getBackground().setAlpha和setAlpha有什么区别

代码

MainActivity

package com.example.test;import android.os.Bundle;import android.app.Activity;import android.util.Log;import android.view.Menu;import android.widget.TextView;public class MainActivity extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        TextView textView1=(TextView)findViewById(R.id.tv1);        textView1.setBackgroundColor(0xFA8072);        textView1.setAlpha(0.2f);        TextView textView2=(TextView)findViewById(R.id.tv2);        textView2.setBackgroundColor(0xFA8072);        float alf=textView2.getAlpha();        int balf=textView2.getBackground().getAlpha();        Log.e("linux","-getAlpha-"+alf+" getBackground.getAlpha "+balf);        TextView textView3=(TextView)findViewById(R.id.tv3);                textView3.setBackgroundColor(0xFA8072);        textView3.getBackground().setAlpha(100);        TextView textView4=(TextView)findViewById(R.id.tv4);                TextView textView5=(TextView)findViewById(R.id.tv5);        TextView textView6=(TextView)findViewById(R.id.tv6);        TextView textView7=(TextView)findViewById(R.id.tv7);        Log.e("linux","---默认-- ");        Log.e("linux","-textView3- "+textView3.getBackground().getAlpha());        Log.e("linux","-textView4- "+textView4.getBackground().getAlpha());        Log.e("linux","-textView5- "+textView5.getBackground().getAlpha());        Log.e("linux","-textView6- "+textView6.getBackground().getAlpha());        Log.e("linux","-textView7- "+textView7.getBackground().getAlpha());        textView4.getBackground().setAlpha(10);        Log.e("linux","--setAlpha(10)-- ");        Log.e("linux","-textView3- "+textView3.getBackground().getAlpha());        Log.e("linux","-textView4- "+textView4.getBackground().getAlpha());        Log.e("linux","-textView5- "+textView5.getBackground().getAlpha());        Log.e("linux","-textView6- "+textView6.getBackground().getAlpha());        Log.e("linux","-textView7- "+textView7.getBackground().getAlpha());        textView5.getBackground().setAlpha(255);        Log.e("linux","---setAlpha(255)-- ");        Log.e("linux","-textView3- "+textView3.getBackground().getAlpha());        Log.e("linux","-textView4- "+textView4.getBackground().getAlpha());        Log.e("linux","-textView5- "+textView5.getBackground().getAlpha());        Log.e("linux","-textView6- "+textView6.getBackground().getAlpha());        Log.e("linux","-textView7- "+textView7.getBackground().getAlpha());        textView6.getBackground().mutate().setAlpha(88);        Log.e("linux","---mutate().setAlpha(88)-- ");        Log.e("linux","-textView3- "+textView3.getBackground().getAlpha());        Log.e("linux","-textView4- "+textView4.getBackground().getAlpha());        Log.e("linux","-textView5- "+textView5.getBackground().getAlpha());        Log.e("linux","-textView6- "+textView6.getBackground().getAlpha());        Log.e("linux","-textView7- "+textView7.getBackground().getAlpha());    }}

activity_main文件

<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"    tools:context=".MainActivity" >    <TextView        android:id="@+id/tv1"        android:layout_width="match_parent"        android:layout_height="30dp"        android:text="@string/hello_world"         android:textSize="15sp"/>    <TextView        android:id="@+id/tv2"        android:layout_width="match_parent"        android:layout_height="30dp"        android:text="@string/hello_world"         android:textSize="15sp" />    <TextView        android:id="@+id/tv3"        android:layout_width="match_parent"        android:layout_height="30dp"        android:text="@string/hello_world"         android:textSize="15sp"        />    <TextView        android:id="@+id/tv4"        android:layout_width="match_parent"        android:layout_height="30dp"        android:text="@string/hello_world"         android:background="#00FFFF"        android:textSize="15sp"        />    <TextView        android:id="@+id/tv5"        android:layout_width="match_parent"        android:layout_height="30dp"        android:text="@string/hello_world"         android:background="#00FFFF"        android:textSize="15sp"        />    <TextView        android:id="@+id/tv6"        android:layout_width="match_parent"        android:layout_height="30dp"        android:text="@string/hello_world"         android:background="#00FF00"        android:textSize="15sp"        />    <TextView        android:id="@+id/tv7"        android:layout_width="match_parent"        android:layout_height="30dp"        android:text="@string/hello_world"         android:background="#00FF00"        android:textSize="15sp"        /></LinearLayout>

效果图

日志

09-07 13:45:36.027: E/linux(14636): -getAlpha-1.0 getBackground.getAlpha 009-07 13:45:36.027: E/linux(14636): ---默认-- 09-07 13:45:36.027: E/linux(14636): -textView3- 009-07 13:45:36.027: E/linux(14636): -textView4- 25509-07 13:45:36.027: E/linux(14636): -textView5- 25509-07 13:45:36.027: E/linux(14636): -textView6- 25509-07 13:45:36.027: E/linux(14636): -textView7- 25509-07 13:45:36.027: E/linux(14636): --setAlpha(10)-- 09-07 13:45:36.027: E/linux(14636): -textView3- 009-07 13:45:36.027: E/linux(14636): -textView4- 909-07 13:45:36.027: E/linux(14636): -textView5- 909-07 13:45:36.027: E/linux(14636): -textView6- 25509-07 13:45:36.037: E/linux(14636): -textView7- 25509-07 13:45:36.037: E/linux(14636): ---setAlpha(255)-- 09-07 13:45:36.037: E/linux(14636): -textView3- 009-07 13:45:36.037: E/linux(14636): -textView4- 25509-07 13:45:36.037: E/linux(14636): -textView5- 25509-07 13:45:36.037: E/linux(14636): -textView6- 25509-07 13:45:36.037: E/linux(14636): -textView7- 25509-07 13:45:36.037: E/linux(14636): ---mutate().setAlpha(88)-- 09-07 13:45:36.037: E/linux(14636): -textView3- 009-07 13:45:36.037: E/linux(14636): -textView4- 25509-07 13:45:36.037: E/linux(14636): -textView5- 25509-07 13:45:36.037: E/linux(14636): -textView6- 8709-07 13:45:36.037: E/linux(14636): -textView7- 255

分析

1、直接setAlpha是改变整个view的透明度
getBackground().setAlpha只是改变view的背景的透明度
2、有背景才能够设置透明度。
必须先设置背景，之后才能设置背景的透明度，不能直接getBackground().setAlpha。

        TextView textView3=(TextView)findViewById(R.id.tv3);        textView3.getBackground().setAlpha(100);        #textView3.setBackgroundColor(0xFA8072);        /*如果代码这么写,会直接报错        Caused by: java.lang.NullPointerException:         'void android.graphics.drawable.Drawable.setAlpha(int)'         on a null object reference        */

3、textView3是不变的;
看到textView4和textView5是一起改变的;
而textView6和textView7又能够不一起改变

更多

1、getBackground().setAlpha导致其他布局背景透明度都改变
http://www.knowsky.com/1049068.html
2、android代码设置背景setBackgroundColor无效
http://blog.csdn.net/u013351986/article/details/44497491

'为什么textView3设置了setAlpha依然没有背景色?'1、效果我们自己定义一个color.xml文件，然后再xml中引用此文件中的颜色，一切顺利。但当我们在代码中如view.setBackgroundColor(R.color.red)来引用我们自定义的xml文件时，是无效的。2、原因颜色4个字节从高到低对应 不透明度 红 绿 蓝 四个分量，0xFFFFFF对应不透明度的分量是0，所以后面不管是什么背景都是透明的。0xFFFFFFFF就是白色了，0x80FFFFFF就是半透明的白色。Color.rgb()会自动把不透明读设成255，而解析字符串颜色"#FFFFFF"也能判断出只给出了三个分量会自动把不透明度设成255。而对于整数0xFFFFFF和0x00FFFFFF是一样的，是完全透明的。3、解决方案：view.setBackgroundColor(Color.rgb(255, 0, 0));

源码

http://download.csdn.net/download/codehxy/9968275