安卓Android Context类实例详解

1.例如下面的代码片段，toast类的第一个参数接受一个context对象：

@override 
 protected dialog oncreatedialog(int id) { 
 switch (id) { 
 case 0: 
 
 builder builder = new alertdialog.builder(this); 
 builder.seticon(r.drawable.ic_launcher); 
 builder.settitle("this is a dialog with some simple text..."); 
 builder.setpositivebutton("ok", 
  new dialoginterface.onclicklistener() { 
  public void onclick(dialoginterface dialog, 
  int whichbutton) { 
  toast.maketext(getbasecontext(), "ok clicked!", 
   toast.length_short).show(); 
  } 
  }); 
 
 builder.setnegativebutton("cancel", 
  new dialoginterface.onclicklistener() { 
  public void onclick(dialoginterface dialog, 
  int whichbutton) { 
  toast.maketext(getbasecontext(), "cancel clicked!", 
   toast.length_short).show(); 
  } 
  }); 
 
 builder.setmultichoiceitems(items, itemschecked, 
  new dialoginterface.onmultichoiceclicklistener() { 
  public void onclick(dialoginterface dialog, int which, 
  boolean ischecked) { 
  toast.maketext( 
   getbasecontext(), 
   items[which] 
   + (ischecked ? " checked!" 
    : " unchecked!"), 
   toast.length_short).show(); 
  } 
  }); 
 return builder.create(); 
 
 } 
 return null; 
 } 
@override
 protected dialog oncreatedialog(int id) {
 switch (id) {
 case 0:

 builder builder = new alertdialog.builder(this);
 builder.seticon(r.drawable.ic_launcher);
 builder.settitle("this is a dialog with some simple text...");
 builder.setpositivebutton("ok",
  new dialoginterface.onclicklistener() {
  public void onclick(dialoginterface dialog,
  int whichbutton) {
  toast.maketext(getbasecontext(), "ok clicked!",
   toast.length_short).show();
  }
  });

 builder.setnegativebutton("cancel",
  new dialoginterface.onclicklistener() {
  public void onclick(dialoginterface dialog,
  int whichbutton) {
  toast.maketext(getbasecontext(), "cancel clicked!",
   toast.length_short).show();
  }
  });

 builder.setmultichoiceitems(items, itemschecked,
  new dialoginterface.onmultichoiceclicklistener() {
  public void onclick(dialoginterface dialog, int which,
  boolean ischecked) {
  toast.maketext(
   getbasecontext(),
   items[which]
   + (ischecked ? " checked!"
    : " unchecked!"),
   toast.length_short).show();
  }
  });
 return builder.create();

 }
 return null;
 }

然而，toast类并没有直接用在activity中，它被用在了alertdialog类中。所以，这里需要通过getbasecontext()方法获取一个context类的实例。

2.在activity中动态地创建一个视图的时候也会遇见context。

例如，如果想通过硬编码动态地创建一个textview：

textview tv = new textview(this); 
textview tv = new textview(this); 

textview的构造器接受一个context对象，因为activity类是context类的子类，所以可以用this关键字来代替这个conext对象。

提示：

    使用this动态地创建视图，例如textview、button，存在一个潜在的风险——内存泄漏。所以，尽可能地使用getapplicationcontext()方法替代this。