50个Android开发技巧(03 自己定义ViewGroup)

Posted 2020-09-09

问题：怎样创建一个例如以下图所看到的的布局？

               图1
(原文地址：http://blog.csdn.net/vector_yi/article/details/24415537)

 你可能会说，利用RelativeLayout和margins就能够实现。的确，例如以下XML代码能够简单地构建一个类似的布局：

<RelativeLayout xmlns:android = "http://schemas.android.com/apk/res/android"
    android:layout_width= "fill_parent"
    android:layout_height= "fill_parent" >

    <View
        android:layout_width ="100dp"
        android:layout_height ="150dp"
        android:background ="#FF0000" />

    <View
        android:layout_width ="100dp"
        android:layout_height ="150dp"
        android:layout_marginLeft ="30dp"
        android:layout_marginTop ="20dp"
        android:background ="#00FF00" />

    <View
        android:layout_width ="100dp"
        android:layout_height ="150dp"
        android:layout_marginLeft ="60dp"
        android:layout_marginTop ="40dp"
        android:background ="#0000FF" />

</RelativeLayout>

效果如图2：

             图2

可是当遇到复杂、要求可变的类似布局时，利用margins可能就会显得操作非常繁杂。

在此，我们来看还有一种创建类似上图布局的方式---自己定义ViewGroup

优点有下面几点：

• 当你将这个布局应用到不同Activity中时更加easy维护
• 能够利用自己定义属性来自己定义ViewGroup中的每一个子View
• 更加简洁可读的XML文件内容
• 假设须要改变margin的时候。不须要手动的去计算每一个子View的margin

一、理解Android绘制一个View的步骤

     关于绘制View的步骤。能够參见Android官方文档：http://developer.android.com/guide/topics/ui/how-android-draws.html

        在此，我们重点来关注ViewGroup的绘制过程：

          1.处理ViewGroup的width和height.

             处理width及height的操作在onMeasure()方法中进行，在此方法内。ViewGroup会依据它的子View来计算自身所占用的布局空间。

          2.布局到页面上

             这点操作在onLayout()方法中进行，在此方法中，ViewGroup会依据从onMeasure()中得到的信息将其每个子View绘制出来。

二、构建CascadeLayout类

         首先在XML布局文件里加入CascadeLayout：

<FrameLayout 
    <!--自己定义命名空间，以便在下文中使用自己定义的属性-->
    xmlns:cascade ="http://schemas.android.com/apk/res/com.manning.androidhacks.hack003"
    xmlns:android= "http://schemas.android.com/apk/res/android"
    android:layout_width= "fill_parent"
    android:layout_height= "fill_parent" >

    <com.manning.androidhacks.hack003.view.CascadeLayout
        android:layout_width ="fill_parent"
        android:layout_height ="fill_parent"
        cascade:horizontal_spacing ="30dp"<!--由于前面加入了cascade命名空间，所以此处能够使用自己定义属性-->
        cascade:vertical_spacing ="20dp" >

        <View
            android:layout_width ="100dp"
            android:layout_height ="150dp"
            cascade:layout_vertical_spacing ="90dp"<!--为子View加入的自己定义属性，将在本文第三部分用到-->
            android:background ="#FF0000" />

        <View
            android:layout_width ="100dp"
            android:layout_height ="150dp"
            android:background ="#00FF00" />

        <View
            android:layout_width ="100dp"
            android:layout_height ="150dp"
            android:background ="#0000FF" />
    </com.manning.androidhacks.hack003.view.CascadeLayout>

</FrameLayout>

要使用这些自己定义的属性，我们必需要定义它。

     在res/values目录下创建一个attrs.xml文件:

<? xml version ="1.0" encoding= "utf-8" ?>
<resources>
    <declare-styleable name= "CascadeLayout" >
        <attr name= "horizontal_spacing" format = "dimension" />
        <attr name= "vertical_spacing" format = "dimension" />
    </declare-styleable>
</resources>

然后。当我们在创建CascadeLayout且没有为其指定horizontal_spacing与vertical_spacing时，须要有一个默认值。

我们将这个默认值预先定义好并存放在res/values目录下的dimens.xml中：

<?
 xml version ="1.0" encoding= "utf-8" ?
>
<resources>
    <dimen name= "cascade_horizontal_spacing" >10dp</dimen>
    <dimen name= "cascade_vertical_spacing" >10dp</dimen>
</resources>

最后，我们须要创建一个名为CascadeLayout的Java类。它继承了ViewGroup并重写了onMeasure()与OnLayout()方法。

1.CascadeLayout的构造函数

public CascadeLayout (Context context, AttributeSet attrs) {
    super( context, attrs);

    TypedArray a = context .obtainStyledAttributes (attrs ,
        R. styleable. CascadeLayout );

    try {
      mHorizontalSpacing = a. getDimensionPixelSize(
          R. styleable. CascadeLayout_horizontal_spacing ,
          getResources ().getDimensionPixelSize (
              R. dimen. cascade_horizontal_spacing ));

      mVerticalSpacing = a. getDimensionPixelSize(
          R. styleable. CascadeLayout_vertical_spacing , getResources ()
              .getDimensionPixelSize (R .dimen .cascade_vertical_spacing ));
    } finally {
      a .recycle ();
    }

2.构建自己定义的LayoutParams类

     LayoutParams类将作为CascadeLayout的内部类存在，它将存储每一个子View的x。y坐标。定义例如以下：

public static class LayoutParams extends ViewGroup .LayoutParams {
    int x;
    int y;

    public LayoutParams( Context context , AttributeSet attrs) {
      super (context , attrs );
    }

    public LayoutParams( int w , int h ) {
      super (w , h );
    }

  }

3.重写onMeasure()方法

     onMeasure()方法将是CascadeLayout类中最关键的部分，这种方法不仅计算整个ViewGroup所占用的布局空间。还将计算出每一个子View所占用的布局空间。

@Override
  protected void onMeasure (int widthMeasureSpec , int heightMeasureSpec ) {
    int width = 0;
    int height = getPaddingTop ();

    final int count = getChildCount ();
    for ( int i = 0; i < count; i++) {
      View child = getChildAt (i );
      measureChild (child , widthMeasureSpec , heightMeasureSpec );
       LayoutParams lp = (LayoutParams ) child .getLayoutParams ();
      width = getPaddingLeft () + mHorizontalSpacing * i;

      lp .x = width;
      lp .y = height;

      width += child .getMeasuredWidth ();
      height += mVerticalSpacing ;
    }

    width += getPaddingRight ();
    height += getChildAt (getChildCount () - 1). getMeasuredHeight ()
        + getPaddingBottom ();

    setMeasuredDimension ( resolveSize( width, widthMeasureSpec ),
        resolveSize( height, heightMeasureSpec ));
  }

4.最后一步，重写onLayout()方法

     代码非常easy，就是让每一个子View都调用layout()方法。

@Override
  protected void onLayout (boolean changed, int l , int t , int r , int b ) {

    final int count = getChildCount ();
    for ( int i = 0; i < count; i++) {
      View child = getChildAt (i );
      LayoutParams lp = ( LayoutParams ) child .getLayoutParams ();

      child .layout (lp .x , lp .y , lp .x + child. getMeasuredWidth (), lp .y
          + child .getMeasuredHeight ());
    }
  }

至此，就利用自己定义的ViewGroup创建了一个和图2一样效果的布局页面。

三、为子View加入自己定义属性

既然费了这么大劲，怎么可能就和之前几行XML代码效果一样？

以下，我们就来为CascadeLayout中的子View加入自己定义属性:

     首先，在之前创建的attrs.xml中加入例如以下代码：

<declare-styleable name="CascadeLayout_LayoutParams">
     <attr name="layout_vertical_spacing" format="dimension" />
</declare-styleable>

由于这个新加入的属性是以 layout_ 开头的。所以它会被加入到LayoutParams中去。

我们能够在之前自己定义的内部类LayoutParams中的构造函数中读取到这个属性，将第一个构造函数改为：

public LayoutParams (Context context, AttributeSet attrs) {
      super (context , attrs );

      TypedArray a = context .obtainStyledAttributes (attrs ,
          R. styleable. CascadeLayout_LayoutParams );
      try {
        verticalSpacing = a
            .getDimensionPixelSize (
                R .styleable .CascadeLayout_LayoutParams_layout_vertical_spacing ,
                -1 );
      } finally {
        a .recycle ();
      }
    }

既然加入了新的自己定义属性。就必须在onMeasure()方法中对其加以处理：

@Override
  protected void onMeasure (int widthMeasureSpec , int heightMeasureSpec ) {
    int width = getPaddingLeft ();
    int height = getPaddingTop ();
    int verticalSpacing ;

    final int count = getChildCount ();
    for ( int i = 0; i < count; i++) {
      verticalSpacing = mVerticalSpacing ;

      View child = getChildAt (i );
      measureChild (child , widthMeasureSpec , heightMeasureSpec );

      LayoutParams lp = ( LayoutParams ) child .getLayoutParams ();
      width = getPaddingLeft () + mHorizontalSpacing * i;

      lp .x = width;
      lp .y = height;

      if (lp .verticalSpacing >= 0 ) {
        verticalSpacing = lp .verticalSpacing ;
      }

      width += child .getMeasuredWidth ();
      height += verticalSpacing ;
    }

    width += getPaddingRight ();
    height += getChildAt (getChildCount () - 1). getMeasuredHeight ()
        + getPaddingBottom ();

    setMeasuredDimension ( resolveSize( width, widthMeasureSpec ),
        resolveSize( height, heightMeasureSpec ));
  }

最后附上完整的CascadeLayout代码：

package com.manning.androidhacks.hack003.view;

import android.content.Context;
import android.content.res.TypedArray;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewGroup;

import com.manning.androidhacks.hack003.R;

public class CascadeLayout extends ViewGroup {

  private int mHorizontalSpacing;
  private int mVerticalSpacing;

  public CascadeLayout(Context context, AttributeSet attrs) {
    super(context, attrs);

    TypedArray a = context.obtainStyledAttributes(attrs,
        R.styleable.CascadeLayout);

    try {
      mHorizontalSpacing = a.getDimensionPixelSize(
          R.styleable.CascadeLayout_horizontal_spacing,
          getResources().getDimensionPixelSize(
              R.dimen.cascade_horizontal_spacing));

      mVerticalSpacing = a.getDimensionPixelSize(
          R.styleable.CascadeLayout_vertical_spacing, getResources()
              .getDimensionPixelSize(R.dimen.cascade_vertical_spacing));
    } finally {
      a.recycle();
    }

  }

  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int width = getPaddingLeft();
    int height = getPaddingTop();
    int verticalSpacing;

    final int count = getChildCount();
    for (int i = 0; i < count; i++) {
      verticalSpacing = mVerticalSpacing;

      View child = getChildAt(i);
      measureChild(child, widthMeasureSpec, heightMeasureSpec);

      LayoutParams lp = (LayoutParams) child.getLayoutParams();
      width = getPaddingLeft() + mHorizontalSpacing * i;

      lp.x = width;
      lp.y = height;

      if (lp.verticalSpacing >= 0) {
        verticalSpacing = lp.verticalSpacing;
      }

      width += child.getMeasuredWidth();
      height += verticalSpacing;
    }

    width += getPaddingRight();
    height += getChildAt(getChildCount() - 1).getMeasuredHeight()
        + getPaddingBottom();

    setMeasuredDimension(resolveSize(width, widthMeasureSpec),
        resolveSize(height, heightMeasureSpec));
  }

  @Override
  protected void onLayout(boolean changed, int l, int t, int r, int b) {

    final int count = getChildCount();
    for (int i = 0; i < count; i++) {
      View child = getChildAt(i);
      LayoutParams lp = (LayoutParams) child.getLayoutParams();

      child.layout(lp.x, lp.y, lp.x + child.getMeasuredWidth(), lp.y
          + child.getMeasuredHeight());
    }
  }

  @Override
  protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
    return p instanceof LayoutParams;
  }

  @Override
  protected LayoutParams generateDefaultLayoutParams() {
    return new LayoutParams(LayoutParams.WRAP_CONTENT,
        LayoutParams.WRAP_CONTENT);
  }

  @Override
  public LayoutParams generateLayoutParams(AttributeSet attrs) {
    return new LayoutParams(getContext(), attrs);
  }

  @Override
  protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
    return new LayoutParams(p.width, p.height);
  }

  public static class LayoutParams extends ViewGroup.LayoutParams {
    int x;
    int y;
    public int verticalSpacing;

    public LayoutParams(Context context, AttributeSet attrs) {
      super(context, attrs);

      TypedArray a = context.obtainStyledAttributes(attrs,
          R.styleable.CascadeLayout_LayoutParams);
      try {
        verticalSpacing = a
            .getDimensionPixelSize(
                R.styleable.CascadeLayout_LayoutParams_layout_vertical_spacing,
                -1);
      } finally {
        a.recycle();
      }
    }

    public LayoutParams(int w, int h) {
      super(w, h);
    }

  }
}

project文件夹结构：


(原文地址：http://blog.csdn.net/vector_yi/article/details/24415537)