.AT_MOST) {
                // Respect AT_MOST value if that was what is called for by measureSpec
                result = Math.min(result, specSize);
            }
        }

        return result;
    }

 

直接看measureWidth()

首先看到的是参数,分别表明宽度和高度的MeasureSpec

android2.2文档中对于MeasureSpec中的说明是:

一个MeasureSpec封装了从父容器传递给子容器的布局需求.

每个MeasureSpec表明了一个宽度,或者高度的说明.

一个MeasureSpec是一个大小跟模式的组合值.一共有三种模式.

A MeasureSpec encapsulates the layout requirements passed from parent to child Each MeasureSpec represents a requirement for either the width or the height.A MeasureSpec is compsized of a size and a mode.There are three possible modes:

 (1)UPSPECIFIED :父容器对于子容器没有任何限制,子容器想要多大就多大.

UNSPECIFIED The parent has not imposed any constraint on the child.It can be whatever size it wants

 (2) EXACTLY

 父容器已经为子容器设置了尺寸,子容器应当服从这些边界,不论子容器想要多大的空间.

EXACTLY The parent has determined and exact size for the child.The child is going to be given those bounds regardless of how big it wants to be.

(3) AT_MOST

 子容器能够是声明大小内的任意大小.

AT_MOST The child can be as large as it wants up to the specified size

MeasureSpec是View类下的静态公开类,MeasureSpec中的值做为一个整型是为了减小对象的分配开支.此类用于

将size和mode打包或者解包为一个整型.

MeasureSpecs are implemented as ints to reduce object allocation.This class is provided to pack and unpack the size,mode tuple into the int

我比较好奇的是怎么样将两个值打包到一个int中,又如何解包.

MeasureSpec类代码以下 :(注释已经被我删除了,由于在上面说明了.)

public static class MeasureSpec {
        private static final int MODE_SHIFT = 30;
        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

        public static final int UNSPECIFIED = 0 << MODE_SHIFT;
        public static final int EXACTLY     = 1 << MODE_SHIFT;
        public static final int AT_MOST     = 2 << MODE_SHIFT;

        public static int makeMeasureSpec(int size, int mode) {
            return size + mode;
        }
        public static int getMode(int measureSpec) {
            return (measureSpec & MODE_MASK);
        }
        public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }  }

我无聊的将他们的十进制值打印出来了:

mode_shift=30,mode_mask=-1073741824,UNSPECIFIED=0,EXACTLY=1073741824,AT_MOST=-2147483648

而后以为也应该将他们的二进制值打印出来,以下:

mode_shift=11110, // 30

mode_mask=11000000000000000000000000000000,

UNSPECIFIED=0, 

EXACTLY=1000000000000000000000000000000, 

AT_MOST=10000000000000000000000000000000

 

MODE_MASK  = 0x3 << MODE_SHIFT //也就是说MODE_MASK是由11左移30位获得的.由于Java用补码表示数值.最后获得的值最高位是1因此就是负数了

对于上面的数值咱们应该这样想,不要把0x3当作3而要当作二进制的11,

而把MODE_SHIFF就当作30.那为何是二进制 的11呢?

呢,由于只有三各模式,若是有四种模式就是111了由于111三个位才能够有四种组合对吧.

咱们这样来看,

UNSPECIFIED=00000000000000000000000000000000, 

      EXACTLY=01000000000000000000000000000000, 

    AT_MOST=10000000000000000000000000000000

也就是说,0,1,2

对应   00,01,10

当跟11想与时  00 &11 仍是获得 00,11&01 -> 01,10&

我以为到了这个份上相信,看我博客的也都理解了.

 return (measureSpec & ~MODE_MASK);应该是 return (measureSpec & (~MODE_MASK));