MeasureSpec介绍

在自定义View和ViewGroup的时候，我们经常会遇到int型的MeasureSpec来表示一个组件的大小，这个变量里面不仅有组件的尺寸大小，还有大小的模式。


这个大小的模式，有点难以理解。在系统中组件的大小模式有三种：


1.精确模式

在这种模式下，尺寸的值是多少，那么这个组件的长或宽就是多少。


2.最大模式

这个也就是父组件，能够给出的最大的空间，当前组件的长或宽最大只能为这么大，当然也可以比这个小。


3.未指定模式

这个就是说，当前组件，可以随便用空间，不受限制。


可能有很多人想不通，一个int型整数怎么可以表示两个东西（大小模式和大小的值），一个int类型我们知道有32位。而模式有三种，要表示三种状态，至少得2位二进制位。于是系统采用了最高的2位表示模式。如图：


最高两位是00的时候表示"未指定模式"。即MeasureSpec.UNSPECIFIED

最高两位是01的时候表示"'精确模式"。即MeasureSpec.EXACTLY

最高两位是11的时候表示"最大模式"。即MeasureSpec.AT_MOST


很多人一遇到位操作头就大了，为了操作简便，于是系统给我提供了一个MeasureSpec工具类。

这个工具类有四个方法和三个常量（上面所示）供我们使用：


//这个是由我们给出的尺寸大小和模式生成一个包含这两个信息的int变量，这里这个模式这个参数，传三个常量中的一个。

public static int makeMeasureSpec(int size, int mode)


//这个是得到这个变量中表示的模式信息，将得到的值与三个常量进行比较。

public static int getMode(int measureSpec)


//这个是得到这个变量中表示的尺寸大小的值。

public static int getSize(int measureSpec)


//把这个变量里面的模式和大小组成字符串返回来，方便打日志

 public static String toString(int measureSpec)


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public static class MeasureSpec {
       private static final int MODE_SHIFT = 30;
       private static final int MODE_MASK  = 0x3 << MODE_SHIFT;
 
       /**
        * Measure specification mode: The parent has not imposed any constraint
        * on the child. It can be whatever size it wants.
        */
       public static final int UNSPECIFIED = 0 << MODE_SHIFT;
 
       /**
        * Measure specification mode: The parent has determined an exact size
        * for the child. The child is going to be given those bounds regardless
        * of how big it wants to be.
        */
       public static final int EXACTLY     = 1 << MODE_SHIFT;
 
       /**
        * Measure specification mode: The child can be as large as it wants up
        * to the specified size.
        */
       public static final int AT_MOST     = 2 << MODE_SHIFT;
 
       /**
        * Creates a measure specification based on the supplied size and mode.
        *
        * The mode must always be one of the following:
        * <ul>
        *  <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li>
        *  <li>{@link android.view.View.MeasureSpec#EXACTLY}</li>
        *  <li>{@link android.view.View.MeasureSpec#AT_MOST}</li>
        * </ul>
        *
        * <p><strong>Note:</strong> On API level 17 and lower, makeMeasureSpec's
        * implementation was such that the order of arguments did not matter
        * and overflow in either value could impact the resulting MeasureSpec.
        * {@link android.widget.RelativeLayout} was affected by this bug.
        * Apps targeting API levels greater than 17 will get the fixed, more strict
        * behavior.</p>
        *
        * @param size the size of the measure specification
        * @param mode the mode of the measure specification
        * @return the measure specification based on size and mode
        */
       public static int makeMeasureSpec(int size, int mode) {
           if (sUseBrokenMakeMeasureSpec) {
               return size + mode;
           } else {
               return (size & ~MODE_MASK) | (mode & MODE_MASK);
           }
       }
 
       /**
        * Extracts the mode from the supplied measure specification.
        *
        * @param measureSpec the measure specification to extract the mode from
        * @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
        *         {@link android.view.View.MeasureSpec#AT_MOST} or
        *         {@link android.view.View.MeasureSpec#EXACTLY}
        */
       public static int getMode(int measureSpec) {
           return (measureSpec & MODE_MASK);
       }
 
       /**
        * Extracts the size from the supplied measure specification.
        *
        * @param measureSpec the measure specification to extract the size from
        * @return the size in pixels defined in the supplied measure specification
        */
       public static int getSize(int measureSpec) {
           return (measureSpec & ~MODE_MASK);
       }
 
       static int adjust(int measureSpec, int delta) {
           return makeMeasureSpec(getSize(measureSpec + delta), getMode(measureSpec));
       }
 
       /**
        * Returns a String representation of the specified measure
        * specification.
        *
        * @param measureSpec the measure specification to convert to a String
        * @return a String with the following format: "MeasureSpec: MODE SIZE"
        */
       public static String toString(int measureSpec) {
           int mode = getMode(measureSpec);
           int size = getSize(measureSpec);
 
           StringBuilder sb = new StringBuilder("MeasureSpec: ");
 
           if (mode == UNSPECIFIED)
               sb.append("UNSPECIFIED ");
           else if (mode == EXACTLY)
               sb.append("EXACTLY ");
           else if (mode == AT_MOST)
               sb.append("AT_MOST ");
           else
               sb.append(mode).append(" ");
 
           sb.append(size);
           return sb.toString();
       }
   }


 



posted @ 2014-01-22 17:55  Cyning  阅读(646)  评论(0)  编辑  收藏  举报