Learning record:
We want the effect , pop up a small message box , automatically disappear after a few seconds
Create a new infrastructure projects:
In MainActivity add the following content:
package com.example.reactest;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
// downward compatible Activity wherein AppCompatActivity is Activity subclass Activity is Android a system provides a base class event
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate (savedInstanceState);
/*
* Load a layout, the layout of the incoming ID R.layout.activity_main
*
* */
setContentView(R.layout.activity_main);
/*
* By button trigger Toast by onCreate add code method
* First through the findViewById () obtained buttonPanel this example operation of element : findViewById (R.id.buttonPanel);
* Since this method returns the view objects , need to be downcast Button objects
* Downcast : downcast: subclass references to parent class object Father f1 = new Son (); // this is called upcasting (upcast ) , now f1 references to a Son Object
* Upcast: references to the parent class subclass object Son s1 = (Son) f1; // this is called downcasting ( downward transition ) , now f1 is pointing Son objects
* */
Button button_Panel = (Button)findViewById(R.id.buttonPanel);
/*
* With setOnClickListener () to register an instance of this listener OnClickListener () but using statement View.OnClickListener ()
* Why you want to add a front View of it, the reason is behind OnClickListener a View inside the class interface, if the direct use of this interface is not found.
* */
button_Panel.setOnClickListener(new View.OnClickListener() {
/*
* Monitor is created , click the button will execute if listening to the onClick () method , so
* Toast function to the onClick () write .
* */
@Override
public void onClick(View v) {
/*
* We want the effect , pop up a small message box , automatically disappear after a few seconds
* Idea : a text box , a timer , a layout position . A display method
* Static method makeText () to create a Toast objects , call the show () The Toast displayed .
* 说明: makeText(参数一[Toast要求的上下文 ], 参数二[Toast显示的文本内容], 参数三[Toast显示的时长])
* 参数一: Context,由于i活动本身就是一个Context对象,因此直接传入 MainActivity.this
* 参数二: 内容
* 参数三: 有两个内置常量 Toast.LENGTH_SHORT Toast.LENGTH_LONG
*
* 引用结束之后记得用 show()方法显示
* */
Toast.makeText(MainActivity.this,"你好啊",Toast.LENGTH_SHORT).show();
}
});
}
}
在布局视图下添加：
<?xml version="1.0" encoding="utf-8"?>
function(){ //交易杠杆 http://www.fx61.com/definitions/muniu/481.html
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<!-- 只要创建任何资源都会在R文件中生成一个资源id
在引用布局的时候会调用R.layout.(Activity)可以得到XXXX.xml布局中的id,然后将这个值传到setContentVies()方法即可-->
<!-- wrap_content 当前宽度只要刚好包含里边的内容就行-->
<!-- match_parent 当前元素和父元素一样宽 相配，相称 -->
<Button
android:id="@+id/buttonPanel"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="1"
tools:ignore="MissingConstraints" />
<!-- 通过button触发Toast 通过onCreate方法中添加代码-->
</androidx.constraintlayout.widget.ConstraintLayout>