Android 中dp 跟px 转换及原理分析

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具体的转换方法如下,网上都是有的,但是自己看了之后感觉还是有点不明不白的,具体为什么呢,可以继续看后面.

/**	 * 根据手机的分辨率从 dp 的单位 转成为 px(像素)	 */	public static int dip2px(Context context, float dpValue) {		final float scale = context.getResources().getDisplayMetrics().density;		return (int) (dpValue * scale + 0.5f);	}	/**	 * 根据手机的分辨率从 px(像素) 的单位 转成为 dp	 */	public static int px2dip(Context context, float pxValue) {		final float scale = context.getResources().getDisplayMetrics().density;		return (int) (pxValue / scale + 0.5f);	}

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首先先列出来相关的方法:

Resources.java

public Resources(AssetManager assets, DisplayMetrics metrics,            Configuration config, CompatibilityInfo compInfo) {        mAssets = assets;        mMetrics.setToDefaults();        mCompatibilityInfo = compInfo;        updateConfiguration(config, metrics);        assets.ensureStringBlocks();}

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DisplayMetrics.java 相关方法

public void setToDefaults() {        widthPixels = 0;        heightPixels = 0;        density =  DENSITY_DEVICE / (float) DENSITY_DEFAULT;        densityDpi =  DENSITY_DEVICE;        scaledDensity = density;        xdpi = DENSITY_DEVICE;        ydpi = DENSITY_DEVICE;        noncompatWidthPixels = widthPixels;        noncompatHeightPixels = heightPixels;        noncompatDensity = density;        noncompatDensityDpi = densityDpi;        noncompatScaledDensity = scaledDensity;        noncompatXdpi = xdpi;        noncompatYdpi = ydpi;    }			 public static int DENSITY_DEVICE = getDeviceDensity();	 	 	  private static int getDeviceDensity() {        // qemu.sf.lcd_density can be used to override ro.sf.lcd_density        // when running in the emulator, allowing for dynamic configurations.        // The reason for this is that ro.sf.lcd_density is write-once and is        // set by the init process when it parses build.prop before anything else.        return SystemProperties.getInt("qemu.sf.lcd_density",                SystemProperties.getInt("ro.sf.lcd_density", DENSITY_DEFAULT));    }		/**     * The reference density used throughout the system.     */    public static final int DENSITY_DEFAULT = DENSITY_MEDIUM;		/**     * Standard quantized DPI for medium-density screens.     */    public static final int DENSITY_MEDIUM = 160;

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在 Resources.java 的构造函数里面调用DisplayMetrics 的setToDefaults 方法,使density 取值为DENSITY_DEVICE / (float) DENSITY_DEFAULT;

使density 最终的值和key:qemu.sf.lcd_density 和default_value:ro.sf.lcd_density 有关系,

系统里面一般并没有额外对key:qemu.sf.lcd_density 赋值,故取值ro.sf.lcd_density,而ro.sf.lcd_density 的值一般是在系统源码里面的设置好的了,比如:system.prop 里面可以设置该值.