理解Activityの不得不說的Context

在開發Android的過程中，總是能遇見Context類或者它的實例。Context類的實例經常被用來提供“應用程序”的引用。例如下面的代碼片段，Toast類的第一個參數接受一個Context對象：

[java]
@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類並沒有直接用在Activity中，它被用在了AlertDialog類中。所以，這裡需要通過getBaseContext()方法獲取一個Context類的實例。

    在Activity中動態地創建一個視圖的時候也會遇見Context。例如，如果想通過硬編碼動態地創建一個TextView：

[java]
TextView tv = new TextView(this); 
TextView tv = new TextView(this); TextView的構造器接受一個Context對象，因為Activity類是Context類的子類，所以可以用this關鍵字來代替這個Conext對象。

提示：

    使用this動態地創建視圖，例如TextView、Button，存在一個潛在的風險——內存洩漏。所以，盡可能地使用getApplicationContext()方法替代this。

引用:http://www.2cto.com/kf/201203/124600.html