android 动态创建控件并设置布局

有时候我们需要在代码中动态创建view,并把它加入到当前的viewGroup中,动态创建view一般使用LayoutInflater或者构造函数,在这里使用构造函数,有三个构造函数可用,比如动态创建TextView,可以使用这三个构造函数:

    TextView(Context context)
    TextView(Context context, AttributeSet attrs)
    TextView(Context context, AttributeSet attrs, int defStyleAttr)

    其中context是当前Activity的Context,attrs是一组属性值,例如layout_width,layout_height等等,defStyleAttr和控件的style有关,本篇暂不考虑.

    context很容易拿到,那么attrs是怎么拿到的呢,查看文档发现官网上有给出:

    XmlPullParser parser = resources.getXml(myResouce);
    AttributeSet attributes = Xml.asAttributeSet(parser);

    resources可以用context.getResources()获取,这是本地资源信息的类,myResouce是资源ID.

    我们在res/layout下创建资源XML文件textview.xml:

        xmlns:android="http://schemas.android.com/apk/res/android"
    android:background="#ff00ff00"
    android:text="hello world!"/>

    拿到这两个参数,现在就可以调用第二个构建函数创建view了吗?嗯,我也是这样想的,那么问题来了,创建view之后发现没有任何显示,这是为什么呢,试着给它写入文本,在创建后调用tv.setText("hello");这回有显示了,背景也不是我们写在XML中的值,这说明attrs没有发挥作用.推断原因可能是attrs中的值不对,调用parser.getAttributeCount()发现返回的是-1,说明XML没有被正确解析!

    想起使用LayoutInflater也是使用XML文件生成了VIEW,或许可以看看它的代码:

    public View inflate(int resource, ViewGroup root) {
        return inflate(resource, root, root != null);
    }

    public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
        if (DEBUG) System.out.println("INFLATING from resource: " + resource);
        XmlResourceParser parser = getContext().getResources().getLayout(resource);//这里和getXml效果一样
        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

    public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            final AttributeSet attrs = Xml.asAttributeSet(parser); //这里也和我们之前做的一样
            Context lastContext = (Context)mConstructorArgs[0];
            mConstructorArgs[0] = mContext;
            View result = root;

            try {
                // Look for the root node. !!这里是我们没有做过的!!
                int type;
                while ((type = parser.next()) != XmlPullParser.START_TAG &&
                        type != XmlPullParser.END_DOCUMENT) {
                    // Empty
                }

                if (type != XmlPullParser.START_TAG) {
                    throw new InflateException(parser.getPositionDescription()
                            + ": No start tag found!");
                }

                final String name = parser.getName();
                ....更多的代码隐藏起来了

            } catch (XmlPullParserException e) {
                InflateException ex = new InflateException(e.getMessage());
                ex.initCause(e);
                throw ex;
            } catch (IOException e) {
                InflateException ex = new InflateException(
                        parser.getPositionDescription()
                        + ": " + e.getMessage());
                ex.initCause(e);
                throw ex;
            } finally {
                // Don't retain static reference on context.
                mConstructorArgs[0] = lastContext;
                mConstructorArgs[1] = null;
            }

            return result;
        }
    }

    这里发现我们在创建attrs时,没有去查找XML的根节点:

   while ((type = parser.next()) != XmlPullParser.START_TAG &&
            type != XmlPullParser.END_DOCUMENT) {
                    // Empty
    }

    现在在代码中加上这段,发现XML的设置起使用了,并且parser.getAttributeCount()也返回了2,正是我们属性的个数.

    总结一下流程:

    XmlPullParser parser = getResources().getXml(R.layout.textview);
    AttributeSet attributes = Xml.asAttributeSet(parser);

    int type;

    while ((type = parser.next()) != XmlPullParser.START_TAG &&
            type != XmlPullParser.END_DOCUMENT) {
                    // Empty
    }   

    TextView tv=new TextView(this,attributes);

    努力了这么久,只为了创建一个View,还不如用LayoutInflater呢,有人这样想吗?完全正确,我之所以这样做是因为我要获取到attributes来进行布局设置,很多例子中,生成一个view之后就直接setContentView去了,这在项目中应该很少这么干的,因为我们还有其他的view要显示,创建的view只是显示在大布局中的一块而已.我们要把这个view加入到其它的布局中去,一般调用的是addView:

    void     addView(View child, int index, ViewGroup.LayoutParams params)  Adds a child view with the specified layout parameters.
    void     addView(View child, ViewGroup.LayoutParams params)  Adds a child view with the specified layout parameters.
    void     addView(View child, int index)   Adds a child view.
    void     addView(View child) Adds a child view.
    void     addView(View child, int width, int height) Adds a child view with this ViewGroup's default layout parameters and the specified width and height.

    特别说明,只有继承了ViewGroup的类才有这些函数,比如RelativeLayout,LinearLayout等.child参数是我们刚刚创建的view,index是我们的view在此layout中的Z序,params是布局参数.

    不同的布局类有不同的布局参数类.它们都继承自ViewGroup.LayoutParams,例如 LinearLayout.LayoutParams, RelativeLayout.LayoutParams等等,关注RelativeLayout.LayoutParams的构造函数:

    RelativeLayout.LayoutParams(Context c, AttributeSet attrs)
    RelativeLayout.LayoutParams(int w, int h)  //可以指定长和宽
    RelativeLayout.LayoutParams(ViewGroup.LayoutParams source)
    RelativeLayout.LayoutParams(ViewGroup.MarginLayoutParams source)

    从上面看出,我们在构造时可以设置view的长和宽,使用ViewGroup.MarginLayoutParams可以设置view的边距,其它的只能通过attrs来设置了,这就是我要创建attrs的目的.使用attrs完全可以精细化操作布局,下面来一个简单的示例,动态创建一个textView,并把它加入到主布局文件的RelativeLayout中,并局中显示:

 //MainActivity.java
 package com.example.helloworld;

 import java.io.IOException;

 import org.xmlpull.v1.XmlPullParser;
 import org.xmlpull.v1.XmlPullParserException;

 import android.app.Activity;
 import android.os.Bundle;
 import android.util.AttributeSet;
 import android.util.Log;
 import android.util.Xml;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.widget.Button;
 import android.widget.RelativeLayout;
 import android.widget.RelativeLayout.LayoutParams;
 import android.widget.TextView;


 public class MainActivity extends Activity {

     private RelativeLayout mRelativeLayout;
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         mRelativeLayout=(RelativeLayout)findViewById(R.id.relativelayout1);
         ((Button)findViewById(R.id.button1)).setOnClickListener(new OnClickListener() {

             @Override
             public void onClick(View v) {
                 // TODO Auto-generated method stub
                 XmlPullParser parser = MainActivity.this.getResources().getXml(R.layout.textview);
                 AttributeSet attributes = Xml.asAttributeSet(parser);
                 int type;
                 try{
                     while ((type = parser.next()) != XmlPullParser.START_TAG &&
                             type != XmlPullParser.END_DOCUMENT) {
                         // Empty
                     }

                     if (type != XmlPullParser.START_TAG) {
                         Log.e("","the xml file is error!\n");
                     }
                 } catch (XmlPullParserException e) {
                     // TODO Auto-generated catch block
                     e.printStackTrace();
                 } catch (IOException e) {
                     // TODO Auto-generated catch block
                     e.printStackTrace();
                 }
                 Log.d("",""+parser.getAttributeCount());
                 TextView tv=new TextView(MainActivity.this,attributes);
                 RelativeLayout.LayoutParams params=new LayoutParams(MainActivity.this,attributes);
                 mRelativeLayout.addView(tv,0,params);
             }
         });
     }

     @Override
     public boolean onCreateOptionsMenu(Menu menu) {
         // Inflate the menu; this adds items to the action bar if it is present.
         getMenuInflater().inflate(R.menu.main, menu);
         return true;
     }

     @Override
     public boolean onOptionsItemSelected(MenuItem item) {
         // Handle action bar item clicks here. The action bar will
         // automatically handle clicks on the Home/Up button, so long
         // as you specify a parent activity in AndroidManifest.xml.
         int id = item.getItemId();
         if (id == R.id.action_settings) {
             return true;
         }
         return super.onOptionsItemSelected(item);
     }
 }

res/layout/activity_main.xml

 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:paddingBottom="@dimen/activity_vertical_margin"
     android:paddingLeft="@dimen/activity_horizontal_margin"
     android:paddingRight="@dimen/activity_horizontal_margin"
     android:paddingTop="@dimen/activity_vertical_margin"
     tools:context="com.example.helloworld.MainActivity" >

     <TextView
         android:id="@+id/textView1"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="@string/hello_world" />

     <Button
         android:id="@+id/button1"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_below="@+id/textView1"
         android:text="Button" />

     <RelativeLayout
         android:id="@+id/relativelayout1"
         android:layout_width="match_parent"
         android:layout_height="200dp"
         android:layout_alignLeft="@+id/button1"
         android:layout_below="@+id/button1"
         android:background="#ffff0000"
         >
     RelativeLayout>

 RelativeLayout>

res/layout/textview.xml

 xml version="1.0" encoding="utf-8"?>
 <TextView
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:layout_centerInParent="true"
     android:background="#ff00ff00"
     android:visibility="visible"
     android:text="hello world!"/>

示例图片: