我刚刚开始学习安卓。以下代码显示了我的第一次尝试。
package com.example.silentphone;
import android.app.Activity;
import android.graphics.drawable.Drawable;
import android.media.AudioManager;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
public class MainActivity extends Activity {
private AudioManager audioManager;
private boolean mPhoneIsSilent;
/*****我想在这里声明它,但如果我这样做,应用程序就会崩溃
ImageView imageView = (ImageView) findViewById(R.id.ringer_icon);
Drawable newPhoneIcon;
**********
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
audioManager = (AudioManager) getSystemService(AUDIO_SERVICE);
checkPhoneStatus();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
public void onClick(View view) {
Button toggleButton = (Button) findViewById(R.id.toggleBtn);
if (mPhoneIsSilent) {
System.out.println("Phone Silent");
// now put the phone in ringer mode
audioManager.setRingerMode(AudioManager.RINGER_MODE_NORMAL);
mPhoneIsSilent = false;
toggleGUI();
} else {
System.out.println("Phone active");
// now put the phone in silent mode
audioManager.setRingerMode(AudioManager.RINGER_MODE_SILENT);
mPhoneIsSilent = true;
toggleGUI();
}
}
public void checkPhoneStatus() {
int ringerMode = audioManager.getRingerMode();
if (ringerMode == AudioManager.RINGER_MODE_SILENT) {
// means the phone is silent
mPhoneIsSilent = true;
} else {
mPhoneIsSilent = false;
}
}
public void toggleGUI() {
/******These two variables need to be declared here.Why cant i declare it at class level.If i declare it at class level the app crashes.s****************************/
ImageView imageView = (ImageView) findViewById(R.id.ringer_icon);
Drawable newPhoneIcon;
/******************************************************************************/
if (mPhoneIsSilent) {
// means the phone is silent so show phone silent image
newPhoneIcon = getResources().getDrawable(R.drawable.speaker_off);
imageView.setImageDrawable(newPhoneIcon);
} else {
newPhoneIcon = getResources().getDrawable(R.drawable.phone_on);
imageView.setImageDrawable(newPhoneIcon);
}
}
}
我的问题是为什么我需要声明
ImageView imageView = (ImageView) findViewById(R.id.ringer_icon);
Drawable newPhoneIcon;
在函数内。为什么我不能在类级别声明它。如果我在类级别声明它,应用程序崩溃。每次调用函数 toggleGui 时,都会创建这两个类的新对象,我想这将是无效的,但我可能是错的。有人可以给我一个很好的解释。谢谢。
活动中的声明不是问题。
应在设置内容视图后调用ImageView
的findViewById
。(活动的布局)。
否则,以下方法应有效:
public class MainActivity extends Activity {
private AudioManager audioManager;
private boolean mPhoneIsSilent;
//declaring these here is not a problem
ImageView imageView;
Drawable newPhoneIcon;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//when you map the imageView, it should be after the setContentView so your activity identifies exactly what to map - for id and component -in your layout
imageView = (ImageView) findViewById(R.id.ringer_icon);
audioManager = (AudioManager) getSystemService(AUDIO_SERVICE);
checkPhoneStatus();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
public void onClick(View view) {
Button toggleButton = (Button) findViewById(R.id.toggleBtn);
if (mPhoneIsSilent) {
System.out.println("Phone Silent");
// now put the phone in ringer mode
audioManager.setRingerMode(AudioManager.RINGER_MODE_NORMAL);
mPhoneIsSilent = false;
toggleGUI();
} else {
System.out.println("Phone active");
// now put the phone in silent mode
audioManager.setRingerMode(AudioManager.RINGER_MODE_SILENT);
mPhoneIsSilent = true;
toggleGUI();
}
}
public void checkPhoneStatus() {
int ringerMode = audioManager.getRingerMode();
if (ringerMode == AudioManager.RINGER_MODE_SILENT) {
// means the phone is silent
mPhoneIsSilent = true;
} else {
mPhoneIsSilent = false;
}
}
public void toggleGUI() {
if (mPhoneIsSilent) {
// means the phone is silent so show phone silent image
newPhoneIcon = getResources().getDrawable(R.drawable.speaker_off);
imageView.setImageDrawable(newPhoneIcon);
} else {
newPhoneIcon = getResources().getDrawable(R.drawable.phone_on);
imageView.setImageDrawable(newPhoneIcon);
}
}
}
http://developer.android.com/reference/android/app/Activity.html#setContentView(整型)
对于 findViewById:
http://developer.android.com/reference/android/app/Activity.html#findViewById(int)
通常你应该创建任何可变变量,而不管它们的使用范围 - 它是OOP封装/松散耦合/高内聚概念,我建议遵循它的原因有很多)
我相信你可以在类级别定义它们,只要确保它们在访问它们之前正确初始化 - 应用程序可能会因为您尝试在定义 imageView 之前调用 toggleGUI。
问候
亚历克斯。
按如下方式声明你的变量类级别
ImageView imageView;
并在 setcontentview(R.layout.your_layout) 之后在 oncreate 方法中初始化它;
imageView = (ImageView) findViewById(R.id.ringer_icon);
不要在 toggleGUI() 方法上初始化它,因为它永远不会在 onCreate() 中调用。
初始化 oncreate() 方法中的所有 UI 元素。