今天學(xué)習(xí)android自定義組件:docs/guide/topics/ui/custom-components.html

其中有兩個(gè)對(duì)布局界面影響很的方法,onDraw(),和onMeasure().

onDraw()比較好理解.onMeasure()就比較難理解一些,也更復(fù)雜些 ,引用文檔中的說法就是:

onMeasure() is a little more involved.

其實(shí)還有另一個(gè)方面的原因就是我對(duì)這個(gè)單詞measure不是很知道,然后果了下詞典,就放了下心,確實(shí)是測(cè)量的意思.

實(shí)現(xiàn)onMeasure()方法基本需要完成下面三個(gè)方面的事情(最終結(jié)果是你自己寫相應(yīng)代碼得出測(cè)量值并調(diào)用view的一個(gè)方法進(jìn)行設(shè)置,告訴給你的view安排位置大小的父容器你要多大的空間.):

1.傳遞進(jìn)來的參數(shù),widthMeasureSpec,和heightMeasureSpec是你對(duì)你應(yīng)該得出來的測(cè)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計(jì)算出來設(shè)置的width和height將被用來渲染組件.應(yīng)當(dāng)盡量在傳遞進(jìn)來的width和height 聲明之間.

雖然你也可以選擇你設(shè)置的尺寸超過傳遞進(jìn)來的聲明.但是這樣的話,父容器可以選擇,如clipping,scrolling,或者拋出異常,或者(也許是用新的聲明參數(shù))再次調(diào)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計(jì)算好了,就應(yīng)該調(diào)用View.setMeasuredDimension(int width,int height)方法,否則將導(dǎo)致拋出異常.

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提提供的一個(gè)自定義View示例中(在API demos 中的 view/LabelView)可以看到一個(gè)重寫onMeasure()方法的

實(shí)例,也比較好理解.

 

直接看measureWidth()

首先看到的是參數(shù),分別代表寬度和高度的MeasureSpec

android2.2文檔中對(duì)于MeasureSpec中的說明是:

一個(gè)MeasureSpec封裝了從父容器傳遞給子容器的布局需求.

每一個(gè)MeasureSpec代表了一個(gè)寬度,或者高度的說明.

一個(gè)MeasureSpec是一個(gè)大小跟模式的組合值.一共有三種模式.

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 :父容器對(duì)于子容器沒有任何限制,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

 父容器已經(jīng)為子容器設(shè)置了尺寸,子容器應(yīng)當(dāng)服從這些邊界,不論子容器想要多大的空間.

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

 子容器可以是聲明大小內(nèi)的任意大小.

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

MeasureSpec是View類下的靜態(tài)公開類,MeasureSpec中的值作為一個(gè)整型是為了減少對(duì)象的分配開支.此類用于

將size和mode打包或者解包為一個(gè)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

我比較好奇的是怎么樣將兩個(gè)值打包到一個(gè)int中,又如何解包.

MeasureSpec類代碼如下 :(注釋已經(jīng)被我刪除了,因?yàn)樵谏厦嬲f明了.)

我無聊的將他們的十進(jìn)制值打印出來了:

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

然后覺得也應(yīng)該將他們的二進(jìn)制值打印出來,如下:

mode_shift=11110, // 30

mode_mask=11000000000000000000000000000000,

UNSPECIFIED=0, 

EXACTLY=1000000000000000000000000000000, 

AT_MOST=10000000000000000000000000000000

 

對(duì)于上面的數(shù)值我們應(yīng)該這樣想,不要把0x3看成3而要看成二進(jìn)制的11,

而把MODE_SHIFF就看成30.那為什么是二進(jìn)制 的11呢?

呢,因?yàn)橹挥腥髂Ｊ?如果有四種模式就是111了因?yàn)?11三個(gè)位才可以有四種組合對(duì)吧.

我們這樣來看,

UNSPECIFIED=00000000000000000000000000000000, 

      EXACTLY=01000000000000000000000000000000, 

    AT_MOST=10000000000000000000000000000000

也就是說,0,1,2

對(duì)應(yīng)   00,01,10

當(dāng)跟11想與時(shí)  00 &11 還是得到 00,11&01 -> 01,10&

我覺得到了這個(gè)份上相信,看我博客的也都理解了.

 return (measureSpec & ~MODE_MASK);應(yīng)該是 return (measureSpec & (~MODE_MASK));