带进度条的APP启动页面

今天想给APP做一个带进度条的启动页面，照着别人的例子总出错。也没看懂，反正我也不知道怎么蒙对的。

我是小白，没搞清楚什么不能在主线程中更新UI等等一大堆原理，总之这样写没报错，记录一下：

new Handler(new Handler.Callback()

会自动生成方法。

activity_start.xml:只是在背景上加了一个进度条:

<?xml version="1.0" encoding="utf-8"?>
<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"
    android:background="@drawable/start"
    tools:context=".StartActivity">
    
    <ProgressBar
        android:id="@+id/progressBar"
        style="@android:style/Widget.ProgressBar.Horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="50dp"
        android:layout_marginLeft="@dimen/l_padding"
        android:layout_marginRight="@dimen/l_padding"
        android:max="100"
        android:progress="50"
        app:layout_constraintEnd_toStartOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />
        
</androidx.constraintlayout.widget.ConstraintLayout>

StartActivity.java:只需要改背景图片、包名、跳转页

package com.example.career;

import androidx.annotation.NonNull;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.Toast;

public class StartActivity extends Activity {

    private ProgressBar progressBar;
    private int mProgress = 0;
    private Handler mHandler;

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

        progressBar = findViewById(R.id.progressBar);

        mHandler = new Handler(new Handler.Callback() {

            @Override
            public boolean handleMessage(@NonNull Message message) {

                if (message.what == 0x111) {

                    progressBar.setProgress(mProgress);

                } else {

         39                     progressBar.setVisibility(View.GONE);
                    Intent intent = new Intent(StartActivity.this, MainActivity.class);  //跳转页
                    startActivity(intent);
                }

                return false;
            }

        });

        //启动线程来执行任务

        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    mProgress = doWork();
                    Message m = new Message();
                    if (mProgress < 100) {
                        m.what = 0x111;
                        mHandler.sendMessage(m);
                    } else {
                        m.what = 0x110;
                        mHandler.sendMessage(m);
                        break;
                    }
                }
            }

            private int doWork() {

                mProgress += Math.random() * 10;

                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                return mProgress;

            }

        }).start();

    }

}