详解嵌套ListView、ScrollView布局显示不全的问题
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在项目开发中,可能经常遇到嵌套ListView、ScrollView的问题,百度一搜,都是现成的代码,而且都是一样的,就是重写onMeasure方法,但是为什么要那么写,估计就没多少人知道了,这里进行深入的剖析一下下,重点看onMeasure方法,代码如下:
/** * Created by hailonghan on 15/5/28. */public class ExpandListView extends ListView { public ExpandListView(Context context) { super(context); } public ExpandListView(Context context, AttributeSet attrs) { super(context, attrs); } public ExpandListView(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); } @TargetApi(Build.VERSION_CODES.LOLLIPOP) public ExpandListView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) { super(context, attrs, defStyleAttr, defStyleRes); } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2 , MeasureSpec.AT_MOST); super.onMeasure(widthMeasureSpec, expandSpec); }}
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看onMeasure()方法,很多同学很疑惑,那个Integer.MAX_VALUE >> 2是咋个意思,MeasureSpec.AT_MOST又是咋个意思,总之就迷迷糊糊把代码拷到项目中了,一运行,好使,立马高兴的不得了,但是原理搞不懂,等别人问的时候,为什么这样写,就是讲不出来。
要搞明白原理,就要搞懂MeasureSpec这个类,这里我贴一下MeasureSpec类的源码,如下
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: *
- *
- {@link android.view.View.MeasureSpec#UNSPECIFIED} *
- {@link android.view.View.MeasureSpec#EXACTLY} *
- {@link android.view.View.MeasureSpec#AT_MOST} *
Note: 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.
* * @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) { final int mode = getMode(measureSpec); if (mode == UNSPECIFIED) { // No need to adjust size for UNSPECIFIED mode. return makeMeasureSpec(0, UNSPECIFIED); } int size = getSize(measureSpec) + delta; if (size < 0) { Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size + ") spec: " + toString(measureSpec) + " delta: " + delta); size = 0; } return makeMeasureSpec(size, mode); } /** * 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(); } }- 1
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代码不是很长,其实里面最重要的是3种模式和3个方法
3种模式
- UNSPECIFIED模式,官方意思是:父布局没有给子布局强加任何约束,子布局想要多大就要多大,说白了就是不确定大小
- EXACTLY模式,官方意思是:父布局给子布局限定了准确的大小,子布局的大小就是精确的,父亲给多大就是多大
- AT_MOST模式,官方意思是:父布局给定了一个最大的值,子布局的大小不能超过这个值,当然可以比这个值小
3个方法
1.public static int makeMeasureSpec(int size, int mode) ,这个方法的作用是根据大小和模式来生成一个int值,这个int值封装了模式和大小信息2.public static int getMode(int measureSpec),这个方法的作用是通过一个int值来获取里面的模式信息3.public static int getSize(int measureSpec),这个方法的作用是通过一个int值来获取里面的大小信息
在Android里面,一个控件所占的模式和大小是通过一个整数int来表示的,这里很多同学就疑惑了,一个int值是怎么来表示模式的大小的,这里来看一张图片:
原来,Android里面把int的最高2两位来表示模式,最低30位来表示大小
private static final int MODE_SHIFT = 30;
- 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;
不确定模式是0左移30位,也就是int类型的最高两位是00
精确模式是1左移30位,也就是int类型的最高两位是01
最大模式是是2左移30位,也就是int类型的最高两位是10
现在在回头看我们之前的代码,如下:
@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2 , MeasureSpec.AT_MOST); super.onMeasure(widthMeasureSpec, expandSpec); }
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讲到这里很多同学就明白了,我们调用了makeMeasureSpec方法,这个方法是用来生成一个带有模式和大小信息的int值的,第一个参数Integer.MAX_VALUE >> 2,这个参数是传的一个大小值,为什么是这个值呢,我们现在已经知道了,我们要生成的控件,它的大小最大值是int的最低30位的最大值,我们先取Integer.MAX_VALUE来获取int值的最大值,然后左移2位就得到这个临界值最大值了
当然,我们在手机上的控件的大小不可能那么大,极限值就那么大,实际肯定比那个小,所以这个模式就得选择MeasureSpec.AT_MOST了,最后将生成的这个大小传递给父控件就可以了,super.onMeasure(widthMeasureSpec, expandSpec),这个函数只改变的是控件的高度,宽度没有改变,实际开发当中不管listview有多少条数据,都能一次性展现出来
- 详解嵌套ListView、ScrollView布局显示不全的问题
- 详解嵌套ListView、ScrollView布局显示不全的问题
- 详解嵌套ListView、ScrollView布局显示不全的问题
- 详解嵌套ListView、ScrollView布局显示不全的问题
- [详解嵌套ListView、ScrollView布局显示不全的问题]
- 详解嵌套ListView、ScrollView布局显示不全的问题
- 详解嵌套ListView、ScrollView布局显示不全的问题
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