findFocus-获得拥有焦点的控件

所有的view控件有一个findFocus方法，这个方法如下

  /**   
     * Find the view in the hierarchy rooted at this view that currently has
     * focus.
     *
     * @return The view that currently has focus, or null if no focused view can
     *         be found.
     */
    public View findFocus() {
        return (mPrivateFlags & PFLAG_FOCUSED) != 0 ? this : null; 
    }

大概意思就是，获得当前试图下，拥有焦点的控件

我们可以验证下它的具体使用

看demo

xml

<LinearLayout 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:orientation="vertical"
    android:padding="20dp"
    tools:context="com.example.testcode.MainActivity" >

    <TextView
        android:id="@+id/tv"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:layout_marginBottom="10dp"
        android:clickable="true"
        android:focusable="true"
        android:textSize="30sp"
        android:focusableInTouchMode="true"
        android:background="@drawable/select"
        android:textColor="#ffffff" />

    <LinearLayout
        android:id="@+id/aaa"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_marginBottom="10dp"
        android:layout_weight="1"
        android:orientation="horizontal" >

        <Button
            android:id="@+id/bt_1"
            android:layout_width="60dp"
            android:layout_height="60dp"
            android:layout_marginLeft="5dp"
            android:background="@drawable/select"
            android:focusableInTouchMode="true"
            android:textColor="#ffffff" />

        <Button
            android:id="@+id/bt_2"
            android:layout_width="60dp"
            android:layout_height="60dp"
            android:layout_marginLeft="5dp"
            android:background="@drawable/select"
            android:focusableInTouchMode="true"
            android:textColor="#ffffff" />

        <Button
            android:id="@+id/bt_3"
            android:layout_width="60dp"
            android:layout_height="60dp"
            android:layout_marginLeft="5dp"
            android:background="@drawable/select"
            android:focusableInTouchMode="true"
            android:textColor="#ffffff" />

        <Button
            android:id="@+id/bt_4"
            android:layout_width="60dp"
            android:layout_height="60dp"
            android:layout_marginLeft="5dp"
            android:background="@drawable/select"
            android:focusableInTouchMode="true"
            android:textColor="#ffffff" />
    </LinearLayout>

    <Button
        android:id="@+id/bt"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:background="@drawable/select" />

</LinearLayout>

activity

package com.example.testcode;
import com.example.testcode.R.id;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;

import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;

public class MainActivity extends Activity {
    private TextView textView;
    private Button button;
    private LinearLayout relativeLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = (TextView) findViewById(R.id.tv);
        button = (Button)findViewById(R.id.bt);
        relativeLayout = (LinearLayout)findViewById(R.id.aaa);
        textView.setText("当前焦点所在view");
        button.setText("显示当前focus");
        
        button.setOnClickListener(new OnClickListener() {
            
            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                View view = relativeLayout.findFocus();
                textView.setText(""+view);
            }
        });
        
    }

    @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);
    }

}

我们看下它的效果

从上面的结果，我们可以得出结论

1.这个方法其实加在viewGroup上比较有作用，它得到的是当前拥有焦点的子控件view

2.如果焦点不在这个控件内的话，返回的是一个null

3.如果你把这个方法加在ListView上的话，返回的是它的item

另外，我们还发现了另外一个跟focus相关的方法，如下

  /**   
     * Find the nearest view in the specified direction that can take focus.
     * This does not actually give focus to that view.
     *
     * @param direction One of FOCUS_UP, FOCUS_DOWN, FOCUS_LEFT, and FOCUS_RIGHT
     *
     * @return The nearest focusable in the specified direction, or null if none
     *         can be found.
     */
    public View focusSearch(@FocusRealDirection int direction) {
        if (mParent != null) {
            return mParent.focusSearch(this, direction);
        } else {
            return null; 
        }     
    }

这个方法，其实是代码获得某个控件获得焦点以后，下一个焦点移动到的控件。如果我们没有对这个控件的焦点进行操作，它就遵循android本身的焦点顺序。如果我们进行了操作，例如

<Button
            android:id="@+id/bt_3"
            android:layout_width="60dp"
            android:layout_height="60dp"
            android:layout_marginLeft="5dp"
            android:background="@drawable/select"
            android:focusableInTouchMode="true"
            android:nextFocusRight="@+id/tv"
            android:textColor="#ffffff" />

增加了nextfocus 属性，我们得到的其实就是这个里面焦点跳转view