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