轉載自： http://www.ityoudao.com/Web/Android_657_2256.html

1、概述

過 去，程序員通常以像素為單位設計計算機用戶界面。例如：圖片大小為80×32像素。這樣處理的問題在于，如果在一個每英寸點數（dpi）更高的新顯示器上 運行該程序，則用戶界面會顯得很小。在有些情況下，用戶界面可能會小到難以看清內容。由此我們采用與分辨率無關的度量單位來開發程序就能夠解決這個問題。 Android應用開發支持不同的度量單位。

2、度量單位含義

dip: device independent pixels(設備獨立像素). 不同設備有不同的顯示效果,這個和設備硬件有關，一般我們為了支持WVGA、HVGA和QVGA 推薦使用這個，不依賴像素。

dp: dip是一樣的

px: pixels(像素). 不同設備顯示效果相同，一般我們HVGA代表320x480像素，這個用的比較多。

pt: point，是一個標準的長度單位，1pt＝1/72英寸，用于印刷業，非常簡單易用；
sp: scaled pixels(放大像素). 主要用于字體顯示best for textsize。

in（英寸）：長度單位。?
mm（毫米）：長度單位。

3、度量單位的換算公式

在android源碼包TypedValue.java中，我們看如下函數：

public static float applyDimension(int unit, float value,

DisplayMetrics metrics)

{

switch (unit) {

case COMPLEX_UNIT_PX:

return value;

case COMPLEX_UNIT_DIP:

return value * metrics.density;

case COMPLEX_UNIT_SP:

return value * metrics.scaledDensity;

case COMPLEX_UNIT_PT:

return value * metrics.xdpi * (1.0f/72);

case COMPLEX_UNIT_IN:

return value * metrics.xdpi;

case COMPLEX_UNIT_MM:

return value * metrics.xdpi * (1.0f/25.4f);

}

return 0;

}

該函數功能：是把各單位換算為像素。

metrics.density：默認值為DENSITY_DEVICE / (float) DENSITY_DEFAULT;

metrics.scaledDensity：默認值為DENSITY_DEVICE / (float) DENSITY_DEFAULT;

metrics.xdpi：默認值為DENSITY_DEVICE;

DENSITY_DEVICE：為屏幕密度

DENSITY_DEFAULT：默認值為160

4、屏幕密度：表示每英寸有多少個顯示點，與分辨率是兩個不同的概念。

Android主要有以下幾種屏：如下表

屏幕 Tyep	寬度 Pixels	高度 Pixels	尺寸 Range(inches)	屏幕密度
QVGA	240	320	2.6-3.0	low
WQVGA	240	400	3.2-3.5	low
FWQVGA	240	432	3.5-3.8	low
HVGA	320	480	3.0-3.5	Medium
WVGA	480	800	3.3-4.0	High
FWVGA	480	854	3.5-4.0	High
WVGA	480	800	4.8-5.5	Medium
FWVGA	480	854	5.0-5.8	Medium
備注	目前android默認的low=120 ；Medium =160； High = 240

5、綜上所述

據px = dip * density / 160，則當屏幕密度為160時，px = dip
根據 google 的建議，TextView 的字號最好使用 sp 做單位，而且查看TextView的源碼可知Android默認使用sp作為字號單位。將dip作為其他元素的單位。

A dimension value defined in XML. A dimension is specified with a number followed by a unit of measure. For example: 10px, 2in, 5sp. The following units of measure are supported by Android:dp Density-independent Pixels - An abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi (dots per inch) screen, on which 1dp is roughly equal to 1px. When running on a higher density screen, the number of pixels used to draw 1dp is scaled up by a factor appropriate for the screen's dpi. Likewise, when on a lower density screen, the number of pixels used for 1dp is scaled down. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. Using dp units (instead of px units) is a simple solution to making the view dimensions in your layout resize properly for different screen densities. In other words, it provides consistency for the real-world sizes of your UI elements across different devices. sp Scale-independent Pixels - This is like the dp unit, but it is also scaled by the user's font size preference. It is recommend you use this unit when specifying font sizes, so they will be adjusted for both the screen density and the user's preference. pt Points - 1/72 of an inch based on the physical size of the screen. px Pixels - Corresponds to actual pixels on the screen. This unit of measure is not recommended because the actual representation can vary across devices; each devices may have a different number of pixels per inch and may have more or fewer total pixels available on the screen. mm Millimeters - Based on the physical size of the screen. in Inches - Based on the physical size of the screen.

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