图像视图-ImageView

（一）

知识点：

（1）imageView.setImageAlpha(Alpha):设置图片透明度

（2）在布局imageView中设置图片位置：android:scaleType="center"

android:scaleType="center"

1.效果图：点击增加图片透明度增加，点击减少图片透明度减少，点击下一张显示下一张图片

2.activity_main.xml

 <?xml version="1.0" encoding="utf-8"?>
 <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: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.administrator.hello3.MainActivity"
     android:orientation="vertical"
     android:layout_gravity="center"
     >

     <LinearLayout
         android:orientation="horizontal"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_gravity="center">
         <Button
             android:id="@+id/btn_add"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:text="增加" />
         <Button
             android:id="@+id/btn_reduce"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:text="减少" />
         <Button
             android:id="@+id/btn_next"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:text="下一张" />
     </LinearLayout>
     <ImageView
         android:layout_gravity="center"
         android:id="@+id/iv"
         android:src="@mipmap/ic_launcher"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content" />
 </LinearLayout>

3.MainActivity.java

 package com.example.administrator.hello3;

 import android.support.v7.app.AppCompatActivity;
 import android.os.Bundle;
 import android.view.View;
 import android.widget.Button;
 import android.widget.ImageView;

 public class MainActivity extends AppCompatActivity {
     private Button button_add,button_reduce,button_next;
     private ImageView imageView;
     private  int alpha=255;
     private int current=0;
     private int[] pics = {R.drawable.pic1,R.drawable.pic2,R.drawable.pic3,R.drawable.pic4};

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);

         setContentView(R.layout.activity_main);

         button_add=(Button)findViewById(R.id.btn_add);
         button_reduce=(Button)findViewById(R.id.btn_reduce);
         button_next=(Button)findViewById(R.id.btn_next);
         imageView=(ImageView) findViewById(R.id.iv);

         MyListener myListener = new MyListener();
         button_add.setOnClickListener(myListener);
         button_reduce.setOnClickListener(myListener);

         button_next.setOnClickListener(new View.OnClickListener() {
             @Override
             public void onClick(View v) {
                 imageView.setImageResource(pics[current++%pics.length]);
             }
         });

     }
     class  MyListener implements View.OnClickListener{
         @Override
         public void onClick(View v) {
             if(v==button_add){
                 alpha+=20;
                 if(alpha>=255){
                     alpha=255;
                 }
             }else if(v==button_reduce) {
                 alpha -= 20;
                 if (alpha <= 0) {
                     alpha = 0;
                 }
             }
             imageView.setImageAlpha(alpha);
         }
     }

 }

3.drawable下面加入自己的图片