背景

开发一款简单的闪光灯应用。

1. 可以通过某个键去控制闪光灯开关（公司是搞Android 设备的，系统定制，因此有权限监听键值）
2. 在闪灯灯页面有个图标用来控制闪光灯开关

闪光灯应用只有一个界面，界面中会显示当前是开或者关。

但是我发现有时候界面中闪光灯状态始终是灰色。

代码

写了一个用于控制闪光灯开启的单例类,FlashLightUtil.

package com.seuic.flashlight.util;

import android.content.Context;
import android.hardware.camera2.CameraCharacteristics;
import android.hardware.camera2.CameraManager;
import android.util.Log;

import com.seuic.flashlight.MyApplication;

import java.lang.ref.WeakReference;


public class FlashLightUtil {
    private volatile static FlashLightUtil instance;
    private boolean isFlashLightOn = false;

    private Context mContext;
    private WeakReference<FlashLightStatusListener> flashLightStatusListener = null;

    private FlashLightUtil(Context context) {
        mContext = context;
    }

    public static FlashLightUtil getInstance() {
        if (instance == null) {
            synchronized (FlashLightUtil.class) {
                if (null == instance) {
                    instance = new FlashLightUtil(MyApplication.mApplication);
                }
            }
        }
        return instance;
    }

    private static final String TAG = "FlashLightUtil";

    public boolean isFlashLightOn() {
        return isFlashLightOn;
    }

    public synchronized void toggleFlashLight() {
        isFlashLightOn = !isFlashLightOn;
        changeFlashLight(isFlashLightOn);
    }

    public void changeFlashLight(boolean open) {
        try {
            //获取CameraManager
            CameraManager mCameraManager = (CameraManager) mContext.getSystemService(Context.CAMERA_SERVICE);
            //获取当前手机所有摄像头设备ID
            String[] ids = mCameraManager.getCameraIdList();
            for (String id : ids) {
                CameraCharacteristics c = mCameraManager.getCameraCharacteristics(id);
                //查询该摄像头组件是否包含闪光灯
                Boolean flashAvailable = c.get(CameraCharacteristics.FLASH_INFO_AVAILABLE);
                Integer lensFacing = c.get(CameraCharacteristics.LENS_FACING);
                if (flashAvailable != null && flashAvailable
                        && lensFacing != null && lensFacing == CameraCharacteristics.LENS_FACING_BACK) {
                    notifyChange(open);
                    //打开或关闭手电筒
                    mCameraManager.setTorchMode(id, open);
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public boolean supportFlashLight() {
        boolean isSupport = false;
        try {
            //获取CameraManager
            CameraManager mCameraManager = (CameraManager) mContext.getSystemService(Context.CAMERA_SERVICE);
            //获取当前手机所有摄像头设备ID
            String[] ids = mCameraManager.getCameraIdList();
            for (String id : ids) {
                CameraCharacteristics c = mCameraManager.getCameraCharacteristics(id);
                //查询该摄像头组件是否包含闪光灯
                Boolean flashAvailable = c.get(CameraCharacteristics.FLASH_INFO_AVAILABLE);
                Integer lensFacing = c.get(CameraCharacteristics.LENS_FACING);
                if (flashAvailable != null && flashAvailable
                        && lensFacing != null && lensFacing == CameraCharacteristics.LENS_FACING_BACK) {
                    //打开或关闭手电筒
                    isSupport = true;
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return isSupport;
    }

    public void setFlashLightStatusListener(FlashLightStatusListener flashLightStatusListener) {
        this.flashLightStatusListener = new WeakReference<>(flashLightStatusListener);
    }

    private void notifyChange(boolean open) {
        if (this.flashLightStatusListener != null) {
            FlashLightStatusListener f = flashLightStatusListener.get();

        }
    }

    public interface FlashLightStatusListener {
        void onStatusChange(boolean isOpen);
    }
}

复制代码

然后一个MainActivity.kt代码如下:

package com.seuic.flashlight

import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.widget.ImageButton
import com.seuic.flashlight.service.PptService
import com.seuic.flashlight.util.FlashLightUtil

class MainActivity : AppCompatActivity() {
    companion object{
        private const val TAG = "MainActivity"
    }
    private val flashLightUtil by lazy { FlashLightUtil.getInstance() }
    private val ivControl by lazy { findViewById<ImageButton>(R.id.iv_control) }
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        ivControl.setOnClickListener {
            flashLightUtil.toggleFlashLight()
        }
        flashLightUtil.setFlashLightStatusListener { isOpen ->
            if (isOpen) {
                ivControl.setBackgroundResource(R.drawable.ic_flash_light_open)
            } else {
                ivControl.setBackgroundResource(R.drawable.ic_flash_light_colse)
            }
        }
        startService(Intent(this.application, PptService::class.java))
    }

    override fun onResume() {
        super.onResume()
        if (flashLightUtil.isFlashLightOn) {
            ivControl.setBackgroundResource(R.drawable.ic_flash_light_open)
        } else {
            ivControl.setBackgroundResource(R.drawable.ic_flash_light_colse)
        }
    }
}
复制代码

我发现代码始终没有走到 ivControl.setBackgroundResource 这个地方。

原因

代码没走到这个地方的原因是因为setFlashLightStatusListener 传入的对象是没有强引用的，因此flashLightStatusListener.get();获取的是一个null对象。一开始之所以使用WeakReference来保存flashLightStatusListener对象是防止强引用了MainActivity的对象，影响MainActivity的回收（MainActivity退出后仍然有一个服务活在后台，持有FlashLightUtil的对象），但是实际上我传入的是一个局部对象而不是MainActivity的属性，此局部的FlashLightStatusListener对象由于没有强引用，因此就被回收了。